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Influence of ligand binding on structure and thermostability of human α 1 -acid glycoprotein
Abstract
Ligand binding of neutral progesterone, basic propranolol, and acidic warfarin to human α 1-acid glycoprotein (AGP) was investigated by Raman spectroscopy. The binding itself is characterized by a uniform conformational shift in which a tryptophan residue is involved. Slight differences corresponding to different contacts of the individual li-gands inside the β-barrel are described. Results are compared with in silico ligand docking into the available crystal structure of deglycosylated AGP using quantum/molecular mechanics. Calculated binding energies are À18.2, À14.5, and À11.5 kcal/mol for warfarin, propranolol, and progesterone, respectively. These calculations are consistent with Raman difference spectroscopy; nevertheless, minor discrepancies in the precise positions of the ligands point to structural differences between deglycosylated and native AGP. Thermal dynamics of AGP with/without bounded warfarin was followed by Raman spectroscopy in a temperature range of 10–95 °C and analyzed by principal component analysis. With increasing temperature, a slight decrease of α-helical content is observed that coincides with an increase in β-sheet content. Above 45 °C, also β-strands tend to unfold, and the observed decrease in β-sheet coincides with an increase of β-turns accompanied by a conformational shift of the nearby disulfide bridge from high-energy trans-gauche-trans to more relaxed gauche-gauche-trans. This major rearrangement in the vicinity of the bridge is not only characterized by unfolding of the β-sheet but also by subsequent ligand release. Hereby, ligand binding alters the protein dynamics, and the more rigid protein–ligand complex shows an improved thermal stability, a finding that contributes to the reported chaperone-like function of AGP.
... The spectra are dominated by the signal from the protein backbone, namely, by the amide I, the broad band at 1655 cm −1 , reflecting the protein secondary structure content (with vibrations of α-helices at lower wavenumbers and β-sheets/β-turns at higher wavenumbers) and the amide III region ~1225-1310 cm −1 (where lower wavenumbers correspond to the signal of β-sheets whereas α-helices lies at higher wavenumbers). Another strong broad band at 1446 cm −1 mostly corresponds to bending vibrations of the CH 2 and CH 3 groups [45]. Other narrow bands mostly reflect vibrations of amino acids with aromatic side chains, such as phenylalanine twisting, ring stretching, and deformations (620, 1003, 1033 cm −1 , respectively) and tyrosine twisting, Fermi resonance doublet, bending, and deformations (645, 825 and 852, 1170, 1610 cm −1 , respectively), and tryptophane (750, 1334, and 1581 cm −1 ) with less-specific vibrational modes of aromatic amino acids (1209, 1581, 1600-1620, 3063 cm −1 ) [45][46][47]. ...
... Another strong broad band at 1446 cm −1 mostly corresponds to bending vibrations of the CH 2 and CH 3 groups [45]. Other narrow bands mostly reflect vibrations of amino acids with aromatic side chains, such as phenylalanine twisting, ring stretching, and deformations (620, 1003, 1033 cm −1 , respectively) and tyrosine twisting, Fermi resonance doublet, bending, and deformations (645, 825 and 852, 1170, 1610 cm −1 , respectively), and tryptophane (750, 1334, and 1581 cm −1 ) with less-specific vibrational modes of aromatic amino acids (1209, 1581, 1600-1620, 3063 cm −1 ) [45][46][47]. If we compared the Raman signal of cytoplasm between the axenically grown and brain-isolated cells, the latter showed lower content of aromatic amino acids (1003, 1335/1360 cm −1 ) for data normalized for amide I band. ...
... If we compared the Raman signal of cytoplasm between the axenically grown and brain-isolated cells, the latter showed lower content of aromatic amino acids (1003, 1335/1360 cm −1 ) for data normalized for amide I band. Additionally, Raman spectra of cytoplasm contained signal of nucleotides and nucleic acids, mostly RNA, and potentially DNA (Fig 6K yellow bands) [45,47]. These complex spectral fingerprints tend to overlap, e.g., the positions assigned to the phosphate and NH 2 groups have similar positions with nucleotides, nucleic acids, some amino acids and also amide III [48][49][50]. ...
Naegleria fowleri, the causative agent of primary amoebic meningoencephalitis (PAM), requires increased research attention due to its high lethality and the potential for increased incidence as a result of global warming. The aim of this study was to investigate the interactions between N. fowleri and host cells in order to elucidate the mechanisms underlying the pathogenicity of this amoeba. A co-culture system comprising human fibrosarcoma cells was established to study both contact-dependent and contact-independent cytopathogenicity. Proteomic analyses of the amoebas exposed to human cell cultures or passaged through mouse brain were used to identify novel virulence factors. Our results indicate that actin dynamics, regulated by Arp2/3 and Src kinase, play a considerable role in ingestion of host cells by amoebae. We have identified three promising candidate virulence factors, namely lysozyme, cystatin and hemerythrin, which may be critical in facilitating N. fowleri evasion of host defenses, migration to the brain and induction of a lethal infection. Long-term co-culture secretome analysis revealed an increase in protease secretion, which enhances N. fowleri cytopathogenicity. Raman microspectroscopy revealed significant metabolic differences between axenic and brain-isolated amoebae, particularly in lipid storage and utilization. Taken together, our findings provide important new insights into the pathogenic mechanisms of N. fowleri and highlight potential targets for therapeutic intervention against PAM.
... [11][12][13]. The molecule comprises 183 amino acids with two disulfide bridges, and about 60% of its secondary structure is believed to consist of βsheets, but it is still not entirely confirmed [14][15][16]. ...
... !! ! + 1.621 (16) (logk 0,RP , logk 0,HSA -intercept values, obtained from correlation between logk and concentration of 2-propranolol in the mobile phase). ...
In recent decades, drug-protein interactions have been widely studied and several methods of analysis of these phenomena have been developed and improved, These can be classified into separation, physical, chromatographic and electrophoretic methods. This review depicts the assumptions and mechanisms of methods from each group, details their strengths and weaknesses, and presents examples of their usage from the literature. Equilibrium dialysis, ultrafiltration, Hummel-Dreyer method or high performance affinity chromatography are given as representative examples, but this issue is far more expanded. Nowadays, increasing attention is paid to the computational methods and molecular modeling which are convenient tools to estimate protein binding affinity based on the physicochemical properties of compounds. To gain a broader overview, the study also examines the protein binding ability and pharmacotherapy of drugs against a range of substrates such as plasma, skin, tissue and human milk.
... The secondary structure of AGP in the N state has been investigated using circular dichroism (CD), Raman, and Fourier-transform infrared spectroscopy to reveal the binding sites of drugs and the structure-function relationships of AGP. 9,[17][18][19][20][21][22] CD spectroscopy has been used to analyze the structure of AGP in lipid membranes because it is very sensitive to local peptide structures and applicable to any size of protein at a low concentration under various experimental conditions, such as in the presence of a membrane. 23 Nishi et al measured the CD spectrum of AGP from 250 to 200 nm in reverse micelles and phosphatidylglycerol (PG) liposome in a mildly acidic condition (pH 4.5) and confirmed that the β-barrel structure of AGP transformed to the α-helix-rich conformation. ...
α1‐Acid glycoprotein (AGP) interacts with lipid membranes as a peripheral membrane protein so as to decrease the drug‐binding capacity accompanying the β→α conformational change that is considered a protein‐mediated uptake mechanism for releasing drugs into membranes or cells. This study characterized the mechanism of interaction between AGP and lipid membranes by measuring the vacuum‐ultraviolet circular‐dichroism (VUVCD) spectra of AGP down to 170 nm using synchrotron radiation in the presence of five types of liposomes whose constituent phospholipid molecules have different molecular characteristics in the head groups (e.g., different net charges). The VUVCD analysis showed that the α‐helix and β‐strand contents and the numbers of segments of AGP varied with the constituent phospholipid molecules of liposomes, while combining VUVCD data with a neural‐network method predicted that these membrane‐bound conformations comprised several common long helix and small strand segments. The amino‐acid composition of each helical segment of the conformations indicated that amphiphilic and positively charged helices formed at the N‐ and C‐terminal regions of AGP, respectively, were candidate sites for the membrane interaction. The addition of 1 M sodium chloride shortened the C‐terminal helix while having no effect on the length of the N‐terminal one. These results suggest that the N‐ and C‐terminal helices can interact with the membrane via hydrophobic and electrostatic interactions, respectively, demonstrating that the liposome‐dependent conformations of AGP analyzed using VUVCD spectroscopy provide useful information for characterizing the mechanism of interaction between AGP and lipid membranes.
... It is apparent that the binding constant for AGP-CuL 2 and AGP-ZnL 2 interaction decreases with increase in temperature. Previously, many ligands have been reported to interact with different protein with a binding affinity in the 10 3 -10 5 M −1 range [8,9,[47][48][49]. ...
Drug-binding and interactions with plasma proteins strongly affect their efficiency of delivery, hence considered as a key factor in determining the overall pharmacological action. Alpha-1-acid glycoprotein (AGP), a second most abundant plasma protein in blood circulation, has unique drug binding ability and involved in the transportation of various compounds. Here, we have investigated the mechanism of interaction between AGP and potential Cu/Zn metallo-drugs of benzimidazole derived organic motifs (CuL2 and ZnL2, where L is Schiff base ligand) by applying integrated spectroscopic, biophysical techniques and computational molecular docking analyses. We found that both the metallo-drugs (CuL2 and ZnL2) were bound at the central cavity of AGP interacting with the residues of lobe I, lobe II as well as lobe III. The binding of metallo-drugs to AGP occurs in 1:1 M ratios. Hydrogen bonding, electrostatic and hydrophobic interactions played a significant role in stabilizing the AGP-metallo-drug complexes. Binding affinities of both the metallo-drugs towards AGP at 298 K were of the order of 104-105 M-1, corresponding to Gibbs free energy of stabilization of approximately -5.50 to -6.62 kcal mol-1. Furthermore, the spectroscopic investigation by circular dichroism and synchronous fluorescence analyses suggest conformational changes in AGP upon the binding of metallic compounds.
The research aims to elucidate how drug interactions affect the activity of L‐asparaginase (L‐ASNase), an essential enzyme in cancer treatment, especially for acute lymphoblastic leukemia (ALL). Understanding these interactions is crucial for optimizing treatment effectiveness and reducing adverse effects. This study explores the intricate molecular interactions and structural dynamics of L‐ASNase upon binding with colchicine. Fluorescence quenching experiments were conducted at various temperatures (298, 303, and 310 K), revealing notable interactions between L‐ASNase and colchicine. These interactions were characterized by a reduction in fluorescence intensity and a blue shift in emission maxima. Additional analyses, including the determination of Stern–Volmer quenching constants ( K SV ), bimolecular quenching rate constants ( k q ), and thermodynamic parameters, indicated a static quenching mechanism with moderate binding affinities ( K a : 1.40–2.71 × 10 ⁴ M ⁻¹ ) across different temperatures. Thermodynamic study suggested positive enthalpy and entropy changes (Δ H ° = −10.26 kcal mol ⁻¹ ; Δ S ° = −14.19 cal mol ⁻¹ K ⁻¹ ), suggesting a spontaneous reaction with negative Δ G ° values (−5.86 to −6.03 kcal mol ⁻¹ ). FRET measurements supported optimal distances ( r and R o ) for FRET occurrence, reinforcing the static quenching mechanism. Molecular docking further supported these findings, revealing a 1:1 stoichiometric binding ratio for L‐ASNase:colchicine and elucidating specific binding orientations and interactions critical for complex stability. Subsequent molecular dynamics simulations spanning 100 ns underscored the stability of the L‐ASNase–colchicine complex, with minimal deviations observed in key structural parameters such as RMSD, RMSF, R g , and SASA. Additionally, spectroscopic analyses, including circular dichroism (CD), synchronous fluorescence, and 3D fluorescence provided insights into the conformational changes and alterations in the microenvironment of aromatic amino acid residues in L‐ASNase upon colchicine binding. Moreover, L‐ASNase activity was slightly reduced by 25% in the presence of colchicine. This comprehensive investigation sheds light on the molecular intricacies of the L‐ASNase–colchicine complex, advancing our understanding of drug–target interactions and offering potential avenues for therapeutic applications.
α1 -Acid glycoprotein (AGP) is a prominent acute phase component of blood plasma and extravascular fluids. As a member of the immunocalins, AGP exerts protective effects against Gram-negative bacterial infections but the underlying molecular mechanisms still need to be elucidated. Notably, the chemical structures of phenothiazine, phenoxazine and acridine type ligands of AGP are similar to those of phenazine compounds excreted by the opportunistic human pathogen Pseudomonas aeruginosa and related bacterial species. These molecules, like pyocyanin, act as quorum sensing-associated virulence factors and are important contributors to bacterial biofilm formation and host colonization. Molecular docking simulations revealed that these agents fit into the multi-lobed cavity of AGP. The binding site is decorated by several aromatic residues which seem to be essential for molecular recognition of the ligands allowing multifold π-π and CH-π interactions. The estimated affinity constants (~105 M-1 ) predict that these secondary metabolites could be trapped inside the β-barrel of AGP which in turn could reduce their cytotoxic effects and disrupt the microbial QS network, facilitating the eradication of bacterial infections.
Glycobiology as a field holds enormous potential for understanding human health and disease. However, few glycobiology studies adequately address the issue of sex differences in biology, which severely limits the conclusions that can be drawn. Numerous CAZymes, lectins, and other carbohydrate-associated molecules have the potential to be differentially expressed and regulated with sex, leading to differences in O-GlcNAc, N-glycan branching, fucosylation, sialylation, and proteoglycan structure, among others. Expression of proteins involved in glycosylation is influenced through hormones, miRNA, and gene dosage effects. In this review, we discuss the benefits of incorporating sex-based analysis in glycobiology research and the potential drivers of sex differences. We highlight examples of where incorporation of sex-based analysis has led to insights into glycobiology. Finally, we offer suggestions for how to proceed moving forward, even if the experiments are already complete. Properly incorporating sex based analyses into projects will substantially improve the accuracy and reproducibility of studies as well as accelerate the rate of discovery in the glycosciences.
The flavin adenine dinucleotide containing Endoplasmic Reticulum Oxidoreductase-1 α (ERO1α) catalyzes the formation of de novo disulfide bond formation of secretory and transmembrane proteins and contributes towards proper protein folding. Recently, increased ERO1α expression has been shown to contribute to increased tumor growth and metastasis in multiple cancer types. In this report we sought to define novel chemical space for targeting ERO1α function. Using the previously reported ERO1α inhibitor compound, EN-460, as a benchmark pharmacological tool we were able to identify a sulfuretin derivative, T151742 which was approximately two-fold more potent using a recombinant enzyme assay system (IC50 = 8.27 ± 2.33 μM) compared to EN-460 (IC50= 16.46 ± 3.47 μM). Additionally, T151742 (IC50 = 16.04 μM) was slightly more sensitive than EN-460 (IC50= 19.35μM) using an MTT assay as an endpoint. Utilizing a cellular thermal shift assay (CETSA), we determined that the sulfuretin derivative T151742 demonstrated isozyme specificity for ERO1α as compared to ERO1β and showed no detectable binding to the FAD containing enzyme LSD-1. T151742 retained activity in PC-9 cells in a clonogenicity assay while EN-460 was devoid of activity. Furthermore, the activity of T151742 inhibition of clonogenicity was dependent on ERO1α expression as CRISPR edited PC-9 cells were resistant to treatment with T151742. In summary we identified a new scaffold that shows specificity for ERO1α compared to the closely related paralog ERO1β or the FAD containing enzyme LSD-1 that can be used as a tool compound for inhibition of ERO1α to allow for pharmacological validation of the role of ERO1α in cancer.
Protein pharmaceuticals have become a significant class of marketed drug products and are expected to grow steadily over the next decade. Development of a commercial protein product is, however, a rather complex process. A critical step in this process is formulation development, enabling the final product configuration. A number of challenges still exist in the formulation development process. This review is intended to discuss these challenges, to illustrate the basic formulation development processes, and to compare the options and strategies in practical formulation development.
The transcriptional activity of the serum response factor (SRF) protein is triggered by its binding to a 10-base-pair DNA consensus sequence designated the CArG box, which is the core sequence of the serum response element (SRE). Sequence-specific recognition of the CArG box by a core domain of 100 amino acid residues of SRF (core-SRF) was asserted to depend almost exclusively on the intrinsic SRE conformation and on the degree of protein-induced SRE bending. Nevertheless, this paradigm was invalidated by a temperature-dependent Raman spectroscopy study of 20-mer oligonucleotides involved in bonding interactions with core-SRF that reproduced both wild type and mutated c-fos SREs. Indeed, the SRE moieties that are complexed with core-SRF exhibit permanent interconversion dynamics between bent and linear conformers. Thus, sequence-specific recognition of the CArG box by core-SRF cannot be explained only in terms of the three-dimensional structure of the SRE. A particular dynamic pairing process discriminates between the wild type and mutated complexes. Specific oscillations of the phosphate charge network of the SRE govern the recognition between both partners rather than an intrinsic set of conformations of the SRE.
Raman microscopy permits structural analysis of protein crystals in situ in hanging drops, allowing for comparison with Raman measurements in solution. Nevertheless, the two methods sometimes reveal subtle differences in structure that are often ascribed to the water layer surrounding the protein. The novel method of drop-coating deposition Raman spectropscopy (DCDR) exploits an intermediate phase that, although nominally "dry," has been shown to preserve protein structural features present in solution. The potential of this new approach to bridge the structural gap between proteins in solution and in crystals is explored here with extrinsic protein PsbP of photosystem II from Spinacia oleracea. In the high-resolution (1.98 Å) x-ray crystal structure of PsbP reported here, several segments of the protein chain are present but unresolved. Analysis of the three kinds of Raman spectra of PsbP suggests that most of the subtle differences can indeed be attributed to the water envelope, which is shown here to have a similar Raman intensity in glassy and crystal states. Using molecular dynamics simulations cross-validated by Raman solution data, two unresolved segments of the PsbP crystal structure were modeled as loops, and the amino terminus was inferred to contain an additional beta segment. The complete PsbP structure was compared with that of the PsbP-like protein CyanoP, which plays a more peripheral role in photosystem II function. The comparison suggests possible interaction surfaces of PsbP with higher-plant photosystem II. This work provides the first complete structural picture of this key protein, and it represents the first systematic comparison of Raman data from solution, glassy, and crystalline states of a protein.
This work introduces a new approach connecting vibrational spectroscopy with homology and energetic molecular modeling of proteins. Combination of both methods can compensate their disadvantages and result in realistic three-dimensional protein models. The approach is most powerful for membrane proteins or glycoproteins with high carbohydrate content where X-ray or NMR analysis is not always successful. Nevertheless, it can also serve as a tool of preliminary analysis of any protein with unknown structure. Power of the approach is demonstrated on human α1-acid glycoprotein. Its predicted structure published in [V. Kopecký Jr. et al., Biochem. Biophys. Res. Commun. 300 (2003), 41-46] is discussed in detail with respect to the approach and its general employment.
Human α1-acid glycoprotein (hAGP) in serum functions as a carrier of basic drugs. In most individuals, hAGP exists as a mixture of
two genetic variants, the F1*S and A variants, which bind drugs with different selectivities. We prepared a mutant of the
A variant, C149R, and showed that its drug-binding properties were indistinguishable from those of the wild type. In this
study, we determined the crystal structures of this mutant hAGP alone and complexed with disopyramide (DSP), amitriptyline
(AMT), and the nonspecific drug chlorpromazine (CPZ). The crystal structures revealed that the drug-binding pocket on the
A variant is located within an eight-stranded β-barrel, similar to that found in the F1*S variant and other lipocalin family
proteins. However, the binding region of the A variant is narrower than that of the F1*S variant. In the crystal structures
of complexes with DSP and AMT, the two aromatic rings of each drug interact with Phe-49 and Phe-112 at the bottom of the binding
pocket. Although the structure of CPZ is similar to those of DSP and AMT, its fused aromatic ring system, which is extended
in length by the addition of a chlorine atom, appears to dictate an alternative mode of binding, which explains its nonselective
binding to the F1*S and A variant hAGPs. Modeling experiments based on the co-crystal structures suggest that, in complexes
of DSP, AMT, or CPZ with the F1*S variant, Phe-114 sterically hinders interactions with DSP and AMT, but not CPZ.
This chapter describes methods for the estimation of protein secondary structure content—in terms of percentage helix, β-strand, and reverse turn—from a least-squares analysis of Raman amide I and amide III spectra. A statistical analysis of these estimates for proteins with known structures is included to establish the degree of confidence that may be placed on results for other proteins. The amide I analysis here is a refinement of earlier work while the amide III analysis is new. Measurement of the amide I spectrum may be restricted to the region between 1500 and 1800 cm-1 in order to save time and increase the signal-to-noise ratio. The spectrum of buffer is subtracted from the amide III region to satisfy the criteria described for the amide I analysis. However, if the protein concentration is sufficiently high (2%) solvent correction may not be necessary, as the spectrum of water in this region is relatively linear. When samples are not particularly fluorescent, good results can be obtained by collecting the spectrum from 1100 to 1500 cm-1 and by fitting a straight line through the minima. The amide III spectrum is then 9-point smoothed using the algorithm by Savitsky and Golay.
Eight ligands were used in this study, four basic, three neutral and one acidic. Their binding to serum alpha 1-acid glycoprotein (orosomucoid) was measured at several temperatures, and the data were analysed together by a general model with three unknowns, number of binding sites, delta H0 and delta S0. The partition coefficients of the ligands were measured in octanol/water and heptane/water systems (log Poct. and log Phep.), and their molecular volumes were calculated by molecular modelling techniques. These structural properties allow determination of polarity parameters (delta log Poct.-hep., lambda oct. and lambda hep.) which encode in different proportions the various polar interactions between the solute and the aqueous and organic phases, i.e. hydrogen-bonding capacity and dipolarity/polarizability. This study shows that good correlations exist between delta H0 or delta S0 and polarity parameters, such that the enthalpic contribution to binding increases with increasing polarity of the ligands, mainly hydrogen-bond-donor acidity, whereas their entropic contribution to binding decreases.
An overview is provided on the development and status of potential energy functions that are used in atomic-level statistical mechanics and molecular dynamics simulations of water and of organic and biomolecular systems. Some topics that are considered are the form of force fields, their parameterization and performance, simulations of organic liquids, computation of free energies of hydration, universal extension for organic molecules, and choice of atomic charges. The discussion of water models covers some history, performance issues, and special topics such as nuclear quantum effects.
Crystals of α1-acid glycoprotein have been grown reproducibly from delipidated protein in the presence of chlorpromazine. The crystals are large hexagonal prisms of space group either P622 or P6222 and the unit cell dimensions are a = b = 101 Å and C = 201 Å. The unit cell is very highly hydrated and is nearly 80% solvent. It contains one molecule of protein per asymmetric unit. The crystals diffract only to low resolution, presumably reflecting the extensive hydration and accompanying disorder.
Secondary and tertiary structures of human blood α1-acid glycoprotein, a member of the lipocalin family, have been studied for the first time by infrared and Raman spectroscopies. Vibrational spectroscopy confirmed details of the secondary structure and the structure content predicted by homology modeling of the protein moiety, i.e., 15% α-helices, 41% β-sheets, 12% β-turns, 8% bands, and 24% unordered structure at pH 7.4. Our model shows that the protein folds as a highly symmetrical all-β protein dominated by a single eight-stranded antiparallel β-sheet. Thermal dynamics in the range 20–70°C followed by Raman spectroscopy and analyzed by principle component analysis revealed full reversibility of the protein motion upon heating dominated by decreasing of β-sheets. Raman difference spectroscopy confirmed the proximity of Trp122 to progesterone binding.
The fluorescence of the tryptophan residues of asialylated human α1-acid glycoprotein (orosomucoid) was investigated in presence of progesterone. Red-edge excitation spectra did not lead to a shift of the fluorescence emission maximum of the fluorophore, i.e., motions of the Trp residues depend on their microenvironment. This was confirmed by anisotropy studies as a function of temperature in the range of 7–35°C (Perrin plot). These two results identical to those obtained in absence of progesterone [J. Albani, Biochim. Biophys. Acta 1291 (1996) 215–220] indicate that binding of progesterone to orosomucoid does not modify the mean residual motion of the Trp residues. Measurement of the anisotropy in a temperature range of −45° to +6°C in a mixture of 80% glycerol–buffer, allows us to determine the frictional resistance to the local rotations of the tryptophan residues [G. Weber, S.F. Scarlata, M. Rholam, Biochemistry 23 (1984) 6785–6788]. The Y-plot analysis of the anisotropy reveals that the mean motion of the two Trp residues buried in the protein core was different from that of the Trp residue of the surface. The average angles of rotations for buried and surface residues were 16° and 21.5° of arc, respectively, instead of 10° and 14° of arc observed in absence of progesterone [J. Albani, Biochim. Biophys. Acta 1291 (1996) 215–220]. Thus, binding of progesterone to orosomucoid increases the free space of rotation of the two classes of Trp residues.
A method is described for the separation into five fractions of the components of human plasma which remained in solution following precipitation of the major plasma proteins by procedures formerly published. All the proteins not previously precipitated were concentrated with the aid of zinc hydroxide (Fraction VI); among them were hitherto little known glycoproteins of low molecular weight. Unknown polysaccharides with specific blood group activity were selectively rendered insoluble with calcium hydroxide (Fraction VII), and amino acids and peptides were adsorbed on an ion exchange resin (Fraction VIII). After concentration of the final solution, the blood constituents were separated into a lipophilic (Fraction IX) and a hydrophilic fraction (Fraction X). In addition a method is described for subfractionation of the proteins precipitated as Fraction VI. An acid glycoprotein has been separated in a homogeneous state, as judged by electrophoretic and ultracentrifugal analyses, over the pH range 1.9 to 9.6. It was isoelectric, in a phosphate buffer solution of ionic strength 0.1, at pH 2.7. Its sedimentation constant, S20, w, in 0.15 M NaCl solution at pH 6.5, extrapolated to zero concentration, was 3.5 S. The chemical composition and physico-chemical properties of the acid glycoprotein differed widely from those of other plasma proteins. Its concentration in normal plasma was 0.5 g. per liter. The acid glycoprotein has been crystallized as a lead salt.
How the interplay between protein structure and internal dynamics regulates protein function is poorly understood. Often, ligand binding, post-translational modifications and mutations modify protein activity in a manner that is not possible to rationalize solely on the basis of structural data. It is likely that changes in the internal motions of proteins have a major role in regulating protein activity, but the nature of their contributions remains elusive, especially in quantitative terms. Here we show that changes in conformational entropy can determine whether protein-ligand interactions will occur, even among protein complexes with identical binding interfaces. We have used NMR spectroscopy to determine the changes in structure and internal dynamics that are elicited by the binding of DNA to several variants of the catabolite activator protein (CAP) that differentially populate the inactive and active DNA-binding domain states. We found that the CAP variants have markedly different affinities for DNA, despite the CAP−DNA-binding interfaces being essentially identical in the various complexes. Combined with thermodynamic data, the results show that conformational entropy changes can inhibit the binding of CAP variants that are structurally poised for optimal DNA binding or can stimulate the binding activity of CAP variants that only transiently populate the DNA-binding-domain active state. Collectively, the data show how changes in fast internal dynamics (conformational entropy) and slow internal dynamics (energetically excited conformational states) can regulate binding activity in a way that cannot be predicted on the basis of the protein's ground-state structure.
Raman spectroscopy has become a versatile tool in protein science and biotechnology. Recent advances in spectral assignments and vibrational theory, examples of use in structural biology and selected industrial applications are discussed. New insights into protein folding, assembly and aggregation were obtained by classical Raman spectroscopy. Raman spectroscopy has been used to characterize intrinsically unstructured proteins. The improved instrument sensitivity made it possible to use Raman difference spectroscopy to characterize enzyme–substrate interactions. Specifically, Raman crystallography has been instrumental in the delineation of protein–ligand interactions with a resolution surpassing that of x-ray diffraction. Numerous applications of Raman spectroscopy to protein analysis in biotechnology and food industry have been facilitated by the new generation of commercial Raman instruments. Copyright © 2005 John Wiley & Sons, Ltd.
Alpha-1-acid glycoprotein (AGP) or orosomucoid (ORM) is a 41-4.3-kDa glycoprotein with a pI of 2.8-3.8. The peptide moiety is a single chain of 183 amino acids (human) or 187 amino acids (rat) with two and one disulfide bridges in humans and rats,respectively. The carbohydrate content represents 45% of the molecular weight attached in the form of five to six highly sialylated complex-type-N-linked glycans. AGP is one of the major acute phase proteins in humans, rats, mice and other species. As most acute phase proteins, its serum concentration increases in response to systemic tissue injury, inflammation or infection, and these changes in serum protein concentrations have been correlated with increases in hepatic synthesis. Expression of the AGP gene is controlled by a combination of the major regulatory mediators, i.e. glucocorticoids and a cytokine network involving mainly interleukin-1 beta (IL-1 beta), tumour necrosis factor-alpha (TNF alpha), interleukin-6 and IL-6 related cytokines. It is now well established that the acute phase: response may take place in extra-hepatic cell types, and may be regulated by inflammatory mediators as observed in hepatocytes. The biological function of AGP remains unknown; however,a number of activities of possible physiological significance, such as various immunomodulating effects, have been described. AGP also has the ability to bind and to carry numerous basic and neutral lipophilic drugs from endogenous (steroid hormones) and exogenous origin; one to seven binding sites have been described. AGP can also bind acidic drugs such as phenobarbital. The immunomodulatory as well as the binding activities of AGP have been shown to be mostly dependent on carbohydrate composition. Finally, the use of AGP transgenic animals enabled to address in vivo, functionality of responsive elements and tissue specificity, as well as the effects of drugs that bind to AGP and will be an useful tool to determine the physiological role of AGP.
Human alpha(1)-acid glycoprotein (AGP), which is comprised of 183 amino acid residues and 5 carbohydrate chains, is a major plasma protein that binds to basic and neutral drugs as well as to steroid hormones. It has a beta-sheet-rich structure in aqueous solution. Our previous findings suggest that AGP forms an alpha-helix structure through an interaction with biomembranes. We report herein on a study of the mechanism of alpha-helix formation in AGP using various modified AGPs. The disulfide reduced AGP (R-AGP) was extensively unfolded, whereas asialylated AGP (A-AGP) maintained the native structure. Intriguingly, reduced and asialylated AGP (RA-AGP) increased the alpha-helix content as observed in the presence of biomembrane models, and showed a significant decrease in ligand binding capacity. This suggests that AGP has an innate tendency to form an alpha-helix structure, and disulfide bonds are a key factor in the conformational transition between the beta-sheet and alpha-helix structures. However, RA-AGP with all histidine residues chemically modified (HRA-AGP) was found to lose the intrinsic ability to form an alpha-helix structure. Furthermore, disulfide reduction of the H172A mutant expressed in Pichia pastoris also caused a similar loss of folding ability. The present results indicate that disulfide bonds and the C-terminal region, including H172 of AGP, play important roles in alpha-helix formation in the interaction of the protein with biomembranes.
Human α(1)-acid glycoprotein (AGP) is an acute phase plasma glycoprotein containing two disulfide bridges. As a member of the lipocalin superfamily, it binds and transports several basic and neutral ligands, but a number of other activities have also been described. Thanks to its binding properties, AGP is also a good candidate for the development of biosensors and affinity chromatography media, and in this context detailed structural information is needed. The structural properties of AGP at different p(2)Hs and under reducing conditions were analysed by FT-IR spectroscopy. The obtained data indicate that AGP, when denatured, does not aggregate at neutral or basic p(2)Hs whilst it does at acidic p(2)Hs. Under reducing conditions the protein is remarkably less thermostable than its oxidized counterpart and presents an enhanced tendency to aggregate, even at neutral p(2)H. A heat-induced molten globule-like state (MG) was detected at 55 °C at p(2)H 7.4 and 5.5. At p(2)H 4.5 the MG occurred at 45 °C with an onset of formation at 40 °C. The MG was not observed under reducing conditions. A lower affinity of chlorpromazine and progesterone for the MG formed at p(2)H 4.5 and 40 °C was observed, suggesting that ligand(s) may be released near the negative surfaces of biological membranes. Furthermore, the reduced AGP displays an enhanced affinity for progesterone, indicating the importance of disulfide bonds for the binding capacity of AGP.
Unglycosylated recombinant human alpha(1)-acid glycoprotein (hAGP) variants (rF1(*)S and rA) were prepared in an E. coli expression system using the Origami B strain and pET-3c vector. Thioredoxin was co-expressed to promote the appropriate folding of hAGP. SDS-PAGE under reducing conditions showed that rF1(*)S and rA migrate as single bands after purification. However, several bands derived from rA were observed under non-reducing conditions because of the high reactivity of a free cystein residue (C149). We therefore prepared a mutant of A variant (C149R-A), and confirmed that this mutant maintained homogeneity. Circular dichroism and intrinsic tryptophan fluorescence spectroscopic analyses indicated that rF1(*)S and C149R-A have almost the same conformational structures as F1(*)S and A purified from serum. Ligand binding experiments using propranolol as a F1(*)S ligand and disopyramide as an A specific ligand indicated that the capacity of rF1(*)S and C149R-A is equivalent to those ligands as well as F1(*)S and A from serum. These results suggest that the oligosaccharide moieties of hAGP have negligible effects on the structural and ligand binding properties of hAGP. Thus, rF1(*)S and C149R-A promise to be useful in studies on the drug binding sites of hAGP.
In vitro chaperone-like activity of the acute-phase component and plasma drug transporter human alpha(1)-acid glycoprotein (AAG) has been shown for the first time. AAG suppressed thermal aggregation of a variety of unrelated enzymatic (e.g., aldolase, catalase, enolase, carbonic anhydrase) and non-enzymatic proteins (beta-lactoglobulin, ovotransferrin) and it also prevented dithiothreitol induced aggregation of insulin. The anti-aggregation ability of AAG was abolished/reduced upon drug binding suggesting that protein-protein interactions established between the lipocalin beta-barrel fold of AAG and hydrophobic surfaces of the stressed proteins are involved in the chaperone-like activity. The results shed some light on the possible biological function of this enigmatic protein and suggest that besides haptoglobin, clusterin, fibrinogen and alpha(2)-macroglobulin AAG can be considered as a novel member of the extracellular molecular chaperones found in human body fluids.
Transcriptional activity of serum response factor (SRF) is dependent on its binding to the CC(A/T)(6)GG box (CArG box) of serum response element (SRE). By Raman spectroscopy, we carried out a comparative analysis, in solution, of the complexes obtained from the association of core-SRF with 20-mer SREs bearing wild-type and mutated c-fos CArG boxes. In case of association with the wild type c-fos CArG box, the complex does not bring out the expected Raman signature of a stable bending of the targeted SRE but keeps a bend-linear conformer oligonucleotide interconversion. The linear conformer population is larger than that of free oligonucleotide. In the core-SRF moiety of the wild-type complex a large spectral change associated with the CO-groups from Asp and/or Glu residues shows that their ionization states and the strength of their interactions decrease as compared to those of mutated non-specific complexes. Structural constraints evidenced on the free core-SRF are released in the wild-type complex and environmental heterogeneities appear in the vicinity of Tyr residues, due to higher water molecule access. The H-bonding configuration of one Tyr OH-group, in average, changes with a net transfer from H-bond acceptor character to a combined donor and acceptor character. A charge repartition distributed on both core-SRF and targeted SRE stabilizes the specific complex, allowing the two partners to experience a variety of conformations.
The tertiary structure of alpha1-acid glycoprotein (AGP) remains unresolved despite its novel function because AGP is a hard target in X-ray and NMR analyses. To elucidate the membrane-induced conformational change of AGP, the vacuum-ultraviolet circular dichroism (VUVCD) spectra of AGP and its constituent sugars were measured down to 160 nm in the presence or absence of phosphoglyceride liposome using a synchrotron-radiation VUVCD spectrophotometer. The secondary-structure contents and numbers of segments of AGP were estimated from the VUVCD spectra of the protein moiety obtained by subtracting the contributions of the glycan moiety. Further, the positions of secondary structures on the amino acid sequence were predicted by combining the VUVCD data with a neural network algorithm. These comprehensive secondary-structure analyses revealed that AGP consists of 11.4% alpha-helices (3 segments) and 39.9% beta-strands (12 segments) in the absence of liposome (pH 4.5), which are close to the proportions in the secondary structure of native AGP (pH 7.4) predicted by homology modeling, and that it consists of 47.5% alpha-helices (7 segments) and 2.7% beta-strands (2 segments) in the presence of liposome (pH 4.5). Detailed characterization of these alpha-helices of AGP bound to liposome suggested that two alpha-helices (residues 15-27 and 161-175) in the N- and C-terminal regions strongly interact with liposome. Most of the progesterone-binding residues of AGP were involved in the sequences transferring from beta-strands to alpha-helices or unordered structures, which coincided with the large decrease in progesterone-binding capacity of liposome-bound AGP. These results provide the first sequence-level information on the membrane-binding mechanism and structure-function relationship of AGP.
Small heat shock proteins are ubiquitously found in all three domains of life, although they are the most poorly conserved family of molecular chaperones. Their involvement in anti-stress mechanisms of the cells have been clearly demonstrated by induction of their expression in response to various environmental and pathological stresses. Small heat shock proteins comprise the most effective chaperone family concerning their unusual capacity of substrate binding. It is well documented that small heat shock proteins associate with unfolding substrate proteins and form large oligomeric complexes to prevent their aggregation and accumulation, that otherwise would impair the normal cell functions. The substrates captured by small heat shock proteins are further refolded to their native state by ATP depended chaperones. During heat stress, the induced expression and activation of the small heat shock proteins, might reflect that this mechanism of protein quality control contributes to acquired thermotolerance in hyperthermophilic archaea, as well.
Alpha(1)-acid glycoprotein (AGP) is an important drug-binding protein in human plasma and, as an acute-phase protein, it has a strong influence on pharmacokinetics and pharmacodynamics of many pharmaceuticals. We report the crystal structure of the recombinant unglycosylated human AGP at 1.8 A resolution, which was solved using the new method of UV-radiation-damage-induced phasing (UV RIP). AGP reveals a typical lipocalin fold comprising an eight-stranded beta-barrel. Of the four loops that form the entrance to the ligand-binding site, loop 1, which connects beta-strands A and B, is among the longest observed so far and exhibits two full turns of an alpha-helix. Furthermore, it carries one of the five N-linked glycosylation sites, while a second one occurs underneath the tip of loop 2. The branched, partly hydrophobic, and partly acidic cavity, together with the presumably flexible loop 1 and the two sugar side chains at its entrance, explains the diverse ligand spectrum of AGP, which is known to vary with changes in glycosylation pattern.
The linear amino acid sequence of the amino-terminal CNBr fragment of α1-acid glycoprotein derived from pooled human plasma was elucidated and proved to consist of 111 residues. For this investigation the amino acid sequences of the peptides and glycopeptides of a chymotryptic and a tryptic digest and some of the peptides and glycopeptides of a peptic hydrolysate of this protein were elucidated. These data together with the amino acid sequence of the carboxyl-terminal CNBr fragment reported earlier completely established the amino acid sequence of α1-acid glycoprotein. The five heteropolysaccharide groups of this protein were demonstrated to be linked N-glycosidically to asparaginyl residues. The number of amino acids between two subsequent carbohydrate units differs considerably. This report is thus the first one in which the sequence of a glycoprotein with such a high number of polysaccharide units is described. The following two findings were very unusual. First, 11 amino acid substitutions were detected. The carboxyl-terminal CNBr fragment possesses ten further amino acid replacements as described earlier, so that in 21 of the 181 residues of the protein, or 12%, such substitutions have occurred. These replacements, except for two, can be explained by single point mutations. Secondly, a significant degree of homology was noted between the amino-terminal 43-residue segment of CNBr-I and the amino terminal of the variable region of the κ-type L chain of human IgG. This homology and that of the carboxyl-terminal region of this glycoprotein with the constant region of the H chain of IgG suggest that α1-acid glycoprotein may represent a protein that is related to the ancestral immunoglobulin.
Laser Raman spectroscopy, like infrared spectroscopy, is a method for determining molecular structure by measuring the energies (frequencies) of molecular vibrations. Although the two methods differ fundamentally in the mechanisms of interaction between radiation and matter, one obtains in both cases a vibrational spectrum consisting of a number of discrete bands, the frequencies and intensities of which are determined by the nuclear masses in motion, the equilibrium molecular geometry, and the molecular force field. An important advantage of Raman over infrared spectroscopy for biological applications is the virtual transparency of water (both H2O and D2O) in the Raman effect. This greatly simplifies the analysis of aqueous solutions and facilitates the investigation of hydrogen-isotope exchange phenomena. Changes in molecular geometry—particularly the conformational transitions characteristic of biological macromolecules—can produce large shifts in Raman band positions, often referred to as frequency shifts, empowering the technique in the diagnosis of protein secondary structure, determination of side-chain configurations, and detection of interacting side-chain groups. Since the molecular geometry and force field may be sensitive to interactions between molecules, the Raman method also has the potential for investigating intermolecular interactions, including the formation of biologically important protein complexes. Raman spectroscopy is gaining wide use as a method for probing protein structure, dynamics, assembly, and recognition.
The function of sialic acid groups at the terminal of sugar chains of human alpha 1-acid glycoprotein (AGP) was investigated with respect to chiral discrimination between optical isomers of basic drugs, using high-performance capillary electrophoresis/frontal analysis (HPCE/FA), a novel analytical method developed for the determination of unbound drug concentration with ultramicrosample volume (100-200 nl). Native human AGP and desialylated AGP were used as test proteins, and propranolol (PRO) and verapamil (VER) were used as model drugs. The unbound concentration of (S)-VER was 1.31 times higher than that of (R)-VER in native AGP solution. This selectivity was not affected by desialylation. Further, enzymatic elimination of galactose residues, which neighbored sialic acid groups, did not change the binding of either isomer of VER. On the other hand, the unbound concentration of (R)-PRO was 1.27 times higher than that of (S)-PRO in native AGP solution. Desialylation caused the unbound concentration of (S)-PRO to rise to the same level of (R)-PRO, resulting in loss of enantioselectivity. Thus, it follows that sialic acid groups of AGP, as a whole, are not responsible for chiral recognition between enantiomers of VER but are involved in enantioselectivity toward the isomers of PRO.
Human alpha1-acid glycoprotein (AAG) is a mixture of at least two genetic variants: the A variant and the F1 and/or S variant or variants, which are encoded by two different genes. In a continuation of previous studies indicating specific drug transport roles for each AAG variant according to its separate genetic origin, this work was designed to (1) determine the affinities of the two main gene products of AAG (i.e., the A variant and a mixture of the F1 and S variants) for 35 chemically diverse drugs and (2) to obtain meaningful 3D-QSARs for each binding site. Affinities were obtained by displacement experiments, leading to qualitative indications about binding site characteristics. In particular, drugs binding selectively to the A variant displayed some common structural features, but this was not seen for the F1*S variants. Three-dimensional QSAR analyses using the CoMFA method yielded a steric model for binding to the A variant, from which a simplified haptophoric model was derived. In contrast, no statistically sound model was found for the F1*S variants, possibly due (among other reasons) to an insufficient number of high affinity ligands in the set.
The study of filamentous virus structure by Raman spectroscopy requires accurate band assignments. In previous work, site- and residue-specific isotope substitutions were implemented to elucidate definitive assignments for Raman bands arising from vibrational modes of the alpha-helical coat protein main chain and aromatic side chains in the class I filamentous phage, fd [Overman, S. A., and Thomas, G. J., Jr. (1995) Biochemistry 34, 5440-5451; Overman, S. A., and Thomas, G. J., Jr. (1998) Biochemistry 37, 5654-5665]. Here, we extend the previous methods and expand the assignment scheme to identify Raman markers of nonaromatic side chains of the coat protein in the native fd assembly. This has been accomplished by Raman analysis of 11 different fd isotopomers selectively incorporating deuterium at specific sites in either alanine, aspartic acid, glutamic acid, glycine, isoleucine, leucine, lysine, serine, or valine residues of the coat protein. Raman markers are also identified for the corresponding deuterated side chains. In combination with previous assignments, the results provide a comprehensive understanding of coat protein contributions to the Raman signature of the fd virion and validate Raman markers assigned to the packaged single-stranded DNA genome. The findings described here show that nonaromatic side chains contribute prolifically to the fd Raman signature, that marker bands for specific nonaromatics differ in general from those observed in corresponding polypeptides and amino acids, and that the frequencies and intensities of many nonaromatic markers are sensitive to secondary and higher-order structures. Nonaromatic markers within the 1200-1400 cm-1 interval also interfere seriously with the diagnostic Raman amide III band that is normally exploited in secondary structure analysis. Implications of these findings for the assessment of protein conformation by Raman spectroscopy are considered.
Changes in structure of alpha1-acid glycoprotein were followed after deglycosylation with neuraminidase, peptide N-glycohydrolase F or with a mixture of exoglycosidases. Partially deglycosylated preparations of alpha1-acid glycoprotein free of sialic acids, one complete saccharide component, sialic acids and one saccharide component and sialic acids and some of the external saccharides were obtained. The effect of these changes in saccharide components on the glycoprotein structure was studied by temperature perturbation difference spectroscopy, fluorescence spectroscopy, fourth-derivative of absorption spectra and spectra of CD. Partial deglycosylation resulted in transformation of the molecule to a more compact state in which phenylalanyl residues were even more buried, tyrosyl residues became more uniform and tryptophyl residues were less exposed. The content of ordered secondary structures decreased. The thermal stability of the molecule was not significantly affected. Removal of one of the five saccharide components from the native molecule had apparently deeper effect than total desialyzation of the glycoprotein.
An overview of the application of Fourier transform infrared spectroscopy for the analysis of the structure of proteins and protein-ligand recognition is given. The principle of the technique and of the spectra analysis is demonstrated. Spectral signal assignments to vibrational modes of the peptide chromophore, amino acid side chains, cofactors and metal ligands are summarized. Several examples for protein-ligand recognition are discussed. A particular focus is heme proteins and, as an example, studies of cytochrome P450 are reviewed. Fourier transform infrared spectroscopy in combination with the various techniques such as time-resolved and low-temperature methods, site-directed mutagenesis and isotope labeling is a helpful approach to studying protein-ligand recognition.
Calcofluor White is a fluorescent probe that interacts with polysaccharides and is commonly used in clinical studies. Interaction between Calcofluor White and carbohydrate residues of alpha1-acid glycoprotein (orosomucoid) was previously studied at low and high concentrations of Calcofluor compared to that of the protein. alpha1-Acid glycoprotein contains 40% carbohydrate by weight and has up to 16 sialic acid residues. At equimolar concentrations of Calcofluor and alpha1-acid glycoprotein, the fluorophore displays free motions [Albani, J. R.; Sillen, A.; Coddeville, B.; Plancke, Y. D.; Engelborghs, Y. Carbohydr. Res. 1999, 322, 87-94], while at high concentration of Calcofluor, its surrounding microenvironment is rigid, inducing the rigidity of the fluorophore itself [Albani, J. R.; Sillen, A.; Plancke, Y. D.; Coddeville, B.; Engelborghs, Y. Carbohydr. Res. 2000, 327, 333-340]. In the present work, red-edge excitation spectra and steady-state anisotropy studies performed on Trp residues in the presence of Calcofluor, showed that the apparent dynamics of Trp residues are not modified. However, deconvoluting the emission spectra with two different methods into different components, reveals that the structure of the protein matrix has been disrupted in the presence of high Calcofluor concentrations.
Secondary and tertiary structures of human blood alpha(1)-acid glycoprotein, a member of the lipocalin family, have been studied for the first time by infrared and Raman spectroscopies. Vibrational spectroscopy confirmed details of the secondary structure and the structure content predicted by homology modeling of the protein moiety, i.e., 15% alpha-helices, 41% beta-sheets, 12% beta-turns, 8% bands, and 24% unordered structure at pH 7.4. Our model shows that the protein folds as a highly symmetrical all-beta protein dominated by a single eight-stranded antiparallel beta-sheet. Thermal dynamics in the range 20-70 degrees C followed by Raman spectroscopy and analyzed by principle component analysis revealed full reversibility of the protein motion upon heating dominated by decreasing of beta-sheets. Raman difference spectroscopy confirmed the proximity of Trp(122) to progesterone binding.
Objective:
Alpha(1)-acid glycoprotein (AAG) is a major binding protein for neutral and basic drugs because of its great drug affinity. AAG has three main genetic variants--F1, S, and A variants. Several attempts have been made to elucidate the differences in compositions of the carbohydrate moiety and structure-function relationships such as drug-binding differences. However, there have been few reports on age- and gender-related differences in compositions or concentrations of the carbohydrate moiety of AAG variants. The aim of this study was to clarify the age- and gender-related differences in carbohydrate concentrations and in drug-binding capacities of AAG glycoforms.
Methods:
The sera used in this study were obtained from 32 healthy subjects (17 men and 15 women, aged 16-84 years). The AAG glycoforms were isolated by hydroxyapatite chromatography. The binding capacity of AAG to disopyramide (DP), which is a basic drug, was determined using the ultrafiltration method. The concentrations of N-acetylneuraminic acid (NeuAc) and monosaccharides in AAG were determined using high-pH anion-exchange chromatography with pulsed-amperometric detection.
Results:
The mean plasma AAG concentration in the female subjects was significantly lower than that in the male subjects (0.67 +/- 0.12 mg/ml, mean +/- SD, in females, n = 15, versus 0.81 +/- 0.17 mg/ml in males, n = 17, P < 0.05), but no age-related differences were found (0.75 +/- 0.18 mg/ml in young subjects, n = 24, versus 0.77 +/- 0.12 mg/ml in older subjects, n = 8, n.s.). However, the degree of branching of the glycan chain in the female subjects was significantly lower than that in the male subjects (1.61 +/- 0.17 mol/mol, mean +/- SD, in females, n = 15, versus 1.75 +/- 0.23 mol/mol in males, n = 17, P < 0.05). There was a significant inverse relationship between the binding capacity of AAG to DP (Cb/AAG) and the degree of branching of the glycan chain. The binding capacity (Cb/AAG) decreased as the degree of branching in AAG glycans increased. The binding capacity (Cb/AAG) in the female subjects was significantly higher than that in the male subjects (2.79 +/- 0.59 mg/g AAG in females, mean +/- SD, n = 15, versus 2.37 +/- 0.29 mg/g AAG in males, n = 17, P < 0.05). CONCLUSION. The degree of branching of the glycan chain in AAG plays an important role in drug-binding capacity. Gender-related differences in drug-binding capacity (Cb/AAG) may be caused by differences in the ratios of the extent of branching of the glycan chain in AAG.
Thermal stability of human alpha(1)-acid glycoprotein and its desialyzed form were studied in the pH range of 1.5-5.2, i.e. about its pI. Circular dichroism, fluorescence and UV-absorption were used to determine the conformational changes and their reversibility in the temperature range 25-80 degrees C. These changes were tested in a three step process-heating, cooling and a second heating. Principal component analysis was applied for analyzing the spectral sets obtained in these experiments. Fully reversible behavior of Trp residues, as characterized by fluorescence spectroscopy, was observed during the heating process at all pH values. Nevertheless, three different types of the protein motion (reversible, irreversible and rearrangement of the protein core) were determined by UV-absorption spectroscopy. Thus, an environment of Tyr and Phe is modified or reversibly rearranged during the heating process in acid media. These types of alpha(1)-acid glycoprotein behavior were not significantly affected by desialyzation.
alpha(1)-Acid glycoprotein (AGP), an acute phase reactant, is extensively glycosylated at five Asn-linked glycosylation sites. In a number of pathophysiological states, including inflammation, rheumatoid arthritis, and cancer, alterations of Asn-linked glycans (N-glycans) have been reported. We investigated alteration of N-glycans at each of glycosylation sites of AGP in the sera of patients with acute and chronic inflammation.
AGP purified from sera was digested with Glu-C and the liberated glycopeptides were isolated by reverse phase HPLC. N-glycans released with peptide N-glycosidase F and followed by neuraminidase treatment were analyzed by matrix-assisted laser desorption ionization-time of flight mass spectrometry.
Site-specific differences in branching structures were observed among N-glycosylation sites 1, 3, 4 and 5. Within the sera of patients with acute inflammation, increases in bi-antennary and decreases in tri- and tetra-antennary structures were observed, as well as increases in alpha1,3-fucosylation, at most glycosylation sites. In the sera of patients with chronic inflammation, increased rates of tri-antennary alpha1,3-fucosylation at sites 3 and 4 and tetra-antennary alpha1,3-fucosylation at sites 3, 4 and 5 were detected. Although there were no significant differences between acute and chronic sera in site directed branching structures, significant differences of alpha1,3-fucosylation were detected in tri-antennary at sites 2, 4 and 5 and in tetra-antennary at sites 3 and 4.
Little variation in the N-glycan composition of the glycosylation sites of AGP was observed among healthy individuals, while the sera of patients with acute inflammation demonstrated increased numbers of bi-antennary and alpha1,3-fucosylated N-glycan structures at each glycosylation site.
The binding of drugs to plasma proteins, such as albumin and alpha1-acid glycoprotein (AGP) is a major determinant in the disposition of drugs. A topology analysis of drug binding sites on HSA and AGP was determined using various methods, including spectroscopy, QSAR, photoaffinity labeling and site directed mutagenesis. Recombinant albumin was found to be useful for rapidly identifying drug binding sites. The binding sites on AGP are not completely separated but are partially overlapped, and Trp, Tyr, Lys and His residues in the drug binding pockets play important roles in this process. Drug displacement is somewhat complex, due to the involvement of multiple effects. The reduced binding in uremic patients may be explained by a mechanism that involves a combination of direct displacement by free fatty acids as well as cascade effects of free fatty acids and unbound uremic toxins for significant inhibition in serum binding. Albumin-containing dialysate is useful for the extracorporeal removal of endogenous toxins and in the treatment of drug overdoses. Oxidized albumin is a useful biomarker for the quantitative and qualitative evaluation of oxidative stress. Interestingly, AGP undergoes a structural transition to a unique structure that differs from the native and denatured states, when it interacts with membranes.
Binding studies between progesterone and alpha1-acid glycoprotein allowed us to demonstrate that the binding site of progesterone contains one hydrophobic tryptophan residue and that the structure of the protein is not altered upon binding. The data obtained at saturated concentrations of progesterone clearly reveal the type of interaction at physiological levels.
Glycosylation is one of the most important post-translational modifications of proteins, and has been widely acknowledged as one of the most important ways to modulate both protein function and lifespan. The acute phase proteins are a major group of serum proteins whose concentration is altered during various pathophysiological conditions. The aim of this paper is to review the structure and functions of the alpha1-acid glycoprotein (AGP). AGP belongs to the subfamily of immunocalins, a group of binding proteins that also have immunomodulatory functions. One of the most interesting features of AGP is that its glycosylation microheterogeneity can be modified during diseases. This aspect is particularly remarkable, since both the immunomodulatory and the binding properties of AGP strongly depend on its carbohydrate composition. For these reasons, AGP can be considered an outstanding model for the study of glycan pattern modification during diseases. This review is focused on the most recent studies on the occurrence of different glycoforms in plasma and tissues and how the appearance of different oligosaccharide patterns during systemic inflammation or diseases can influence AGP's biological functions. The first part of the review will describe the structure of AGP and the several biological functions identified so far for this protein. The second part will be devoted to the post-translational modifications of the oligosaccharides micro-heterogeneity of AGP caused by pathological states. A critical evaluation of the impact of different AGP glycoforms on both its transport and anti-inflammatory features, and how the modifications of the glycan pattern can be utilized in clinical biochemistry, is also discussed.
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