The conformational stability (delta G) of muscle acylphosphatase, a small alpha/beta globular protein, has been determined as a function of temperature, urea concentration, and pH. A combination of thermally induced and urea-induced unfolding, monitored by far-UV circular dichroism, was used to define the conformational stability over a wide range of temperature. Through analysis of all these data, the heat capacity change upon unfolding (delta Cp) could be estimated, allowing the determination of the temperature dependence of the main thermodynamic functions (delta G, delta H, delta S). Thermal unfolding in the presence of urea made it possible to extend such thermodynamic analysis to examine these parameters as a function of urea concentration. The results indicate that acylphosphatase is a relatively unstable protein with a delta G(H2O) of 22 +/- 1 kJ mol-1 at pH 7 and 25 degrees C. The midpoints of both thermal and chemical denaturation are also relatively low. Urea denaturation curves over the pH range 2-12 have allowed the pH dependence of delta G to be determined and indicate that the maximum stability of the protein occurs near pH 5.5. While the dependence of delta G on urea (the m value) does not vary with temperature, a significant increase has been found at low pH values, suggesting that the overall dimensions of the unfolded state are significantly affected by the number of charges within the polypeptide chain. The comparison of these data with those from other small proteins indicates that the pattern of conformational stability is defined by individual sequences and not by the overall structural fold.
"A protein with a destabilized globular structure, the F94L mutant of AcP, has been converted into amyloid fibrils in the absence of denaturant and under conditions of pH that were previously found to be optimal for the stability and enzymatic activity of the wild-type protein (Chiti et al., 1998b). The amyloid fibrils formed under these conditions are similar to those formed by the wild-type protein under denaturing conditions and indeed to those formed by proteins associated with human amyloidotic diseases. "
[Show abstract][Hide abstract] ABSTRACT: Acylphosphatase can be converted in vitro, by addition of trifluoroethanol (TFE), into amyloid fibrils of the type observed in a range of human diseases. The propensity to form fibrils has been investigated for a series of mutants of acylphosphatase by monitoring the range of TFE concentrations that result in aggregation. We have found that the tendency to aggregate correlates inversely with the conformational stability of the native state of the protein in the different mutants. In accord with this, the most strongly destabilized acylphosphatase variant forms amyloid fibrils in aqueous solution in the absence of TFE. These results show that the aggregation process that leads to amyloid deposition takes place from an ensemble of denatured conformations under conditions in which non-covalent interactions are still favoured. These results support the hypothesis that the stability of the native state of globular proteins is a major factor preventing the in vivo conversion of natural proteins into amyloid fibrils under non-pathological conditions. They also suggest that stabilizing the native states of amyloidogenic proteins could aid prevention of amyloidotic diseases.
The EMBO Journal 05/2000; 19(7):1441-9. DOI:10.1093/emboj/19.7.1441 · 10.43 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: An open reading frame encoding a putative acylphosphatase was found in Drosophila melanogaster. The corresponding gene product shows 40% identity and 22 additional amino acid residues at the C-terminus as compared to muscle- and common-type human acylphosphatases. Moreover, all the residues involved in the catalytic mechanism of vertebrate enzymes are conserved in the D. melanogaster acylphosphatase. The D. melanogaster protein and a deletion mutant, similar in length to vertebrate acylphosphatases, were produced by cloning the corresponding cDNA in Escherichia coli. The wild-type enzyme is a protein with a well-established three-dimensional fold and a markedly reduced conformational stability as compared to vertebrate isoenzymes. The specific activity of the enzyme is significantly lower than that found in vertebrate enzymes though the substrate binding capability is basically unaltered. The deletion of 22 residues does not cause a significant change in k(cat), while affecting the apparent binding parameters. This work suggests that the genes encoding the vertebrate enzymes originate from an ancestor gene by duplication and subsequent evolution.
[Show abstract][Hide abstract] ABSTRACT: The folding of a 98 residue protein, muscle acylphosphatase (AcP), has been studied using a variety of techniques including circular dichroism, fluorescence and NMR spectroscopy following transfer of chemically denatured protein into refolding conditions. A low-amplitude phase, detected in concurrence with the main kinetic phase, corresponds to the folding of a minor population (13%) of molecules with one or both proline residues in a cis conformation, as shown from the sensitivity of its rate to peptidyl prolyl isomerase. The major phase of folding has the same kinetic characteristics regardless of the technique employed to monitor it. The plots of the natural logarithms of folding and unfolding rate constants versus urea concentration are linear over a broad range of urea concentrations. Moreover, the initial state formed rapidly after the initiation of refolding is highly unstructured, having a similar circular dichroism, intrinsic fluorescence and NMR spectrum as the protein denatured at high concentrations of urea. All these results indicate that AcP folds in a two-state manner without the accumulation of intermediates. Despite this, the folding of the protein is extremely slow. The rate constant of the major phase of folding in water, kfH2O, is 0.23 s-1 at 28 degreesC and, at urea concentrations above 1 M, the folding process is slower than the cis-trans proline isomerisation step. The slow refolding of this protein is therefore not the consequence of populated intermediates that can act as kinetic traps, but arises from a large intrinsic barrier in the folding reaction.
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