Crystal structure of auracyanin, a "blue" copper protein from the green thermophilic photosynthetic bacterium Chloroflexus aurantiacus
ABSTRACT Auracyanin B, one of two similar blue copper proteins produced by the thermophilic green non-sulfur photosynthetic bacterium Chloroflexus aurantiacus, crystallizes in space group P6(4)22 (a=b=115.7 A, c=54.6 A). The structure was solved using multiple wavelength anomalous dispersion data recorded about the CuK absorption edge, and was refined at 1.55 A resolution. The molecular model comprises 139 amino acid residues, one Cu, 247 H(2)O molecules, one Cl(-) and two SO(4)(2-). The final residual and estimated standard uncertainties are R=0.198, ESU=0.076 A for atomic coordinates and ESU=0.05 A for Cu---ligand bond lengths, respectively. The auracyanin B molecule has a standard cupredoxin fold. With the exception of an additional N-terminal strand, the molecule is very similar to that of the bacterial cupredoxin, azurin. As in other cupredoxins, one of the Cu ligands lies on strand 4 of the polypeptide, and the other three lie along a large loop between strands 7 and 8. The Cu site geometry is discussed with reference to the amino acid spacing between the latter three ligands. The crystallographically characterized Cu-binding domain of auracyanin B is probably tethered to the periplasmic side of the cytoplasmic membrane by an N-terminal tail that exhibits significant sequence identity with known tethers in several other membrane-associated electron-transfer proteins.
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ABSTRACT: The thermodynamic parameters of the conformational transition occurring at low pH (acid transition, AT) in blue copper proteins, involving protonation and detachment from the Cu(I) ion of one histidine ligand, have been determined electrochemically for spinach and cucumber plastocyanins, Rhus vernicifera stellacyanin, cucumber basic protein (CBP), and Paracoccus versutus amicyanin. These data were obtained from direct protein electrochemistry experiments carried out at varying pH and temperature. For all species but CBP, the overall conformational change turns out to be exothermic. The entropy change is remarkably species-dependent. In particular, we found that (i) the balance of bond breaking/formation favors the acid transition in plastocyanins, which show remarkably negative DeltaH degrees '(AT) values, and (ii) the transition enthalpy turns out to be much less negative (or even positive) for the two phytocyanins (stellacyanin and CBP): for these species, the transition turns out to be observable thanks to the favorable (positive) entropy change. Thus, it is apparent that the thermodynamic "driving force" for this transition is enthalpic for the plastocyanins and entropic for the phytocyanins. Amicyanin is an intermediate case in which both enthalpic and entropic terms favor the transition. Under the assumption that the transition entropy originates from solvent reorganization effects, which are known to involve compensative enthalpy and entropy changes, the free energy change of the transition would also correspond to the enthalpy change due to bond breaking/formation in the first coordination sphere of the metal and in its immediate environment. Indeed, this term turns out to be very similar for the proteins investigated, in line with the conservation of the Cu(I)-His bond strengths in these species, except for amicyanin, for which the greater exothermicity of the transition can be ascribed to peculiar features of the active site.Biochemistry 01/2003; 41(48):14293-8. DOI:10.1021/bi026564s · 3.19 Impact Factor
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ABSTRACT: Research on photosynthetic electron transfer closely parallels that of other electron transfer pathways and in many cases they overlap. Thus, the first bacterial cytochrome to be characterized, called cytochrome c (2), is commonly found in non-sulfur purple photosynthetic bacteria and is a close homolog of mitochondrial cytochrome c. The cytochrome bc (1) complex is an integral part of photosynthetic electron transfer yet, like cytochrome c (2), was first recognized as a respiratory component. Cytochromes c (2) mediate electron transfer between the cytochrome bc (1) complex and photosynthetic reaction centers and cytochrome a-type oxidases. Not all photosynthetic bacteria contain cytochrome c (2); instead it is thought that HiPIP, auracyanin, Halorhodospira cytochrome c551, Chlorobium cytochrome c555, and cytochrome c (8) may function in a similar manner as photosynthetic electron carriers between the cytochrome bc (1) complex and reaction centers. More often than not, the soluble or periplasmic mediators do not interact directly with the reaction center bacteriochlorophyll, but require the presence of membrane-bound intermediates: a tetraheme cytochrome c in purple bacteria and a monoheme cytochrome c in green bacteria. Cyclic electron transfer in photosynthesis requires that the redox potential of the system be delicately poised for optimum efficiency. In fact, lack of redox poise may be one of the defects in the aerobic phototrophic bacteria. Thus, large concentrations of cytochromes c (2) and c' may additionally poise the redox potential of the cyclic photosystem of purple bacteria. Other cytochromes, such as flavocytochrome c (FCSD or SoxEF) and cytochrome c551 (SoxA), may feed electrons from sulfide, sulfur, and thiosulfate into the photosynthetic pathways via the same soluble carriers as are part of the cyclic system.Photosynthesis Research 02/2003; 76(1-3):111-26. DOI:10.1023/A:1024910323089 · 3.19 Impact Factor
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ABSTRACT: The reversible formal potentials of auracyanin A and auracyanin B, two closely related "blue" copper proteins from the photosynthetic bacterium Chloroflexus aurantiacus, have been determined by protein film voltammetry in the range 4<or=pH<or=9. At pH 7 in 0.1 M NaCl, the values of for auracyanin A and auracyanin B are 205+/-7 mV and 215+/-7 mV, respectively, versus the standard hydrogen electrode. In both cases there is a smooth but non-sigmoidal change in from approximately 190 mV at pH 9 to approximately 240 mV at pH 4. The small changes in as a function of pH indicate that auracyanin A and auracyanin B differ from those "blue" copper proteins in which the Cu site in the reduced (Cu(I)) state switches to a redox-inhibited form at low pH. For auracyanin A, the results obtained by protein film voltammetry are closely similar to those obtained by the conventional spectroelectrochemical method. The findings are discussed in relation to the putative role of auracyanin in biological electron transfer.JBIC Journal of Biological Inorganic Chemistry 03/2003; 8(3):306-17. DOI:10.1007/s00775-002-0416-5 · 3.16 Impact Factor