Crystal structure of auracyanin, a "blue" copper protein from the green thermophilic photosynthetic bacterium Chloroflexus aurantiacus

Auracyanin B, one of two similar blue copper proteins produced by the therm ophilic green non-sulfur photosynthetic bacterium Chloroflexus aurantiacus, crystallizes in space group P6(4)22 (a = b = 115.7 Angstrom, c = 54.6 Angs trom). The structure was solved using multiple wavelength anomalous dispers ion data recorded about the CuK absorption edge, and was refined at 1.55 An gstrom resolution. The molecular model comprises 139 amino acid residues, o ne Cu, 247 H2O molecules, one Cl- and two SO42-. The final residual and est imated standard uncertainties are R = 0.198, ESU = 0.076 Angstrom for atomi c coordinates and ESU = 0.05 Angstrom for Cu-ligand bond lengths, respectiv ely. The auracyanin B molecule has a standard cupredoxin fold. With the exc eption of an additional N-terminal strand, the molecule is very similar to that of the bacterial cupredoxin, azurin. As in other cupredoxins, one of t he 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 discuss ed with reference to the amino acid spacing between the latter three ligand s. The crystallographically characterized Cu-binding domain of auracyanin B is probably tethered to the periplasmic side of the cytoplasmic membrane b y an N-terminal tail that exhibits significant sequence identity with known tethers in several other membrane-associated electron-transfer proteins. ( C) 2001 Academic Press.