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 thermophilic green non-sulfurphotosynthetic bacterium Chloroflexus aurantiacus, crystallizes in space group P 6422(a=b=115.7 Å, c=54.6 Å). The structure was solved usingmultiple wavelength anomalous dispersion data recorded about the Cu K absorption edge, and was refined at1.55 Å resolution. The molecular model comprises 139 amino acid residues, one Cu, 247 H2O molecules, oneCl− and two SO42−. The final residual and estimated standard uncertainties are R=0.198, ESU=0.076 Å for atomic coordinates and ESU=0.05 Å for Cu---ligandbond lengths, respectively. The auracyanin B molecule has a standard cupredoxin fold. With the exception of an additionalN-terminal strand, the molecule is very similar to that of the bacterial cupredoxin, azurin. As in other cupredoxins, one of the Culigands lies on strand 4 of the polypeptide, and the other three lie along a large loop between strands 7 and 8. The Cu sitegeometry is discussed with reference to the amino acid spacing between the latter three ligands. The crystallographicallycharacterized Cu-binding domain of auracyanin B is probably tethered to the periplasmic side of the cytoplasmic membrane by anN-terminal tail that exhibits significant sequence identity with known tethers in several other membrane-associatedelectron-transfer proteins.