Anion complexes of Cu(II) and Co(II) bovine carbonic anhydrase as models for the copper site of blue copper proteins

1 The presence of two intense transitions in the optical absorption spectrum of the sulfide and 2-mercaptoethanol complexes of Cu(II) and Co(II)-substituted bovine carbonic anhydrase suggests that charge-transfer interactions between sulfur and an acceptor group of the protein play an important role in the stabilization of these complexes. 2 The spectra of Co(II) bovine carbonic anhydrase sulfides are very similar to the spectrum of Co(II) stellacyanin whilst the spectra of the corresponding Cu(II) enzymes are considerably different. A possible explanation is that Cu(II) is pentacoordinate in native stellacyanin unlike Cu(II) bovine carbonic anhydrase sulfides and Co(II) enzymes. Tetrahedral Co(II) stellacyanin is proposed as a model of the reduced copper site.