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 suggest 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.