Copper coordination in blue proteins

The spectroscopic and electrochemical properties of blue copper proteins ar e strikingly different from those of inorganic copper complexes in aqueous solution. Over three decades ago this unusual behavior was ascribed to cons trained coordination in the folded protein; consistent with this view, crys tal structure determinations of blue proteins have demonstrated that the li gand positions are essentially unchanged on reduction as well as in the apo protein. Blue copper reduction potentials are tuned to match the particular function of a given protein by exclusion of water from the metal site and strict control of the positions of axial ligands in the folded structure. E xtensive experimental work has established that the reorganization energy o f a prototypal protein, Pseudomonas aeruginosa azurin, is similar to0.7 eV, a value that is much lower than those of inorganic copper complexes in aqu eous solution. The lowered reorganization energy in the protein, which is a ttributable to constrained coordination, is critically important for functi on, since the driving forces for electron transfer often are low (similar t o0.1 eV) between blue copper centers and distant (>10 Angstrom) donors and accepters.