High-potential states of blue and purple copper proteins

Abstract

Electrochemical measurements show that there are high-potential states of two copper proteins, Pseudomonas aeruginosa azurin and Thermus thermophilus Cu A domain; these perturbed states are formed in guanidine hydrochloride (GuHCl) solution in which the proteins are still blue (azurin) and purple (Cu A). In each case, the high-potential state forms reversibly. Absorption (azurin, Cu A), visible circular dichroism (azurin, Cu A), resonance-Raman (Cu A), and EPR (Cu A) spectra indicate that the structure of the oxidized copper site of each high-potential form is very similar to that of the native protein. It is proposed that GuHCl perturbs one or more H-bonds in the blue or purple copper active site, thereby allowing Cu(I) to adopt a more favorable coordination structure than that in the rigid cavity of the native protein.