REDUCTION POTENTIALS OF BLUE AND PURPLE COPPER PROTEINS IN THEIR UNFOLDED STATES - A CLOSER LOOK AT RACK-INDUCED COORDINATION

Cyclic voltammetry has been used to determine the reduction potentials of blue (Pseudomonas aeruginosa azurin) and purple (Thermus thermophi lus Cu-A domain) copper proteins unfolded by guanidine hydrochloride. These Cu(II/I) potentials [456 (azurin); 453 (Cu-A) mV vs., NHE] are h igher than those of the folded proteins, The downshift of the potentia l in the folded state can be accounted for by assuming that rack-induc ed axial coordination stabilizes Cu(II) relative to Cu(I) in a protein -encapsulated active site.