Enhanced rate of intramolecular electron transfer in an engineered purple Cu_A azurin

The recent expression of an azurin mutant where the blue type 1 copper site is replaced by the purple Cu(A) site of Paracoccus denitrificans cytochrome c oxidase has yielded an optimal system for examining the unique electron mediation properties of the binuclear Cu(A) center, because both type 1 and Cu(A) centers are placed in the same location in the protein while all other structural elements remain the same. Long-range electron transfer is induced between the disulfide radical anion, produced pulse radiolytically, and the oxidized binuclear Cu(A) center in the purple azurin mutant. The rate constant of this intramolecular process, k(ET) = 650 ± 60 s^-1 at 298 K and pH 5.1, is almost 3-fold faster than for the same process in the wild-type single blue copper azurin from Pseudomonas aeruginosa (250 ± 20 s^-1), in spite of a smaller driving force (0.69 eV for purple Cu(A) azurin vs. 0.76 eV for blue copper azurin). The reorganization energy of the Cu(A) center is calculated to be 0.4 eV, which is only 50% of that found for the wild-type azurin. These results represent a direct comparison of electron transfer properties of the blue and purple Cu(A) sites in the same protein framework and provide support for the notion that the binuclear purple Cu(A) center is a more efficient electron transfer agent than the blue single copper center because reactivity of the former involves a lower reorganization energy.