Direct measurements of electron transfer (ET) within a protein-protein complex with a redesigned interface formed by physiological partner proteins myoglobin (Mb) and cytochrome b5 (b5) reveal interprotein ET rates comparable to those observed within the photosynthetic reaction center. Brownian dynamics simulations show that Mb in which three surface acid residues are mutated to lysine binds b5 in an ensemble of configurations distributed around a reactive most-probable structure. Correspondingly, charge-separation ET from a photoexcited singlet zinc porphyrin incorporated within Mb to the heme of b5 and the follow-up charge-recombination exhibit distributed kinetics, with median rate constants, kf s = 2.1 × 109 second-1 and k bs = 4.3 × 1010 second-1, respectively. The latter approaches that for the initial step in photosynthetic charge separation, k = 3.3 × 1011 second-1.
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