Electron Transfer to Ferricytochrome c : Reaction with Hydrated Electrons and Conformational Transitions Involved

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Abstract
The reaction of horse-heart cytochrome c with hydrated electrons has been studied by the pulseradiolysis technique. In neutral solution, the ferriheme group was reduced in a bimolecular reaction that takes place at a rate equal to that of the decay of the e - aq , and approaches the diffusion-controlled limit. This reduction is assigned mainly to a direct reaction, proceeding via the exposed edge of the porphyrin projecting into the cytochrome c crevice. The reaction absorption spectrum observed 20 μsec after an electron pulse was very similar, yet blue-shifted relative to, the difference spectrum between the reduced and oxidized forms of cytochrome c . However, this shift vanishes in a slow monomolecular reaction, which seems to reflect the conformational relaxation of the protein to the final equilibrium state of its reduced form. In alkaline solutions, the transition of cytochrome c molecules into an irreducible conformation causes a proportionate decrease in the amount of ferricytochrome c reduced in the direct reaction. The rate of conformational transition of the protein into the reactive form is now the limiting step for a substantial part of the reduction that takes place via this slow monomolecular reaction. Pseudomonas cytochrome c 551 which, in contrast to horse-heart cytochrome c , is a negatively charged protein at neutral pH reacts with e - aq at a rate lower than does the horse-heart protein. The reduction of the heme group follows that of the e - aq decay with a small, yet significant, delay.