Electron self-exchange cytochrome

R. G. Herbert and S. H. Northrup, Brownian simulation of cytochrome C551 association and electron self-exchange, J. Mol. Liq., 41 (1989) 207-222. 

The value of the rate constant obtained for the heme undecapeptide, MPll (1.3 x 10 s ), is comparable to those of other model compounds.The rate constants for model hemes and the heme c undecapeptide are approximately a factor of 10 larger than those found in cytochromes with 80-90 amino acids. This argues further that the heme exposure to the solvent seems not to be a major factor controlling the rate constants for electron self-exchange in cytochromes. 

Concur, D. W., Hill, H. A. O., Moore, G. R., Whitford, D., Williams, R. J. P., The Modulation of Cytochrome C Electron Self-Exchange by Site-Specific Chemical Modification and Anion Binding , FEBS Utt. 206 (1986) 15-19. 

ABSTRACT. A theoretical study of the electron self-exchange in porphyrins and in cytochrome c, and of the free-energy dependence of poq)hyrin-cytochrome c systems ows that these reactions are not easily amenable to an explanation in the framework of the theory of Marcus. On the other hand the intersecting-state model can be used to calculate the self-exchange rates and ] ovide an useful rationalization of the free-energy relationsh obs ed in these systems. 

It is convenient to consider, in the first place, the application of ISM to the electron self-exchange in synthetic models of cytochrome c, in order to avoid the difficulties introduced by the AG dependence and the structural complexity. For the couple iron tetraphenylporphyrin bis-1-methylimidazole, FeTPP(l-MeIm)2 /+ the electron self-exchange rate of 8.1x10 7 -1 s... 

The electron self-exchange in iron porphyrins follows a pattern close to the one of iron-tiisphenanthroline or bipyiidine complexes. The self-exchange in cytochrome c has a higher AG in part due to the nature of the Fe-S bond in the transition state, but its distance dependence is similar to that observed in other proteins. The free-energy relationships observed in porphyrin-cytochrome c systems depend on the nature of the reactant electronic state and, relative to the reactants, may exhibit a more relaxed transition state than the reactions that occur in soludoa... 

The electron self-exchange rate constant and activation parameters have been determined for trypsin-solubilized bovine liver microsomal cytochrome bs, and the reorganizational energies have been evaluated. The rate constant increases with ionic strength from 2.6x 10 s at 0.1 Af to... 

In the following sections the effect of pressure on different types of electron-transfer processes is discussed systematically. Some of our work in this area was reviewed as part of a special symposium devoted to the complementarity of various experimental techniques in the study of electron-transfer reactions (124). Swaddle and Tregloan recently reviewed electrode reactions of metal complexes in solution at high pressure (125). The main emphasis in this section is on some of the most recent work that we have been involved in, dealing with long-distance electron-transfer processes involving cytochrome c. However, by way of introduction, a short discussion on the effect of pressure on self-exchange (symmetrical) and nonsymmetrical electron-transfer reactions between transition metal complexes that have been reported in the literature, is presented. 

The electrostatics-corrected self-exchange rate for plastocyanin based on Co(phen)33+ is 2.6 X 103 M 1 sec 1. The kncorr value for plastocyanin based on the cytochrome c cross reaction, 5 X 105 M"1 sec 1, is substantially smaller than the uncorrected value (3 X 107 M 1 sec 1). Taking either value, however, it is apparent that both cytochrome c and Co-(phen)33+ are better electron transfer agents for plastocyanin than is Fe(EDTA)2". 

The simplest electron transfer reaction that cytochrome c can undergo, at least in principle, is the self-exchange reaction. The rate of this... 

Figure 2 Dependence of the mean square electronic coupling on distance between two porphyrin rings in the cytochrome self-exchange ET reaction (41). For each distance, system conformations were sampled using MD and the coupling was computed for each conformation at the extended Huckel level. The black line marked XEI(P, W) shows the water-mediated coupling for comparison, the red line marked XEI(P) shows the coupling computed for the same protein conformation in vacuum. Conformational snapshots typical for the three coupling regimes are shown.

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