Biological electron transfer reactions, general

The experiments referred to in this chapter have also assisted the theoretical analysis of the role of temperature and pressure on biological electron transfer reactions. In the case of cytochrome c, an empirical approach could show that the heme iron is screened more efficiently from surface charges in the oxidized state (37). In general, solute-solvent interactions are directly influenced by temperature and pressure, and these interactions will affect the electrostatic interaction energies, which can be accounted for in terms of changes in the dielectric constant of both the solute and the solvent. Such interactions are of major importance in the understanding of electron transfer processes. 

Mechanism (20) provides a general framework for characterizing the origin of donor and acceptor specificity in biological electron transfer reactions. In sequential order, discrimination between transfer to correct and incorrect electron acceptors may occur at Steps 1,2, and/or 3. 

It has been appreciated for some time that protein-bound iron-sulfur clusters are important in biological electron transfer reactions and, reasonably, they can be found in a variety of proteins associated with redox systems. Although data are incomplete, the extent to which they are effective in any particular system (i.e., their measured reduction potential) appears to be a function of their position (e.g., buried vs. exposed) in the protein. However, in general, as shown in Figure 12.2, the 4Fe-4S cluster is surrounded by protein-bound cysteine residues, forming sulfur-iron bonds external to the cluster, holding it in place. 

Given the extensive literature on electron transfer reactions, we have not attempted to provide an exhaustive and historically complete collection of references. In keeping with the general nature of the review, we have tried to cite representative references which serve to illustrate particular points. A number of excellent reviews on electron transfer theory and biological electron transfer have recently appeared (7-9) more detailed information on many of the topics may be found in these sources. 

Two factors appear important in establishing sequential selective electron transfer in biological chains of this type. One is the arrangement of free-energy differences AG between chain components, so that the flow of electrons is thermodynamically downhill (Fig. 9.3). The second factor involves the use of distance and the effect of distance on the rate of electron transfer in providing selectivity. This latter factor has attracted much attention, and there have been many studies of the effect of distance on the rate of electron transfer reactions in both biological systems and model systems. We can write a general expression based on the Marcus treatment for the rate of electron transfer ... 

General metabolic significance. Flavin coenzyme dependent enzymes catalyze one-electron-transfer reactions, dehydrogenase reactions, reduction of O2 to H2O2, hydroxylations, etc. - numerous flavin enzymes dealing with biological oxidation. 

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