Biological redox proteins, oxidation-reduction potentials

Iron-sulfur proteins belong to the class of electron-transport proteins [29]. They contain an iron sulfur cluster, e.g. [4Fe-4S], which shuttles between different oxidation states. The structure of the cluster is quite consistent among a series of these proteins, but their redox potentials vary widely. Synthetic models of iron-sulfur proteins have been designed [30] to investigate the factors that determine the reduction potential of the core and to mimic other biologically... 

Ferredoxins and Rieske proteins employ a (Fe )2/Fe Fe redox couple for biological electron transfer reactions. Within the protein, the two iron atoms are rendered inequivalent, even in the hilly oxidized (Fe )2 state, by the surrounding protein environment Within a synthetic cluster, however, both iron atoms are typically equivalent, as may be expected from the symmetry of the overall complex. Table 4 shows reduction potentials for selected analog clusters. 

The simple constitution of the active center, iron and sulfur, contrasts with the diversified role played by these proteins in key biological oxidation-reduction processes, such as carbon, hydrogen, sulfur and nitrogen metabolism, using a very wide range of redox potentials (+ 350 mV in photosynthetic bacteria to — 600 mV in chloro-plasts). 

The NO/NO+ and NO/NO- self-exchange rates are quite slow (42). Therefore, the kinetics of nitric oxide electron transfer reactions are strongly affected by transition metal complexes, particularly by those that are labile and redox active which can serve to promote these reactions. Although iron is the most important metal target for nitric oxide in mammalian biology, other metal centers might also react with NO. For example, both cobalt (in the form of cobalamin) (43,44) and copper (in the form of different types of copper proteins) (45) have been identified as potential NO targets. In addition, a substantial fraction of the bacterial nitrite reductases (which catalyze reduction of NO2 to NO) are copper enzymes (46). The interactions of NO with such metal centers continue to be rich for further exploration. 

Flavin coenzymes are usually bound tightly to proteins and cycle between reduced and oxidized states while attached to the same protein molecule. In a free unbound coenzyme the redox potential is determined by the structures of the oxidized and reduced forms of the couple. Both riboflavin and the pyridine nucleotides contain aromatic ring systems that are stabilized by resonance. Part of this resonance stabilization is lost upon reduction. The value of E° depends in part upon the varying amounts of resonance in the oxidized and reduced forms. The structures of the coenzymes have apparently evolved to provide values of E° appropriate for their biological functions. 

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