Cytochrome electron transfer with transition metals

Cytochrome c, a small heme protein (mol wt 12,400) is an important member of the mitochondrial respiratory chain. In this chain it assists in the transport of electrons from organic substrates to oxygen. In the course of this electron transport the iron atom of the cytochrome is alternately oxidized and reduced. Oxidation-reduction reactions are thus intimately related to the function of cytochrome c, and its electron transfer reactions have therefore been extensively studied. The reagents used to probe its redox activity range from hydrated electrons (I, 2, 3) and hydrogen atoms (4) to the complicated oxidase (5, 6, 7, 8) and reductase (9, 10, 11) systems. This chapter is concerned with the reactions of cytochrome c with transition metal complexes and metalloproteins and with the electron transfer mechanisms implicated by these studies. 

The many redox reactions that take place within a cell make use of metalloproteins with a wide range of electron transfer potentials. To name just a few of their functions, these proteins play key roles in respiration, photosynthesis, and nitrogen fixation. Some of them simply shuttle electrons to or from enzymes that require electron transfer as part of their catalytic activity. In many other cases, a complex enzyme may incorporate its own electron transfer centers. There are three general categories of transition metal redox centers cytochromes, blue copper proteins, and iron-sulfur proteins. 

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. 

Iodosylbenzene has been extensively utilized over the last few years for its ability to cleanly transfer oxene oxygen atoms to metals and possibly generate high-valent reactive metal-oxo species. PhIO has been successfully used instead of 02+NADPH in conjunction with cytochrome P-450 to hydroxylate alkanes,81 and has found a variety of interesting applications as a two-electron oxidant in the presence of first-row transition metal porphyrins. 

This is a remarkable reaction because the transition metal chemistry of N2O is sparse, especially with copper. Most N2O reductases are soluble, periplasmic homodimers however, there are examples of membrane-associated enzymes. " The best characterized N2O reductases are from Paracoccus denitrificans, Pseudomonas nautica, and Pseudomonas stutzeri, and most of the information presented here is derived from experiments on these enzymes. Where comparable data are available, N2O reductases from various organisms appear to be fairly similar, with the exception of the enzyme from Wolinella succinogenes, as noted above. The crystal stractmes of N2O reductase from P. nautica and more recently from P. denitrificans show two distinct copper clusters per subunit a bis-thiolate bridged dinuclear electron-transfer site (Cua), which is analogous to the Cua site in cytochrome c oxidase see Cyanide Complexes of the Transition Metals), and a novel four-copper cluster ligated by seven histidines, the catalytic copper site (Cuz), where N2O is thought to bind and be reduced. Cuz was proposed to be a copper-histidine cluster on the basis of the presence of nine strictly conserved histidine residues, and this was supported by a H NMR study that identified two non-CuA associated resonances that were assigned as copper-histidine N-H protons. ... 

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