Molecular geometry of cytochrome c and its peroxidase: a model for biological electron transfer.

Although the respiratory electron transport chain was discovered (or more accurately, rediscovered) by David Keilin in 1925, progress toward understanding its operation at the molecular level has been painfully slow. Compare, for example, how far we have come in characterizing the intimate details of the citric acid cycle during roughly the same time span. The reason for this state of affairs is well known: the machinery of the electron transport chain consists of an extraordinarily complicated array of membrane bound, multi-subunit protein-lipid structures that can be dissected only with great difficulty. I have seen estimates that more than 20 metalcontaining redox centres are involved. Much of this complexity must arise from the way in which the machinery conserves almost all of the energy released in the reduction of oxygen to water by causing ATP to be synthesized. It appears to be generally accepted now that this energy conservation is accomplished by coupling electron transport to proton translocation, so that protons are pumped across the mitochondria1 inner membrane, out of the mitochondrial interior, and that the resulting proton gradient then drives ATP synthesis at other membrane-bound ATPase sites. The one protein component of the electron transport chain that has been most thoroughly examined is cytochrome c, a small soluble electron carrier protein which accepts electrons one at a time from Complex I11 and delivers them to the terminal oxidase, Complex IV. The latter in turn, as the final step in the process, somehow transfers four electrons to a bound 0, molecule and releases two water molecules. While the function of the terminal oxidase sounds simple enough, evidently that function requires a large, complicated molecular assemblage. It has only recently been established, for example, that the mammalian oxidase, Complex IV, probably consists of 12 polypeptide chains containing four redox centres (Merle & Kadenbach, 1980). Although I shall not further discuss the electron transport chain itself, I have introduced my subject in this way to justify the more than five years of effort our laboratory has put into the X-ray crystallographic structure determination of an otherwise really rather obscure haem-containing enzyme, yeast cytochrome c peroxidase, which will be my main focus. What I shall be telling you about, essentially, is how the structure of cytochrome c peroxidase has suggested a working model for haem-catalysed reductive cleavage of the oxygen-xygen bond and electron transfer between protein-bound redox centres, both of which are central features of the respiratory electron transport chain. Details may be found in Poulos et al. (1978, 1980) and Poulos & Kraut (1980a,b). Cytochrome c peroxidase catalyses the oxidation of reduced cytochrome c by hydrogen peroxide and alkyl hydroperoxides, and let me admit at the outset that its biological function is still obscure. However, the enzyme is relatively small (293 amino acid residues), water soluble and readily crystallized. But most importantly, the cytochrome c peroxidase molecule may in certain respects provide clues as to how the terminal oxidase works and in particular how it interacts with cytochrome c. Thus, although we don't know why it's there, cytochrome c The Eighth Keilin Memorial Lecture