Cytochrome c oxidase: A short review

Abstract

For some years now research on cytochrome c oxidase (EC1931) has been gaining momentum [see 1–3 for reviews]. Largely this is due to the important role this enzyme plays in cellular respiration but also it reflects the availability of new and sophisticated techniques which afford the opportunity of making rapid and significant progress in our understanding of both the structure and mechanism of the enzyme. Cytochrome c oxidase is one of a small class of enzymes capable of catalysing the reduction of molecular oxygen to water. This reaction involves coupling the single electron donors of the mitochondrial respiratory chain to the 4 electron acceptor, dioxygen. The mechanism thus includes oxygen binding and activation, electron transfer steps and stabilisation of potentially harmful oxygen intermediates. Also, as protons are taken up from solution to produce water this terminal step in the respiratory chain is important for the maintenance of a proton gradient across the mitochondrial membrane and thus to ATP synthesis. The complex and varied functions of which cytochrome c oxidase is capable are reflected in its structure which is itself complex and asymmetric. The enzyme isolated from eukaryote sources is made up of a number [7–12] of polypeptide subunits [4, 5] assembled to form a complex which spans the inner mitochondrial membrane [6, 7]. This complex contains four metal centres, two copper atoms and two haem a groups, all of which appear to be associated with the largely hydrophobic subunits. A wide range of spectroscopic and kinetic techniques have now been applied to probe the nature of the metal sites and the route of electron transfer. The consensus view is that electrons enter the complex through one of the haem a groups (cytochrome a) and are rapidly transferred to a copper atom (CuA). Electrons subsequently pass to a binuclear centre consisting of a copper atom (CuB) and a haem a group (cytochrome a3) in close association and which act as the oxygen binding site. The properties of these metal sites and the nature of their immediate environments have now been partially elucidated [e.g. 8–10]. The mechanism of reaction and the nature of bound intermediates have been investigated by coupling spectroscopic methods with low temperature trapping techniques thus allowing intermediates with short life times at in vivo temperatures to be captured and studied [11, 12]. A short review of the structure of cytochrome c oxidase and the nature of the metal sites will be presented together with an outline of the catalytic cycle as postulated from a recent EPR measurements [13]. Experiments were performed in which samples of the enzyme during ‘turnover’, and whilst being monitored by optical methods, were rapidly frozen and prepared for EPR spectroscopy. In this way the optical and EPR signals associated with the metal sites could be related to each other and to the level of reduction maintained during steady-state. This technique has yielded information regarding which of the many known derivatives of the enzymes are populated during catalysis. Attention will also be drawn to site-site interactions and to recent proposals suggesting that interconversion between forms of the enzyme (possibly conformational variants) play a role in the regulation of activity [14].