The metal centers of cytochrome c oxidase: Structure and function

Abstract Considerable progress has been made in recent years on the structure of the metal centers in cytochrome c oxidase. Most of these studies have naturally focused on the ligands of the metal center as they play a prominent role in electron transfer, oxygen reduction and possibly also energy conservation. The most unambiguous structural information on the ligands has emerged from EPR/ENDOR studies, particularly when these studies are undertaken in conjunction with isotopically substituted cytochrome c oxidases prepared by incorporating selectively isotopically substituted amino acids into the protein via biosynthetic procedures. These results will be reviewed. Good progress has also been made towards elucidating the mechanism of dioxygen reduction. Preliminary evidence for a mechanism involving both a peroxo and a ferryl intermediate will be presented. The possible structural differences at the dioxygen reduction site between the resting oxidized enzyme and the pulsed enzyme will also be discussed. While impressive progress has been made toward understanding the structure of the metal centers of cytochrome oxidase and their roles in dioxygen reduction, comparatively little is known about the mechanisms by which the free energy of this reaction is conserved in the form of a transmembrane electrochemical potential gradient. It appears likely that the protons consumed in the reduction of dioxygen to water are derived from the matrix side of the mitochondrial membrane. Since the electrons used in this reaction originate from the intermembrane space of the mitochondrion, a transmembrane electrochemical potential gradient will be generated. However, the spatial dispositions of the metal centers in the membrane profile (or along the transmembrane electrochemical potential profile) are not sufficiently known to allow one to pinpoint the electron transfer step(s) which will lead to energy conservation by this mechanism. The available data which bear upon this question will be discussed briefly and new experimental approaches will be suggested. Another means of conserving the available energy is pumping protons from the mitochondrial matrix to the intermembrane space. Evidence that the enzyme is, in fact, a proton pump will be reviewed, as will some of the current models for the mechanism of this pumping. The relative merits of each of the metal centers as the site of proton pumping will be considered. The implications of electron transfer rate theory for the mechanism of proton pumping and the positioning of the metal sites in the transmembrane electrochemical potential profile will be explored. It is suggested that CuA in the most suitable candidate for the proton pump; available evidence for this hypothesis will be presented. The cytochrome oxidase-catalyzed dioxygen reduction reaction will probably not release energy in uniform increments. Thus, it might be expected that some elctron-transfer steps will not lead to proton pumping, particularly under conditions of high membrane potential. This possibility has not been adequately appreciated by investigators who attempt to assign fixed proton/electron stoichiometrics to the cytochrome oxidase reaction. This question will be discussed with reference to available information on the thermodynamics of this reaction. Simple mechanisms by which the enzyme might adapt to a changing membrane potential will be described.