Spectral and Kinetic Equivalence of Oxidized Cytochrome c Oxidase as Isolated and “Activated” by Reoxidation

Abstract

The spectral and kinetic characteristics of two oxidized states of bovine heart cytochrome c oxidase (CcO) have been compared. The first is the oxidized state of enzyme isolated in the fast form (O) and the second is the form that is obtained immediately after oxidation of fully reduced CcO with O 2 (O H ). No observable differences were found between O and O H states in: (i) the rate of anaerobic reduction of heme a 3 for both the detergent-solubilized enzyme and for enzyme embedded in its natural membraneous environment, (ii) the one-electron distribution between heme a 3 and Cu B in the course of the full anaerobic reduction, (iii) the optical and (iv) EPR spectra. Within experimental error of these characteristics both forms are identical. Based on these observations it is concluded that the reduction potentials and the ligation states of heme a 3 and Cu B are the same for CcO in the O and O H states.Mitochondrial cytochrome c oxidase (CcO) 2 transfers electrons from cytochrome c to dioxygen. The reduction of O 2 to water is associated with the generation of a transmembrane proton gradient. Two processes contribute to the development of this gradient. The first is the oxidation of ferrocytochrome c from the cytosolic side of the inner mitochondrial membrane coupled to proton consumption for water formation from the matrix compartment of mitochondria. The second process is proton pumping during which protons are translocated from the matrix space to the cytosolic side of the membrane (1).In CcO four redox centers participate in electron transfer (ET) and reduction of O 2 to H 2 O. Three of these centers, heme a, heme a 3 , and copper ion, called Cu B , are in subunit I and a dinuclear copper center Cu A , is located in subunit II (2). Cu A serves as the acceptor of electrons either from ferrocytochrome c or from artificial electron donors (3-7). Electrons received by the oxidase are rapidly distributed between Cu A and heme a on the microsecond time scale (4, 5, 7-10). ET continues further to the catalytic center composed of heme a 3 and Cu B where O 2 is reduced to water.The part of the catalytic cycle during which electrons are delivered to CcO, prior to reaction with O 2 , is usually referred to as the reductive phase. In the subsequent oxidative phase the reduced enzyme reacts with O 2 . It was proposed (11) and demonstrated later that proton pumping takes place in both the reductive and oxidative phases (12)(13)(14). Two protons are pumped in the oxidative phase when fully reduced CcO is oxidized by oxygen (12,14). Subsequently, if this freshly oxidized enzyme is reduced by two electrons two additional protons are translocated across the membrane (12, 14).However, proton translocation in the reductive phase is only observed with freshly oxidized enzyme. This reoxidized enzyme appears to be in a metastable state, which relaxes relatively slowly in the absence of electron donors to the stable oxidized form on a time scale of seconds. These two forms of oxidized enzyme, CcO as it exists immediately after ox...

Computational study of the activated O H state in the catalytic mechanism of cytochrome c oxidase

“…A previous spectroscopic study on the bovine heart enzyme revealed no difference between the two states (27). However, in that study the observed rate of reduction of the BNC in the presumed O H state was much slower than reported in ref.…”

“…It has turned out to be difficult to trap the metastable O H state to study its structure, lifetime, and how it differs from state O (27,28). However, electron photo-injection experiments indeed showed that Cu B has a raised redox potential in the O H state: the E m,7 is at least 100 mV higher than the corresponding potentials of hemes a and a 3 (25,29 Resonance Raman spectroscopy studies by Rousseau and coworkers (31) have established an intermediate beyond the F state, but before state O.…”

Protonation of the binuclear active site in cytochrome c oxidase decreases the reduction potential of CuB

“…In Ref. [10] it was argued that at least one equilibrium measurement of the Cu B reduction potential [4] should be quite uncertain, since it is based on an uncertain estimate of the amount of reduced copper, and that the calculated reduction potentials for Cu(II) and Fe(III) may be compatible with the observations in that experiment.…”

“…However, so far there are no well founded suggestions for possible structural differences between two oxidized states that may lead to different reduction potentials for Cu B . Neither has it been possible to establish two states with different spectroscopic properties [4].…”

“…The exergonicity of each of the four reduction steps is determined by the difference between the reduction potential of the donor cytochrome c (0.25 V) and the proton coupled reduction potential of the particular active site cofactor reduced in each step. Experimental information indicates that the equilibrium reduction potentials for Cu(II) and Fe(III) are in the range 0.28-0.35 V [2][3][4][5], which means that the two reduction steps from O to R (see Fig. 1) should be only slightly exergonic already without the gradient.…”