The effect of inhibitors on the oxygen kinetics of cytochrome c oxidase.

1. The oxygen kinetics of purified beef heart cytochrome c oxidase were investigated. 2. The effect of addition of various fixed concentrations of the inhibitors CO, HN3, HCOOH, HCN and H2S on the double reciprocal plot of respiration rate against oxygen concentration was studied. 3. CO is strictly competitive, azide and formate are uncompetitive, and cyanide and sulfide are non-competitive inhibitors towards oxygen. 4. Binding constants for the various inhibitors from secondary plots of the oxygen kinetics at pH 7.4 are: CO: Ki = 0.32 micronM, azide: Ki = 33 micronM; formate: Ki = 15 mM; cyanide: Ki = 0.2 micronM and sulfide: Ki = 0.2 micronM. 5. The possible significance of these results in the elucidation of the reaction mechanism is discussed.     

Semiconduction as the mechanism of the cytochrome oxidase reaction. Low activation energy of semiconduction measured for cytochrome oxidase protein. Solid state theory of cytochrome oxidase predicts observed kinetic peculiarities.

Cytochrome oxidase protein has a measured activation energy of semiconduction much smaller than that of other proteins, falling within the range of the activation energy of the cytochrome oxidase reaction in solution. Many kinetic peculiarities of the cytochrome oxidase reaction difficult to explain by mass-action theories are easily accounted for if semiconduction is assumed to be the controlling mechanism.     

The inhibition of cytochrome oxidase by diaminomaleonitrile

Diaminomaleonitrile, a tetramer of cyanide, was examined as a possible antagonist to cyanide inhibition of cytochrome oxidase (EC 1.9.3.1). This compound was found to inhibit cytochrome oxidase in vitro; however, despite their structural similarities, diaminomaleonitrile and cyanide inhibit cytochrome oxidase by different mechanisms and bind to the enzyme at different sites. Diaminomaleonitrile inhibition of cytochrome oxidase is described in terms of a partially competitive mechanism. Biological oxidation of diaminomaleonitrile may lead to the formation of cyanide.     

The mechanism of reaction of fully reduced membrane-bound cytochrome oxidase with oxygen at 176K.

1. The results of non-linear optimization studies on the mechanism of reaction of fully reduced cytochrome oxidase with O2 at 176K are presented. The analysis is carried out on data obtained by means of dual-wavelength multi-channel spectroscopy at three wavelength pairs (604-630, 608-630 and 830-940 nm) and at three O2 concentrations (60, 200 and 1180 micron). The only model that satisfies the triple requirement of a standard deviation within the standard error of the experimental data, good determination of the optimized parameters and a random distribution of residuals is a three-species sequential mechanism. 2. On the basis of the optimized values of the relative absorption coefficients of the intermediates at each wavelength obtained from the present paper together with data from low-temperature trapping, e.p.r. and magnetic-susceptibility studies, the possible valence states of the metal centres in each of the intermediates are discussed.     

The mechanism of reaction of ferricyanide-pretreated mixed-valence-state membrane-bound cytochrome oxidase with oxygen at 173 K.

1. The results of non-linear optimization studies on the mechanism of reaction of ferricyanide-pretreated mixed-valence-state cytochrome oxidase with O2 at 173 K are presented. The analysis is carried out on data obtained by means of dual-wavelength multi-channel spectroscopy at four wavelength pairs (444-463 nm, 604-630 nm, 608-630 nm and 830-940 nm) and at two O2 concentrations (360 micron and 520 micron). The only model that satisfies the triple requirement of a standard deviation within the standard error of the experimental data, a random distribution of residuals and good determination of the optimized parameters, is a three-intermediate sequential mechanism. 2. On the basis of the optimized values of the relative absorption coefficients of the intermediates at each wavelength obtained from the present paper together with data from optical wavelength scanning and e.p.r. spectroscopy obtained by low-temperature trapping studies, the possible valence states of the metal centres in each of the intermediates are discussed.     

X-ray structure and the reaction mechanism of bovine heart cytochrome c oxidase

X-ray structure of bovine heart cytochrome c oxidase in the fully oxidized state shows a peroxide bridging between Fe2+ and Cu2+ in the O2 reduction site. The bond distances for Fe-O and Cu-O are 2.52 and 2.16 A, respectively. The structure is consistent with antiferromagnetic coupling between the two metals, which has long been known and to recent redox titration results [J. Biol. Chem. 274 (1999) 33403]. The trigonal planer coordination of Cu1+ in the O2 reduction site is consistent with the very weak interaction between Cu1+ and O2 bound at Fe2+ revealed by time-resolved resonance Raman investigations. One of the three histidine imidazoles coordinated to the Cu ion in the O2 reduction site fixes a tyrosine phenol group near the O2 reduction site with the direct covalent link between the two groups. The structure suggests that the phenol group is the site for donating protons to the bound O2. Redox-coupled conformational change in an extramembrane loop indicates that an aspartate (Asp51) in the loop apart from the O2 reduction site is the site for proton pumping.