Cytochrome oxidase reaction

M. Wikstrom and J.E. Morgan. 1992. The dioxygen cycle Spectral, kinetic, and thermodynamic characteristics of ferryl and peroxy intermediates observed by reversal of the cytochrome oxidase reaction J. Biol. Chem. 267 10266-10273. (PubMed)... 

The assumption of a bound peroxide is compatible with the fact that H202 has not been detected in the cytochrome oxidase reaction. 

Sjogren, T. and J. Hajdu (2001). Structure of the bound dioxygen species in the cytochrome oxidase reaction of cytochrome cdl nitrite reductase. 

Perhaps the most convincing argument in favor of a direct role of copper in the cytochrome oxidase reaction comes from studies done with the electron spin resonance spectrometer. Such studies demonstrate that some of the copper ions undergo valency changes during the oxidation-reduction process. The site of action of the copper in the electron transport chain would then be as follows ... 

By the procedure described in the foregoing section, cytochrome a is purified and no other cytochrome components are observed spectro-photometrically, so that the sample is sufficiently pure to investigate the mechanism of the cytochrome oxidase reaction. However, there is sometimes a small absorption shoulder at about 430 m/A. To remove... 

Eichel et al. (1950), Green and Crane (1957), and Okunuki et al. (1957) have reported that their cytochrome oxidase and cytochrome a preparations contain high concentrations of copper as well as of heme iron. On the other hand, many investigators (Cohen and Elvehjem, 1934 Yoshikawa, 1937 Gallagher, 1956) have observed from dietary experiments that copper-deficient tissues and yeast have a low cytochrome oxidase activity and a decreased heme a content. From the above results, it has been considered that the copper in the cytochrome oxidase of typical cytochrome systems plays an important role in the cytochrome oxidase reaction. However, there is no direct evidence to support the above role of copper. In order to determine whether the copper participates in the cytochrome oxidase reaction, the iron and copper contents of various preparations of cytochrome a were first analyzed. The results are summarized in Table IV. 

It is most interesting that phenylhydrazine and hydrazine inhibit the cytochrome oxidase reaction in a competitive way with cytochrome c. This is the first report of such an inhibitor acting competitively with cytochrome c and specifically with cytochrome oxidase. 

Figure 15 shows that the greater the modification, the more slowly does the reduced band of cytochrome a appear. Within a few minutes, however, reduced cytochrome c containing 20 to 30% acetylation of the total -NHj groups of lysine can reduce cytochrome a at 80% or more of the rate of that of native cytochrome c. However the system reducing cytochrome a does not consume oxygen, as shown in Fig. 16. This is very significant, because it was shown that cytochrome c has a coenzymic function in the cytochrome oxidase reaction in addition to its well-known function as an electron carrier. Since the former function of cytochrome c...

In short, it is reasonable to conclude that the cytochrome oxidase reaction is due to the reduction of molecular oxygen. 

Superoxide is formed (reaction 1) in the red blood cell by the auto-oxidation of hemoglobin to methemo-globin (approximately 3% of hemoglobin in human red blood cells has been calculated to auto-oxidize per day) in other tissues, it is formed by the action of enzymes such as cytochrome P450 reductase and xanthine oxidase. When stimulated by contact with bacteria, neutrophils exhibit a respiratory burst (see below) and produce superoxide in a reaction catalyzed by NADPH oxidase (reaction 2). Superoxide spontaneously dismu-tates to form H2O2 and O2 however, the rate of this same reaction is speeded up tremendously by the action of the enzyme superoxide dismutase (reaction 3). Hydrogen peroxide is subject to a number of fates. The enzyme catalase, present in many types of cells, converts... 

Boulatov R. 2004. Understanding the reaction that powers this world Biomimetic studies of respiratory O2 reduction by cytochrome oxidase. Pure Appl Chem 76 303. 

P. Mitchell (Nobel Prize for Chemistry, 1978) explained these facts by his chemiosmotic theory. This theory is based on the ordering of successive oxidation processes into reaction sequences called loops. Each loop consists of two basic processes, one of which is oriented in the direction away from the matrix surface of the internal membrane into the intracristal space and connected with the transfer of electrons together with protons. The second process is oriented in the opposite direction and is connected with the transfer of electrons alone. Figure 6.27 depicts the first Mitchell loop, whose first step involves reduction of NAD+ (the oxidized form of nicotinamide adenosine dinucleotide) by the carbonaceous substrate, SH2. In this process, two electrons and two protons are transferred from the matrix space. The protons are accumulated in the intracristal space, while electrons are transferred in the opposite direction by the reduction of the oxidized form of the Fe-S protein. This reduces a further component of the electron transport chain on the matrix side of the membrane and the process is repeated. The final process is the reduction of molecular oxygen with the reduced form of cytochrome oxidase. It would appear that this reaction sequence includes not only loops but also a proton pump, i.e. an enzymatic system that can employ the energy of the redox step in the electron transfer chain for translocation of protons from the matrix space into the intracristal space. 

Adults require 1-2 mg of copper per day, and eliminate excess copper in bile and feces. Most plasma copper is present in ceruloplasmin. In Wilson s disease, the diminished availability of ceruloplasmin interferes with the function of enzymes that rely on ceruloplasmin as a copper donor (e.g. cytochrome oxidase, tyrosinase and superoxide dismutase). In addition, loss of copper-binding capacity in the serum leads to copper deposition in liver, brain and other organs, resulting in tissue damage. The mechanisms of toxicity are not fully understood, but may involve the formation of hydroxyl radicals via the Fenton reaction, which, in turn initiates a cascade of cellular cytotoxic events, including mitochondrial dysfunction, lipid peroxidation, disruption of calcium ion homeostasis, and cell death. 

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