Dioxygen functional reduction

The terminal oxidase in an energy-transducing, cytochrome-based electron-transport system maintains electron flow by coupling cytochrome oxidation to dioxygen (O2) reduction. Members of this protein class are referred to as cytochrome oxidases they carry out Oj-binding and redox chemistry at transition metal-containing active sites. Although iron is the most commonly used metal and may occur as a protoheme or iron-chlorin species in the protein, this section is concerned only with mitochondrial cytochrome oxidase, which contains 2 mol of Cn and 2 mol of heme a bound Fe per function unit. Biochemistry of the protein will not be considered here, instead the focus will be on the stmcture of the metal centers, on the reactions they catalyze and on models for these centers. 

Collman JP, Boulatov R, Sunderland CJ. 2003a. Functional and structural analogs of the dioxygen reduction site in terminal oxidases. In Kadish KM, Smith KM, Guilard R, editors. The Porphyrin Handbook. Boston Academic Press, p. 1. 

It is rather difficult to study function 2 separately, as the catalyst generally presents, simultaneously, functions 1 and/or function 3. Nevertheless, the mild interaction of HC with N02 can be approached through the direct N02/HC reaction, even in the absence of dioxygen. It has to be compared to the total oxidation of HC in the presence of oxygen, as it is a competitive reaction for the HC consumption. In contrast, it will be very interesting to compare the two catalytic pathways of elementary steps, and reaction intermediates, in both oxidation reactions (mild and total oxidation of reductant). 

Regulation of the total oxidation of reductants (HC, CxIIvO.) by 02, taking into account the difference of temperature between their mild oxidation by NOz and their total oxidation by dioxygen. This point requires the choice of a total oxidation function, but not too much active. 

Fig. 22.8. Energy yields for various anaerobic (top) and aerobic (bottom) metabolisms during mixing of a subsea hydrothermal fluid with seawater, expressed as a function of temperature, per kg of hydrothermal water. Energy yields for acetoclastic methanogenesis and acetotrophic sulfate reduction under oxic conditions are hypothetical, since microbes from these functional groups are strict anaerobes and cannot live in the presence of dioxygen.

The Functional Model. Synthesis of a chemically similar or dissimilar molecule that will mimic the desired function of the enzyme be it dioxygen transport, electron transport, isomerization, or small molecule reduction. 

Estabrook, R.W., Hildebrandt, A.G., Baron, J., Netter, K.J. and Leibman, K. (1971) A new spectral intermediate associated with cytochrome P-450 function in liver microsomes. Biochemical and Biophysical Research Communications, 42 (1), 132-139. Pompon, D. and Coon, M.J. (1984) On the mechanism of action of cytochrome P-450. Oxidation and reduction of the ferrous dioxygen complex of liver microsomal cytochrome P-450 by cytochrome b5. Journal of Biological Chemistry, 259 (24), 15377-15385. Hildebrandt, A. and Estabrook, R.W. (1971) Evidence for the participation of cytochrome b 5 in hepatic microsomal mixed-function oxidation reactions. Archives of Biochemistry and Biophysics, 143 (1), 66-79. 

The modern era of the study of the species derived from the reduction of molecular dioxygen began with the discovery by McCord and Fridovich of the enzymic function of erythrocuprein a cupro-enzyme first isolated from erythrocytes. The reaction catalyzed by erythrocuprein in which superoxide anions dismute. 

Cytochrome P-450 is a hemoprotein that is distributed widely in nature in animal tissue and organelles, and in plants and microoganisms. Its main function lies in the monooxygenation of lipophilic substances, in which two electrons are taken up from NADPH with the reduction of one atom of dioxygen to water and insertion of the other into the substrate (equation 79). 

If a reacting solution is acid quenched then a small amount of hydrazine, derived probably from an intermediate reduction product, is formed. The rate of ammonia formation is linear in PN2 and depends on the square of vanadium concentration, but the actual rate is a function of dioxygen pressure, metal contaminants, etc. The mechanism is believed to follow the pattern discussed above, with dinitrogen bound between two dinuclear pairs of vanadium(II) ions (232, 233), but the precise identity of the fixing species remains a mystery. 

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