Is H2O2 Involved in Electron Transport Between Cytochromes a and a3 in Cytochrome Oxidase?
The molecular mechanism by which cytochrome oxidase translocates protons across the inner mitochondrial membrane is unknown. Since the four redox centres in the oxidase (cytochromes a and a 3, CuA and CuB) are electron carriers, it has been assumed that the mechanism of proton pumping cannot involve direct ligand conduction, and must therefore be indirect, for example by a redox-linked Bohr effect (Papa, 1976). However, it has been pointed out that involvement of H2O2 as an intermediate in the oxidase reaction permits the formulation of theoretical O-loops or cycles, which could give rise to the observed stoichiometry, by a direct mechanism (Mitchell et al, 1985). The simplest such model is an O-loop (Fig.1), where two molecules of H2O2 sequentially generated at “centre A”, diffuse to “centre O”, from where they sucessively reduce oxygen at “centre B” to H2O2 and then to H2O. This model makes the radical prediction of two sites of oxygen interaction with the oxidase, whereas spectroscopically only one site (associated with cytochrome a 3) has ever been demonstrated.
KeywordsCytochrome Oxidase Alpha Band Redox Centre Oxygen Removal Pulse Form
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