The active site of the plant alternative oxidase: structural and mechanistic considerations

In the present review we seek to provide an up-to-date view on the molecular nature of the active site of the plant alternative oxidase which has been postulated to comprise of a binuclear iron centre. A three-dimensional model of the catalytic centre of the oxidase is presented which is based on the active site structure of the free radical component of ribonucleotide reductase and methane monooxygenase. The model indicates that a highly conserved carboxylate (Glu-270) occupies a central position within the proposed di-iron centre as it co-ordinates both iron atoms. The expression of an alternative oxidase protein in Schizosaccharomyces pombe in which Glu-270 was mutated to asparagine yields an inactive protein. The implications of this in relation to the structural model of the active site of the oxidase suggests that Glu-270 is essential for catalytic alternative oxidase activity. A kinetic mechanism is suggested which accounts for the full reduction of dioxygen to water catalysed by a single di-iron centre.