A kinetic model for the regulation of electron transfer through the cyanide-resistant pathway in plant mitochondria

Abstract

The appearance of a voltametric technique for measuring the redox poise of the ubiquinone pool during the course of electron transfer (Moore et al. (1988) FEBS Lett. 235: 76–80) has facilitated attempts to understand the regulation of electron flow between the main and the cyanide-resistant (alternative) electron transfer chains in isolated plant mitochondria. In the present study, a two-step reduction model describing electron flow between the ubiquinone pool and the alternative oxidase is developed based upon ‘Q-pool’ assumptions. The steady-state rate equation that results from this model is used to simulate data describing the relationship between Q-pool redox poise and alternative pathway activity in mitochondria isolated from several plant sources. These simulations provide a better fit to the observed relationship than those associated with previous models. The simulations also indicate that variations in the observed relation between Q-pool reduction state and alternative pathway activity among different plant mitochondria is mainly associated with differences in the rates of reaction between the reduced oxidase and oxidized ubiquinone, and further suggest that the reduction of oxygen by the alternative oxidase proceeds via the initial formation of a four-electron reduced enzyme. The derived kinetic rate equation for the two-step reduction model also explains the source of the low apparent K m for oxygen of the alternative oxidase and provides some insights into conditions under which the alternative pathway velocity should display a greater dependence upon oxygen concentration.