Conformational changes and possible structure of the oxoglutarate translocator of rat-heart mitochondria revealed by the kinetic study of malate and oxoglutarate uptake.

Initial rates of the exchanges [14C]malateout:malatein, ([14C]oxoglutarate + malate)out:malatein and ([14C]malate + oxoglutarate)out:malatein catalysed by the oxoglutarate carrier of rat-heart mitochondria have been studied under conditions where internal and external substrates may be varied. It is shown that contrary to external oxoglutarate which induces a conformational change of the translocator subunit to which it binds, external malate does not induce conformational changes during its binding and is a Michaelian substrate. The study of the effect of external malate on the rate of oxoglutarate uptake shows that external malate and external oxoglutarate are competitive. External oxoglutarate affects the catalytic rate constant of malate uptake in a modulated way. After substrate binding, the exchange reaction between an external dicarboxylate and an internal dicarboxylate is accompanied by conformational changes. The particular form of the rate equation strongly suggests that during a first step the external substrate bound to an external binding subunit at the external surface of the membrane, and the internal substrate bound to an internal binding subunit at the internal surface of the membrane, are transferred to a catalytic subunit (channel?) deeper in the membrane. Two models, one with a single channel, and the other with several associated channels, are proposed. It is demonstrated that a binding subunit which has transferred its substrate to a catalytic subunit is left in a conformation which does not depend on the substrate that has 'passed through it'. It is also demonstrated that all the catalytic subunits are identical. These theoretical deductions allow a simple description of the complicated effect that external oxoglutarate has on the rate of malate uptake. The fact that all the external binding subunits are equivalent regarding external malate binding and that all the catalytic subunits are identical support the view that the mitochondrial preparation contains a single species of oxoglutarate translocator and not an isozymic mixture.