Kinetics of the coupled reaction catalysed by a fusion protein of yeast mitochondrial malate dehydrogenase and citrate synthase

The mechanistic implications of the kinetic behaviour of a fusion protein o f mitochondrial malate dehydrogenase and citrate synthase have been reanaly sed in view of predictions based on experimentally determined kinetic param eter values for the dehydrogenase and synthase activities of the protein. T he results show that the time-course of citrate formation from malate in th e coupled reaction catalysed by the fusion protein can be most satisfactori ly accounted for in terms of a free-diffusion mechanism when consideration is taken to the inhibitory effects of NADH and oxaloacetate on the malate d ehydrogenase activity. The effect of aspartate aminotransferase on the coup led reaction is likewise fully consistent with that expected for a free-dif fusion mechanism. It is concluded that no tenable kinetic evidence is avail able to support the proposal that the fusion protein catalyses citrate form ation from malate by a mechanism involving channelling of the intermediate oxaloacetate.