Functions of the membrane-associated and cytoplasmic malate dehydrogenasesin the citric acid cycle of Corynebacterium glutamicum

Like many other bacteria, Corynebacterium glutamicum possesses two types of L-malate dehydrogenase, a membrane-associated malate:quinone oxidoreductas e (MQO; EC 1.1.99.16) and a cytoplasmic malate dehydrogenase (MDH; EC 1.1.1 .37) The regulation of MDH and of the three membrane-associated dehydrogena ses MQO, succinate dehydrogenase (SDH), and NADH dehydrogenase was investig ated. MQO, MDH, and SDH activities are regulated coordinately in response t o the carbon and energy source for growth. Compared to growth on glucose, t hese activities are increased during growth on lactate, pyruvate, or acetat e, substrates which require high citric acid cycle activity to sustain grow th. The simultaneous presence of high activities of both malate dehydrogena ses is puzzling. MQO is the most important malate dehydrogenase in the phys iology of C, glutamicum, A mutant with a site-directed deletion in the mqo gene does not grow on minimal medium. Growth can be partially restored in t his mutant by addition of the vitamin nicotinamide, in contrast, a double m utant lacking MQO and MDH does not grow even in the presence of nicotinamid e, Apparently, MDH is able to take over the function of MQO in an mqo mutan t, but this requires the presence of nicotinamide in the growth medium. It is shown that addition of nicotinamide leads to a higher intracellular pyri dine nucleotide concentration, which probably enables MDH to catalyze malat e oxidation, Purified MDH from C,glutamicum catalyzes oxaloacetate reductio n much more readily than malate oxidation at physiological pH, In a reconst ituted system with isolated membranes and purified MDH, MQO and MDH catalyz e the cyclic conversion of malate and oxaloacetate, leading to a net oxidat ion of NADH. Evidence is presented that this cyclic reaction also takes pla ce in vivo. As yet, no phenotype of an mdh deletion alone was observed, whi ch leaves a physiological function for MDH in C,glutamicum obscure.