Dynamic assembly of malate dehydrogenase-citrate synthase multienzyme complex in the mitochondria

The tricarboxylic acid (TCA) cycle enzymes malate dehydrogenase (MDH1) and citrate synthase (CIT1) form a multienzyme complex, referred to as a metabolon, that channels intermediate oxaloacetate between their reaction centers. Given that the MDH1-CIT1 metabolon enhances pathway reactions in vitro, its dynamic assembly is hypothesized to contribute to TCA cycle regulation in response to cellular metabolic demands. Here, we demonstrated that yeast mitochondrial MDH1 and CIT1 dissociated when aerobic respiration was suppressed by the Crabtree effect and associated when the respiratory activity was enhanced by acetate. Pharmacological TCA cycle inhibition dissociated the complex, whereas electron transport chain inhibition enhanced the interaction. The multienzyme complex assembly was related to the mitochondrial matrix acidification and oxidation, as well as cellular levels of malate, fumarate, and citrate. These factors significantly affected the MDH1-CIT1 complex affinity in vitro. Especially, variations in buffer pH within the physiological pH range between 6.0 and 7.0 in the mitochondrial matrix significantly impacted the MDH1-CIT1 affinity. These results demonstrate the dynamic association and dissociation of the MDH1-CIT1 metabolon and its relationship with respiratory activity, supporting metabolon dynamics as an integral factor in metabolic regulation governed by multiple factors such as mitochondrial pH and metabolite levels.