Malate synthesis in the chloroplasts of C₃ plants

Vascular plants distribute reducing power among organelles, cells, and tissues in the form of malate, a stable metabolite from which organisms can quickly generate the strong reductants NADH and NADPH. Current thought is that malate in chloroplasts derives either from the import and reduction of oxaloacetate or from the import of malate into chloroplasts via an exchange with glutamate. This article proposes that plastid malate may also derive from rubisco-mediated catabolism of RuBP to pyruvate, followed by the putative conversion of pyruvate to malate through two alternative pathways. One pathway involves the enzymes phosphogluconate dehydrogenase and malic enzyme, and the other involves PEP carboxylase and malate dehydrogenase. These enzymes have metal-binding sites that accommodate either manganese or magnesium, but association with manganese favors malate generation. Therefore, plants may shift the balance between manganese and magnesium activities in chloroplasts to enhance malate generation for nitrate and sulfate assimilation into amino acids at the expense of carbon fixation and carbohydrate accumulation. This regulatory mechanism may help plants maintain protein homeostasis as atmospheric CO₂ rises in the coming decades.