NADP⁺‐malate dehydrogenase in C₃‐plants: Regulation and role of a light‐activated enzyme

The thioredoxin‐dependent light/dark modulation system of the chloroplast is described as a prerequisite enabling the flexible control of fluxes through the various parts of the CO 2 ‐fixation pathway. Both the rapid turnover of the reduced thiol‐containing form of the respective target enzyme, and the metabolite effect upon the reductive enzyme modulation, allow rapid adjustment of the amount of active species to the actual requirements. The structural basis of the regulation of chloroplast NADP + ‐malate dehydrogenase (EC 1.1.1.82) is described in more detail. The modulable plastid enzyme is characterized by two sequence extensions not present in any other known NADP + ‐ and/or NAD + ‐specific malate dehydrogenase. The NADP + ‐malate dehydrogenase of C 3 ‐plants is part of the “malate valve”, which catalyzes the export of reducing equivalents in the form of malate from the chloroplast only when the NADPH to NADP + ratio is high, thus poising the NADPH to ATP ratio required for optimal carbon reduction in the light. The mode of regulation of other light/dark modulated enzymes is discussed.