Structural basis for the light regulation of chloroplast NADP malate dehydrogenase

The activity of chloroplast NADP-malate dehydrogenase (NADP-MDH; EC 1.1.1.8 2) in both C-3 and C-4 plants is regulated by light intensity, In darkness, the activity of the enzyme can be less than 1% of the maximal activity fou nd at high light intensities. The extent of activation in the light is dyna mic, responding rapidly to changes in light intensity and adapting to chang es in photosynthetic rate, Enzyme activation is caused by thioredoxin-catal yzed reduction of two regulatory disulfide bonds, while inactivation is acc omplished by thioredoxin-catalyzed re-oxidation, In the case of NADP-MDH, t he coenzyme substrates NADP(+) and NADPH modify the rate of this interconve rsion and seem to be important to the extent of activation in vivo. The rec ent determination of the X-ray structure of the oxidized, dark form of NADP -MDH from the C-4 plants Flaveria bidentis and Sorghum shows how oxidation of a disulfide bond can inactivate the enzyme. This review discusses the va rious structural features of NADP-MDH that seem to be responsible for the r egulatory properties of the enzyme and emphasizes that large changes of act ivity can be accomplished by multiple, small, reinforcing changes rather th an a single large change in a signal molecule concentration.