Light-activation of NADP-malate dehydrogenase: A highly controlled processfor an optimized function

The chloroplastic nicotinamide adenine dinucleotide phosphate-malate dehydr ogenase (NADP-MDH) (EC 1.1.1.82), a key enzyme of photosynthetic carbon ass imilation of the C-4 NADP-malic enzyme type plants, is strictly regulated b y light through the ferredoxin-thioredoxin system. It is inactive in the da rk, in the oxidized form, and activated in the light by the reduction of sp ecific regulatory disulfides, A site-directed mutagenesis approach allowed localization of the regulatory disulfides in the N- and C-terminal sequence extensions conserved in all the light-regulated chloroplastic malate dehyd rogenases. These extensions do not exist in the permanently active NAD-depe ndent MDHs (EC 1.1.1.37). Biochemical characterization of the mutants and e limination of negative charges at the C-terminus provided evidence for auto -inhibition of the oxidized enzyme by its C-terminal end through interactio n with the active site and showed that the more compact structure of the ox idized dimer was linked to the presence of the N-terminal disulfide, The re cently published 3-dimensional structures of the oxidized enzyme confirmed the location of the regulatory disulfides and fully support the auto-inhibi tion hypothesis, Indeed, the C-terminus is trapped inside the active site, interacting with active-site residues, and the N-termini are inserted at th e dimer contact area where they are bound by hydrophobic interactions with both subunits. The physiological function of such complex regulation is dis cussed.