A PREDICTION OF THE 3-DIMENSIONAL STRUCTURE OF MAIZE NADP+-DEPENDENT MALATE-DEHYDROGENASE WHICH EXPLAINS ASPECTS OF LIGHT-DEPENDENT REGULATION UNIQUE TO PLANT ENZYMES

A model has been built for the plant NADP-malate dehydrogenase from Zea mays, a key enzyme in photosynthesis, which undergoes light-dependent regulation. The model was based on sequence and presumed structural homology to the known three-dimensional structure of mammalian porcine cytosolic NAD-malate dehydrogenase. A cystine-loop present in an extended C-terminal region of plant NADP-malate dehydrogenases was modelled using molecular mechanics and computer graphical methods, based on the assumption that a disulphide bridge exists in the inactive form of the enzyme between Cys351 and Cys363. The predicted conformation of the intact C-terminal cystine-loop suggests that the extended polypeptide will bind in the active centre and inhibit enzyme activity. Another ionizable cysteine residue in the active site is predicted to control the charge of the catalytic His215 and might be responsible for the uniquely tight binding of the positively charged nicotinamide ring of NADP+ in this and other C4 and C3 plant NADP-malate dehydrogenases.