NADP-malate dehydrogenase from Chlamydomonas:: prediction of new structural determinants for redox regulation by homology modelling

The function of a gene closely linked to nitrate assimilation loci from Chlamydomonas reinhardtii has been investigated. Gene expression analysis shows that its mRNA accumulation is modulated by light, carbon source and adaptation to light/dark cyclic conditions of growth. A full-length cDNA was isolated for the light-regulated transcript, and sequence characterization indicates that it encodes the NADP-malate dehydrogenase from C reinhardtii (NADP-MDH;Cr). The primary structure of NADP-MDH;Cr is closely related to plant, mossfern and algal NADP-malate dehydrogenases, and shares structural determinants for chloroplast targeting, cofactor binding and catalysis. Sequence conservation extends to the carboxy end of the protein, where plant and mossfern enzymes have two cysteines and an acidic C-terminus with a critical role for regulation of NADP-MDH activity by the thioredoxin/ferredoxin system. Accordingly, incubation with DTT activates NADP-MDH enzyme in cell-free extracts from C reinhardtii. Like NADP-malate dehydrogenases from two other green algae, the N-terminal extension of NADP-MDH;Cr lacks two thiol residues whose reduction constitutes the rate-limiting step in the activation reaction of plant enzymes. Homology-based 3D modelling of NADP-MDH;Cr, the first structure predicted for NADP-malate dehydrogenase from a lower eukaryote, evidences close positioning of two new cysteines in an accessible region of the protein surface. These results suggest that the algal enzyme has a different arrangement of regulatory disulfide bridges, which might involve the existence of new mechanisms that control functioning of the malate valve, the main system to export reducing power from the chloroplast of plant cells.