NMR structures of thioredoxin m from the green alga Chlamydomonas reinhardtii

Chloroplast thioredoxin in from the green alga Chlamydomomas reinhardtii is very efficiently reduced in vitro and in vivo in the presence of photoredu ced ferredoxin and a ferredoxin dependent ferredoxin-thioredoxin reductase, Once reduced, thioredoxin m has the capability to quickly activate the NAD P malate dehydrogenase (EC 1.1.1.82) a regulatory enzyme involved in an ene rgy-dependent assimilation of carbon dioxide in C4 plants. This activation is the result of the reduction of two disulfide bridges by thioredoxin m, t hat are located at the N- and C-terminii of the NADP malate dehydrogenase. The molecular structure of thioredoxin m was solved using NMR and compared to other known thioredoxins, Thioredoxin m belongs to the prokaryotic type of thioredoxin, which is divergent from the eukaryotic-type thioredoxins al so represented in plants by the h (cytosolic) and f (chloroplastic) types o f thioredoxins, The dynamics of the molecule have been assessed using N-15 relaxation data and are found to correlate well with regions of disorder fo und in the calculated MMR ensemble. The results obtained provide a novel ba sis to interpret the thioredoxin dependence of the activation of chloroplas t NADP-malate dehydrogenase, The specific catalytic mechanism that takes pl ace in the active site of thioredoxins is also discussed on the basis of th e recent new understanding and especially in the light of the dual general acid-base catalysis exerted on the two cysteines of the redox active site. It is proposed that the two cysteines of the redox active site may insulate each other from solvent attack by specific packing of invariable hydrophob ic amino acids. (C) 2000 Wiley-Liss, Inc.