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NADP-Malate Dehydrogenase from Unicellular Green Alga Chlamydomonas reinhardtii. A First Step toward Redox Regulation?¹

Institut de Biotechnologie des Plantes, Unité Mixte de Recherche 8618 Centre National de la Recherche Scientifique, Université Paris-Sud, 91405 Orsay cedex, France (S.D.L., E.K., E.I.-B., M.M.-M.); Departamento de Bioquímica y Biología Molecular, Faculdad de Veterinaria, 10071–Cáceres, Spain (A.Q., F.M., J.M.C., M.I.I.); and Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas, 79409–1061 (M.H., D.B.K.)

The determinants of the thioredoxin (TRX)-dependent redox regulation of the chloroplastic NADP-malate dehydrogenase (NADP-MDH) from the eukaryotic green alga Chlamydomonas reinhardtii have been investigated using site-directed mutagenesis. The results indicate that a single C-terminal disulfide is responsible for this regulation. The redox midpoint potential of this disulfide is less negative than that of the higher plant enzyme. The regulation is of an all-or-nothing type, lacking the fine-tuning provided by the second N-terminal disulfide found only in NADP-MDH from higher plants. The decreased stability of specific cysteine/alanine mutants is consistent with the presence of a structural disulfide formed by two cysteine residues that are not involved in regulation of activity. Measurements of the ability of C. reinhardtii thioredoxin f (TRX f) to activate wild-type and site-directed mutants of sorghum (Sorghum vulgare) NADP-MDH suggest that the algal TRX f has a redox midpoint potential that is less negative than most those of higher plant TRXs f. These results are discussed from an evolutionary point of view.

² These authors contributed equally to this paper.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.052670.

^* Corresponding author; e-mail miginiac{at}ibp.u-psud.fr; fax 33(0)–1–39–15–34–24.

Received August 31, 2004; returned for revision October 20, 2004; accepted October 20, 2004.

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