Molecular Mechanisms of the Resistance to Hydrogen Peroxide of Enzymes Involved in the Calvin Cycle from Halotolerant Chlamydomonas sp. W80

cDNA clones encoding NADP+-glyceraldehyde-3-phosphate dehydrogenase (NADP+-GAPDH) and sedoheptulose-1,7-bisphosphatase (SBPase) were isolated and characterized from halotolerant Chlamydomonas sp. W80 (C. W80) cells. The cDNA clone for NADP+-GAPDH encoded 369 amino acid residues, preceded by the chloroplast transit peptide (37 amino acid residues). The cDNA clone for SBPase encoded 351 amino acids with the chloroplast transit peptide. The activities of NADP+-GAPDH and SBPase from C. W80 cells were resistant to H2O2 up to 1 mM, as distinct from spinach chloroplastic thiol-modulated enzymes. The illumination to the dark-adapted cells and dithiothreitol treatment to the crude homogenate had little effect on the activities of NADP+-GAPDH and SBPase in C. W80. Modeling of the tertiary structures of NADP+-GAPDH and SBPase suggests that resistance of the enzymes to H2O2 in C. W80 is due to the different conformational structures in the vicinity of the Cys residues of the chloroplastic enzymes between higher plant and C. W80 cells.