Determination of the regulatory disulfide bonds of NADP-dependent malate dehydrogenase from Pisum sativum by site-directed mutagenesis

The light-mediated reversible activation of NADP-dependent malate dehydrogenase (NADP-MDH) from Pisum sativum can be simulated in vitro by reducing the inactive oxidized enzyme with dithiothreitol. Since the gross structure and the dimeric state of the enzyme are unaffected by the state of oxidation, the redox modulation cannot be attributed to inter-subunit disulfide bridges. In order to identify intra-chain cystine cross bridges that might be candidates responsible for the activation reaction, site-directed mutagenesis experiments were performed, substituting alanine for up to four exposed cysteine residues. Mutants were expressed in freshly transformed EcoB cells and purified to homogeneity. As indicated by the activation behavior (by dithiothreitol-mediated thioldisulfide exchange), disulfides C23-C28 in the N-terminal and C364-C376 in the C-terminal part of the polypeptide chain are involved in the light-induced modulation of the activity of the wild type enzyme. A mutant of the enzyme lacking the N-terminal 45 residues confirms this result. Electrophoretic mobility and FPLC prove the wild type enzyme and its mutants to be dimeric; differences refer to the packing of the N- and C-terminal portions of the enzyme in its oxidized and reduced state. The kinetics of the redox modulation differ, depending on the solvent conditions and the mode of activation. After elimination of the N-terminal disulfide bond, sigmoidal activation profiles are no longer observed, suggesting a slow conformational rearrangement in the N-terminal portion of the wild type enzyme to be rate-limiting in the course of reductive activation. For the wild type, this finding can be mimicked in the presence of non-denaturing concentrations of guanidinium-chloride.

Cloning, site-specific mutagenesis, expression and characterization of full-length chloroplast NADP-malate dehydrogenase from Pisum sativum

Chloroplast NADP-dependent malate dehydrogenase is regulated by a dithiol redox reaction. The assignment of the groups involved, requires the primary structure of the enzyme to be known. Using the polymerase chain reaction and the cDNA library of Pisum sativum, the sequence of the enzyme and its targeting signal was determined. The gene was cloned in Escherichia coli JM83 and expressed in E. coli JM83 and E. coli B at high yield. The determination of the physical properties of the gene product proves the recombinant protein to be indistinguishable from the enzyme purified from the plant. This holds true, in spite of the fact that the plant enzyme lacks 11 N-terminal residues. The lengths of the complete polypeptide chain of the recombinant enzyme and its transit peptide are 388 and 53 residues, respectively. The comparison of the sequences of the mature enzyme with those of known chloroplast NADP-MDH shows 83-95% identity, but with mitochondrial or bacterial MDH only approximately 20%. Reduction of the (inactive) oxidized enzyme with dithiothreitol allows mimicking of the in vivo activation. The reaction follows a consecutive second-order-kinetics mechanism. Guanidinium chloride (GdmCl) at concentrations below 0.4 M leads to a significant activation of the oxidized form of the enzyme. At [GdmCl] = 0.4-0.46 M, both oxidized and reduced NADP-MDH show highly cooperative changes in the hydrodynamic and spectral properties, indicating the synchronous breakdown of the quaternary, tertiary and secondary structures. Site-directed mutations C23A and C28A do not quench the regulatory properties of the enzyme; additional substitution of alanine for Cys206 and Cys376 renders the enzyme equally active in both the reduced and the oxidized state. Therefore, one can consider these residues, either alone or in combination with Cys23 and Cys28, as responsible for enzyme activation.