Molecular properties and thioredoxin-mediated activation of spinach chloroplastic NADP-malate dehydrogenase

Effects of N-terminal truncations upon chloroplast NADP-malate dehydrogenases from pea and spinach

Using the purification procedure of Fickenscher and Scheibe (Biochim. Biophys. Acta 749 (1983), 249-254) and a modification of the method, we produced a series of NADP-MDH forms from spinach and pea-leaf extracts that were characterized by a stepwise shortening of the N-terminal sequences. Limited proteolysis of the enzymes resulted in the generation of even shorter forms. Immunoprecipitation of the NADP-MDH from crude extracts revealed that the sequences of the intact enzymes from pea, spinach and maize started at a position (Ser) identical with that established for the Sorghum enzyme (Crétin, C., et al. (1990) Eur. J. Biochem. 192, 299-303). Spinach NADP-MDH isolated by conventional methods was shown to represent the intact form. Thus, the kinetic, regulatory and structural properties of the various truncated forms could be compared with those of an intact form. Removal of 5 or 11 amino acids, as occurred during isolation of the pea NADP-MDH, was without any significant effect. The enzymes were all dimeric and still exhibited the characteristic redox-regulatory properties. However, removal of 31 and 37 amino acids using aminopeptidase K resulted in the formation of active monomers characterized by only slightly lowered affinities towards the substrates, a shift of their pH optimum from 8 to 7, the loss of oxaloacetate inhibition and an increased maximal velocity. Although these forms lacked most or all of the N-terminal extra-peptide, including the 2 cysteines involved in redox-modification, they were still sensitive to the redox-potential. However, the low concentration of thiol required for immediate and complete restoration of any lost activity (40 mM beta-mercaptoethanol) suggested that this reaction might not be relevant for redox-regulation in vivo.

Thioredoxin activation of phosphoribulokinase in a chloroplast multi-enzyme complex

The activation process of spinach phosphoribulokinase by thioredoxin f has been studied with the enzyme in a free, isolated state, or integrated in a multi-enzyme complex. The time periods required for enzyme activation are always smaller and the maximal enzyme velocities are always greater when chloroplast phosphoribulokinase is included in the multi-enzyme complex than when it is in the isolated state. Comparative kinetic studies show that phosphoribulokinase extracted from the complex behaves exactly as in the isolated state. The reduced form of the kinase, whatever it has been included in the complex or isolated from the chloroplasts, are deactivated by oxidized thioredoxins. In the absence of thioredoxin f however, the reduced form of the isolated enzyme undergoes spontaneous oxidation whereas the reduced kinase included in the multi-enzyme complex is stable. 'Unspecific' proteins such as bovine serum albumin do not provide any protection of the kinase against autooxidation, whereas 'homologous' specific proteins such as ribulose-1,5-bisphosphate carboxylase/oxygenase dramatically decrease the rate of this autooxidation process. These results therefore support the view that interactions between phosphoribulokinase and the other components of the multi-enzyme complex play an important role in the modulation of the activity of this enzyme. The possible part of these interactions in the control of the Calvin cycle is discussed.

Maize NADP-malate dehydrogenase: cDNA cloning, sequence, and mRNA characterization

We report here the isolation, characterization and nucleotide sequence of clones encoding the maize chloroplastic NADP-malate dehydrogenase (NADP-MDH) which functions in the C4 cycle of photosynthesis. A nearly full-length NADP-MDH cDNA clone was isolated using antibodies against the purified protein. This clone hybridizes to a 1600 base mRNA that is eight times more abundant in light-grown maize leaves than in etiolated leaves. Transcription in leaves begins 230 bp upstream of the predicted start of translation, as shown by primer extension experiments. The encoded amino acid sequence predicts that NADP-MDH is synthesized as a preprotein of 432 amino acids (46 865 Da) which is processed into a mature protein of 375 amino acids (40 934 Da) with removal of a 57 amino acid transit peptide (5 931 Da). We identify regions of similarity between the maize NADP-MDH and other MDH polypeptides.

NADP-malate dehydrogenase from leaves of Zea mays: purification and physical, chemical, and kinetic properties

NADP-malate dehydrogenase (NADP-MDH) from leaves of Zea mays has been purified and has a specific activity of 600-1000 mumol/min/mg protein. The native, inactive form of the enzyme is an 87.4-kDa, dimeric protein with a sedimentation coefficient of 5.5 S and a Stokes' radius of 3.62 nm. Isofocus analysis reveals the native enzyme preparation to contain two proteins of pI 4.88 and 4.90. The uv-visible absorbance spectrum reveals no chromophores on the protein. The inactive form of the enzyme contains three thiols and three disulfides per subunit. 2-Mercaptoethanol can reduce two of the three subunit disulfides without concomitant activation of the enzyme. Treating the enzyme with dithiothreitol reduces all three subunit disulfides and fully activates the enzyme. These results show that NADP-MDH activation is dependent on the reduction of a critical disulfide bond. The enzyme can use both NADPH and NADH for oxaloacetate (OAA) reduction and NADP and NAD for malate oxidation at the following measured specific activities (eu/mg protein) at pH 8.5 in Tris buffer: NADPH plus OAA (690), NADH plus OAA (260), NADP plus malate (82), and NAD plus malate (37). These activities vary as a function of pH and buffer composition. Km values for the substrate pairs are NADPH (24 microM) plus OAA (56 microM); NADH (0.83 mM) plus OAA (61 microM); NADP (73 microM) plus malate (32 mM); and NAD (0.80 mM) plus malate (29 mM). The enzyme shows allosteric kinetics for NADP with a Hill number of 1.56. The enzyme is substrate-inhibited by malate for both NADP- and NAD-dependent activities.

Structural, immunological and kinetic comparisons of NADP-dependent malate dehydrogenases from spinach (C3) and corn (C4) chloroplasts

This paper compares structural, immunological and kinetic properties of corn (C4) and spinach (C3) NADP-malate dehydrogenases. These chloroplastic enzymes are regulated in vivo by thiol-disulfide interchange. Both in their oxidized (inactive) and reduced (active) states these enzymes have a dimeric structure with molecular masses for the subunit ranging from 28 kDa to 38 kDa according to the procedure used for the determination. These enzymes are thus structurally related. The use of specific antibodies showed that they are also immunologically related although not identical. Finally both enzymes showed close kinetic properties with comparable kcat and Km. Since C4 plants have approximately ten times more NADP-malate dehydrogenase activity than C3 plants, these data suggest that the differences in activities are probably related to the enzyme content of each plant type.

Affinity chromatography, on fructose-bisphosphatase-Sepharose, of two chloroplastic thioredoxins F. Purification and comparative molecular properties

A new method of purification of chloroplastic thioredoxins has been presented. This method is based on affinity chromatography on fructose-bisphosphatase--Sepharose columns. Two thioredoxin, fA and fB, may be extracted and purified to homogeneity from the same leaf extract. Whereas fA is monomeric and has an Mr of 11 400 +/- 500, fB is dimeric with an Mr of 18 000 +/- 600. The dimer dissociates in two halves in the ultracentrifuge under the effect of high pressures. Raising the ionic strength results in the same effect. Thioredoxins fA and fB activate to similar extents chloroplastic fructose bisphosphatase and NADP--malate dehydrogenase. Chloroplastic sedoheptulose bisphosphatase is activated by thioredoxin fB but not by thioredoxin fA.