Factors affecting the oligomeric state of NADP-malic enzyme from maize and wheat tissues: a chemical crosslinking study.

The different aggregational states of maize and wheat NADP-malic enzyme as affected by pH, temperature and various metabolites have been studied by the combined use of intersubunit crosslinking and denaturing polyacrylamide gel electrophoresis. The association/dissociation equilibrium is a pH-dependent process: pH values above 8.0 promote the tetramer formation, while lowering the pH shifts the equilibria towards dimers and monomers. Below pH 6.0, most molecules exist as monomers. In the same way, the temperature governs the equilibria between the different oligomeric states. As the temperature is lowered from 42 to 0 degrees C, a progressive dissociation into dimers and monomers is observed. Excess enthalpies are negative in all cases, but the overall process demands an input of Gibb's free energy. Consequently, the protein dissociation is an entropy-driven process. The presence of Mg2+ or glycerol induces aggregation in both enzymes, while increasing the ionic strength produces the opposite effect. The results suggest that changes in the equilibria between monomer, dimer and tetramer of NADP-malic enzyme could be the molecular basis for an effective regulation of the enzyme activity in vivo.