Functional Roles of the Tetramer Organization of Malic Enzyme

Malic enzyme has a dimer of dimers quaternary structure in which the dimer interface associates more tightly than the tetramer interface. In addition, the enzyme has distinct active sites within each subunit. The mitochondrial NAD(P)+-dependent malic enzyme (m-NAD(P)-ME) isoform behaves cooperatively and allosterically and exhibits a quaternary structure in dimer-tetramer equilibrium. The cytosolic NADP+-dependent malic enzyme (c-NADP-ME) isoform is noncooperative and nonallosteric and exists as a stable tetramer. In this study, we analyze the essential factors governing the quaternary structure stability for human c-NADP-ME and m-NAD(P)-ME. Site-directed mutagenesis at the dimer and tetramer interfaces was employed to generate a series of dimers of c-NADP-ME and m-NAD(P)-ME. Size distribution analysis demonstrated that human c-NADP-ME exists mainly as a tetramer, whereas human m-NAD(P)-ME exists as a mixture of dimers and tetramers. Kinetic data indicated that the enzyme activity of c-NADP-ME is not affected by disruption of the interface. There are no significant differences in the kinetic properties between AB and AD dimers, and the dimeric form of c-NADP-ME is as active as tetramers. In contrast, disrupting the interface of m-NAD(P)-ME causes the enzyme to be less active than wild type and to become less cooperative for malate binding; the kcat values of mutants decreased with increasing Kd,24 values, indicating that the dissociation of subunits at the dimer or tetramer interfaces significantly affects the enzyme activity. The above results suggest that the tetramer is required for a fully functional m-NAD(P)-ME. Taken together, the analytical ultracentrifugation data and the kinetic analysis of these interface mutants demonstrate the differential role of tetramer organization for the c-NADP-ME and m-NAD(P)-ME isoforms. The regulatory mechanism of m-NAD(P)-ME is closely related to the tetramer formation of this isoform.