Properties of leaf NAD malic enzyme from plants with C 4 pathway photosynthesis

Abstract

C 4 acid decarboxylation in one group of C 4-pathway species is mediated by an NAD malic enzyme. This paper reports on the partial purification and properties of this enzyme from three species of this group, Atriplex spongiosa, Amaranthus edulis, and Panicum miliaceum. Depending upon the conditions, the Atriplex spongiosa enzyme was 5–30% as active with NADP compared with NAD but the enzyme from the other species was specific for NAD. The enzyme from each species had an absolute requirement for Mn 2+ that could not be replaced by Mg 2+, and activity was increased several fold by low concentrations of either CoA or acetyl CoA. For the enzyme from Atriplex spongiosa and Amaranthus edulis, there was cooperativity for malate binding and the activators CoA and acetyl CoA functioned to increase the affinity of malate for the enzyme. The Hill coefficients for malate binding were approximately 2 and 4, respectively. However, with the enzyme from Panicum miliaceum, cooperative binding of malate was not apparent and activators operated by increasing V rather than the affinity for malate. Bicarbonate inhibited the enzyme from Atriplex spongiosa and Amaranthus edulis and its effect was inversely related to the concentrations of malate, NAD, and activators. The possible significance of these various allosteric effects on the regulation of the enzyme in vivo is discussed. Reactant concentrations and other conditions required for maximum activity are reported.