Form of Inorganic Carbon Involved as a Product and as an Inhibitor of C 4 Acid Decarboxylases Operating in C 4 Photosynthesis

Abstract

These studies demonstrated that CO(2) rather than HCO(3) (-) is the inorganic carbon metabolite produced by the C(4) acid decarboxylases involved in C(4) photosynthesis (chloroplast located NADP malic enzyme, mitochondrial NAD malic enzyme, and cytosolic phosphoenolpyruvate [PEP] carboxykinase). The effect of varying CO(2) or HCO(3) (-) as a substrate for the carboxylation reaction catalyzed by these enzymes or as inhibitors of the decarboxylation reaction was also determined. The K(m)CO(2) was 1.1 millimolar for NADP malic enzyme and 2.5 millimolar for PEP carboxykinase. For these two enzymes the velocity in the carboxylating direction was substantially less than for the decarboxylating direction even with CO(2) concentrations at the upper end of the range of expected cellular levels. Activity of NAD malic enzyme in the carboxylating direction was undetectable. The decarboxylation reaction of all three enzymes was inhibited by added HCO(3) (-). For NADP malic enzyme CO(2) was shown to be the inhibitory species but PEP carboxykinase and NAD malic enzyme were apparently inhibited about equally by CO(2) and HCO(3) (-).