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Metabolism and Enzymology

Form of Inorganic Carbon Involved as a Product and as an Inhibitor of C₄ Acid Decarboxylases Operating in C₄ Photosynthesis

Division of Plant Industry, CSIRO, GPO Box 1600, Canberra City A.C.T. 2601 Australia

These studies demonstrated that CO₂ rather than HCO₃^– is the inorganic carbon metabolite produced by the C₄ acid decarboxylases involved in C₄ photosynthesis (chloroplast located NADP malic enzyme, mitochondrial NAD malic enzyme, and cytosolic phosphoenolpyruvate [PEP] carboxykinase). The effect of varying CO₂ or HCO₃^– as a substrate for the carboxylation reaction catalyzed by these enzymes or as inhibitors of the decarboxylation reaction was also determined. The K_mCO₂ 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 CO₂ 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 HCO₃^–. For NADP malic enzyme CO₂ was shown to be the inhibitory species but PEP carboxykinase and NAD malic enzyme were apparently inhibited about equally by CO₂ and HCO₃^–.