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

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RESUMO

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 KmCO2 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−.

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