XVI International Botanical Congess

The regulatory and catalytic properties of PEPC are markedly modulated by bicarbonate. When bicarbonate in the assay medium was raised from 0.05 mM to 10 mM the activity of PEPC increased by two-fold and the affinity of PEPC to Mg2+ increased by >2 fold, while Km for PEP was not much effected. In contrast, Ki for malate increased by 3 fold and Ka for G-6-P raised by four-fold as bicarbonate concentration was raised from 0.05 to 10 mM. Thus, bicarbonate desensitizes PEPC to both malate and G-6-P. These changes were manifested in both dark- as well as light-forms of the enzyme and appear to be independent of phosphorylation. We suggest that bicarbonate acts as not only a substrate but also a regulatory of enzyme.