Enzyme Regulation in C4 Photosynthesis

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Abstract
Activities catalyzing the synthesis of fructose-2,6-biphosphate (fructose-6-phosphate,2-kinase or Fru-6-P,2K) and its breakdown (fructose-2,6-bisphosphatase of Fru-2,6-P2ase) were identified in leaves of corn, a C4 plant. Fru-6-P,2K anf Fru-2,6-P2ase were both localized mainly, if not entirely, in the leaf mesophyll cells. A partially purified preparation containing the 2 activities revealed that the kinase and phosphatase were regulated by metabolite effectors in a manner generally similar to their counterparts in C3 species. Corn Fru-6-P,2K was activated by Pi and fructose-6-phosphate, and was inhibited by 3-phosphoglycerate and dihydroxyacetone phosphate. Fru-2,6-P2ase was inhibited by its products, fructose-6-phosphate and Pi. Unlike its spinach equivalent, corn Fru-2,6-P2ase was also inhibited by 3-phosphoglycerate and, less effectively, by dihydroxyacetone phosphate. The C4 Fru-6-P,2K and Fru-2,6P2ase were also quite sensitive to inhibition by phosphoenolpyruvate, and each enzyme was also selectively inhibited by certain other metabolites.