Metabolite-mediated catalyst conversion of PFK and PFP

Metabolites known to occur in the cytosol of photosynthetic leaf cells were found to mediate the reversible conversion of pyrophosphate—D-fructose-6-phosphate 1-phosphotransferase (PFP) to phosphofructokinase (PFK) in partially purified preparations from spinach leaves. Preincubation of PFP with fructose 2,6-bisphosphate, ATP or fructose 6-phosphate converted PFP to PFK. The reverse reaction (PFK → PFP) was promoted by UDP-glucose plus pyrophosphate. These conversions in catalytic capability were accompanied by changes in molecular mass and charge. The results are in accord with the view that the alterations in PFP and PFK activity, provisionally called ‘metabolite-mediated catalyst conversion’, represent a regulatory mechanism to direct left cytosolic carbon flux in either the biosynthetic or degradatory direction

Enzyme Regulation in C(4) Photosynthesis : Identification and Localization of Activities Catalyzing the Synthesis and Hydrolysis of Fructose-2,6-Bisphosphate in Corn Leaves

Activities catalyzing the synthesis of fructose-2,6-bisphosphate (fructose-6-phosphate,2-kinase or Fru-6-P,2K) and its breakdown (fructose-2,6-bisphosphatase or Fru-2,6-P(2)ase) were identified in leaves of corn (Zea mays), a C(4) plant. Fru-6-P,2K and Fru-2,6-P(2)ase were both localized mainly, if not entirely, in the leaf mesophyll cells. A partially purified preparation containing the two activities revealed that the kinase and phosphatase were regulated by metabolite effectors in a manner generally similar to their counterparts in C(3) species. Thus, corn Fru-6-P,2K was activated by inorganic phosphate (Pi) and fructose-6-phosphate, and was inhibited by 3-phosphoglycerate and dihydroxyacetone phosphate. Fru-2,6-P(2)ase was inhibited by its products, fructose-6-phosphate and Pi. However, unlike its spinach equivalent, corn Fru-2,6-P(2)ase was also inhibited by 3-phosphoglycerate and, less effectively, by dihydroxyacetone phosphate. The C(4) Fru-6-P,2K and Fru-2,6-P(2)ase were also quite sensitive to inhibition by phosphoenolpyruvate, and each enzyme was also selectively inhibited by certain other metabolites