Regulation of Photosynthetic Sucrose Synthesis by Fructose 2,6-bisphosphate

Abstract

Fructose 2,6-bisphosphate (F2,6P₂) is a signal metabolite, first found in liver where it has an important role in controlling glycolysis and gluconeogensis. F2,6P₂ is present in spinach leaves (Cseke et al., 1982) where it is located in the cytosol with concentrations of 5–20 μM (Stitt et al., 1983b). The cytosolic fructose 1,6-bisphosphatase (FBPase) is inhibited by F2,6P₂ (Cseke et al., 1982, Stitt et al., 1982). More detailed studies (see these Proceedings, Heldt et al.) have shown that F2,6P₂ alters the cytosolic FBPase from hyperbolic substrate dependence (Km FBP = 3 μM) to sigmoidal dependence with a much lower substrate affinity (S _0.5 FBP = 0.1 and 0.25 mM in the presence of 2 and 10 μM F2,6P₂ respectively. This means that the cytosolic FBPase, which catalyses the first irreversible step in the conversion of trioseP to sucrose in the cytosol, would be particularly sensitive to control by alterations in the concentrations of F2,6P₂ and FBP.