A special fructose bisphosphate functions as a cytoplasmic regulatory metabolite in green leaves

Abstract

Fructose 2,6-bisphosphate (Fru-2,6-P2), a regulatory metabolite discovered in animal cells and recently reported to occur in etiolated seedlings, was present in the cytoplasmic fraction of leaves of spinach and peas [Spinacia oleracea cv. Hipack and Pisum sativum cv. Progress Number 9] (typical C3 plants, in which a 3-C carboxylic acid is a major early photosynthetic product). At concentrations approximating those calculated to occur physiologically, Fru-2,6-P2 modulated 2 enzymes of the leaf cytoplasm: fructose-1,6-bisphosphatase (EC 3.1.3.11), a key enzyme of sugar synthesis, was competitively inhibited by Fru-2,6-P2 and pyrophosphate-linked phosphofructokinase ([PPi] D-fructose-6-phosphate 1-phosphotransferase, EC 2.7.1.90), a cytoplasmic enzyme that now seems important in glycolysis of C3 plants, was activated by Fru-2,6-P2. There was no indication of a role for Fru-2,6-P2 in photosynthesis of either chloroplasts or oxygenic prokaryotes. Evidently, Fru-2,6-P2 functions in the regulation of glycolysis and gluconeogenesis (carbohydrate synthesis) in the cytoplasm of leaves of C3 plants.