The Control of Photosynthetic Carbon Metabolism

Strong regulation of photosynthetic metabolism is maintained by controlled key enzymes within the carbon reduction cycle, on biosynthetic paths leading from it, and within the chloroplast membrane. Of these, the most important are ribulose diphosphate carboxylase, controlling the initial entry of carbon dioxide, and fructose diphosphatase, operating as a portal between carbohydrate metabolism and other biosynthesis, and perhaps playing some role in the selective diffusion of sugar phosphates through the outer chloroplast membrane. Intracellular pH and concentrations of magnesium ion and inorganic pyrophosphate seem to be candidates for controlling factors in some of these processes. These and other control effects have been demonstrated by determinations of metabolite concentrations in green cells and chloroplasts in vivo by tracer techniques. Along biosynthetic paths, pyruvate kinase, which controls the conversion of phosphoenolpyruvic acid to pyruvate, and sucrose phosphate synthetase, which controls sucrose synthesis, play important roles. 61 references, 6 figures.