Regulation of ribulose 1,5-bisphosphate carboxylase/oxygenase by metabolites

Abstract Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) and other enzymes and their common metabolites form a system regulated by feedforward and feedback mechanisms that is directly involved in assimilation of carbon. The interaction of these components in the chloroplast gives rise to properties, which can not be predicted from the study of isolated components alone. Experiments were conducted to explain how the observed pattern of system level regulation is consistent with the individual characteristics of Rubisco measured in vitro. In vitro Rubisco is responsive to a wide range of ribulose 1,5-bisphosphate and phosphoglycerate concentrations, similar to those found in leaves under field conditions. Metabolite regulation of carbon assimilation in vivo involves several processes. Among these are negative co-operativity of ribulose 1,5-bisphosphate binding by Rubisco, competitive inhibition of Rubisco by phosphoglycerate and inorganic phosphate, a high Rubisco active site concentration in the chloroplast, and maintenance of a constant phosphate concentration in the chloroplast resulting in reciprocal changes in the concentrations of ribulose 1,5-bisphosphate, phosphoglycerate and inorganic phosphate. High-light-grown plants exhibited higher assimilation rates than those growing under moderate light levels, but had similar Rubisco concentrations. Larger ribulose 1,5-bisphosphate/phosphoglycerate ratios in plants grown outdoors are responsible for the higher Rubisco activities and greater rates of carbon fixation observed under these conditions.