REGULATION OF RIBULOSE 1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE BY METABOLITES

Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) and other en zymes and their common metabolites form a system regulated by feedforw ard and feedback mechanisms that is directly involved in assimilation of carbon, The interaction of these components in the chloroplast give s rise to properties, which can not be predicted from the study of iso lated components alone, Experiments were conducted to explain how the observed pattern of system level regulation is consistent with the ind ividual characteristics of Rubisco measured in vitro, In vitro Rubisco is responsive to a wide range of ribulose 1,5-bisphosphate and phosph oglycerate concentrations, similar to those found in leaves under fiel d conditions, Metabolite regulation of carbon assimilation in vivo inv olves several processes. Among these are negative co-operativity of ri bulose 1,5-bisphosphate binding by Rubisco, competitive inhibition of Rubisco by phosphoglycerate and inorganic phosphate, a high Rubisco ac tive site concentration in the chloroplast, and maintenance of a const ant phosphate concentration in the chloroplast resulting in reciprocal changes in the concentrations of ribulose 1,5-bisphosphate, phosphogl ycerate and inorganic phosphate, High-light-grown plants exhibited hig her assimilation rates than those growing under moderate light levels, but had similar Rubisco concentrations, Larger ribulose 1,5-bisphosph ate/phosphoglycerate ratios in plants grown outdoors are responsible f or the higher Rubisco activities and greater rates of carbon fixation observed under these conditions.