Regulation of Rubisco, Anionic Modulators, Specificity and Mechanism o f Action o f Dianionic EffectorsCatalysis and regulation o f C 0 2 fixation differ in a characteristic manner in their response to anionic modifiers and the polarity of the reaction medium. Monovalent inorganic anions inhibit catalysis and C 0 2-activation of the D-ribulose 1,5-bisphosphate carboxylase/oxygenase from spinach, whereas the activity and binding o f NADPH and effector sugar phosphates are affected only at appreciably higher concentrations. In contrast such modulators with a dianion structure stimulate C 0 2 fixation by an increase of the affinity o f the enzyme for the activator C 0 2 and stabilization of the reactive carbamate. Structure-activity studies revealed a broad specificity o f the enzyme for these regulatory effects. Essentially amino groups are involved in these processes. Certain organic solvents, as methanol or acetone, stimulate C 0 2 fixation by a similar modification of the C 0 2 activation centers, as induced by dianionic effectors. These results infer that such effects are due to a decrease of the polarity at the regulatory centers of the enzyme and a concomitant change of the pK of the active lysine responsible for the binding o f the activator C 0 2. A correlation of effector binding and activity demonstrates that already low, non-saturating concentrations o f such modifiers induce high activation levels of the carboxylase and prevent the dissociation o f the activated ternary complex. It is discussed that the central problem concerning the catalytical competence o f D-ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the presence of active site directed dianionic effectors can be solved kinetically.

No abstract

No abstract

No abstract

No abstract