Characteristic Features of the Regulatory Functions of the ᴅ-Ribulose 1,5-Bisphosphate Carboxylase/Oxygenase from Spinach
Vater, Joachim; Gaudszun, Thomas; Lange, Benita; Erdin, Nazmiye; Salnikow, Johann
Catalysis and regulation of CO2 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 CO2-activation of the ᴅ-ribulose 1,5-bisphosphate carboxylase/oxygenase from spinach, whereas the activity and binding of NADPH and effector sugar phosphates are affected only at appreciably higher concentrations. In contrast such modulators with a dianion structure stimulate CO2 fixation by an increase of the affinity of the enzyme for the activator CO2 and stabilization of the reactive carbamate. Structure-activity studies revealed a broad specificity of the enzyme for these regulatory effects. Essentially amino groups are involved in these processes. Certain organic solvents, as methanol or acetone, stimulate CO2 fixation by a similar modification of the CO2 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 of the activator CO2. A correlation of effector binding and activity demonstrates that already low, non-saturating concentrations of such modifiers induce high activation levels of the carboxylase and prevent the dissociation of the activated ternary complex. It is discussed that the central problem concerning the catalytical competence of ᴅ-ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the presence of active site directed dianionic effectors can be solved kinetically.
