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.

Es wurde die Hydrierung von Benzol, Toluol, p‐Xylol, Äthylbenzol und Mesitylen in der Gasphase an einem Nickelkatalysator untersucht. Aktivität und Aktivierungsenergie der Reaktion nehmen in der angeführten Reihenfolge ab. Als Mechanismus der Hydrierung wird ein als π‐Komplex chemisorbierter Benzolring diskutiert.