The activation of ribulose-1,5-bisphosphate carboxylase by carbon dioxide and magnesium ions. Equilibria, kinetics, a suggested mechanism, and physiological implications.
Ribulose-1,5-bisphosphate carboxylase was activated by incubation with CO2 and Mg2++, and inactivated upon removal of CO2 and Mg2+ by gel filtration. The activation process involved CO2 rather than HCO3-. The activity of the enzyme was dependent upon the preincubation concentrations of CO2 and Mg2+ and upon the preincubation pH, indicating that activation involved the reversible formation of an equilibrium complex of enzyme-CO2-Mg. The initial rate of activation was linearly dependent upon the CO2 concentration but independent of the Mg2+ concentration. Kinetic analyses indicated that the enzyme reacted first with CO2 in a rate-determining and reversible step, followed by a rapid reaction with Mg2+ to form an active ternary complex (see eq 1 in text). The pseudo-first order rate constant, kobsd, for the activation process at constant pH was derived: kobsd=k1[CO2] + (k2k4/k3[Mg2+]). Experimentally, kobsd was shown to be linearly dependent upon the CO2 concentration and inversely dependent upon the Mg2+ concentration. The activity of the enzyme after preincubation to equilibrium at constant concentrations of CO2 and Mg2+ increased as the preincubation pH was raised, indicating that CO2 reacted with an enzyme group whose pK was distinctly alkaline. It is proposed that the activation of ribulose-1, 5-biphosphate carboxylane involves the formation of a carbamate.
D001639:Bicarbonates / Q000494:pharmacology; D002245:Carbon Dioxide / Q000494:pharmacology*; D002262:Carboxy-Lyases / Q000378:metabolism*; D004789:Enzyme Activation / Q000187:drug effects; D006863:Hydrogen-Ion Concentration; D008274:Magnesium / Q000494:pharmacology*; D008433:Mathematics; D010944:Plants / Q000201:enzymology*; D012273:Ribulose-Bisphosphate Carboxylase / Q000378:metabolism*; D013997:Time Factors
D001639:Bicarbonates; D002245:Carbon Dioxide; D002262:Carboxy-Lyases; D012273:Ribulose-Bisphosphate Carboxylase; D008274:Magnesium