Effect of pH on the role of Mg2+ and Mn2+ on Phycomyces isocitrate lyase kinetics.
Mg2+ and Mn2+ function with the same partial mixed-type activation/inhibition mechanism, in which the metal isocitrate complex is the true substrate of Phycomyces isocitrate lyase. Binding of Mg2+ or Mn2+ to the activation site normally contributes significantly to the mechanism of catalysis. Whereas both ions activate catalysis at pH 7.3, at pH 8.5, Mg2+ ions behaved as inhibitors (beta < 1) and Mn2+ ions continued to function as activators. The binding of Mg2+ or Mn2+ to the activator site is virtually independent of the pH value. The affinity of the non-activated form of the enzyme for the Mg(2+)-isocitrate complex decreased (Ksa increased 20-fold) as pH was raised, but for Mn2+ ions the affinity of the activated enzyme for the Mn(2+)-isocitrate complex decreased 86-fold. The ion moiety of the metal-ion-isocitrate complex appears to be involved in the formation of the active enzyme-substrate complex from the non-activated enzyme.
Rúa
J
J
Departamento de Bioquímica y Biología Molecular, Universidad de León, Spain.
Soler
J
J
Busto
F
F
de Arriaga
D
D
D002384:Catalysis; D006863:Hydrogen-Ion Concentration; D007522:Isocitrate Lyase / Q000378:metabolism*; D007523:Isocitrates / Q000378:metabolism; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism / Q000494:pharmacology*; D008345:Manganese / Q000378:metabolism / Q000494:pharmacology*; D010800:Phycomyces / Q000201:enzymology*
D007523:Isocitrates; D008345:Manganese; C034219:isocitric acid; D007522:Isocitrate Lyase; D008274:Magnesium