EFFECTS 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/inh ibition mechanism, in which the metal isocitrate complex is the true s ubstrate of Phycomyces isocitrate lyase. Binding of Mg2+ or Mn2+ to th e 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 activato r site is virtually independent of the pH value. The affinity of the n on-activated form of the enzyme for the Mg2+-isocitrate complex decrea sed (Ksa increased 20-fold) as pH was raised, but for Mn2+ ions the af finity of the activated enzyme for the Mn2+ isocitrate complex decreas ed 86-fold. The ion moiety of the metal-ion-isocitrate complex appears to be involved in the formation of the active enzyme-substrate comple x from the non-activated enzyme.