title: Role of metal ions in activity and site:site heterogeneity in isocitrate lyase of castor seedling endosperm.
abstract: Time-dependent inactivation of isocitrate lyase with EDTA, ATP, and ADP exhibits biphasic kinetics in which half of the initial activity is lost in a fast and the remaining half in a slow phase. Each phase exhibits first-order kinetics. When Mg2+ ions are added to an enzymatically inactive "apo-protein" obtained by EDTA dialysis of isocitrate lyase, the activity regained exhibits anticooperativity with respect to Mg2+ binding. The data suggest the presence of two classes of Mg2+ binding sites of equal specific activity, but having different affinities for Mg2+. It is proposed that the site:site heterogeneity in isocitrate lyase is a consequence of its quaternary structure constraints, and is independent of any of the known ligands.
mesh_terms: D000244:Adenosine Diphosphate / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000494:pharmacology; D001665:Binding Sites; D004492:Edetic Acid / Q000494:pharmacology; D004789:Enzyme Activation / Q000187:drug effects; D004793:Enzyme Reactivators; D007522:Isocitrate Lyase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D008670:Metals / Q000494:pharmacology*; D008956:Models, Chemical; D007652:Oxo-Acid-Lyases / Q000378:metabolism*; D010944:Plants / Q000201:enzymology*
chemical_list: D004793:Enzyme Reactivators; D008670:Metals; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D004492:Edetic Acid; D007652:Oxo-Acid-Lyases; D007522:Isocitrate Lyase; D008274:Magnesium