Role of metabolites and significance of SH groups in the action of NADP+-linked isocitrate dehydrogenase of urdbean seeds (Phaseolus mungo L.)

Abstract

NADP+-linked-isocitrate dehydrogenase (EC 1.1.1.42) is a key enzyme of the Tricarboxylic Acid Cycle (TCA) and has been purified from urdbean seeds and it is inhibited by ATP in a competitive manner having inhibitor constant (Ki 1.32 mM. Phosphoenol-pyruvate, an energy rich compound plays an important role in the regulation of this enzyme and this metabolite inhibited the enzyme activity of NADP+-linked-isocitrate dehydrogenase of urdbean with inhibitor constant (Ki 2.66 mM in a competitive manner. The mode of inhibition by various metabolites of Krebs cycle has been carried out and found that oxaloacetate and succinate inhibit ICDH urdbean enzyme in a competitive manner with respect to isocitrate and their Ki values are found to be 7.27 and 10.67 mM, respectively. Citrate inhibits the urdbean ICDH enzyme non competitively with Ki revalue equal to 3.33 mM. The SH groups play a important role in the activity of NADP+-linked-isocitrate dehydrogenase and blocking of this group with SH-reagents, leads to inactivation of urdbean ICDH enzyme. With excess iodoacetamide (1.00 mM) and N-ethylmaleimide (4.0 mM) inhibition of this enzyme follows first order kinetics, suggesting that there are four reactive SH groups per mole of enzyme which are equally reactive and there is no site- site interaction among the tetrameric isoicitrate dehydrogenase of urdbean. © 2011 Academic Journals Inc.