Studies on isocitrate lyase from the facultative autotroph Thiobacillus novellus

Purification and characterization of Acinetobacter calcoaceticus isocitrate lyase

Acinetobacter calcoaceticus is capable of growing on acetate or compounds that are metabolized to acetate. During adaptation to growth on acetate, A. calcoaceticus B4 exhibits an increase in NADP(+)-isocitrate dehydrogenase and isocitrate lyase activities. In contrast, during adaptation to growth on acetate, Escherichia coli exhibits a decrease in NADP(+)-isocitrate dehydrogenase activity that is caused by reversible phosphorylation of specific serine residues on this enzyme. Also, in E. coli, isocitrate lyase is believed to be active only in the phosphorylated form. This phosphorylation of isocitrate lyase may regulate entry of isocitrate into the glyoxylate bypass. To understand the relationships between these two isocitrate-metabolizing enzymes and the metabolism of acetate in A. calcoaceticus B4 better, we have purified isocitrate lyase to homogeneity. Physical and kinetic characterization of the enzyme as well as the inhibitor specificity and divalent cation requirement have been examined.

Escherichia coli isocitrate lyase: properties and comparisons

The glyoxylate cycle was first discovered during studies on bacteria and fungi with the ability to grow on acetate or ethanol as the sole carbon source. Isocitrate lyase, the first enzyme unique to the glyoxylate cycle, has been studied in numerous prokaryotic and eukaryotic organisms. However, information on this enzyme from Escherichia coli is limited. We have recently reported the purification and in vitro phosphorylation of this enzyme. In the present study we have examined and characterized a variety of inhibitors, the divalent cation requirement and the amino acid composition of E. coli isocitrate lyase and compared these results to those obtained with other organisms.

Formation of enzyme-bound carbanion intermediate in the isocitrate lyase-catalyzed reaction: enzymatic reaction of tetranitromethane with substrates and its dependence on effector, pH, and metal ions

Isocitrate lyase of germinating castor seed endosperm catalyzes the reactions of succinate and of isocitrate (but not of glyoxylate) with tetranitromethane (TNM), giving rise to the nitroform anion (C-(NO2)3), analogous to the reaction of TNM with carbanions (O.P. Malhotra and U.N. Dwivedi, 1984, Ind. J. Biochem. Biophys. 21, 65-67). The kinetics of this reaction have been investigated under a variety of conditions. At a fixed TNM concentration, the initial rate of reaction exhibits a hyperbolic saturation of the enzyme with isocitrate. The reaction with succinate, however, shows "negative cooperativity" in succinate saturation and the data are consistent with the existence of two sets of succinate binding sites of unequal affinity ("tight" and "loose" sites). Equal reaction rates are observed at enzyme-saturating concentrations of succinate and isocitrate. In every case, the rate of reaction is proportional to the TNM concentration. In the presence of alpha-ketoglutarate, hyperbolic saturation curves are obtained for all the substrates (TNM and succinate or TNM and isocitrate). In the presence of this effector the Km of succinate and TNM are independent of the concentration of the second substrate. On the other hand, sets of parallel straight lines are obtained in the double-reciprocal plots for the enzymatic reaction of TNM with isocitrate in the presence of alpha-ketoglutarate. Studies on the effect of pH on the isocitrate lyase-catalyzed reactions of TNM with succinate, TNM with isocitrate, and succinate with glyoxylate in the absence as well as in the presence of alpha-ketoglutarate show that the proton behaves as an uncompetitive inhibitor in all these reactions, suggesting the presence of a "masked" basic group at the enzyme site, which is protonated in the presence of substrate only. The pKa value of this group lies in the range 6.7-6.9. The enzymatic reactions of TNM with succinate and isocitrate exhibit identical Mg2+ ion dependence. From a comparison of the data on the enzymatic reactions of TNM with the corresponding results on the physiological reaction catalyzed by this enzyme, it has been suggested that an ion pair intermediate (E+ X S-, in which E, S, and S- stand for enzyme, succinate, and succinate carbanion, respectively) lies on the pathway of catalysis by isocitrate lyase.

Role of metal ions in activity and site:site heterogeneity in isocitrate lyase of castor seedling endosperm

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.