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

Role of Mg2+ in the structure and activity of maize (Zea mays L.) isocitrate lyase: indications for hysteretic behaviour

The role of Mg2+ in the structure and activity of maize isocitrate lyase has been studied by CD, limited proteolysis, protection by ligands against inactivation, and activity measurements at various metal concentrations. From CD and trypsinolysis experiments, the existence of high-affinity binding sites for Mg2+ was demonstrated, and a KdME of 200 microM was determined. Both free enzyme (E) and enzyme molecules with high-affinity sites occupied (ME) are catalytically competent, the former showing 40% of the activity of the latter. Mg2+ thus acts as a non-essential activator. A second Mg2+-binding site with a KdMEM of 6 mM was revealed from protection experiments by increasing Mg2+ concentrations against inactivation. From activity measurements at different Mg2+ concentrations, the affinity of the enzyme for the Mg2+-isocitrate complex (MI) was determined to be KdE(MI) = 9 microM. Maize isocitrate lyase was shown to display hysteretic behaviour. Filling the low-affinity binding sites with Mg2+ induces a conformational change in the high-affinity binding sites resulting in an even higher affinity for Mg2+ (KdME* = 40 microM). On lowering the Mg2+ concentration again, the enzyme only responds slowly: the time needed for all enzyme molecules to return to the conformation at which KdME is 200 microM was found to be 60 min. Finally it was shown that the high-affinity binding site for Mg2+ is not formed at low (4 degrees C) temperature.

Isocitrate lyase from Phycomyces blakesleeanus. The role of Mg2+ ions, kinetics and evidence for two classes of modifiable thiol groups

Isocitrate lyase was purified from Phycomyces blakesleeanus N.R.R.L. 1555(-). The native enzyme has an Mr of 240,000. The enzyme appeared to be a tetramer with apparently identical subunits of Mr 62,000. The enzyme requires Mg2+ for activity, and the data suggest that the Mg2(+)-isocitrate complex is the true substrate and that Mg2+ ions act as a non-essential activator. The kinetic mechanism of the enzyme was investigated by using product and dead-end inhibitors of the cleavage and condensation reactions. The data indicated an ordered Uni Bi mechanism and the kinetic constants of the model were calculated. The spectrophotometric titration of thiol groups in Phycomyces isocitrate lyase with 5.5'-dithiobis-(2-nitrobenzoic acid) gave two free thiol groups per subunit of enzyme in the native state and three in the denatured state. The isocitrate lyase was completely inactivated by iodoacetate, with non-linear kinetics. The inactivation data suggest that the enzyme has two classes of modifiable thiol groups. The results are also in accord with the formation of a non-covalent enzyme-inhibitor complex before irreversible modification of the enzyme. Both the equilibrium constants for formation of the complex and the first-order rate constants for the irreversible modification step were determined. The partial protective effect of isocitrate and Mg2+ against iodoacetate inactivation was investigated in a preliminary form.

Isocitrate lyase from Pinus pinea. Characterization of its true substrate and the action of magnesium ions

We found that the Mg-isocitrate complex is the true substrate for pine isocitrate lyase and that magnesium acts as a non-essential activator. Both the non-activated and the activated enzyme forms are catalytically active. Our model is consistent with the presence of two Mg-binding sites with different affinities: an activator site with high affinity in addition to the catalytic site with lower affinity. This may result in a complex, fine regulation of isocitrate lyase activity by magnesium. The affinity of the free enzyme for isocitrate is very low. Moreover, free isocitrate does not bind to the activated enzyme, nor it can yield a catalytically active form by binding to an enzyme species whose catalytic site has already been bound by magnesium.

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.