Thermodynamic interactions in the glutamate dehydrogenase-NADPH-oxalylglycine complex

Glutamate pathways of the beta-cell and the control of insulin secretion

Pancreatic beta-cells secrete insulin in response to circulating glucose, thereby maintaining euglycemia. Inside the beta-cell, glucose is transformed into intracellular signals stimulating exocytosis. While calcium is an obligatory messenger, this ion is not sufficient to promote the full secretory response. Accordingly, glucose metabolism produces the additive factor glutamate that participates to an amplifying pathway of the calcium signal. Although intracellular glutamate potentiates insulin secretion, extracellular glutamate may activate ionotropic receptors. As a consequence of such activation, insulin exocytosis is slowed down. Therefore, for the beta-cell glutamate is a double-edged sword, an amplifying pathway and a negative feedback, illustrating the principle of homeostasis.

Competitive inhibition of glutamate dehydrogenase reaction

Irrespective of their pyridine nucleotide specificity, all glutamate dehydrogenases share a common chemical mechanism that involves an enzyme bound 'iminoglutarate' intermediate. Three compounds, structurally related to this intermediate, were tested for the inhibition of purified NADP-glutamate dehydrogenases from two Aspergilli, as also the bovine liver NAD(P)-glutamate dehydrogenase. 2-Methyleneglutarate, closely resembling iminoglutarate, was a potent competitive inhibitor of the glutamate dehydrogenase reaction. This is the first report of a non-aromatic structure with a better glutamate dehydrogenase inhibitory potency than aryl carboxylic acids such as isophthalate. A suitably located 2-methylene group to mimic the iminium ion could be exploited to design inhibitors of other amino acid dehydrogenases.

Exact analysis of heterotropic interactions in proteins: Characterization of cooperative ligand binding by isothermal titration calorimetry

Intramolecular interaction networks in proteins are responsible for heterotropic ligand binding cooperativity, a biologically important, widespread phenomenon in nature (e.g., signaling transduction cascades, enzymatic cofactors, enzymatic allosteric activators or inhibitors, gene transcription, or repression). The cooperative binding of two (or more) different ligands to a macromolecule is the underlying principle. To date, heterotropic effects have been studied mainly kinetically in enzymatic systems. Until now, approximate approaches have been employed for studying equilibrium heterotropic ligand binding effects, except in two special cases in which an exact analysis was developed: independent binding (no cooperativity) and competitive binding (maximal negative cooperativity). The exact analysis and methodology for characterizing ligand binding cooperativity interactions in the general case (any degree of cooperativity) using isothermal titration calorimetry are presented in this work. Intramolecular interaction pathways within the allosteric macromolecule can be identified and characterized using this methodology. As an example, the thermodynamic characterization of the binding interaction between ferredoxin-NADP+ reductase and its three substrates, NADP+, ferredoxin, and flavodoxin, as well as the characterization of their binding cooperativity interaction, is presented.

The location of active site opening and closing events in the prehydride transfer phase of the oxidative deamination reaction catalyzed by bovine liver glutamate dehydrogenase using a novel pH jump approach

A pH jump approach has been developed and used to locate the opening and closing events occurring in the time course of the beef liver glutamate dehydrogenase catalyzed reaction. A comparison of the pH jump results, the resolved component time courses, and the 340 nm fluorescence signal suggests the existence and location on the reaction time course of a previously unreported prehydride transfer complex.