Glutamate dehydrogenase: Chemical modification and ligand binding

Characterization of tryptophan environments in glutamate dehydrogenases from temperature-dependent phosphorescence

Tryptophan room temperature phosphorescence in solution was detected in glutamic dehydrogenase from bovine liver and Escherichia coli with lifetimes of 1.2 and 0.65 s, respectively. Although these enzymes possess three and five tryptophanyl residues per polypeptide chain, respectively, the temperature dependence of the phosphorescence quantum yield estimates that the room temperature emission is due, in either case, to a single residue. Long triplet-state lifetimes and very small rates of O2 quenching indicate that these tryptophanyl side chains are embedded in a highly inflexible internal region of the macromolecule. Aided by sequence homology with dehydrogenases of known structure and theoretical predictions of secondary structure [Wootton, J.C. (1974) Nature (London) 252, 542-546; Brett, M., Chambers, G.K., Holder, A. A., Fincham, J.R.S., & Wootton, J.C. (1976) J. Mol. Biol. 106, 1-22], the phosphorescing tryptophans have been tentatively placed in the catalytic coenzyme binding domain of each enzyme. The particular sensitivity of the triplet-state lifetime in probing local changes in conformation provides a strong indication that within the time window of phosphorescence measurements the six subunits in the hexameric enzymes are equivalent. Furthermore, while in the bovine enzyme this parameter is markedly affected by the interaction with ligands which have a functional role, the constancy of the phosphorescence lifetime at various degrees of polymerization suggests that the association process is not accompanied by important conformational changes in the macromolecule.

Development and characterization of a polymer gel with an immobilized enzyme to measure L-glutamate

Glutamate dehydrogenase (GDH) is used in an enzyme electrode to measure L-glutamate. GDH is covalently immobilized in a hydrophilic, permeable, and semirigid gel produced by the copolymerization of polyacrylamide and N-acryloxysuccinimide. Experimental conditions necessary to retain GDH in the gel with high efficiency and minimum denaturation are optimized. The abilities of enzymatic cofactors and coenzymes, NADH, NAD, ATP, ADP, GTP, and ZnCl2, to protect the enzyme during immobilization are explored. Under optimum experimental procedures an enzyme-containing gel is produced that is reproducible and long lasting in its functional behavior. The gel responds to the presence of L-glutamate with high velocity, the delay being less than 500 ms; high specificity, being 1000-fold more responsive to L-glutamate than D-glutamate, D- or L-aspartate, and N-acetylhistidine; and high sensitivity, a concentration of about 3 microM can be measured.

The binding of NADH and NADPH to bovine-liver glutamate dehydrogenase. Spectroscopic characterisation

1. The binding of NADH and NADPH to bovine liver glutamate dehydrogenase has been studied using circular dichroism (CD) and fluorescence spectroscopy, under comparable conditions of enzyme concentration. Spectroscopic titrations have been analysed using non-linear least-squares techniques to determine dissociation constants and parameters for the different chromophoric portions of the bound coenzymes, according to different models for the binding equilibria. 2. For NADPH, enhancement of dihydronicotinamide fluorescence and positive CD at 340 nm reflect binding of the coenzyme at the active site (site I) via the dihydronicotinamide moiety with a dissociation constant K1 of 50 microM; negative CD at 260 nm reflects the binding of a second molecule of coenzyme at the non-active site (site II) via the adenine moiety with a KII of 650 microM. 3. For NADH, the same spectroscopic assignment is made; however, site II affinity is much greater (KII 20 microM) and exceeds the affinity at the active site (KI 50 microM). 4. In no case was spectroscopic evidence obtained with either coenzyme to support the concept of non-identical active site binding as three sites IA and three sites IB per hexamer. 5. Coenzyme affinities are approximately twofold higher in triethanolamine buffer (0.05 + 0.1 M KCl, pH 7.6) compared with phosphate buffer (0.1 M at pH 7.0). 6. ADP enhances the CD of bound NADPH at 340 nm by up to twofold; phosphate ion also enhances the CD and binds with a Kd of 60-80 mM. GTP causes strong negative CD at 340 nm but only for the NADPH bound at site II. These results are consistent with independent binding sites for ADP (site II) and GTP (site III); phosphate ion is apparently able to affect certain properties of all three sites.

The reaction of bovine glutamate dehydrogenase with periodate-oxidised ADP

1. The reactive analogue oADP produced by periodate oxidation of ADP has been studied as a potential affinity label for the enzyme bovine glutamate dehydrogenase, using circular dichroism (CD) difference spectroscopy to monitor specific binding. 2. The analogue binds stoichiometrically, rapidly and reversibly to the adenine nucleotide binding site with Kd approximately equal to 12 microM (20 degrees C, pH 7) with characteristic intensification of the adenine nucleotide CD at 260 nm. 3. This complex is unstable and decays with a half-life of about 1.5 h; the analogue then becomes attached as a Schiff base to a number of subsidiary sites, including the enzyme active site, with partial inactivation of the enzyme. 4. Depending upon initial concentration of oADP, the enzyme activity is progressively lost during the slow reaction; following borohydride reduction, up to four molecules of analogue are bound/subunit. 5. Protection against loss of enzyme activity is afforded by the coenzyme NAD+ plus glutarate or L-hydroxyglutarate (an effective inhibitor), or by glutarate alone, but not by NAD+ alone. 6. Spectroscopic and protection studies indicate that after the decay of the specific CD signal, the enzyme retains the capacity to bind ADP, but that this is progressively lost in parallel with decay of enzymic activity. 7. The results are consistent with proximity or functional interaction between the adenine nucleotide site and the coenzyme binding portion of the active site. 8. Thus oADP does not act as a true affinity label for the adenine nucleotide binding site, but the reaction subsequent to binding at that site shows some specificity directed towards the active site.

The immobilization technique or solid-phase approach as an aid in investigating the interaction of subunits in oligomeric enzymes, notably of dehydrogenases

The use of immobilization techniques or a solid-phase approach, allows several questions concerning interactions between sub-units of oligomeric enzymes, particularly dehydrogenases to be, answered are the monomers of ADH and LDH active per se, or are interactions necessary for their activity? Are the sub-units (such as those of ADH) identical from a kinetic point of view? Is the increase in activity due to the addition of ligands (as for GDH), the result of the formation of large aggregates, which appear simultaneously, or is this association a secondary effect? A technique using affinity chromatography, with the help of bifunctional nucleotide derivatives, is presented. This technique could prove useful for topographical studies, or provide new information on the spatial arrangements of the sub-units.

Immobilization of L-glutamate dehydrogenase into soluble cross-linked polymers. ADP effect and electron microscopy studies

Active soluble cross-linked L-glutamate dehydrogenase (L-glutamate: NAD(P)+ oxidoreductase (deaminating), EC 1.4.1.3) albumin polymers were produced. Electron microscopic studies and kinetic properties were studied with the polymer in solution and compared with previous published data about the enzyme immobilized inside proteic films (Barbotin, J.N. and Breuil, M. (1978) Biochim. Biophys. Acta 525, 18--27). The glutaraldehyde effect on activity yield, ADP and beta-NAD+ protection, stability and pH rate profile were studied and discussed. Apparent Michaelis constants were determined with soluble polymers produced with or without ADP during the grafting process. Experiments were performed on the regulatory properties of immobilized glutamate dehydrogeanse showing the decrease of ADP activation and GTP inhibition as compared to the free form. In other respects, electron microscopy observations showed morphological differences between the two populations of soluble polymers produced in presence of ADP, obtained after gel filtration on Sepharose 6B. Linear aggregates of high molecular weight and classical soluble polymers were obtained. Similar Km values and regulatory properties were exhibited by the two forms, demonstrating the absence of interdependence between the allosteric control and the polymerization of enzyme monomers.