Glutamate Decarboxylase

I.U.B.: 4.1.1.15
L-Glutamate 1-carboxy-lyase

Enzymatic Reaction (image will open in a new window) 

Glutamate decarboxylase catalyses the removal of the carboxyl group adjacent to the α-amino group from L-glutamic acid to produce γ-aminobutyric acid and carbon dioxide.

Glutamate decarboxylase from E. coli has been isolated to a state of crystalline purity, uniform by ultra-centrifugation and disc gel electrophoresis (Strausbach et al. 1967).

Characteristics of Glutamate Decarboxylase from E. coli:

Molecular weight: 310,000 (Strausbauch and Fischer 1970a).

Compositon: The enzyme is a hexamer of approximately 50,000 dalton subunits, each with one molecule of pyridoxal phosphate (Fischer 1970).

Optimum pH: 3.8 (Shukuya and Schwert 1960; Strausbauch and Fischer 1970). Many assays have been described employing pH 4.0 - 4.5. The Worthington preparation, assayed as described below, is more active at pH 5.0. See also Berry et al. (1981).

Specificity: Glutamate decarboxylase is unusually specific, showing significant activity only on L-glutamic acid and α-methyl glutamic acid (Fonda 1972). The same report shows the following compounds to be neither substrates nor inhibitors: D-glutamate, D- and L-aspartate, α-amino adipic and α-aminopimelic acids. Pure L-glutamine is not a substrate.

Inhibitors: Fonda (1972) reported the following compounds to inhibit the enzyme: L-isoglutamic acid, and aliphatic dicarboxylic acids, especially glutaric, pimelic and adipic acids. Kuo and Rando (1981) report irreversible inhibition by α-(fluoromethyl)glutamic acid.

Activators: Pyridoxal phosphate is a necessary but firmly bound coenzyme. Because of the reversible configurational change at about pH 5.5 (O'Leary and Brummund 1974), chloride ions may be stimulatory to activity and acetate ions may inhibit. The crystalline enzyme has a maximum specific activity of 100 - 120 u/mg (pH 4.5, 38°C).

Stability: Many reports have described a more rapid loss of activity of dilute enzyme solutions in the cold than at room temperature. (Shukuya and Schwert 1960; Strausbauch and Fischer 1970).