Glucose-6-Phosphate Dehydrogenase

I.U.B.: 1.1.1.49
D-glucose-6-phosphate:NADP⁺ 1-oxidoreductase

Glucose-6-phosphate dehydrogenase (GPDH) catalyzes the following reaction:

Extensively purified glucose-6-phosphate dehydrogenases have been obtained from yeasts, E. coli, and various mammalian tissues, and these are generally considered to be coenzyme specific for NADP. The glucose-6-phosphate dehydrogenase of certain microorganisms, Leuconostoc mesenteroides ( DeMoss 1955; Olive and Levy 1967), Acetobacter suboxydans (Cheldelin 1961) and Pseudomonas aeruginosa (Lessie and Neidhardt 1967), for example, are known to exhibit a dual coenzyme specificity, utilizing either NAD or NADP.

Characteristics of GPDH from L. mesenteroides:

Molecular weight: 104,000 (Olive and Levy 1971), comprised of two subunits of approximately 55,000 (Ishaque et al. 1974).

Composition: The amino acid composition has been reported and an unusual feature, for dehydrogenases at least, is the complete absence of cysteine/cystine residues (Ishaque et al. 1974). An essential lysine is indicated to be at the active site (Milhausen and Levy 1975).

Optimum pH: 7.8, Tris⋅HCl buffer, 0.05 M and NAD as coenzyme.

Constants: Partial specific volume, sedimentation coefficient, absorption spectrum and other physical data have been reported by Olive and Levy (1971). Kinetic constants and mechanisms have been described by Olive, Geroch and Levy (1971).

Extinction Coefficient: = 11.5, pH 7.2 in 0.1 M Tris⋅HCl (Olive and Levy 1971).

Isoelectric point: pH 4.6 (Olive and Levy 1971).

Activators: Mg²⁺ has not been demonstrated to be an absolute requirement for the Leuconostoc enzyme. HCO^3- at concentrations up to 0.3 M produces a slight stimulation (Olive and Levy 1967).

Specificity: Coenzyme: Either NAD or NADP will serve as coenzyme with the intrinsic reaction velocity with NAD being approximately 1.8 times greater than with NADP (Olive and Levy 1967).

Substrate: D-glucose-6-phosphate is considered to be the natural substrate, although D-glucose reacts slowly (Metzger et al. 1972). F-6-P, FDP, G-1-P, and ribose-5-P are not considered to be substrates (DeMoss 1955).

Inhibitors: Pyridoxal 5'-phosphate inhibits competitively with respect to G-6-P and noncompetitively with respect to NAD or NADP; 1-fluoro-2,4-dinitrobenzene inhibits irreversibly (Milhausen and Levy 1975). At low concentrations palmitoyl-CoA and steroids such as dehydroepiandrosterone have no effect (Olive and Levy 1967), Kawaguchi and Block (1974) report that only high concentrations of palmitoyl-CoA inhibit the L. mesenteroides enzyme, while Coe and Hsu (1973) indicate that various acyl-CoA's and ATP are inhibitory. Cho and Joshi (1989) indicate that inactivation of G6PD by Al⁺³ is due to the conformational change induced by Al⁺³ binding.

Stability: The Leuconostoc mesenteroides glucose-6-phosphate dehydrogenase is a relatively stable enzyme in solution.