title: Reductive and oxidative half-reactions of glutathione reductase from Escherichia coli.
abstract: Glutathione reductase catalyzes the reduction of glutathione disulfide by NADPH and has a redox active disulfide and an FAD cofactor in each monomer. In the reductive half-reaction, FAD is reduced by NADPH and electrons pass from the reduced flavin to the redox active disulfide. The oxidative half-reaction is dithiol-disulfide interchange between the enzyme dithiol and glutathione disulfide. We have investigated the reductive and oxidative half-reactions using wild-type glutathione reductase from Escherichia coli and in an altered form of the enzyme in which the active site acid-base catalyst, His439, has been changed to an alanine residue (H439A). H439A has 0.3% activity in the NADPH/GSSG assay. The replacement affects both the oxidative half-reaction, as expected, and the reductive half-reaction--specifically, the passage of electrons from reduced flavin to the disulfide. Reduction of H439A by NADPH allows direct observation of flavin reduction. The NADPH-FAD charge transfer complex is formed in the dead time. Reduction of FAD, at a limiting rate of 250 s-1, is observed as a decrease at 460 nm and an increase at 670 nm (FADH(-)-NADP+ charge transfer). Subsequent passage of electrons from FADH- to the disulfide (increase at 460 nm and a decrease at 670 nm) is very slow (6-7 s-1) and concentration independent in H439A. The monophasic oxidative half-reaction is very slow, as expected for reduced H439A.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Veterans Affairs Medical Center, Department of Biological Chemistry, Ann Arbor, Michigan.
authors.forename: P
authors.initials: P
authors.lastname: Rietveld
authors.forename: L D
authors.initials: LD
authors.lastname: Arscott
authors.forename: A
authors.initials: A
authors.lastname: Berry
authors.forename: N S
authors.initials: NS
authors.lastname: Scrutton
authors.forename: M P
authors.initials: MP
authors.lastname: Deonarain
authors.forename: R N
authors.initials: RN
authors.lastname: Perham
authors.forename: C H
authors.initials: CH
authors.lastname: Williams
mesh_terms: D000409:Alanine / Q000378:metabolism; D001665:Binding Sites; D004926:Escherichia coli / Q000201:enzymology*; D005980:Glutathione Reductase / Q000235:genetics / Q000378:metabolism*; D006639:Histidine / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D016297:Mutagenesis, Site-Directed; D009249:NADP / Q000378:metabolism; D010084:Oxidation-Reduction
chemical_list: D006639:Histidine; D009249:NADP; D005980:Glutathione Reductase; D000409:Alanine