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Role of the glutamyl alpha-carboxylate of the substrate glutathione in the catalytic mechanism of human glutathione transferase A1-1.
- Source :
-
Biochemistry [Biochemistry] 2001 Dec 25; Vol. 40 (51), pp. 15835-45. - Publication Year :
- 2001
-
Abstract
- The Glu alpha-carboxylate of glutathione contributes to the catalytic function of the glutathione transferases. The catalytic efficiency of human glutathione transferase A1-1 (GST A1-1) in the conjugation reaction with 1-chloro-2,4-dinitrobenzene is reduced 15 000-fold if the decarboxylated analogue of glutathione, dGSH (GABA-Cys-Gly), is used as an alternative thiol substrate. The decrease is partially due to an inability of the enzyme to promote ionization of dGSH. The pK(a) value of the thiol group of the natural substrate glutathione decreases from 9.2 to 6.7 upon binding to GST A1-1. However, the lack of the Glu alpha-carboxylate in dGSH raised the pK(a) value of the thiol in the enzymatic reaction to that of the nonenzymatic reaction. Furthermore, K(M)(dGSH) was 100-fold higher than K(M)(GSH). The active-site residue Thr68 forms a hydrogen bond to the Glu alpha-carboxylate of glutathione. Introduction of a carboxylate into GST A1-1 by a T68E mutation increased the catalytic efficiency with dGSH 10-fold and reduced the pK(a) value of the active site bound dGSH by approximately 1 pH unit. The altered pK(a) value is consistent with a catalytic mechanism where the carboxylate contributes to ionization of the glutathione thiol group. With Delta(5)-androstene-3,17-dione as substrate the efficiency of the enzyme is decreased 24 000-fold while with 4-nitrocinnamaldehyde (NCA) the decrease is less than 150-fold. In the latter reaction NCA accepts a proton and, unlike the other reactions studied, may not be dependent on the Glu alpha-carboxylate for deprotonation of the thiol group. An additional function of the Glu alpha-carboxylate may be productive orientation of glutathione within the active site.
- Subjects :
- Binding Sites genetics
Catalysis
Dinitrochlorobenzene chemistry
Dinitrochlorobenzene metabolism
Glutathione analogs & derivatives
Glutathione Transferase genetics
Humans
Hydrogen-Ion Concentration
Isoenzymes chemistry
Isoenzymes genetics
Isoenzymes metabolism
Kinetics
Models, Chemical
Mutagenesis, Site-Directed
Oligopeptides chemistry
Oligopeptides metabolism
Protein Binding genetics
Substrate Specificity genetics
Tyrosine chemistry
Tyrosine genetics
Carboxylic Acids chemistry
Glutamic Acid chemistry
Glutathione chemistry
Glutathione metabolism
Glutathione Transferase chemistry
Glutathione Transferase metabolism
Subjects
Details
- Language :
- English
- ISSN :
- 0006-2960
- Volume :
- 40
- Issue :
- 51
- Database :
- MEDLINE
- Journal :
- Biochemistry
- Publication Type :
- Academic Journal
- Accession number :
- 11747461
- Full Text :
- https://doi.org/10.1021/bi010429i