Ligand effects on the fluorescence properties of tyrosine-9 in alpha 1-1 glutathione S-transferase.
A conserved tyrosine plays a critical role in catalysis by mammalian glutathione S-transferases (GSTs) of the alpha-, mu-, and pi-classes, by forming a hydrogen bond to and stabilizing the thiolate form of glutathione. The hydrogen bonding properties of this tyrosine in the rat A1-1 GST (Tyr-9), in the absence and presence of ligands, have been studied by steady state and time-resolved fluorescence spectroscopy. In order to achieve this, the single tryptophan (Trp 21) found in the rat A1-1 GST has been replaced with the fluorometrically silent phenylalanine (W21F). Additionally, a double mutant lacking this tryptophan and the catalytic tyrosine (W21F:Y9F) has been constructed, and these mutants have been used as probes of ligand effects at Tyr-9. A comparison of the correlated excitation--emission spectra of the W21F mutant and the W21F-Y9F indicates that a red-shifted emission component is contributed by Tyr-9 with excitation bands at 255 and 300 nm, in the ligand-free enzyme. The pH-dependence of the intensity of these spectral cross-peaks is consistent with an active site tyrosine with a pKa of 8.1-8.3. Upon addition of GSH, the red-shifted component is quenched. Multifrequency phase/modulation fluorescence experiments qualitatively demonstrate that GSH causes a decrease in the average excited state lifetime on the red-edge of the spectrum of W21F but not of the W21F:Y9F spectrum. Steady state correlated difference spectra (W21F-W21F:Y9F) have been used to obtain a model for the excitation-emission correlated spectrum of Tyr-9, which indicates that Tyr-9 is heterogeneous at pH 7.5, with properties of both tyrosinate and "normal tyrosine". The tyrosinate fraction is eliminated, and the blue-shifted component becomes more intense upon addition of GSH conjugates, indicating that the weak hydrogen bond between Tyr-9 and thioethers has little charge-transfer character. The S-methyl GSH yields an "anomalous" spectrum at pH 7.5, which retains cross-peaks consistent with ionized tyrosinate. These results indicate that, in the absence of ligand, Tyr-9 forms a strongly polarized hydrogen bond or a fraction of the phenolic hydroxyl group is partially deprotonated. However, when a GSH conjugate with a sufficiently large hydrophobic group occupies the H-site, Tyr-9 is fully protonated, with little charge-transfer character.
Dietze
E C
EC
University of Washington, Medicinal Chemistry, Seattle, Washington 98195-7610, USA.
Wang
R W
RW
Lu
A Y
AY
Atkins
W M
WM
D000818:Animals; D001665:Binding Sites; D005978:Glutathione / Q000378:metabolism; D005982:Glutathione Transferase / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism*; D006860:Hydrogen Bonding; D007527:Isoenzymes / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism*; D007700:Kinetics; D008024:Ligands; D008322:Mammals; D016297:Mutagenesis, Site-Directed; D010649:Phenylalanine; D017354:Point Mutation; D051381:Rats; D011994:Recombinant Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism; D013050:Spectrometry, Fluorescence; D014443:Tyrosine
D007527:Isoenzymes; D008024:Ligands; D011994:Recombinant Proteins; D014443:Tyrosine; D010649:Phenylalanine; D005982:Glutathione Transferase; D005978:Glutathione