Catalysis and Site-Specific Modification of Glutathione Transferases Enabled by Rational Design
Abstract: This thesis describes the rational design of a novel enzyme, a thiolester hydrolase, derived from human glutathione transferase (GST) A1-1 by the introduction of a single histidine residue. The first section of the thesis describes the design and the determination of the reaction mechanism. The design was based on the crystal structure of human GST A1-1 complexed with S-benzylglutathione. The resulting enzyme, A216H, catalyzed the hydrolysis of the non-natural substrate GSB, a thiolester of glutathione and benzoic acid. The reaction followed saturation kinetics with a kcat of 0.00078 min-1 and KM of 5 ?M. The rate constant ratio, (kcat/KM)/kuncat, was found to be more than 107 M-1. The introduction of a single His residue in position 216 opened up a novel reaction pathway in human GST A1-1 and is a nice example of catalytic promiscuity. The substrate requirements were investigated and A216H was found to be selective since only two out of 18 GS-thiolesters tested were substrates for A216H. The reaction mechanism of the A216H-catalyzed hydrolysis of GSB was determined and found to proceed via an acyl intermediate at Y9. The hydrolysis was catalyzed by H216 that acts as a general base and the deacylation was found to be the rate-determining step. The Y9-intermediate could be selectively trapped by oxygen nucleophiles and primary alcohols, in particular 1-propanol and trifluoroethanol, were the most efficient. In addition, saturation kinetics was obtained in the acyl transfer reaction with 1-propanol indicating the presence of a second binding site in A216H.The second section of this thesis describes the site-specific covalent modification of human GST A1-1. The addition of GSB to the wild-type protein results in a site-specific benzoylation of only one tyrosine residue, Y9, out of ten present in the protein (one out of totally 51 nucleophiles). The reaction was tested with five GST classes (Alpha, Mu, Pi, Theta and Omega) and found to be specific for the Alpha class isoenzymes. The covalent modification reaction was further refined to target a single lysine residue, K216, providing a more stable linkage in the form of an amide bond. The reaction was found to be versatile and approximately 50% of the GS-thiolesters tested acylated K216, including a fluorophore.
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