CONTRIBUTION OF AMINO-ACID RESIDUE-208 IN THE HYDROPHOBIC BINDING-SITE TO THE CATALYTIC MECHANISM OF HUMAN GLUTATHIONE TRANSFERASE-A1-1

Glutathione transferases (GSTs) catalyze the nucleophilic attack of th e thiolate of glutathione on a variety of noxious, often hydrophobic, electrophiles. The interactions responsible for the binding of glutath ione have been deduced in great detail from the 3-dimensional structur es that have been solved for three different GSTs, each a member of a distinct structural class. However, the interactions of the electrophi lic substrates with these enzymes are still largely unexplored. The co ntribution of the active-site Met208 to aromatic and benzylic chloride substitution reactions catalyzed by human class Alpha GST A1-1 has be en evaluated by comparison of wild-type enzyme with variants mutated i n position 208. The results show that the amino acid residue at positi on 208 primarily affects the aromatic substitution reaction, tested wi th 1-chloro-2,4-dinitrobenzene as substrate, possibly by interacting w ith the delocalized negative charge of the substituted ring structure in the transition state.