Structural basis for catalytic activity of a silkworm Delta-class glutathione transferase

Background: Glutathione transferase (GST) catalyzes glutathione conjugation, a major detoxification pathway for xenobiotics and endogenous substances. Here, we determined the crystal structure of a Delta-class GST from Bombyx mori (bmGSTD) to examine its catalytic residues. Methods: The three-dimensional structure of bmGSTD was resolved by the molecular replacement method and refined to a resolution of 2.0 Å. Results: Structural alignment with a Delta-class GST of Anopheles gambiae indicated that bmGSTD contains 2 distinct domains (an N-terminal domain and a C-terminal domain) connected by a linker. The bound glutathione localized at the N-terminal domain. Putative catalytic residues were changed to alanine by site-directed mutagenesis, and the resulting mutants were characterized in terms of catalytic activity using glutathione and 1-chloro-2,4-dinitrobenzene, a synthetic substrate of GST. Kinetic analysis of bmGSTD mutants indicated that Ser11, Gln51, His52, Ser67, and Arg68 are important for enzyme function. General significance: These results provide structural insights into the catalysis of glutathione conjugation in B. mori by bmGSTD.