An electron-sharing network involved in the catalytic mechanism is functionally conserved in different glutathione transferase classes.
In Anopheles dirus glutathione transferase D3-3, there are electrostatic interactions between the negatively charged glutamyl alpha-carboxylate group of glutathione, the positively charged Arg-66, and the negatively charged Asp-100. This ionic interaction is stabilized by a network of hydrogen bonds from Ser-65, Thr-158, Thr-162, and a conserved water-mediated contact. This alternating ionic bridge interaction between negatively and positively charged residues stabilized by a network of hydrogen bonding we have named an electron-sharing network. We show that the electron-sharing network assists the glutamyl alpha-carboxylate of glutathione to function as a catalytic base accepting the proton from the thiol group forming an anionic glutathione, which is a crucial step in the glutathione transferase (GST) catalysis. Kinetic studies demonstrate that the mutation of electron-sharing network residues results in a decreased ability to lower the pKa of the thiol group of glutathione. Although the residues that contribute to the electron-sharing network are not conserved in the primary sequence, structural characterizations indicate that the presence of the network can be mapped to the same region in all GST classes. A structural diversification but functional conservation suggests a significant role for the electron-sharing network in catalysis as the purpose was maintained during the divergent evolution of GSTs. This network appears to be a functionally conserved motif that contributes to the "base-assisted deprotonation" model suggested to be essential for the glutathione ionization step of the catalytic mechanism.
Winayanuwattikun
Pakorn
P
Institute of Molecular Biology and Genetics, Mahidol University, Salaya Campus, Nakhon Pathom 73170, Thailand.
Ketterman
Albert J
AJ
D000818:Animals; D000852:Anopheles / Q000201:enzymology; D020134:Catalytic Domain / Q000235:genetics; D017124:Conserved Sequence; D004583:Electrons; D005978:Glutathione / Q000737:chemistry* / Q000378:metabolism; D005982:Glutathione Transferase / Q000737:chemistry* / Q000145:classification* / Q000235:genetics / Q000378:metabolism; D006358:Hot Temperature; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D057927:Hydrophobic and Hydrophilic Interactions; D007700:Kinetics; D016297:Mutagenesis, Site-Directed; D017434:Protein Structure, Tertiary; D013379:Substrate Specificity
D005982:Glutathione Transferase; D005978:Glutathione