Glutamate-64, a newly identified residue of the functionally conserved electron-sharing network contributes to catalysis and structural integrity of glutathione transferases

In Anopheles dirus glutathione transferase D3-3, position 64 is occupied by a functionally conserved glutamate residue, which interacts directly with the gamma-glutamate moiety of GSH (glutathione) as part of an electron-sharing network present in all soluble GSTs (glutathione transferases). Primary sequence alignment of all GST classes suggests that Glu(64) is one of a few residues that is functionally conserved in the GST superfamily. Available crystal structures as well as consideration of the property of the equivalent residue at position 64, acidic or polar, suggest that the GST electron-sharing motif can be divided into two types. Electrostatic interaction between the GSH glutamyl and carboxylic Glu(64), as well as with Arg(66) and Asp(100), was observed to extend the electron-sharing motif identified previously. Glu(64) contributes to the catalytic function of this motif and the 'base-assisted deprotonation' that are essential for GSH ionization during catalysis. Moreover, this residue also appears to affect multiple steps in the enzyme catalytic strategy, including binding of GSH, nucleophilic attack by thiolate at the electrophilic centre and product formation, probably through active-site packing effects. Replacement with non-functionally-conserved amino acids alters initial packing or folding by favouring aggregation during heterologous expression. Thermodynamic and reactivation in vitro analysis indicated that Glu(64) also contributes to the initial folding pathway and overall structural stability. Therefore Glu(64) also appears to impact upon catalysis through roles in both initial folding and structural maintenance.