A putative glutathione-binding site in T4 glutaredoxin investigated by site-directed mutagenesis

A glutathione monomer has been docked into the active site cleft of T4 glutaredoxin (previously called T4 thioredoxin) using molecular graphics. The central part of the cleft is formed by the side chain of Tyr-16 on one side and the residues Thr-64, Met-65, and Pro-66 on the other. The entire glutathione molecule fits well into the cleft. A cis-peptide bond between the residues Met-65 and Pro-66 allows glutathione to bind in an anti-parallel fashion to residues 64-66. Hydrogen bonds can be formed between Met-65 and the glutathione cysteine. This binding positions the glutathione sulfur atom ideally for reaction with the glutaredoxin disulfide. In the model, glutathione can form a hydrogen bond to the hydroxyl group of Tyr-16. Charged interactions at opposite ends of the binding cleft are provided by His-12 and Asp-80. The negatively charged alpha-carboxyl group of glutathione may interact with a positive helix dipole of the protein. Fifteen mutant T4 glutaredoxins have been produced and assayed for glutathione binding by determining thioltransferase activity. Mutant proteins with substitutions in the sides of the cleft (Tyr-16, Pro-66) exhibited the most marked decreases in thioltransferase activity. Mutation of His-12 to a serine decreases the catalytic efficiency whereas substitution of Asp-80 by serine increases the catalytic efficiency. A double mutant, D80S;H12S, has much less affinity for glutathione than either single mutant. Substitution of Cys-14 produces an inactive protein, whereas C17S retains some thioltransferase activity.