Probing the stereochemistry of the active site of gamma-glutamyl transpeptidase using sulfur derivatives of l-glutamic acid

Abstract

Gamma-glutamyl transpeptidase (GGT) catalyses the transfer of a γ-glutamyl moiety from a donor substrate to different acceptors, such as amino acids and water. GGT is known to display relatively low stereospecificity with respect to the α-stereocentre of its donor substrates. In this study we have studied its stereospecificity with respect to the stereocentre at the δ-position of different analogues of L-glutamic acid. Notably, L-methionine sulfoxide is well-recognised whereas L-methionine sulfone and L-methionine sulfoximine are not. Furthermore, when the synthetic γ-diastereoisomers of L-methionine sulfoxide were separated and tested, it was discovered that GGT shows remarkable stereospecificity at the γ-position, binding the S_CS_S diastereoisomer with a K_i of 3.5 mM, whereas the S_CR_S diastereoisomer is not recognised. Finally, using a sulfoxide as a new pharmacophore for GGT, we have synthesized and tested an analogue of glutathione to obtain a very promising competitive inhibitor with a K_i of (53 ± 3) µM.