Inhibition of human folylpolyglutamate synthetase by diastereomeric phosphinic acid mimics of the tetrahedral intermediate

Phosphorus-containing pseudopeptides, racemic at the C-terminal α-carbon, are potent mechanism-based inhibitors of folylpolyglutamate synthetase (FPGS). They are mimics of the tetrahedral intermediate postulated to form during FPGS-catalyzed biosynthesis of poly(γ-l-glutamates). In the present paper, the FPGS inhibitory activity of each diastereomer coupled to three heterocycles is reported. The high Rf pseudopeptide containing the 5,10-dideazatetrahydropteroyl (DDAH4Pte) heterocycle is most potent (Kis = 1.7 nM). While the heterocyclic portion affects absolute FPGS inhibitory potency, the high Rf species is more potent in each pair containing the same heterocycle. This species presumably has the same stereochemistry as the natural folate polyglutamate, i.e., (l-Glu-γ-l-Glu). Unexpectedly, the low Rf (presumed l-Glu-γ-d-Glu) species are only slightly less potent (<30-fold) than their diastereomers. Further study of this phenomenon comparing l-Glu-γ-l-Glu and l-Glu-γ-d-Glu dipeptide-containing FPGS substrates shows that <1% contamination of commercial d-Glu precursors by l-Glu may give misleading information if l-Glu-γ-l-Glu substrates have low Km values.