Design and synthesis of trans-2-substituted-cyclopropane-1-carboxylic acids as the first non-natural small molecule inhibitors of O-acetylserine sulfhydrylase

O-Acetylserine sulfhydrylase (isoform A, OASS-A) is a pyridoxal-5'-phosphate-dependent enzyme responsible for cysteine biosynthesis in many pathological microorganisms. It is proposed that inhibition of OASS-A could represent a novel strategy to overcome bacterial resistance to antibiotics. A class of 2-substituted-cyclopropane-1-carboxylic acids was synthesized, based on structural determinants grasped by analyzing a group of synthetic pentapeptides known to efficiently bind OASS-A from Haemophilus influenzae (HiOASS-A). The cyclopropane derivatives were submitted to a binding affinity assay with HiOASS-A and three of them, with K-diss in the low micromolar range, showed higher affinity than the most active synthetic pentapeptide. Thus, in this communication we report the first example of potent non-natural small molecule inhibitors of HiOASS-A. In addition, a molecular modelling study suggested a possible inhibition mechanism, through which the new cyclopropane ligands block HiOASS-A. Noteworthily, the novel, small-sized, non-peptidic inhibitors retain the structural motifs of the bulky peptides, which are relevant for the enzyme inhibition.