Inhibition studies of ketol-acid reductoisomerases from pathogenic microorganisms

Ketol-acid reductoisomerase (KARI), the second enzyme in the branched-chain amino acid (BCAA) biosynthesis pathway, is an emerging target for the discovery of biocides. Here, we demonstrate that cyclopropane-1,1-dicarboxylate (CPD) inhibits KARIs from the pathogens Mycobacterium tuberculosis (Mt) and Campylobacter jejuni (Cj) reversibly with K_i values of 3.03 μM and 0.59 μM, respectively. Another reversible inhibitor of both KARIs, Hoe 704, is more potent than CPD with K_i values of 300 nM and 110 nM for MtKARI and CjKARI, respectively. The most potent inhibitor tested here is N-hydroxy-N-isopropyloxamate (IpOHA). It has a K_i of ~26 nM for MtKARI, but binds rather slowly (k_on ~900 M⁻¹s⁻¹). In contrast, IpOHA binds more rapidly (k_on ~7000 M⁻¹s⁻¹) to CjKARI and irreversibly.