Slow-tight-binding inhibition of enoyl-acyl carrier protein reductase from Plasmodium falciparum by triclosan

Abstract

Triclosan is a potent inhibitor of FabI (enoyl-ACP reductase, where ACP stands for acyl carrier protein), which catalyses the last step in a sequence of four reactions that is repeated many times with each elongation step in the type II fatty acid biosynthesis pathway. The malarial parasite Plasmodium falciparum also harbours the genes and is capable of synthesizing fatty acids by utilizing the enzymes of type II FAS (fatty acid synthase). The basic differences in the enzymes of type I FAS, present in humans, and type II FAS, present in Plasmodium, make the enzymes of this pathway a good target for antimalarials. The steady-state kinetics revealed time-dependent inhibition of FabI by triclosan, demonstrating that triclosan is a slow-tight-binding inhibitor of FabI. The inhibition followed a rapid equilibrium step to form a reversible enzyme–inhibitor complex (EI) that isomerizes to a second enzyme–inhibitor complex (EI*), which dissociates at a very slow rate. The rate constants for the isomerization of EI to EI* and the dissociation of EI* were 5.49×10−2 and 1×10−4 s−1 respectively. The Ki value for the formation of the EI complex was 53 nM and the overall inhibition constant Ki* was 96 pM. The results match well with the rate constants derived independently from fluorescence analysis of the interaction of FabI and triclosan, as well as those obtained by surface plasmon resonance studies [Kapoor, Mukhi, N. Surolia, Sugunda and A. Surolia (2004) Biochem. J. 381, 725–733].