Mechanism of triclosan inhibition of bacterial fatty acid synthesis

Triclosan is a broad-spectrum antibacterial agent that inhibits bacterial f atty acid synthesis at the enoyl-acyl carrier protein reductase (FabI) step . Resistance to triclosan in Escherichia coli is acquired through a missens e mutation in the fabI gene that leads to the expression of FabI[G93V], The specific activity and substrate affinities of FabI[G93V] are similar to Fa bI. Two different binding assays establish that triclosan dramatically incr eases the affinity of FabI for NAD(+). In contrast, triclosan does not incr ease the binding of NAD(+) to FabI[G93V]. The x-ray crystal structure of th e FabI-NAD(+)-triclosan complex confirms that hydrogen bonds and hydrophobi c interactions between triclosan and both the protein and the NAD(+) cofact or contribute to the formation of a stable ternary complex, with the drug b inding at the enoyl substrate site. These data show that the formation of a noncovalent "bi-substrate" complex accounts for the effectiveness of tricl osan as a FabI inhibitor and illustrates that mutations in the FabI active site that interfere with the formation of a stable FabI-NAD(+)-triclosan te rnary complex acquire resistance to the drug.