X-ray Structural Analysis of Plasmodium falciparum Enoyl Acyl Carrier Protein Reductase as a Pathway toward the Optimization of Triclosan Antimalarial Efficacy

Stephanie G Valderramos,Han-Chun Tsai,Juan-Carlos Valderramos,David P Jacobus,Hong-Ming Shieh,Min Yu,Joel S Freundlich,Feng Wang,Louis J Nkrumah,David A Fidock,Guy A Schiehser,William R Jacobs,John W Anderson,T.R Santha Kumar,Mack Kuo,James C Sacchettini

doi:10.1074/jbc.m701813200

Abstract

The x-ray crystal structures of five triclosan analogs, in addition to that of the isoniazid-NAD adduct, are described in relation to their integral role in the design of potent inhibitors of the malarial enzyme Plasmodium falciparum enoyl acyl carrier protein reductase (PfENR). Many of the novel 5-substituted analogs exhibit low micromolar potency against in vitro cultures of drug-resistant and drug-sensitive strains of the P. falciparum parasite and inhibit purified PfENR enzyme with IC50 values of <200 nM. This study has significantly expanded the knowledge base with regard to the structure-activity relationship of triclosan while affording gains against cultured parasites and purified PfENR enzyme. In contrast to a recent report in the literature, these results demonstrate the ability to improve the in vitro potency of triclosan significantly by replacing the suboptimal 5-chloro group with larger hydrophobic moieties. The biological and x-ray crystallographic data thus demonstrate the flexibility of the active site and point to future rounds of optimization to improve compound potency against purified enzyme and intracellular Plasmodium parasites.

Highlights

New, highly potent antimalarials that are orally bioavailable, affordable, safe, and unencumbered by existing resistance mechanisms [2]
The final reaction in the FAS-II pathway is catalyzed by the enoyl acyl carrier protein reductase enzyme (PfENR in P. falciparum, known as PfFabI), which mediates the NADH-dependent reduction of trans-2-enoyl-ACP to acyl-ACP
A possible correlation among these biological activities is supported by the observation that triclosan produces a dose-dependent inhibition of [14C]acetate incorporation into fatty acids by cultured P. falciparum parasites and [14C]malonyl-CoA incorporation into parasite extracts freed from host red blood cells [7] and inhibits a P. falciparum FAS-II in vitro reconstituted system [13]

Summary

Introduction

New, highly potent antimalarials that are orally bioavailable, affordable, safe, and unencumbered by existing resistance mechanisms [2]. Chhibber et al [16] described the synthesis and activity, versus both purified PfENR and cultured parasites, of triclosan analogs with primarily hydrophilic 5-substituents.

Results

Conclusion