Fabl enzyme

In addition to its broad-spectrum biocidal activity, triclosan (22) displays reversible inhibition of E. coli Fabl with a picomolar K, for binding the enzyme-cofactor complex [4]. Triclosan entry results in the reordering of a loop of amino acids close to the active site, making it a slow, tight-binding inhibitor [41]. 

Among the few reported inhibitors, the optimized HTS lead phe-nylimidazole 39 is an interesting candidate against S. pneumoniae with nanomolar FabK inhibition and an MIC90 of 4 pg/mL [59]. The same team hybridized 29, a FabI pyridine inhibitor with 39 to provide 40, in an attempt to realize a compound with a dual mode of action. Although inhibition of both enzymes as well as sub-pg/ mL MIC against S. pneumoniae is retained, the Fabl-related antibacterial activity against S. aureus is lost [60]. 

The last step in the fatty acid biosynthetic pathway is catalyzed by enoyl-acyl carrier protein (ACP) reductase, which is responsible for reduction of the double bond in the enoyl-ACP derivative (Heath and Rock, 1995 Payne et al., 2002). While fabl genes encode enoyl-ACP reductases (FabI enzymes) in S. aureus and E. coli, an alternative enoyl-ACP reductase, FabK, replaces the function of Fabl in a number of bacterial species such as Streptococcus pneumoniae (Heath and Rock, 2000). More interestingly, a number of bacterial species (such as Enterococcus faecalis and Pseudomonas aeruginosa) possess both the Fabl and FabK enzymes (Heath and Rock, 2000). To discover Fabl-specific antibacterial inhibitors, Payne and colleagues at GlaxoSmithKline (GSK) developed assays for various versions of enoyl-ACP reductases (Payne et al., 2002 Seefeld et al., 2003) based on the following reaction scheme ... 

Four enzymes participate in each iterative cycle of chain elongation (Fig. 3). The acetoacyl-ACP formed from the initiating FabH condensation is reduced by an NADPH-dependent P-ketoacyl-ACP reductase (fabG), and a water molecule is then removed by a P-hydroxyacyl-ACP dehydrase (fabA otfabZ). The last step is catalyzed by enoyl-ACP reductase (fabl or fabK) to form a saturated acyl-ACP, which serves as the substrate for another condensation reaction or when the chain length reaches 16-18 carbons is utilized for membrane phospholipid synthesis. p-Ketoacyl-ACP synthase I or II (fabB or fabF) initiates additional... 

The final step in each round of fatty acyl elongation in E. coli is the NADH-dependent reduction of the trans double bond, catalyzed by the homotetrameric (subunit mass of 29 kDa) NADH-dependent enoyl-ACP reductase I (encoded by fabl). The Fabl amino acid sequence is similar (34% identical) to the product of a gene (called inhA) from mycobacteria. InhA is involved in mycolic acid biosynthesis (A. Banerjee, 1994). The synthesis of these unusual 70-80 carbon mycobacterial acids requires a pathway composed of enzymes essentially identical to those of fatty acid synthesis. Missense mutations within the inhA gene result in resistance to the anti-tuberculosis drugs, isoniazid and ethionamide. The crystal structures of Fabl and InhA have been solved, and are virtually superimposable for most... 

Both the biosynthetic and degradative fatty acid cycles contain two oxidoreductases each. In the biosynthetic pathway, the /3-ketoacyl-ACP formed by the KAS enzymes is reduced by an NADPH-dependent reductase, encoded by the fabG gene in E. coli. Following a dehydration step, the resulting enoyl-ACP is reduced by an enoyl-ACP reductase, encoded by xS fahlin E. coli. FabI is an NADH-dependent reductase, and both FabI and FabG are members of the SDR superfamily. Not all bacteria utilize FabI as their enoyl-ACP reductase, and currently, three other enzymes that include FabV, FabL, and FabK are known. Both FabV and FabL are also members of the SDR family however, the flavin-dependent enoyl-ACP reductase FabK is not. 

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