Crystal structure of hexanoyl-CoA bound to β-ketoacyl reductase FabG4 of Mycobacterium tuberculosis

Fab Gs, or beta-oxoacyl reductases, are involved in fatty acid synthesis. The reaction entails NADPH/NADH-mediated conversion of beta-oxoacyl-ACP (acyl-carrier protein) into beta-hydroxyacyl-ACP. HMwFabGs (high-molecular-weight FabG) form a phylogenetically separate group of FabG enzymes. FabG4, an HMwFabG from Mycobacteriuni tuberculosis, contains two distinct domains, an N-terminal 'flavodoxintype' domain and a C-terminal oxoreductase domain. The catalytically active C-terminal domain utilizes NADH to reduce beta-oxoacyl-CoA to beta-hydroxyacyl-CoA. In the present study the crystal structures of the FabG4 NADH binary complex and the FabG4-NAD(+) -hexanoyl-CoA ternary complex have been determined to understand the substrate specificity and catalytic mechanism of FabG4. This is the first report to demonstrate how FabG4 interacts with its coenzyme NADH and hexanoyl-CoA that mimics an elongating fattyacyl chain covalently linked with CoA. Structural analysis shows that the binding of hexanoyl-CoA within the active site cavity of FabG significantly differs from that of the C-16 fattyacyl substrate bound to mycobacterial FabI [InhA (enoyl-ACP reductase)]. The ternary complex reveals that both loop I and loop II interact with the phosphopantetheine moiety of CoA or ACP to align the covalently linked fattyacyl substrate near the active site. Structural data ACP inhibition studies indicate that FabG4 can accept both CoA- and ACP-based fattyacyl substrates. We have also shown that in the FabG4 dimer Arg(146) and Arg(445) of one monomer interact with the C-terminus of the second monomer to play pivotal role in substrate association and catalysis.