Crystal Structure of the Mycobacterium tuberculosisβ-Ketoacyl-Acyl Carrier Protein Synthase III*

Mycolic acids (α-alkyl-β-hydroxy long chain fatty acids) cover the surface of mycobacteria, and inhibition of their biosynthesis is an established mechanism of action for several key front-line anti-tuberculosis drugs. In mycobacteria, long chain acyl-CoA products (C14–C26) generated by a type I fatty-acid synthase can be used directly for the α-branch of mycolic acid or can be extended by a type II fatty-acid synthase to make the meromycolic acid (C50–C56))-derived component. An unusual Mycobacterium tuberculosisβ-ketoacyl-acyl carrier protein (ACP) synthase III (mtFabH) has been identified, purified, and shown to catalyze a Claisen-type condensation between long chain acyl-CoA substrates such as myristoyl-CoA (C14) and malonyl-ACP. This enzyme, presumed to play a key role in initiating meromycolic acid biosynthesis, was crystallized, and its structure was determined at 2.1-Å resolution. The mtFabH homodimer is closely similar in topology and active-site structure to Escherichia coli FabH (ecFabH), with a CoA/malonyl-ACP-binding channel leading from the enzyme surface to the buried active-site cysteine residue. Unlike ecFabH, mtFabH contains a second hydrophobic channel leading from the active site. In the ecFabH structure, this channel is blocked by a phenylalanine residue, which constrains specificity to acetyl-CoA, whereas in mtFabH, this residue is a threonine, which permits binding of longer acyl chains. This same channel in mtFabH is capped by an α-helix formed adjacent to a 4-amino acid sequence insertion, which limits bound acyl chain length to 16 carbons. These observations offer a molecular basis for understanding the unusual substrate specificity of mtFabH and its probable role in regulating the biosynthesis of the two different length acyl chains required for generation of mycolic acids. This mtFabH presents a new target for structure-based design of novel antimycobacterial agents.