Crystal structure of FadA2 thiolase from Mycobacterium tuberculosis and prediction of its substrate specificity and membrane-anchoring properties.

Tuberculosis (TB) is one of the leading causes of human death caused by Mycobacterium tuberculosis (Mtb). Mtb can enter into a long-lasting persistence where it can utilize fatty acids as the carbon source. Hence, fatty acid metabolism pathway enzymes are considered promising and pertinent mycobacterial drug targets. FadA2 (thiolase) is one of the enzymes involved in Mtb's fatty acid metabolism pathway. FadA2 deletion construct (ΔL136-S150) was designed to produce soluble protein. The crystal structure of FadA2 (ΔL136-S150) at 2.9 Å resolution was solved and analysed for membrane-anchoring region. The four catalytic residues of FadA2 are Cys99, His341, His390 and Cys427, and they belong to four loops with characteristic sequence motifs, i.e., CxT, HEAF, GHP and CxA. FadA2 is the only thiolase of Mtb which belongs to the CHH category containing the HEAF motif. Analysing the substrate-binding channel, it has been suggested that FadA2 is involved in the β-oxidation pathway, i.e., the degradative pathway, as the long-chain fatty acid can be accommodated in the channel. The catalysed reaction is favoured by the presence of two oxyanion holes, i.e., OAH1 and OAH2. OAH1 formation is unique in FadA2, formed by the NE2 of His390 present in the GHP motif and NE2 of His341 present in the HEAF motif, whereas OAH2 formation is similar to CNH category thiolase. Sequence and structural comparison with the human trifunctional enzyme (HsTFE-β) suggests the membrane-anchoring region in FadA2. Molecular dynamics simulations of FadA2 with a membrane containing POPE lipid were conducted to understand the role of a long insertion sequence of FadA2 in membrane anchoring.