IpdAB, a virulence factor inMycobacterium tuberculosis, is a cholesterol ring-cleaving hydrolase.Crowe, A.M., Workman, S.D., Watanabe, N., Worrall, L.J., Strynadka, N.C.J., Eltis, L.D.
(2018) Proc Natl Acad Sci U S A 115: E3378-E3387
- PubMed: 29581275
- DOI: https://doi.org/10.1073/pnas.1717015115
- Primary Citation of Related Structures:
6CO6, 6CO9, 6COJ, 6CON
- PubMed Abstract:
Mycobacterium tuberculosis ( Mtb ) grows on host-derived cholesterol during infection. IpdAB, found in all steroid-degrading bacteria and a determinant of pathogenicity, has been implicated in the hydrolysis of the last steroid ring. Phylogenetic analyses revealed that IpdAB orthologs form a clade of CoA transferases (CoTs). In a coupled assay with a thiolase, IpdAB transformed the cholesterol catabolite ( R )-2-(2-carboxyethyl)-3-methyl-6-oxocyclohex-1-ene-1-carboxyl-CoA (COCHEA-CoA) and CoASH to 4-methyl-5-oxo-octanedioyl-CoA (MOODA-CoA) and acetyl-CoA with high specificity ( k cat / K m = 5.8 ± 0.8 × 10 4 M -1 ⋅s -1 ). The structure of MOODA-CoA was consistent with IpdAB hydrolyzing COCHEA-CoA to a β-keto-thioester, a thiolase substrate. Contrary to characterized CoTs, IpdAB exhibited no activity toward small CoA thioesters. Further, IpdAB lacks the catalytic glutamate residue that is conserved in the β-subunit of characterized CoTs and a glutamyl-CoA intermediate was not trapped during turnover. By contrast, Glu105 A , conserved in the α-subunit of IpdAB, was essential for catalysis. A crystal structure of the IpdAB·COCHEA-CoA complex, solved to 1.4 Å, revealed that Glu105 A is positioned to act as a catalytic base. Upon titration with COCHEA-CoA, the E105A A variant accumulated a yellow-colored species (λ max = 310 nm; K d = 0.4 ± 0.2 μM) typical of β-keto enolates. In the presence of D 2 O, IpdAB catalyzed the deuteration of COCHEA-CoA adjacent to the hydroxylation site at rates consistent with k cat Based on these data and additional IpdAB variants, we propose a retro-Claisen condensation-like mechanism for the IpdAB-mediated hydrolysis of COCHEA-CoA. This study expands the range of known reactions catalyzed by the CoT superfamily and provides mechanistic insight into an important determinant of Mtb pathogenesis.
Department of Microbiology and Immunology, Life Sciences Institute, The University of British Columbia, Vancouver, BC, Canada, V6T 1Z3.