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 (kcat/Km = 5.8 ± 0.8 × 104 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, Glu105A, 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 Glu105A is positioned to act as a catalytic base. Upon titration with COCHEA-CoA, the E105AA variant accumulated a yellow-colored species (λmax = 310 nm; Kd = 0.4 ± 0.2 μM) typical of β-keto enolates. In the presence of D2O, IpdAB catalyzed the deuteration of COCHEA-CoA adjacent to the hydroxylation site at rates consistent with kcat 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.
Keywords: carbon bond cleavage; coenzyme A transferase; enzymology; steroid catabolism; tuberculosis.