The steroid side-chain-cleaving aldolase Ltp2-ChsH2 is a thiolase superfamily member with a radically repurposed active site.

An aldolase from the bile acid-degrading actinobacterium catalyzes the C-C bond cleavage of an isopropyl-CoA side chain from the D-ring of the steroid metabolite 17-hydroxy-3-oxo-4-pregnene-20-carboxyl-CoA (17-HOPC-CoA). Like its homolog from , the aldolase is a protein complex of Ltp2 with a DUF35 domain derived from the C-terminal domain of a hydratase (ChsH2) that catalyzes the preceding step in the pathway. We determined the structure of the Ltp2-ChsH2 complex at 1.7 Å resolution using zinc-single anomalous diffraction. The enzyme adopts an αββα organization, with the two Ltp2 protomers forming a central dimer, and the two ChsH2 protomers being at the periphery. Docking experiments suggested that Ltp2 forms a tight complex with the hydratase but that each enzyme retains an independent CoA-binding site. Ltp2 adopted a fold similar to those in thiolases; however, instead of forming a deep tunnel, the Ltp2 active site formed an elongated cleft large enough to accommodate 17-HOPC-CoA. The active site lacked the two cysteines that served as the nucleophile and general base in thiolases and replaced a pair of oxyanion-hole histidine residues with Tyr-246 and Tyr-344. Phenylalanine replacement of either of these residues decreased aldolase catalytic activity at least 400-fold. On the basis of a 17-HOPC-CoA -docked model, we propose a catalytic mechanism where Tyr-294 acts as the general base abstracting a proton from the D-ring hydroxyl of 17-HOPC-CoA and Tyr-344 as the general acid that protonates the propionyl-CoA anion following C-C bond cleavage.