Structural and functional analysis of a bile salt hydrolase from the bison microbiome

The bile salt hydrolases (BSHs) are significant constituents of animal microbiomes. An evolving appreciation of their roles in health and disease has established them as targets of pharmacological inhibition. These bacterial enzymes belong to the N-terminal nucleophile superfamily and are best known to catalyze the deconjugation of glycine or taurine from bile salts to release bile acid substrates for transformation and or metabolism in the gastrointestinal tract. Here, we identify and describe the BSH from a common member of the Plains bison microbiome, Arthrobacter citreus (BSH Ac ). Steady-state kinetic analyses demonstrated that BSH Ac is a broad-spectrum hydrolase with a preference for glycine-conjugates and deoxycholic acid (DCA). Second-order rate constants ( k cat / K M ) for BSH Ac -catalyzed reactions of relevant bile salts—glyco- and tauro-conjugates of cholic acid and DCA— varied by ∼30-fold and measured between 1.4 × 10 5 and 4.3 × 10 6 M -1 s -1 . Interestingly, a pan-BSH inhibitor named AAA-10 acted as a slow irreversible inhibitor of BSH Ac with a rate of inactivation ( k inact ) of ∼2 h -1 and a second order rate constant ( k inact / K I ) of ∼24 M -1 s -1 for the process. Structural characterization of BSH Ac reacted with AAA-10 showed covalent modification of the N-terminal cysteine nucleophile, providing molecular details for an enzyme-stabilized product formed from this mechanism-based inhibitor's α-fluoromethyl ketone warhead. Structural comparison of the BSHs and BSH:inhibitor complexes highlighted the plasticity of the steroid-binding site, including a flexible loop that is variable across well-studied BSHs.