Electrostatic Interactions Dictate Bile Salt Hydrolase Substrate Preference

The human intestines are colonized
by trillions of microbes,
comprising
the gut microbiota, which produce diverse small molecule metabolites
and modify host metabolites, such as bile acids, that regulate host
physiology. Biosynthesized in the liver, bile acids are conjugated
with glycine or taurine and secreted into the intestines, where gut
microbial bile salt hydrolases (BSHs) deconjugate the amino acid to
produce unconjugated bile acids that serve as precursors for secondary
bile acid metabolites. Among these include a recently discovered class
of microbially conjugated bile acids (MCBAs), wherein alternative
amino acids are conjugated onto bile acids. To elucidate the metabolic
potential of MCBAs, we performed detailed kinetic studies to investigate
the preference of BSHs for host-conjugated bile acids and MCBAs. We
identified a BSH that exhibits positive cooperativity uniquely for
MCBAs containing an aromatic side chain. Further molecular modeling
and phylogenetic analyses indicated that the BSH preference for aromatic
MCBAs is due to a substrate-specific cation–π interaction
and is predicted to be widespread among human gut microbial BSHs.

人体肠道定植着数以万亿计的微生物，这些微生物构成肠道菌群（gut microbiota），可产生多种小分子代谢物，并修饰诸如调控宿主生理功能的胆汁酸等宿主代谢物。胆汁酸由肝脏生物合成，随后与甘氨酸或牛磺酸结合并分泌进入肠道；肠道微生物来源的胆汁盐水解酶（bile salt hydrolases，BSHs）可水解其共轭结合的氨基酸，生成游离胆汁酸，而游离胆汁酸是次级胆汁酸代谢物的前体。在这类代谢产物中，还包含一类新近发现的微生物结合型胆汁酸（microbially conjugated bile acids，MCBAs），这类物质是将其他氨基酸结合至胆汁酸上形成的。为阐明MCBAs的代谢潜力，我们开展了精细的动力学研究，以探究BSHs对宿主结合型胆汁酸与MCBAs的底物偏好性。我们发现了一种BSH，其仅对带有芳香族侧链的MCBAs表现出正协同性。进一步的分子建模与系统发育分析表明，BSH对芳香族MCBAs的偏好性源于底物特异性的阳离子-π相互作用，且该特征预计广泛存在于人类肠道微生物的BSHs中。