# Chemical Proteomics Reveals Regulation of Bile Salt Hydrolases via Oxidative Post-translational Modifications

## TL;DR

This study shows that bile salt hydrolase (BSH) activity in gut bacteria is regulated by reversible oxidative modifications, which could impact bile acid metabolism and gut health.

## Contribution

The discovery of reversible oxidative post-translational modification of BSH catalytic cysteine and the development of a chemoproteomic platform to profile these modifications in the gut microbiome.

## Key findings

- The catalytic cysteine in BSH exists in multiple oxidation states, including sulfenic acid, which inactivates the enzyme.
- A chemoproteomic platform using a sulfenic acid-reactive probe was developed to profile BSH modifications in the gut microbiome.
- Some gut BSHs are reversibly inactivated via cysteine oxidation, suggesting a regulatory mechanism for BSH activity.

## Abstract

The gut microbiome is the vast, diverse ecosystem of
microorganisms
that inhabits the human intestines and provides numerous essential
functions for the host. One such key role is the metabolism of primary
bile acids that are biosynthesized in the host liver into a plethora
of secondary bile acids produced by gut bacteria. These metabolites
serve as both antimicrobial and chemical signaling agents within the
host. The critical microbial enzyme that plays a gatekeeping role
in secondary bile acid metabolism is bile salt hydrolase (BSH), a
cysteine hydrolase that is primarily known for its deconjugating and
reconjugating activities on bile acid substrates. Despite the crucial
nature of these biotransformations, regulation of BSH activity is
not well understood. Here, we found that the catalytic cysteine 2
(Cys2) within the BSH active site exists in multiple sulfur oxidation
states including sulfenic acid (Cys-SOH). Importantly, we show this
reversible oxidative post-translational modification (oxPTM) ablates
BSH catalytic activity. We have leveraged this discovery to develop
a chemoproteomic platform featuring a sulfenic acid-reactive bile
acid probe to profile BSH Cys2 oxPTMs throughout the gut microbiome.
Our results reveal that though most gut microbiota-associated BSHs
exist in the active Cys2-SH state, some are preferentially and reversibly
inactivated in the Cys2-SOH state. This reversible oxidation of Cys2
may serve as a general mechanism to regulate BSH activity in vivo in response to a changing physiological environment.