# Commensal gut bacteria employ de-chelatase HmuS to harvest iron from heme

## TL;DR

Gut bacteria use a de-chelatase enzyme called HmuS to extract iron from heme under anaerobic conditions.

## Contribution

The study identifies HmuS as a de-chelatase that enables anaerobic bacteria to harvest iron from heme, a mechanism previously unknown.

## Key findings

- HmuS deconstructs heme into protoporphyrin IX and Fe(II) under anaerobic conditions.
- The cryo-EM structure of HmuS reveals conserved heme and cation binding sites.
- HmuS is encoded in the hmu operon, which is widespread in gut metagenomes and other anaerobic ecosystems.

## Abstract

Iron is essential for almost all organisms, which have evolved different strategies for ensuring a sufficient supply from their environment and using it in different forms, including heme. The hmu operon, primarily found in Bacteroidota and ubiquitous in gastrointestinal tract metagenomes of healthy humans, encodes proteins involved in heme acquisition. Here, we provide direct physiological, biochemical, and structural evidence for the anaerobic removal of iron from heme by HmuS, a membrane-bound, NADH-dependent de-chelatase that deconstructs heme to protoporphyrin IX (PPIX) and Fe(II). Heme can serve as the sole iron source for the model gastrointestinal bacterium Bacteroidetes thetaiotaomicron, when active HmuS is present. Heterologously expressed HmuS was isolated with bound heme molecules under saturating conditions. Its cryo-EM structure at 2.6 Å resolution revealed binding of heme and a pair of cations at distant sites. These sites are conserved across the HmuS family and chelatase superfamily, respectively. The proposed structure-based mechanism for iron removal by HmuS is chemically analogous to the chelatases in both unrelated heme biosynthetic pathways and homologous enzymes in the biosynthetic pathways for chlorophyll and vitamin B12, although the reaction proceeds in the opposite direction. Taken together, our study identifies a widespread mechanism via which anaerobic bacteria can extract nutritional iron from heme.

Gastrointestinal tract bacteria require iron for survival, with heme as an abundant source in their environment. This work identifies a mechanism for anaerobic extraction of iron from heme mediated by a de-chelatase that is ubiquitously encoded in healthy human gastrointestinal metagenomes and abundant in other anaerobic ecosystems.

Bacteroidetes thetaiotaomicron, an anaerobic gut commensal, transforms heme into protoporphyrin IX and Fe(II).Its hmuS gene encodes the enzyme required for iron extraction from heme.The cryo-electron microscopy structure of HmuS reveals binding sites for heme and cations.

Bacteroidetes thetaiotaomicron, an anaerobic gut commensal, transforms heme into protoporphyrin IX and Fe(II).

Its hmuS gene encodes the enzyme required for iron extraction from heme.

The cryo-electron microscopy structure of HmuS reveals binding sites for heme and cations.

De-chelatase HmuS mediates anaerobic extraction of iron from heme in gastrointestinal bacteria.