# Structural insights into lacto‐N‐biose I recognition by a family 32 carbohydrate‐binding module from Bifidobacterium bifidum

## TL;DR

This study reveals how Bifidobacterium bifidum captures lacto-N-biose I from breast milk, supporting infant gut health through microbial symbiosis.

## Contribution

The study provides structural and functional insights into the CBM32 domain's specific recognition of lacto-N-biose I in Bifidobacterium bifidum.

## Key findings

- CBM32 binds lacto-N-biose I with a dissociation constant of 98 μm.
- The crystal structure of CBM32 complexed with lacto-N-biose I reveals the molecular basis of its recognition.
- LnbB's CBM32 domain plays a key role in capturing lacto-N-biose I, supporting microbial symbiosis in the infant gut.

## Abstract

Bifidobacterium bifidum, a predominant colonizer of the infant gut, utilizes lacto‐N‐biose I (LNB), a prominent component of human milk oligosaccharides (HMOs), through a dedicated metabolic pathway. Among a diverse set of extracellular glycosidases involved in HMO degradation, lacto‐N‐biosidase (LnbB) plays a pivotal role by releasing LNB. We investigated the structure and function of the carbohydrate‐binding module family 32 (CBM32) domain located at the C‐terminus of the glycoside hydrolase family 20 catalytic domain in LnbB. Isothermal titration calorimetry showed that CBM32 binds LNB with a dissociation constant (K
d) of 98 μm. The crystal structure of the CBM32 complexed with LNB reveals the molecular basis for its specific recognition.

Impact statementBifidobacteria are beneficial gut microbes, and infant‐associated strains establish symbiosis by degrading human milk oligosaccharides. This study uncovers the molecular mechanism by which Bifidobacterium bifidum captures lacto‐N‐biose I, a key disaccharide, functioning as a cross‐feeder that promotes the growth of other bifidobacteria and supports the infant gut ecosystem.

Bifidobacteria are beneficial gut microbes, and infant‐associated strains establish symbiosis by degrading human milk oligosaccharides. This study uncovers the molecular mechanism by which Bifidobacterium bifidum captures lacto‐N‐biose I, a key disaccharide, functioning as a cross‐feeder that promotes the growth of other bifidobacteria and supports the infant gut ecosystem.

Bifidobacterium bifidum establishes symbiosis with infants by metabolizing lacto‐N‐biose I (LNB) from human milk oligosaccharides (HMOs). The extracellular multidomain enzyme LnbB drives this process, releasing LNB via its catalytic glycoside hydrolase family 20 (GH20) lacto‐N‐biosidase domain. Its carbohydrate‐binding module family 32 (CBM32) domain binds LNB (K
d = 98 μm), and the crystal structure of the CBM32–LNB complex reveals the molecular basis of recognition.