# Tangled Up in Fibers: How a Multidomain Lytic Polysaccharide Monooxygenase Binds Its Chitin Substrate

## TL;DR

This paper explores how a specific enzyme from Vibrio cholerae binds to chitin fibers, revealing how it coats and modifies the substrate to support bacterial microcolony formation.

## Contribution

The study introduces a novel method to stabilize and characterize the interaction between a multidomain LPMO and chitin, revealing structural and functional insights.

## Key findings

- GbpA binds rapidly to chitin fibers and smooths their surface.
- GbpA binding leads to the formation of protein–chitin clumps with multiple enzyme molecules.
- The enzyme's interaction with chitin supports bacterial microcolony formation.

## Abstract

Lytic polysaccharide monooxygenases (LPMOs) are redox
enzymes that
bind to and oxidize insoluble carbohydrate substrates such as chitin
or cellulose. This class of enzymes has attracted considerable attention
due to their ability to convert biomaterials of high abundance into
oligosaccharides that can be useful for producing biofuels and bioplastics.
However, processes at the interface between solution and insoluble
substrates represent a major challenge to biochemical and structural
characterization. Here, we investigated the four-domain LPMO from Vibrio cholerae, N-acetyl glucosamine binding
protein A (GbpA), to elucidate how it docks onto its insoluble substrate
with its two terminal domains. First, we developed a protocol that
allowed GbpA and chitin to form a stable complex in suspension, overcoming
incompatibilities of the two binding partners with respect to pH.
After determining the small-angle neutron scattering (SANS) contrast
match point for chitin (47% D2O), we characterized the
mesoscale structure of GbpA in complex with chitin by SANS and complemented
the results with negative-stain electron microscopy. We found that
GbpA binds rapidly to chitin, where it coats the chitin fibers and
smooths their surface. In some locations, GbpA binding induces the
formation of protein–chitin clumps containing a large number
of GbpA molecules. Together, this suggests how the secretion of GbpA
efficiently prepares the ground for microcolony formation by the bacteria.