Structural Features of a Bacteroidetes-Affiliated Cellulase Linked with a Polysaccharide Utilization Locus

Area of Science:

• Biochemistry
• Structural Biology
• Microbiology

Background:

• A gene-centric analysis of cow rumen metagenomes identified a novel polysaccharide utilization locus.
• The primary enzyme, AC2aCel5A, was classified as a glycoside hydrolase (GH) family 5 endo-cellulase, suggesting cellulolytic activity.

Purpose of the Study:

• To determine the three-dimensional structure of AC2aCel5A.
• To characterize the enzymatic activities and substrate specificity of AC2aCel5A.

Main Methods:

• X-ray crystallography was used to obtain high-resolution structures (1.8 Å and 2.4 Å).
• Enzymatic assays were performed to assess activity on various polysaccharides.
• Site-directed mutagenesis was employed to create a catalytically inactive mutant for co-crystallization studies.

Main Results:

• The structure revealed the typical (β/α)8-barrel fold of GH family 5 enzymes with conserved catalytic glutamates.
• AC2aCel5A demonstrated activity exclusively on insoluble cellulose and linear β-(1,4)-linked glucans.
• Co-crystallization showed cellotriose bound in the active site, with a hydrophobic clamp potentially interacting with sugars.
• The enzyme's active-site cleft is narrower than related enzymes, suggesting adaptation for cellulose degradation over branched polysaccharides like xylans or mannans.

Conclusions:

• AC2aCel5A is a specialized endo-cellulase adapted for efficient breakdown of cellulose in the cow rumen.
• Its narrow active-site cleft restricts activity to linear glucans, differentiating it from broader-acting GH5 family members.
• This structural and functional characterization provides valuable insights into the enzymatic machinery for cellulose degradation in ruminant digestion.