An evolutionarily distinct family of polysaccharide lyases removes rhamnose capping of complex arabinogalactan proteins.Munoz-Munoz, J., Cartmell, A., Terrapon, N., Basle, A., Henrissat, B., Gilbert, H.J.
(2017) J Biol Chem 292: 13271-13283
- PubMed: 28637865
- DOI: https://doi.org/10.1074/jbc.M117.794578
- Primary Citation of Related Structures:
5NO8, 5NOA, 5NOK
- PubMed Abstract:
The human gut microbiota utilizes complex carbohydrates as major nutrients. The requirement for efficient glycan degrading systems exerts a major selection pressure on this microbial community. Thus, we propose that this microbial ecosystem represents a substantial resource for discovering novel carbohydrate active enzymes. To test this hypothesis we screened the potential enzymatic functions of hypothetical proteins encoded by genes of Bacteroides thetaiotaomicron that were up-regulated by arabinogalactan proteins or AGPs. Although AGPs are ubiquitous in plants, there is a paucity of information on their detailed structure, the function of these glycans in planta , and the mechanisms by which they are depolymerized in microbial ecosystems. Here we have discovered a new polysaccharide lyase family that is specific for the l-rhamnose-α1,4-d-glucuronic acid linkage that caps the side chains of complex AGPs. The reaction product generated by the lyase, Δ4,5-unsaturated uronic acid, is removed from AGP by a glycoside hydrolase located in family GH105, producing the final product 4-deoxy-β-l-threo-hex-4-enepyranosyl-uronic acid. The crystal structure of a member of the novel lyase family revealed a catalytic domain that displays an (α/α) 6 barrel-fold. In the center of the barrel is a deep pocket, which, based on mutagenesis data and amino acid conservation, comprises the active site of the lyase. A tyrosine is the proposed catalytic base in the β-elimination reaction. This study illustrates how highly complex glycans can be used as a scaffold to discover new enzyme families within microbial ecosystems where carbohydrate metabolism is a major evolutionary driver.
the Architecture et Fonction des Macromolécules Biologiques, CNRS, Aix-Marseille University, F-13288 Marseille, France.