Understanding enzyme-substrate interactions in Carbohydrate Esterase Family 15

Carbohydrate Esterase Family 15 (CE15) is a rather small family, comprising approximately 200 members, which was established in CAZy (www.cazy.org) in 2006. The family was created following the characterization of a glucuronoyl esterase (GE) from the fungus Schizophyllum commune [1], which was shown to cleave methyl moieties ester-linked to the O6 position of glucuronic acid. CE15 enzymes are proposed to cleave ester linkages between lignin and glucuronoxylan, so-called lignin-carbohydrate complexes (LCCs), which are important features in biomass recalcitrance. We recently characterized ten new GEs from three bacterial species and solved the structures of two of these, essentially doubling both the biochemical and structural data available for the family [2]. An in-depth understanding of how CE15 enzymes interact with their complex substrates is still lacking, as only one structure with a monosaccharide ligand has been solved to date [3]. To address this, we have pursued solving new GE structures and obtaining protein-ligand complex structures. The studies have resulted in a novel structure exhibiting features with prominent inserts surrounding the active site, suggesting different specificities between bacterial and fungal GEs. In addition, we have solved the first structures of a CE15 enzyme with larger ligands, which gives direct evidence of how these enzymes interact with the different parts of its proposed physiological LCC substrates. Combined with kinetic characterizations, these new investigations greatly add to the knowledge of enzyme-substrate interactions in CE15 and enhances how these enzymes may act in natural conditions, which could aid in industrial biomass conversion. ______________[1]\ua0 Španikov\ue1 S.; Biely P. FEBS Lett. 2006, 580, 4597-4601.[2]\ua0 Arnling B\ue5\ue5th J.; Mazurkewich S.; Knudsen R.M.; Poulsen J.N.; Olsson L.; Lo Leggio L.; Larsbrink J. Biotechnol Biofuels. 2018, 11, 213-226.[3]\ua0 Charavgi M.D.; Dimarogona M.; Topakas E.; Christakopoulos P.; Chrysina E.D. Acta Crystallogr. D Biol. Crystallogr. 2013, 69, 63-73