Axe7A: A Novel CE7 Acetylxylan Esterase for Biomass Degradation.

As a clean and readily available agricultural residue, corn bran may have the potential of becoming a source for new C5 biofuel products or for the manufacture of food ingredients. These novel uses require partial or complete degradation of the biomass to its constituent monomers. Corn bran originates from a graminaceous plant having a primary cell wall, a complex macromolecular composites whose degradation by microorganisms requires a large and diverse panel of hydrolytic enzymes. Moreover, the degradation and fermentation of complex polysaccharides by microbiota is essential for many ecosystem-processes including nutrient cycling and herbivores nutrition. Among the different structural polysaccharides composing the plant cell walls, xylan is the second most abundant one and its degradation involves the removal of O-acetyl esters decorating the O-2 and O-3 positions of the D-xylanopyranosyl moiety. A transcriptomic analysis of a gene cluster, present in the ruminal anaerobe Prevotella ruminicola 23, revealed a repertoire of xylanolytic enzymes with different functions and cellular localization [1]. Among them Axe7A, an acetylxylan esterase belonging to the CAZy CE7 carbohydrate esterase family, has been established [2]. Interestingly Axe7A amino acidic sequence includes, besides the characteristic catalytic domain, an additional 100 residues long N-terminal domain no previously found in the CE7 family members, showing low similarity (around 10% identity) with any other structurally characterized protein annotated in the PDB. In order to gain insights into the functional role of its N-terminal domain, the crystal structure of Axe7A has been determined at 2.6 Å resolution (A). Axe7A shares the 32 hexameric doughnut-shape (B) commonly found in CE7 deacetylases [3], while the unique protomer architecture has been disclosed in the CE7 family, for the first time. The N-terminal domain, characterized by a ?-sandwich fold, is actively engaged in the oligomer formation. We are further investigating, by SAXS, the behaviour of Axe7A in solution aiming at validate the crystallographic oligomeric assembly in nearly-physiological conditions.