Solution structure of the CBM10 cellulose binding module from Pseudomonas xylanase A

Plant cell wall hydrolases generally have a modular structure consisting of a catalytic domain linked to one or more noncatalytic carbohydrate-binding modules (CBMs), whose common function is to attach the enzyme to the polym eric substrate. Xylanase A from Pseudomonas fluorescens subsp, cellulosa (P f Xyn10A) consists of a family 10 catalytic domain, an N-terminal family II a cellulose-binding module, and an internal family 10 cellulose-binding mod ule. The structure of the 45-residue family 10 CBM has been determined in s olution using NMR. It consists of two antiparallel beta-sheets, one with tw o strands and one with three, with a short or-helix across one face of the three-stranded sheet. There is a high density of aromatic residues on one s ide of the protein, including three aromatic residues (Tyr8, Trp22, and Trp 24), which are exposed and form a flat surface on one face, in a classical polysaccharide-binding arrangement. The fold is closely similar to that of the oligonucleotide/oligosaccharide-binding (OB) fold, but appears to have arisen by convergent evolution, because there is no sequence similarity, an d the presumed binding sites are on different faces.