Recognition of the Helical Structure of β-1,4-Galactan by a New Family of Carbohydrate-binding Modules

• Published in: The Journal of biological chemistry 2010-11, Vol.285 (46), p.35999-36009
• Main Authors: Cid, Melissa, Pedersen, Henriette Lodberg, Kaneko, Satoshi, Coutinho, Pedro M., Henrissat, Bernard, Willats, William G.T., Boraston, Alisdair B.
• Format: Article
• Language: English
• Subjects: Bacterial Proteins - chemistry; Bacterial Proteins - genetics; Bacterial Proteins - metabolism; Binding Sites; Calorimetry; Carbohydrate-binding Module; Carbohydrates - chemistry; Crystal Structure; Crystallography, X-Ray; Galactan; Galactans - chemistry; Galactans - metabolism; Galactose; Galactose - chemistry; Galactose - metabolism; Glycobiology and Extracellular Matrices; Glycoside Hydrolase; Glycoside Hydrolases - chemistry; Glycoside Hydrolases - genetics; Glycoside Hydrolases - metabolism; Microarray; Microarray Analysis; Microscopy, Fluorescence; Models, Molecular; Oligosaccharides - chemistry; Oligosaccharides - metabolism; Pectin; Plant; Polysaccharides - chemistry; Polysaccharides - metabolism; Protein Binding; Protein Folding; Protein Structure, Secondary; Protein Structure, Tertiary; Thermotoga maritima; Thermotoga maritima - enzymology
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M110.166330

The microbial enzymes that depolymerize plant cell wall polysaccharides, ultimately promoting energy liberation and carbon recycling, are typically complex in their modularity and often contain carbohydrate-binding modules (CBMs). Here, through analysis of an unknown module from a Thermotoga maritima endo-β-1,4-galactanase, we identify a new family of CBMs that are most frequently found appended to proteins with β-1,4-galactanase activity. Polysaccharide microarray screening, immunofluorescence microscopy, and biochemical analysis of the isolated module demonstrate the specificity of the module, here called TmCBM61, for β-1,4-linked galactose-containing ligands, making it the founding member of family CBM61. The ultra-high resolution x-ray crystal structures of TmCBM61 (0.95 and 1.4 Å resolution) in complex with β-1,4-galactotriose reveal the molecular basis of the specificity of the CBM for β-1,4-galactan. Analysis of these structures provides insight into the recognition of an unexpected helical galactan conformation through a mode of binding that resembles the recognition of starch.