Polysaccharide-degrading enzymes are generally modular proteins that contain non-catalytic carbohydrate-binding modules (CBMs), which potentiate the activity of the catalytic module. CBMs have been grouped into sequence-based families, and three-dimensional structural data are available for half of these families ...
Polysaccharide-degrading enzymes are generally modular proteins that contain non-catalytic carbohydrate-binding modules (CBMs), which potentiate the activity of the catalytic module. CBMs have been grouped into sequence-based families, and three-dimensional structural data are available for half of these families. Clostridium thermocellum xylanase 11A is a modular enzyme that contains a CBM from family 6 (CBM6), for which no structural data are available. We have determined the crystal structure of this module to a resolution of 2.1 A. The protein is a beta-sandwich that contains two potential ligand-binding clefts designated cleft A and B. The CBM interacts primarily with xylan, and NMR spectroscopy coupled with site-directed mutagenesis identified cleft A, containing Trp-92, Tyr-34, and Asn-120, as the ligand-binding site. The overall fold of CBM6 is similar to proteins in CBM families 4 and 22, although surprisingly the ligand-binding site in CBM4 and CBM22 is equivalent to cleft B in CBM6. These structural data define a superfamily of CBMs, comprising CBM4, CBM6, and CBM22, and demonstrate that, although CBMs have evolved from a relatively small number of ancestors, the structural elements involved in ligand recognition have been assembled at different locations on the ancestral scaffold.
Related Citations:
Homologous Xylanases from Clostridium Thermocellum: Evidence for Bi-Functional Activity, Synergism between Xylanase Catalytic Modules and the Presence of Xylan-Binding Domains in Enzyme Complexes Fernandes, A., Fontes, C.M.G.A., Gilbert, H.J., Hazelwood, G.P., Fernandes, T.H., Ferreira, L.M.A., Gilbert, H.J. (1999) Biochem J 342: 105
Organizational Affiliation:
Laboratoire d'Architecture et de Fonction des Macromolécules Biologiques, IBSM, CNRS Marseille and University Aix-Marseille I & II, 31 Chemin Joseph Aiguier, 13402 Marseille Cedex 20, France. czjzek@afmb.cnrs-mrs.fr
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