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Carbohydrate Binding Module Family 41
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Modules from family CBM41 bind to alpha-glucans including starch (amylopectin), glycogen, amylose (linear alpha-1,4-linked glucose), and pullulan (alpha-1,6-linked maltotriose), and shorter alpha glucan oligosaccharides derived from these polysaccharides including maltose, maltotriose, longer maltooligosaccharides up to DP7, glucosyl-maltotriose and glucosyl-maltotriosyl-maltotriose . CBM41 modules are specific for alpha-1,4-linked glucose chains and may accommodate a linear alpha-1,6-linked glucose moiety.
CBM41s are mainly associated with pullulanases and other starch/glycogen debranching enzymes of family GH13. CBM41s are shown to direct the enzyme onto alpha-1,4-glucan chains to situate the catalytic machinery towards alpha-1,6-branch points . The majority of CBM41s are found in bacteria, including several pathogenic bacterial species such as Streptococcus, Klebsiella and Bacillus [1, 3]. They are also found in eukaryotic red and green algae. An updated list of CBM41 family members can be found in the CAZy Database
There are several X-ray crystal structures of CBM41 modules of which the majority are in complex with carbohydrate ligand. All adopt a common beta-sandwich configuration with an immunoglobulin (Ig)-like fold. A concave-shaped binding groove is formed on the side of the protein molecule to accommodate the helical structure of alpha-1,4-linked maltooligosaccharides [3, 4]. Typically two solvent exposed tryptophan residues form hydrophobic stacking interactions with the primary glucose molecule, with a third tryptophan creating a platform for interacting with longer maltooligosaccharide chains. The binding groove is made up of 4 binding subsites that interact with up to 4 intra-chain alpha-1,4-linked glucose molecules, classifying them as Type B CBMs. The CBM41 module from Thermotoga maritima was shown to accommodate either an alpha-1,4 or alpha-1,6-linked glucose residue in the fourth subsite, demonstrating that there is room for flexibility in the linkage that can be accommodated at this site .
The overall structural scaffold and mode of alpha-glucan recognition of CBM41 is similar to other starch-binding CBM families, which include CBM20, CBM21, CBM25, CBM26, CBM34, and CBM48. Although these different starch-binding module families have very little amino-acid sequence similarity to each other, that fact that they share almost identical modes of starch-binding suggests a common evolution towards maltooligosaccharide recognition by all starch-binding CBM families .
Structural data are available for several full-length pullulanases and glycogen-debranching enzymes containing both catalytic modules and associated CBMs in complex with alpha-glucan substrates which has provided details on how modularity contributes to the overall function of these enzymes. For example, the x-ray crystal structure of full length glycogen-debranching enzyme SpuA from Streptococcus pneumoniae revealed that the first of two dual, tandemly arranged N-terminal CBM41 modules directly participates in binding alpha-1,6-linked glucose branch points within the active site of the C-terminal GH13 catalytic module . This is the first demonstration that a CBM directly participates in substrate binding which has so far has only been found to occur within CBM41-containing pullulanases. The second CBM41 of SpuA is available to interact with an adjacent alpha-glucan chain, suggesting a possible disruptive role for these CBMs in loosening granular glycogen and increasing the substrate availability for the catalytic module.
- First Identified
Family 41 CBMs were previously known as X28 modules. They were first classified as a CBM in 2004 after demonstrating alpha-glucan binding by an N-terminal X28 module from Thermotoga maritima pullulanase PulA 
- First Structural Characterization
The first structure of CBM41 was revealed in 2006 in the x-ray crystal structure of full-length pullulanase from Klebsiella pneumoniae .
- Lammerts van Bueren A, Finn R, Ausió J, and Boraston AB. (2004) Alpha-glucan recognition by a new family of carbohydrate-binding modules found primarily in bacterial pathogens. Biochemistry. 43, 15633-42. DOI:10.1021/bi048215z |
- Lammerts van Bueren A, Ficko-Blean E, Pluvinage B, Hehemann JH, Higgins MA, Deng L, Ogunniyi AD, Stroeher UH, El Warry N, Burke RD, Czjzek M, Paton JC, Vocadlo DJ, and Boraston AB. (2011) The conformation and function of a multimodular glycogen-degrading pneumococcal virulence factor. Structure. 19, 640-51. DOI:10.1016/j.str.2011.03.001 |
- van Bueren AL, Higgins M, Wang D, Burke RD, and Boraston AB. (2007) Identification and structural basis of binding to host lung glycogen by streptococcal virulence factors. Nat Struct Mol Biol. 14, 76-84. DOI:10.1038/nsmb1187 |
- van Bueren AL and Boraston AB. (2007) The structural basis of alpha-glucan recognition by a family 41 carbohydrate-binding module from Thermotoga maritima. J Mol Biol. 365, 555-60. DOI:10.1016/j.jmb.2006.10.018 |
- Christiansen C, Abou Hachem M, Janecek S, Viksø-Nielsen A, Blennow A, and Svensson B. (2009) The carbohydrate-binding module family 20--diversity, structure, and function. FEBS J. 276, 5006-29. DOI:10.1111/j.1742-4658.2009.07221.x |
- Mikami B, Iwamoto H, Malle D, Yoon HJ, Demirkan-Sarikaya E, Mezaki Y, and Katsuya Y. (2006) Crystal structure of pullulanase: evidence for parallel binding of oligosaccharides in the active site. J Mol Biol. 359, 690-707. DOI:10.1016/j.jmb.2006.03.058 |