Carbohydrate Binding Module Family 80

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• Author: ^^^Immacolata Venditto^^^
• Responsible Curator: ^^^Harry Gilbert^^^

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CAZy DB link

https://www.cazy.org/CBM80.html

Ligand specificities

CBM80 is a small bacterial family comprising around 96 amino acids and identified in the Ruminococcus flavefaciens cellulosome [1]. CBM80 displays specificity for β-1,4- and mixed linked β-1,3-1,4-glucans, with some members also binding to β-1,4-mannans. CBM80 is a component of an enzyme that contains catalytic module derived from GH5_4 (CBM80_RfGH5-1/2, and CBM80_RfGH5) with endo-β1,4-glucanases activity. CBM80 is also component of an enzyme that contains GH5_7 catalytic module with β1,4-mannanase activity. The only family member characterized is CBM80_RfGH5-1/2 and the dual specificity of this CBM is consistent with the catalytic modules of the enzymes that hydrolyze β-glucans (GH5_4) or β-mannans (GH5_7) [2]. CBM80_RfGH5-1/2 binds galactomannan in addition to the β-glucans with affinities in the range of 10⁴ to 10⁵ M^-1. Additionally, this CBM binds mannotetraose and not cellotetraose.

Structural Features

Figure 1. Crystal structure of CBM80_RfGH5-1/2. The residues that contribute to ligand recognition are shown.

The three-dimensional structure of CBM80_RfGH5-1/2 (5fu3) was solved using single-wavelength anomalous diffraction (SAD) methods and selenomethionyl protein. The apo structure of CBM80_RfGH5-1/2 (Figure 1) and in complex with mannohexaose and cellohexaose was solved to a resolution of 1.0 Å, 1.4 Å and 1.5 Å respectively [2]. CBM80_RfGH5-1/2 has a β-sandwich fold and contains two β-sheets, 1 and 2, respectively (Figure 1) [2]. The β-sheet 2 of CBM80_RfGH-5-1/2 presents a planar hydrophobic surface with a parallel orientation of Trp453 and Trp489 and a perpendicular orientation of a third aromatic residue, Trp490. The mannohexaose CBM80_RfGH5-1/2 complex revealed electron density for mannohexaose along the hydrophobic surface of β-sheet 2 [2]. The structure of CBM80_RfGH5-1/2 in complex with cellohexaose revealed electron density for only three glucose units [2].

Functionalities

CBM80, component of enzyme that contains GH5_7 and GH5_4 catalytic modules, binds β-glucans and β-mannans. Examples of CBMs that recognize both β-1,4-glucans and β-1,4-mannans are found in families CBM16 [3] and CBM29 [4]. The key residues implicated in ligand binding were identified by site-direct mutagenesis. Alanine substitution of Trp453 and Trp489 revealed a complete abrogation of binding to β-glucans and β-mannans, showing the importance of tryptophan residues in ligand recognition. CBMs that bind to β-1,4-glycans typically contain three aromatic residues that make apolar interactions with sugars [4]. The predicted polar interactions between the protein and β-glycans have very little influence on affinity.

Family Firsts

First Identified

CBM80_RfGH5-1/2 from Ruminococcus flavefaciens [2].

First Structural Characterization

The first 3D crystal structure solved was CBM80_RfGH5-1/2 [2].

References

1. Rincon MT, Dassa B, Flint HJ, Travis AJ, Jindou S, Borovok I, Lamed R, Bayer EA, Henrissat B, Coutinho PM, Antonopoulos DA, Berg Miller ME, and White BA. (2010). Abundance and diversity of dockerin-containing proteins in the fiber-degrading rumen bacterium, Ruminococcus flavefaciens FD-1. PLoS One. 2010;5(8):e12476. DOI:10.1371/journal.pone.0012476 |
2. Venditto I, Luis AS, Rydahl M, Schückel J, Fernandes VO, Vidal-Melgosa S, Bule P, Goyal A, Pires VM, Dourado CG, Ferreira LM, Coutinho PM, Henrissat B, Knox JP, Baslé A, Najmudin S, Gilbert HJ, Willats WG, and Fontes CM. (2016). Complexity of the Ruminococcus flavefaciens cellulosome reflects an expansion in glycan recognition. Proc Natl Acad Sci U S A. 2016;113(26):7136-41. DOI:10.1073/pnas.1601558113 |
3. Bae B, Ohene-Adjei S, Kocherginskaya S, Mackie RI, Spies MA, Cann IK, and Nair SK. (2008). Molecular basis for the selectivity and specificity of ligand recognition by the family 16 carbohydrate-binding modules from Thermoanaerobacterium polysaccharolyticum ManA. J Biol Chem. 2008;283(18):12415-25. DOI:10.1074/jbc.M706513200 |
4. Charnock SJ, Bolam DN, Nurizzo D, Szabó L, McKie VA, Gilbert HJ, and Davies GJ. (2002). Promiscuity in ligand-binding: The three-dimensional structure of a Piromyces carbohydrate-binding module, CBM29-2, in complex with cello- and mannohexaose. Proc Natl Acad Sci U S A. 2002;99(22):14077-82. DOI:10.1073/pnas.212516199 |