Carbohydrate Binding Module Family 81

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• Author: ^^^Marcelo Liberato^^^
• Responsible Curator: ^^^Fabio Squina^^^

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CAZy DB link

https://www.cazy.org/CBM81.html

Ligand specificities

The family CBM81 was first described on September 12, 2016 [1]. According to CAZy, another two members were characterized in literature since then. The first CBM81 (named CBM_E1) was identified from sugar cane soil metagenome library [2], as part of a GH5 endoglucanase, where the catalytic module and the CBM are connected by a 32 amino acids (serine-rich) linker. The CBM_E1 interaction with soluble ligands was determined by Isothermal Titration Calorimetry, resulting in the highest affinity with barley β-glucan (Ka of 1.4 x 10-4 M-1), followed by cellohexaose (1.2 x 10-4 M-1), xyloglucan (0.5 x 10-4 M-1) and cellopentaose (0.4 x 10-4 M-1). The protein did not show any affinity for xylan and oligosaccharides such as cellotetraose, mannohexaose, and xylohexaose. The thermodynamic parameters indicated that the CBM_E1 binding to ligands is enthalpically driven, which is a typical characteristic of Type B CBMs [3]. On the other hand, based on pull-down assays with insoluble carbohydrates, the CBM_E1 was able to bind to Avicel, but not to Bacterial Microcrystalline Cellulose (BMCC), which is characteristic of Type A CBMs. The Avicel is composed of about 40% of amorphous regions [4], these disordered regions of the polysaccharide should be the probable CBM_E1 binding region.

Structural Features

The CBM81 is the first CBM family to exhibit mixing characteristics from Type A and Type B. The Type A CBMs bind to the surface of crystalline polysaccharides (such as cellulose and chitin) through CH-pi interactions between the aromatic residues and the monosaccharide’s units from carbohydrates [5]. Although the planar surface of this CBM81 member is composed of aromatic residues, similar to Type A CBMs, one of the tryptophans has the indole ring perpendicular to the oligosaccharide chain, leading to a hydrogen bond instead of a hydrophobic interaction. This observation explains the enthalpically driven binding between the CBM and the ligand, defining the classification of CBM_E1 as a Type B.

Functionalities

The CBM_E1 was demonstrated to bind amorphous regions of cellulose, as well as beta-glucan, xyloglucan and cello-oligosaccharides. All these ligands are typical substrates of endoglucanases, which are the enzymes linked to the CBM81 members deposited so far. The CBM81 can enhance endoglucanases activity by approximating the catalytic domain to the substrate [6]. However, this effect was not experimentally demonstrated for this family.

Family Firsts

First Identified

The CBM81 was identified as part of a GH5 endoglucanase, originated from an uncultured microorganism (metagenomics) [1].

First Structural Characterization

The first crystal structures of the family CBM81 were from CBM_E1, in absence and presence of the substrate cellopentaose [1].

References

1. Campos BM, Liberato MV, Alvarez TM, Zanphorlin LM, Ematsu GC, Barud H, Polikarpov I, Ruller R, Gilbert HJ, Zeri AC, and Squina FM. (2016). A Novel Carbohydrate-binding Module from Sugar Cane Soil Metagenome Featuring Unique Structural and Carbohydrate Affinity Properties. J Biol Chem. 2016;291(45):23734-23743. DOI:10.1074/jbc.M116.744383 |
2. Alvarez TM, Paiva JH, Ruiz DM, Cairo JP, Pereira IO, Paixão DA, de Almeida RF, Tonoli CC, Ruller R, Santos CR, Squina FM, and Murakami MT. (2013). Structure and function of a novel cellulase 5 from sugarcane soil metagenome. PLoS One. 2013;8(12):e83635. DOI:10.1371/journal.pone.0083635 |
3. Georgelis N, Yennawar NH, and Cosgrove DJ. (2012). Structural basis for entropy-driven cellulose binding by a type-A cellulose-binding module (CBM) and bacterial expansin. Proc Natl Acad Sci U S A. 2012;109(37):14830-5. DOI:10.1073/pnas.1213200109 |
4. Hall M, Bansal P, Lee JH, Realff MJ, and Bommarius AS. (2010). Cellulose crystallinity--a key predictor of the enzymatic hydrolysis rate. FEBS J. 2010;277(6):1571-82. DOI:10.1111/j.1742-4658.2010.07585.x |
5. Gilbert HJ, Knox JP, and Boraston AB. (2013). Advances in understanding the molecular basis of plant cell wall polysaccharide recognition by carbohydrate-binding modules. Curr Opin Struct Biol. 2013;23(5):669-77. DOI:10.1016/j.sbi.2013.05.005 |
6. Hervé C, Rogowski A, Blake AW, Marcus SE, Gilbert HJ, and Knox JP. (2010). Carbohydrate-binding modules promote the enzymatic deconstruction of intact plant cell walls by targeting and proximity effects. Proc Natl Acad Sci U S A. 2010;107(34):15293-8. DOI:10.1073/pnas.1005732107 |