Carbohydrate Binding Module Family 16

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• Author: Maria Matard-Mann
• Responsible Curator: ^^^Elizabeth Ficko-Blean^^^

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CAZy DB link

https://www.cazy.org/CBM16.html

Ligand specificities

Family 16 CBMs (CAZy - CBM16) are found essentially in bacteria (with the exception of some CBM16 members in archaea). They are also found associated with catalytic modules belonging mainly to 4 families of CAZymes: GH5 mannanases [1, 2], GH16 kappa carrageenases [3, 4, 5], GH18 chitinases [6] and PL18 alginate lyases [7, 8]. Binding to glucomannan and kappa-carrageenan has been demonstrated [1, 2, 5]. CBM16 binding to glucomannan (mixed β-1,4-linked polymer contains both glucose and mannose) has been studied by mean of ITC (isothermal titration calorimetry) analysis and X-ray crystallography of complexes with pentomannan and pentoglucan [1, 2]. Conversely, binding to kappa-carrageenan has been shown by a double-blind approach using polysaccharide microarrays [5].

Structural Features

CBM16 is a type B CBM family, with a characteristic concave cleft, allowing the binding of substrate longer than triose. The ligand binding cleft shows some promiscuity as it can accommodate both pentoses (containing glucose and mannose), but only in the context of planar polymer like β-1,4-glucans, and not helical β-1,3-glucans [1]. The crystallographic structure determination of both CBMs from Caldanaerobius polysaccharolyticus (formerly Thermoanaerobacterium polysaccharolyticum) ManA revealed the importance of two aromatic residues in the binding cleft, as long as two stretches of polar residues on both sides of the cleft [1]. Affinity studies of targeted mutants for the predicted key resides confirmed the importance of two tryptophans (Trp-20 and Trp-125), and two glutamines (Gln-81 and Gln-93) [2].

Based on sequence similarity and conservation of secondary structure elements it has been proposed that, along with the CBM4, CBM17, CBM22 and CBM27 families, they form a superfamily [9].

Functionalities

In the Man5A of Caldanaerobius polysaccharolyticus, the deletion of both its CBM16s severely impairs the ability of the catalytic module (GH5) to bind cellulose [10].

In the case of CgkA from Zobellia galactanivorans, the presence of the CBM16 is not required for the enzymatic activity on kappa-carrageenan, but has been shown to take part in the processive mechanism of the catalytic module (GH16) [4].

Even if frequently found within the gene coding for alginate lyase from family PL18, it is absent in the mature form of the enzyme, and no role in alginate degradation has been found up to now [8]. A chaperone function of this N-terminal module has been proposed after observation that its deletion hindered the correct folding and activity of the catalytic module [7].

Family Firsts

First Identified

Cloning of Man5A GH5 by Cann et al. in 1999 reaveled the presence of two CBM16 tandem associated in C-terminal side. Their deletion resulted in failure of the catalytic module to bind to cellulose column, and significant loss of both mannanase and carboxy methylcellulase activities [10].

First Structural Characterization

Bae et al. solved the first structures of CBM16 family: both modules of Caldanaerobius polysaccharolyticus Man5A, (PDBID: 2zew, 2zez), and two complexes of CBM16-1, one with cellopentaose (PDBID: 2zex) and one with mannopentaose (PDBID: 2zey)[1].

References

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3. Barbeyron T, Gerard A, Potin P, Henrissat B, and Kloareg B. (1998). The kappa-carrageenase of the marine bacterium Cytophaga drobachiensis. Structural and phylogenetic relationships within family-16 glycoside hydrolases. Mol Biol Evol. 1998;15(5):528-37. DOI:10.1093/oxfordjournals.molbev.a025952 |