Carbohydrate Binding Module Family 73

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• Author: ^^^Zarah Forsberg^^^
• Responsible Curator: ^^^Gustav Vaaje-Kolstad^^^

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CAZy DB link

https://www.cazy.org/CBM73.html

Ligand specificities

Family 73 CBMs are modules of approximately 60 residues that are appended to bacterial enzymes associated to chitin degradation [1, 2, 3]. Binding to amorphous and crystalline α- and β-chitin has been demonstrated [1, 2].

Structural Features

Currently no NMR or crystal structure is available for CBM73s, but circular dichroism experiments of the CBM from the Vibrio cholera GlcNAc-binding protein (GbpA, VcLPMO10B) indicated a β-sheet containing structure [2]. Sequence alignment shows that the CBM73 are distantly related to chitin-binding modules belonging to Family 5 CBMs (that are Type A CBMs). Despite low sequence similarity, conserved aromatic amino acids of the CBM5s responsible for substrate-binding [4] align well with similar residues in the CBM73s [1, 3]. Two additional aromatic residues are found in CBM73s compared to CBM5s. Compatibly, a somewhat lower Kd for α-chitin was observed for the C-terminal CBM73 of the Cellvibrio japonicus LPMO (CjLPMO10A) relative to its internal CBM5 [1] (Fig. 1).

Figure 1. (A) Multiple sequence alignment of CBM5s (grey) and CBM73s (yellow) from Cellvibrio japonicus (Cj), Cellvibrio mixus (Cm) and Streptomyces griseus (Sg). Conserved aromatic residues are labelled white on a blue background and the disulfide in CBM5s are shown above the alignment (S-S). (B) Modular organization of C. japonicus CBM73 containing enzymes. (C) Comparative binding of the CBMs from the C. japonicus LPMO (CjLPMO10A). Dissociation constants of 5.3 µM and 4.3 µM was obtained for α-chitin for the CBM5 and the CBM73, respectively [1]. The figure was adapted from Forsberg et al. [1].

Functionalities

CBM73s are found in Gram-negative bacteria from the genus of Proteobacteria and are covalently attached to chitin degrading enzymes such as GH18 and GH19 chitinases [2, 5], AA10 chitin-oxidizing lytic polysaccharide monooxygenases [1, 2] and often in combination with a CBM5 chitin-binding module. In chitin degrading enzymes from C. japonicus, the CBM73s are found internally as well as in the N- or C-terminus (Fig. 1). The CBM73 from CjLPMO10A (together with the CBM5) strongly promotes targeting and binding of crystalline α- and β-chitin as the LPMO domain alone binds weakly to its substrate. Removal of the two CBMs (CBM5 and CBM73) in CjLPMO10A reduces the lifetime of the catalytic AA10 domain and decreases the overall product yield. A CBM73 has also been found appended to a serine protease/peptidoglycan hydrolase from Vibrio vulnificus. Truncation of the two CBMs (CBM5 and CBM73) resulted in reduced peptidoglycan hydrolyzing activity but did not affect the protease activity [6].

Family Firsts

First Identified

Family CBM73 was first found as a C-terminal module of the tri-modular Cellvibrio japonicus chitin-oxidizing LPMO (AA10-CBM5-CBM73) [1].

First Structural Characterization

Hitherto, no structural information is available

References

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