In molecular biology, a carbohydrate-binding module (CBM) is a protein domain found in carbohydrate-active enzymes (for example glycoside hydrolases).

[1] CBMs are classified into numerous families, based on amino acid sequence similarity.

[2] CBMs of microbial glycoside hydrolases play a central role in the recycling of photosynthetically fixed carbon through their binding to specific plant structural polysaccharides.

[4] CBMs are the most common non-catalytic modules associated with enzymes active in plant cell-wall hydrolysis.

Many putative CBMs have been identified by amino acid sequence alignments but only a few representatives have been shown experimentally to have a carbohydrate-binding function.

Carbohydrate-binding module family 2 (CBM2) contains two conserved cysteines - one at each extremity of the domain - which have been shown [6] to be involved in a disulfide bond.

[7][8][9] Carbohydrate-binding module family 3 (CBM3) is involved in cellulose binding [10] and is found associated with a wide range of bacterial glycosyl hydrolases.

[12] Multidimensional heteronuclear nuclear magnetic resonance (NMR) spectroscopy was used to determine the tertiary structure of the 152 amino acid N-terminal cellulose-binding domain from C. fimi 1,4-beta-glucanase CenC (CBDN1).

The dockerin domains are believed to be responsible for the assembly of a multiprotein cellulase/hemicellulase complex, similar to the cellulosome found in certain anaerobic bacteria.

[17][18] In anaerobic bacteria that degrade plant cell walls, exemplified by Clostridium thermocellum, the dockerin domains of the catalytic polypeptides can bind equally well to any cohesin from the same organism.

More recently, anaerobic fungi, typified by Piromyces equi, have been suggested to also synthesise a cellulosome complex, although the dockerin sequences of the bacterial and fungal enzymes are completely different.

[20] Carbohydrate-binding module family 12 (CBM12) comprises two beta-sheets, consisting of two and three antiparallel beta strands respectively.

[21] Carbohydrate-binding module family 15 (CBM15), found in bacterial enzymes, has been shown to bind to xylan and xylooligosaccharides.

[24] Sequence and structural conservation in families CBM17 and CBM28 suggests that they have evolved through gene duplication and subsequent divergence.

Sequence and structural conservation in families CBM17 and CBM28 suggests that they have evolved through gene duplication and subsequent divergence.

[4][25] Carbohydrate-binding module family 32 (CBM32) binds to diverse substrates, ranging from plant cell wall polysaccharides to complex glycans.