Type C CBMs

Notable Type C CBMs are the CBM Family 9 (p-sandwich folded),one member of which has been shown rigorously to bind the reducing end of oligo-and polysaccharides in an enthalpy-driven process.  

CBM Family 13 adopt the p-trefoil fold first encountered with the ricin B (carbohydrate-binding) chain. It contains 12 strands of p-sheet arranged in such a way as to provide three independent sugar binding sites with different specificities, and again binding is enthalpy driven. 

The fairly small Family 18 CBMs ( 40 residues) possess the hevein fold associated with chitin binding in higher plants despite their small size, they can accommodate chitotetraoside essentially on the outside of the domain. Related is another chitin-binding CBM Family, 14. 

Various eukaryotic organisms produce proteins which hydrolyse the carbon-pyridinium linkage of NAD in a way reminiscent of the hydrolysis of 

Sir2 protein deacetylases (sirtuins) use an acetyl lysine residue as an oxygen nucleophile to displace nicotinamide from NAD in a second step the imidate intermediate is hydrolysed to 1-O-acetyl ADP ribose and a lysine residue. A study of the effect of acyl substitution in the acyl lysine yielded a p value of — 1.9 for the first step, equivalent to a of 1, an extraordinarily 

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The Type C CBMs were first discovered as lectins with small-sugar-binding activity and have only subsequently been included as CBMs due to their discovery in a number of glycoside hydrolases. This family of CBMs is found exclusively in xylanases. Differences between Type B and Type C CBMs can be small, but the hydrogen-bonding network between protein and ligand is more extensive in Type C CBMs than in Type B modules. 

Functionally, CBMs can be divided into three types those which bind to the crystalline regions of solid polysaccharides and show weak, if any affinity for soluble carbohydrates (Type A) those which bind polysaccharide-chains, whether as soluble oligosaccharides or as amorphous regions of insoluble polysaccharides (Type B) and those which bind small sugar molecules (Type C). Unsurprisingly, there is considerable overlap between Type C CBMs and lectins, molecules which bind, but do not transform, sugars. 

As with Type A CBMs, the interactions of planar aromatic residues play a role in determining function, but in conjunction with other interactions, specifically hydrogen bonding. The aromatic interactions are often of the stacking type in which C-H bonds in electron-deficient systems interact with the cloud of 71 electrons of an (electron-rich) aromatic system the well-known... 

All Type B function is associated with a so-called p-sandwich fold this comprises two p-sheets, each consisting of three to six antiparallel p-strands. With one exception (a CBM2a from C.fimi) all of the CBMs with a p-sandwich fold have a structural Ca " ion. The first CBD that bound preferentially to amorphous, rather than crystalline, cellulose i.e. that had Type B functionality) to be discovered was a CBM 4 attached to the N-terminus of a GH 9 endoglucanase from Cellulomonas fimv, it bound cellooligosaccharides as well as insoluble cellulose. The isolated module was a 152-residue peptide, whose structure, p-sandwich, was solved by NMR spectroscopy the active site was a cleft with many potential hydrogen-bonding groups. ... 

Based on the macroscopic nature of the target ligand, a classification of CBMs in three types, A, B or C, has been proposed. Table 1 presents the CBM families and corresponding type, A B or C. It should be noted that CBMs with type A and B-topology are found in the same family, while others still remains to be classified. 

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