Saccharide-binding sites

Derewenda Z, Yariv J, Helliwell JR, Kalb AJ, Dodson EJ, Papiz MZ, Wan T, Campbell J. The structure of the saccharide-binding site of concanavalin A. EMBO J 1989 8 2189-2193. 

We tested whether the saccharide binding site on the upper rim could communicate with the metal binding site on the lower rim through the calix[4]arene cavity. The CD band weakened with increasing concentrations of Na+, Mg2+, or Ca2+. 78 On the other hand, when K+, Rb+, or Cs+ was added, the CD intensities increased and the spectral shape was changed. 78 For a number of reasons, we consider the new band to be attributable to the l,2-alternate-3- D-glucose-metal complexes. Therefore, this system is a unique example that exhibits both negative and positive allosteric interactions between metal ions and saccharides in a calix[4]arene host. 

Using equilibrium dialysis, Stein and coworkers186 found that there are approximately two saccharide binding-sites per LcH-A molecule with Ka = 230 30 M-1 for D-mannose and Ka = 100 24 M l for methyl a-D-glucopyranoside. These are extremely low binding-constants (see Section IX, Table XXV). 

Wright (1987) found hve clusters of more than three water molecules in a 1.8 A resolution analysis of the wheat germ agglutinin dimer. The four saccharide binding sites of the dimer showed similarity in ordered water structure. 

The effect of the changes in the upper part of the substrate cleft appears to be that sites A and B are blocked off, largely a consequence of the replacement of Ala-107 (see Section VI) in lysozyme with Tyr (or His in the rabbit). These changes make it unlikely that sites A and B would remain attractive saccharide binding sites in a-lactalbumin. However, as discussed in Section VI, 107 is not always Ala in lysozyme. 

The Chelate Effect Multivalent Ligands That Can Span Two Saccharide Binding Sites Within the ConA Tetramer Are the Most Potent Inhibitors... 

The function of CBM Family 20 (sometimes called SBD, starch binding domains) is particularly well understood in the case where it is attached to a GH 15 catalytic module, as in the Aspergillus spp. glucoamylase. The SBD contains two saccharide binding sites, each of which relies on a Trp residue to interact with the hydrophobic portion of the amylose unit. Mutation of either Trp residue gives an enzyme which continues to bind to soluble starch, but no... 

Like ricin, abrin is a type 2 ribosome inactivating protein with A and B chains linked by a disulphide bond. The abrin A chain comprises 251 amino acid residues compared to 267 in the ricin A chain, both having three folding structural domains and a molecular weight of approximately 30 kDa. The abrin B chain has a molecular weight of 35 kDa with 60% of its amino acid residues identical to those of ricin s B chain. Both B chains have two saccharide binding sites which are highly conserved between the two toxins. 

Fig. 3. Substrate binding site on Taka-amylase A deduced from electron density difference maps with the enzyme-maltose complex and model building. The seven saccharide binding sites are numbered. Presumed catalytic amino acids Asp-206 and Asp-297 surround the sessile glycoside bond. Glu-230 is considered as a possible catalytic amino acid as well because of its proximity to the reaction center. Adapted from Matsuura et al. (262) with permission from J. Biochem (Tokyo).

An interesting point is that manganese, as well as other divalent transition metals, plays an essential role in saccharide binding in concanavalin A, a member of the plant proteins which are also known as hemagglutinins or plant lectins. Concanavalin A occurs in quantities of > 5% DM in legumes (e.g., the Jack bean, Canavalia ensi-formis). The function of plant lectins in saccharide binding sites is unknown, but a defense mechanism may be involved (Kalb (Gilboa) et al. 2000). 

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