Lectins hydrophobic binding site

Most considerations of plant lectin function revolve around the carbohydrate binding activities of these proteins. Yet, as discussed in a previous section of this chapter, additional activities are associated with some of these lectins. These activities must also be taken into account in considering the possible roles of these proteins. For example, a major class of plant hormones, the cytokinins, are derivatives of adenine that bind with high affinity to the hydrophobic binding sites found on some of the legume seed lectins and at least one seed-like vegetative tissue lectin [20, 30]. It is of interest that the specificity of this site, when present, appears to have... 

Amino acids involved in binding M6P are shown in O Fig. 8. The phosphate moiety is coordinated by His 105, Asn 104, Asp 103 and divalent cation. Each hydroxyl of Man is hydrogen-bonded to binding site residues, while no interactions between the hydrophobic carbohydrate backbone and aromatic side chains are observed [148]. The multiple contacts between lectin and M6P, in contrast to that observed for many lectins which mediate cell surface interactions, are reflected in pM binding constants, reminiscent of bacterial arabinose-binding protein which functions as a sugar transport protein [149]. 

The X-ray structure of the Erythrina corallodendwn lectin complexed with lactose was solved at 2.6 A resolution. It was determined that only the galactose moiety of the lactose ligand was situated within the binding site it is stabilized by seven hydrogen bonds and numerous hydrophobic interactions [50]. 

The most potent inhibitor described thus far is the type A disaccharide GalNAc(al-3)Gal. However, the addition of an a-L-fucosyl group to the Gal, as occurs in the type A trisaccharide, greatly diminished its activity [168]. A hydrophobic site on soybean agglutinin was identified by its interaction with A-dansylgalactosamine [169]. The soybean lectin possesses four carbohydrate binding sites per molecule, one for each subunit [169]. 

Qy-boronate derivatives have the ability to form covalent bonds with d,v-diol sugars at alkaline pH. Unlike lectins, which typically bind only the terminal sugar in a complex carbohydrate, boronates can bind any physically accessible CM-diol on a glycoprotein.23-25 When hydrophobic boronate derivatives are used, the hydrophobicity of each complexation site is enhanced. To the extent that the protein components of a sample are differentially glycosylated, this can provide a tool for exploiting those differences. 

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