The enzymatic lectin of field bean (Dolichos lablab): Salt assisted lectin–sugar interaction.

Field bean seed contains a Gal/GalNAc lectin (DLL-II) that exhibits associated polyphenol oxidase (PPO)
activity and does not bind to its sugar specific affinity matrix. The molecular basis for this lack of binding
is not known. The DLL-II gene was therefore cloned and its sequence analyzed. A conserved aromatic residue
in the sugar binding site required for a stacking interaction with the apolar backbone of Gal is
replaced by His in DLL-II, which explains the lack of binding. However, specific sugar binding is achieved
by including (NH4)2SO4 in the buffer. Interestingly two other salts of the Hofmeister series, K2HPO4 and
Na2SO4 also assist binding to immobilized galactose. In the presence of (NH4)2SO4 the surface hydrophobicity
of DLL-II and dissociation constant for 8-anilino 1-naphthalene sulfonic acid were enhanced three
fold. This increased surface hydrophobicity in the presence of salt is probably the cause for assisted sugar
binding in legume lectins that lack aromatic stacking interactions. Accordingly, two other lectins which
lack the conserved aromatic residue show similar salt assisted binding. The salt concentrations required
for Gal/GalNAc binding are not physiologically relevant in vivo, suggesting that the role of DLL-II per se in
the seed is primarily that of a PPO purportedly for plant defense.