Porphyrin binding to jacalin is facilitated by the inherent plasticity of the carbohydrate-binding site: novel mode of lectin-ligand interaction

The crystal structure of the complex of meso-tetrasulfonatophenylporphyrin (H₂TPPS) with jack fruit (Artocarpus integriflora) agglutinin (jacalin) has been determined at 1.8 Å resolution. A porphyrin pair is sandwiched between two symmetry-related jacalin monomers in the crystal, leading to a cross-linking network of protein molecules. Apart from the stacking interactions, H₂TPPS also forms hydrogen bonds, some involving water bridges, with jacalin at the carbohydrate-binding site. The residues that are involved in rendering galactopyranoside specificity to jacalin undergo conformational adjustments in order to accommodate the H₂TPPS molecule. The water molecules at the carbohydrate-binding site of jacalin cement the jacalin-porphyrin interactions, optimizing their complementarity. Interactions of porphyrin with jacalin are relatively weak compared with those observed between galactopyranoside and jacalin, perhaps because the former largely involves water-mediated hydrogen bonds. While H₂TPPS binds to jacalin at the carbohydrate-binding site as in the case of ConA, its mode of interaction with jacalin is very different. H₂TPPS does not enter the carbohydrate-binding cavity of jacalin. Instead, it sits over the binding site. While the porphyrin binding is mediated by replicating the hydrogen-bonding network of mannopyranoside through the sulfonate atoms in the case of ConA, the plasticity associated with the carbohydrate-binding site accommodates the pluripotent porphyrin molecule in the case of jacalin through an entirely different set of interactions.