Abstract
The C-type lectins SPL-1 and SPL-2 from the bivalve Saxidomus purpuratus are composed of A and B chains and of two B chains, respectively. They bind specific carbohydrates containing acetamido groups, such as N-acetylglucosamine (GlcNAc) and N-acetylgalactosamine (GalNAc), in a Ca(2+)-independent manner. Unlike ordinary C-type lectins, which require Ca(2+) ions for carbohydrate recognition, these lectins recognize specific carbohydrates mainly through interactions with the acetamido group without Ca(2+) ions, even though Ca(2+) enhances the binding affinity of these lectins, especially SPL-1. In the present study, the crystal structure of the SPL-1-GlcNAc complex in the presence of Ca(2+) revealed that the binding of SPL-1 to GlcNAc is stabilized by hydrogen bonds to the water molecule(s) coordinating Ca(2+), whereas in ordinary C-type lectins Ca(2+) directly forms coordinate bonds to the hydroxy groups of carbohydrates. These differences may also allow SPL-1 and SPL-2 to recognize both GlcNAc and GalNAc, which have different orientations of the 4-hydroxy group.