Novel carbohydrate‐recognition mode of the invertebrate C‐type lectin SPL‐1 from Saxidomus purpuratus revealed by the GlcNAc‐complex crystal in the presence of Ca2

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 Ca2+‐independent manner. Unlike ordinary C‐type lectins, which require Ca2+ ions for carbohydrate recognition, these lectins recognize specific carbohydrates mainly through interactions with the acetamido group without Ca2+ ions, even though Ca2+ 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 Ca2+ revealed that the binding of SPL‐1 to GlcNAc is stabilized by hydrogen bonds to the water molecule(s) coordinating Ca2+, whereas in ordinary C‐type lectins Ca2+ 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. The crystal structure of the C‐type lectin SPL‐1 complexed with N‐acetylglucosamine and Ca2+ reveals Ca2+‐mediated enhancement of the carbohydrate‐binding affinity of SPL‐1, which can recognize specific carbohydrates without Ca2+ in a manner distinct from that of ordinary C‐type lectins.