Collectins are animal calcium dependent lectins that
target the carbohydrate structures on invading pathogens,
resulting in the agglutination and enhanced clearance of
the microorganism. These proteins form trimers that may
assemble into larger oligomers. Each polypeptide chain
consists of four regions: a relatively short N-terminal
region, a collagen like region, an α-helical coiled-coil,
and the lectin domain. Only primary structure data are
available for the N-terminal region, while the most important
features of the collagen-like region can be derived from
its homology with collagen. The structures of the α-helical
coiled-coil and the lectin domain are known from crystallographic
studies of mannan binding protein (MBP) and lung surfactant
protein D (SP-D). Carbohydrate binding has been structurally
characterized in several complexes between MBP and carbohydrate;
all indicate that the major interaction between carbohydrate
and collectin is the binding of two adjacent carbohydrate
hydroxyl group to a collectin calcium ion. In addition,
these hydroxyl groups hydrogen bond to some of the calcium
amino acid ligands. While each collectin trimer contains
three such carbohydrate binding sites, deviation from the
overall threefold symmetry has been demonstrated for SP-D,
which may influence its binding properties. The protein
surface between the three binding sites is positively charged
in both MBP and SP-D.