The Structures of Crystalline Complexes of Human Serum Amyloid P Component with Its Carbohydrate Ligand, The Cyclic Pyruvate Acetal of Galactose

Two monoclinic (P21) crystal forms of human serum amyloid P component (SAP) in complex with the 4,6-pyruvate acetal of β-d-galactose (MOβDG) were prepared. Structure analysis by molecular replacement and refinement at 2.2 Å resolution revealed that crystal form 1 (a=95.76 Å, b=70.53 Å, c=103.41 Å, β=96.80°) contained a pentamer in the asymmetric unit with a structure very similar to that of the published search model. The mode of ligand co-ordination was also similar except that four of the five subunits showed bound ligand with an additional H-bond between O1 of the galactose and the side-chain of Lys79. One sub-unit showed no bound ligand and a vacant calcium site close to a crystal contact. The 2.6 Å resolution structure of crystal form 2 (a=118.60 Å, b=109.10 Å, c=120.80 Å and β=95.16°) showed ten sub-units in the asymmetric unit, all with two bound calcium ions and ligand. The most extensive protein–protein interactions between pentamers describe an AB face-to-face interaction involving 15 ion pairs that sandwiches five molecules of bound MOβDG at the interface.