Two monoclinic (P21) crystal forms of human serum amyloid P component (SAP) in complex with the 4,6-pyruvate acetal of b-D-galactose (MObDG) were prepared. Structure analysis by molecular replacement and refinement at 2.2 A ° resolution revealed that crystal form 1 (a 1/4 95:76 A; b 1/4 70:53 A; c 1/4 103:41 A; b 1/4 96:808) 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 A ° resolution structure of crystal form 2 (a 1/4 118:60 A; b 1/4 109:10 A; c 1/4 120:80 A and b 1/4 95:168) 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 MObDG at the interface
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