Putative APC-binding sites at the surface of the PS Gla domain. The PS Gla domain is shown in red, the TSR in purple, and the EGF1 domain in white. Ca2+ ions are depicted as yellow spheres. The PS Gla domain is thought to insert into the membrane surface through its N-terminal ω-loop (approximately the first 10 residues). Solvent-exposed amino acid residues that differ significantly from the corresponding residues in prothrombin are shown in blue, and the nature of the substitution is indicated at each position (eg, human PS has a Gln at position 10 whereas human prothrombin has a Lys). Face 1 comprises amino acids Gln10, Asn33, Asp34, Pro35, Gla36, and Tyr39. Face 2 comprises amino acids Leu21, Asn23, Lys24, Arg28, Asn33, Asp34, Pro35, Tyr41, and Leu45. Residues at positions 33, 34, and 35 are at the interface and are, therefore, present on both faces. In the present study, Asn33 and Pro35 was considered part of face 1, whereas Asp34 was considered part of face 2.