Molecular model based on ceruloplasmin structure. A secondary structure drawing, using MOLSCRIPT,31 of relative positions of homologous factor VIII residues in the human ceruloplasmin structure27 is presented. The two β-barrel structures that compose the A2 domain (residues 380 through 711 in factor VIII) are viewed in a vertical presentation (perpendicular to the main axes of the barrel structures) on the right. Arg⁵⁹³ is drawn in ball and stick form extending down from a helical portion of the top loop of the second β-barrel structure of the A2 domain. The second β-barrel of the A1 domain (residues 183-329) is on the left to show the packing of the A2 domain's epitope side loop (residues 484-508)16 that is predominantly between the A1 and A2 domains as shown in the darkly shaded ribbon (residues 482-501). In ceruloplasmin, corresponding side loops from A1 and A3 domains pack between the β-barrels and may stabilize A domain interactions. Broad arrows indicate the orientation of β-pleated sheets; spiral ribbons, α-helices; narrow ribbon cords, turns and coil structures.