Ribbon diagram of the A domains of factor Va. The 3 A domains of FV are shown (green, A1; magenta, A2; and white, A3) while the 2 cleavage sites for APC visible in this predicted structure are highlighted in blue (Arg306 and Arg505). Ala221 is in red, Glu275 (possibly involved in a salt bridge although the orientation of the side chain is not very well defined) is in magenta, and 2 cysteine residues forming a disulfide bond (Cys220-Cys301) are in yellow. Possible binding sites for FXa are shown in yellow (the ribbon is colored yellow and the residue numbers boxed).