Biological forms of FV and FVa with different B-domain fragments. Shown are the domain structures of FV, thrombin-activated FVa (FVa_IIa), FXa-activated FVa (FVa_Xa), and platelet FV/FVa, which is a mixture of multiple FVa species containing B domains of varying lengths, as well as full-length FV. The species shown are intended to indicate the heterogeneity and do not represent all of the platelet FVa species present or their relative abundance. The heavy chain (HC: green), light chain (LC: purple), and B domain (B: orange) are indicated. The heavy and light chains of FVa are linked through a calcium bridge (yellow diamonds). The basic and acidic regions of the B domain are indicated in blue and red, respectively. A form of FVa that contains the B-domain basic region but lacks the acidic region has not been identified.