FV circulates in the blood as an inactive procofactor composed of the following domains: A1-A2-B-A3-C1-C2. The B domain maintains FV in its inactive state via an electrostatic interaction between its basic region (BR) and acidic region (AR) (Bos MH & Camire RM, 2012; Camire RM, 2016; Dahlbäck B et al., 2017; Ruben EA et al., 2021; Mohammed BM et al., 2024). During the amplification phase, FV is activated by proteolytic cleavage at Arg737, Arg1046, and Arg1573 catalyzed by thrombin (factor IIa), releasing the inhibitory B domain (Monkovic DD & Tracy PB, 1990; Orfeo T et al., 2004; reviewed by Camire RM & Bos MHA, 2009). The B domain of FV is cleaved sequentially (Steen M & Dahlbäck B 2002; Dahlbäck B, 2023). This process begins with proteolysis at Arg737 and Arg1046, generating partially activated FVa intermediate forms that retain the acidic region (AR) (not shown here). The final cleavage at Arg1573 completely removes the B domain, fully converting FV into its activated form, FVa, which binds FXa with high-affinity (Bunce MW et al., 2013; Bos MHA & Camire RM, 2012; Ruben EA et al., 2021; Mohammed BM et al., 2023, 2024). The FVa:FXa complex, known as the prothrombinase complex, is assembled in the (Ca²⁺)-dependent manner on the cell surface, where FVa:FXa converts prothrombin (factor II, FII) to thrombin (FIIa), creating a positive feedback loop (Mann KG et al., 1988; reviewed by Stoilova-McPhie S, 2021).