Coagulation factor V (FV, encoded by the F5 gene) 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). Activation of FV involves proteolytic cleavage at Arg737, Arg1046, and Arg1573, releasing the inhibitory B domain, which separates the heavy FV (29–737) and light FV (1574–2224) chains (Monkovic DD & Tracy PB, 1990; Thorelli E et al., 1997; Orfeo T et al., 2004; Toso R & Camire RM 2004; Schuijt TJ et al., 2013; Maag A et al., 2022; reviewed by Camire RM & Bos MHA, 2009). The B domain of FV is cleaved sequentially, generating partially activated FVa intermediate forms that retain the acidic region (AR) (Steen M & Björn Dahlbäck B 2002; Dahlbäck B, 2023). This process begins with cleavage at Arg737 and Arg1046, catalyzed during the initiation phase of coagulation by either activated factor X (FXa) or trace amounts of thrombin. This cleavage removes the N-terminal region of the B domain (FV (738-1046)), disrupting the inhibitory BR-AR interaction, exposing the AR, and enhancing the prothrombinase activity of FV intermediates. At this stage, partially cleaved FV intermediates are procoagulant-active but remain regulated by tissue factor pathway inhibitor α (TFPIα), which prevents full activation of FV, inhibiting its prothrombinase cofactor activity (Wood JP et al., 2013; van Doorn P et al., 2017; Petrillo T et al., 2021; Mohammed BM et al., 2024). The final cleavage at Arg1573 (catalyzed mostly by thrombin) completely removes the B domain, fully converting FV into its activated form, FVa, which exhibits high-affinity binding to FXa (Bunce MW et al., 2013; Bos MHA & Camire RM, 2012; Ruben EA et al., 2021; Mohammed BM et al., 2023, 2024). Meizothrombin, an intermediate in prothrombin (FII) activation, may also convert FV to FVa (Tans G et al., 1994; Orfeo T et al., 2004; reviewed by Camire RM & Bos MHA, 2009). However, cleavage by meizothrombin occurs at a slower rate compared to thrombin (FIIa) due to the covalent linkage of its N-terminal pro-piece, which impairs the function of anion-binding exosite 2 (ABE2), a critical exosite for efficient FV activation (Bradford HN & Krishnaswamy S, 2019).

This Reactome event describes proteolytic cleavage of the FVa intermediate form at Arg1573 catalyzed by trace amounts of thrombin (FIIa) during the initiation of coagulation (Monkovic DD & Tracy PB, 1990; Orfeo T et al., 2004; Corral-Rodríguez MA et al., 2011; Ruben EA et al., 2021; Mohammed BM et al., 2024; reviewed by Camire RM & Bos MHA, 2009). The C-terminal region of the inhibitory B domain, FV (1047-1573), is released from the FV light chain (FV (1574-2224), resulting in the formation of an FVa heterodimer composed of a heavy chain FV (29–737) and a light chain FV (1574–2224). While FXa preferentially cleaves FV at Arg737 and Arg1046, producing FV intermediates during the initiation phase, it may also contribute to FV processing at Arg1573, albeit less efficiently than thrombin (not shown here) (Monkovic DD & Tracy PB, 1990; Thorelli E et al., 1997; Schuijt TJ et al., 2013). Activated FVa binds FXa in the presence of calcium ions to form the prothrombinase complex (FVa:FXa) on the cell surface, where FVa:FXa converts factor II (FII) to thrombin (FIIa) (Mann KG et al., 1988; reviewed by Stoilova-McPhie S, 2021). Lacking the AR, FVa is no longer subject to inhibition by TFPIα, though it remains sensitive to inactivation by activated protein C (APC), which cleaves FVa at Arg334 and Arg534 in the A2 domain (Santamaria S et al., 2017; Petrillo T et al., 2021; Ruben EA et al., 2021).