Activated protein C (APC) cleaves peptide bonds in activated factor V (factor Va), converting it to an inactive form (factor Vi). APC proteolysis involves cleavage of the factor Va heavy chain at Arg-334 (306 if signal peptide is not included) and Arg-534 (506 with no signal peptide) (Nicolaes et al. 1985). Most factor Va molecules are initially cleaved at Arg-534, yielding a partially active intermediate, followed by complete inactivation through cleavage at Arg-334 (Kalafatis et al. 1994). Protein S stimulates the cleavage at Arg334 ~20-fold (Rosing et al. 1995) and also counteracts the protective effect of factor Xa on Arg534 cleavage (Norstrom et al. 2006). A mutation of the APC cleavage sites in Fv at Arg-534Gln a.k.a. FVLeiden is the most common identifiable hereditary risk factor for venous thrombosis among Caucasians (Camire 2011).

Protein S also enhances the APC-mediated inactivation of factor VIIIa (van de Poel et al. 2001). Protein S and factor V act as synergistic cofactors in the APC-mediated inactivation of factor VIIIa (Shen & Dahlback 1994, Somajo et al. 2014).