In the presence of Ca²⁺ and phospholipids, factor IXa (FIXa) forms a complex with factor VIIIa, known as "tenase" or "X-ase" complex, which catalyzes the formation of activated factor X (FXa) with high efficiency (Fang H et al., 2019; Freato N et al., 2021; Childers KC et al., 2022; Bos MHA et al., 2024). The amino terminal part of the heavy chain of FX, the FX activation peptide (183-234), is released (this peptide has no known function) (Rawala-Sheikh R et al., 1990; Panteleev MA et al., 2004). The binding of Na+ to FIXa enhances its protease activity (Schmidt AE et al., 2005; Vadivel K et al., 2019). Studies using physiologic surfaces provide evidence for coordinated binding interactions of the enzyme, cofactor and substrate to discrete surface structures. For example, the presence of both active site-modified FIXa and FX increased both the number and the affinity of binding sites on activated platelets for FVIIIa (Ahmad SS and Walsh PN, 2002; Ahmad S, 2003).