Bz-Ile-Glu-Gly-Arg-pNA·HCl (S-2222) is a tetrapeptide pNA chromogenic substrate with the molecular formula C₃₂H₄₄ClN₉O₉ and a molecular weight of 734.2.Its peptide sequence mimics the native cleavage site of prothrombin and delivers outstanding specificity toward activated coagulation factor Xa (FXa), making it the gold-standard substrate for in vitro FXa detection. FXa hydrolyzes the substrate to liberate free yellow pNA, and enzyme activity is quantified by monitoring absorbance changes.