A characteristic property of vitamin K-dependent plasma protein Z

Evidence is presented for rapid, limited proteolysis of protein Z by α-thrombin. This α-thrombin-catalyzed proteolysis of protein Z occurred at a single peptide linkage, between Arg-365 and Gly-366, located in the COOH-terminal portion. The resulting NH₂-terminal large fragment (PZt) and the COOH-terminal peptide (C-peptide) were isolated and chemically characterized. The C-peptide consisted of 31 amino acid residues including one galactosamine-type Thr residue and was assigned to the position from Gly-366 to the COOH-terminal residue of Val-396 in protein Z. The NH₂-terminal large fragment, PZt, constituted the remainder of protein Z. The abilities to bind calcium of intact protein Z, PZt, and the derivative of protein Z devoid of the NH₂-terminal γ-carboxyglutamic acid (Gla) domain (Gla-domainless), prepared with the known chymotrypsin treatment, were examined by equilibrium dialysis. The results indicated that intact protein Z and PZt contain four calcium binding sites with dissociation constants of 0.1 mM. Moreover, the Scatchard plot analysis showed positive cooperativity, suggesting the presence of at least two initial sites for calcium binding. In contrast, the Gla-domainless protein Z had no calcium binding site, indicating that the domain of protein Z functional for calcium binding occurs within the NH₂-terminal Gla domain. This differed from factor X, factor IX, protein S, and protein C, all of which contain one or two calcium binding site(s) independent on their Gla-domains.