Which protein is a vitamin K–dependent anticoagulant produced in the liver?

Which protein is a vitamin K–dependent anticoagulant produced in the liver?

• A. Protein C
• B. Factor II
• C. Factor VII
• D. Factor IX

Answer: (A)

The concept tested is how vitamin K–dependent proteins regulate coagulation, specifically anticoagulant proteins produced by the liver. Protein C is secreted by the liver as a zymogen and requires vitamin K–mediated gamma-carboxylation to function properly. It is activated on the endothelial surface by thrombin bound to thrombomodulin, becoming Activated Protein C. APC, with its cofactor Protein S, then inactivates factors Va and VIIIa, reducing thrombin generation and producing an anticoagulant effect. Among the options, Protein C is the only one that acts as an anticoagulant in this vitamin K–dependent way. The other factors listed (II, VII, IX) are vitamin K–dependent procoagulant factors produced in the liver that promote coagulation rather than inhibit it.

The concept tested is how vitamin K–dependent proteins regulate coagulation, specifically anticoagulant proteins produced by the liver. Protein C is secreted by the liver as a zymogen and requires vitamin K–mediated gamma-carboxylation to function properly. It is activated on the endothelial surface by thrombin bound to thrombomodulin, becoming Activated Protein C. APC, with its cofactor Protein S, then inactivates factors Va and VIIIa, reducing thrombin generation and producing an anticoagulant effect. Among the options, Protein C is the only one that acts as an anticoagulant in this vitamin K–dependent way. The other factors listed (II, VII, IX) are vitamin K–dependent procoagulant factors produced in the liver that promote coagulation rather than inhibit it.