Protein S is a vitamin K-dependent, potent neutral anticoagulant plasma glycoprotein. Protein S has a molecular weight of approximately 70 kDa and concists of a single polypeptide chain. The concentration of protein S in human plasma is 25 µg/ml, with a half-life of approximately two days. About 40% of Protein S in plasma is in a free form, whereas 60% is complexed with C4b-binding Protein (C4BP). Only the free Protein S functions as a cofactor to antigen presenting cells (APC) in the APC-dependent degradation of Factor Va and Factor VIIIa.
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