Protein S (Last Updated - 3/29/2005)

-plasma zymogen
-synthesized in liver (along with Protein C & Factors II (Prothrombin), VII, IX & X)
-its activation depends on calcium binding by its Gla (Gamma-Carboxyglutamate) residues
-once activated --> degrades Factors Va (Common Pathway) & VIIIa (Intrinsic Pathway) but to a lesser extent than protein C
-leads to decreased rates of activation of Factors X & II (Prothrombin) to activated Xa & IIa (Thrombin) --> anti-coagulation
-if deficient in Proteins C or S --> hypercoagulable state
-1/2-Life:
     ~30 hrs.

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References:

Goodman & Gilman's: The Pharmacologic Basis Of Therapeutics - 10th Edition - 2001. Chapter 55.