Prothrombin (factor II)
Common pathway · Vitamin K dep. Half-life 60-72 h
Prothrombin is the vitamin K-dependent zymogen that docks on the anionic platelet surface through its Gla domain and is cleaved twice by prothrombinase to release thrombin. Its long half-life of about 60-72 hours is what sets the three to five day delay before warfarin is fully effective.
Traced from the start
- Subendothelial collagen and von Willebrand factor
- Activated platelet phospholipid surface
- Prothrombin (factor II)
Detail
- Vitamin K dependent
- Yes; the Gla domain binds Ca2+ and phosphatidylserine
- Cleaved by
- Prothrombinase (Xa-Va-Ca2+-phospholipid) at two arginine bonds, releasing prothrombin fragment 1.2 and thrombin
- Half-life
- About 60-72 hours, the longest of the vitamin K-dependent factors
- Replaced by
- Prothrombin complex concentrate, which contains factors II, VII, IX and X with proteins C and S, for warfarin reversal in major bleeding
When it goes wrong
Prothrombin G20210A variant in the 3' untranslated region, which raises the plasma prothrombin level
The second commonest inherited thrombophilia in people of European descent (about 2%), carrying roughly a 2-3 fold venous thromboembolism risk
Acquired hypoprothrombinaemia with a lupus anticoagulant, usually in a child after a viral illness or in SLE
The unusual antiphospholipid patient who bleeds rather than clots: prolonged PT and APTT with a low factor II level
Practise this structure
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