Thrombin Physiology Pathophysiology. Licari and Kovacic. 2009. JVECC Flashcards
What are the parts of the prothrombinase complex?
- FVa
- FXa
- Calcium
- phospholipids (e.g., from platelets or other cells)
List 6 Vitamin K dependent factors
- II, VII, IX, X
- Protein C and S
What part of the Vitamin-K dependent factors needs Vitamin K for its production
domain
The Gla domain (NH2-terminal domain of gamma-carboxyglutamic acid) is formed from glutamic acid in the presence of vitamin K-dependent carboxylase
How does Vitamin K antagonism affect Thrombin and Prothrombin specifically?
Gla domain needed for binding to phospholipid cell membranes –> part of prothrombinase complex –> inhibits coagulation
Thrombin has an A- and B-chain which one of these has reported binding sites important for coagulation?
B-chain
A-chain has no documented function
What are the 4 functional binding sites of thrombin?
- Sodium binding site
- Active site
- Exosite I
- Exosite II
What is the function of the thrombin Na binding site?
determines whether thrombin acts pro- or anticoagulant
Na ion bound -> favor coagulation
Absence of Na ion -> primarily anticoagulant activity
What is the main function of the Active Site of thrombin?
- Principal binding site of antithrombin and Protein C
- binds fibrinogen together with Exosite I
What is the main function of Exosite I of thrombin?
- Binds Fibrinogen together with the Active Site
- binding site for thrombomodulin
What is the main function of Exosite II of thrombin?
binding site for heparin -> low molecular weight heparin cannot bind well here
What initiates the coagulation “cascade”?
endothelial damage -> TF expressed and contacts circulating FVII
Explain the “extrinsic” and “common” pathway
TF and FVII interact -> FVIIa -> FX to FXa -> FV to FVa -> prothrombin to thrombin
What is an intermediate form between prothrombin and thrombin?
meizothrombin
- local adrenergic receptor activity -> vasoconstriction
- activation of platelets
- fibrinogen cleavage
Explain the thrombin burst
- initially small amounts of thrombin are produced via the TF (extrinsic) pathway
- thrombin activates platelets and VIIIa is released
- thrombin causes XI to XIa activation
- initiated more FXa and FVa activation
- FVa + FXa + phospholipids + Ca = prothrombinase complex -> thrombin burst
What binding sites of thrombin does fibrinogen bind to?
Active site and exosite I
What is factor XIIIa?
fibrin-stabiliing factor
activated by thrombin
forms bonds between fibrin glutamate and lysine residue -> cross-linking of the clot
Why does FXII deficiency rarely cause clinical bleeding?
with thrombin activating FXI, FXIIa is not needed for this activation
thrombin bypasses FXII
What is TFPI?
Tissue factor pathway inhibitor -> inhibits TF-FVIIa-Ca2+ complex
What factor are hemophilia B patients deficient in?
FIX
What binding site of thrombin does thrombomodulin bind to?
Exosite I
How does the Sodium binding site affect thrombomodulin activity?
affinity of thrombomodulin to exosite I decreases if Na ion bound at Sodium binding site
i.e., Na ion presence procoagulant
What is TAFI, how is it activated, and how does it work?
thrombin activatable fibrinolysis inhibitor (TAFI)
* activated by thrombomodulin-thrombin complex
* removes terminal lysine residue of fibrin that normally facilitates binding of plasminogen and TPA -> less plasmin -> less fibrinoysis
How is Protein C activated?
PC binds to endothelial PC receptor (EPCR) -> interacts with Active Site of thrombin-thrombomodulin complex -> APC
What is APC’s cofactor?
Protein S
What factors does APC inactivate?
- inactivates FVa and FVIIIA
- inactivates PAI-1
i.e., APC is anticoagulant and profibrinolytic
What factors does antithrombin inactivate?
FIIa (thrombin)
FXa and FIXa
How can APC and AT potentiate each other?
APC inactivates FVa and FVIIIa
AT inactivates FIIa, FXa and FIXa
when FXa-FVa and FIXa-FVIIIa complexes form these are more resistant to AT inactivation
APC inactivation reduces these complex formation due to less substrate
What receptors does thrombin interact with?
protease activated receptors (PARs)
= G-protein coupled receptors
* highly expressed on platelets
* PAR-1, PAR-3, and PAR-4
How does thrombin stimulate angiogenesis?
thrombin upregulates VEGF (vascular endothelial growth factor) -> initiates angiogenesis
By what mechanisms can thrombin cause vasodilation?
- sitmulates nitric oxide release
- stimulates histamin and serotonin release
- simulates PGI2 release
Explain how cytokines (e.g., TNF-alpha) may have an procoagulant effect
reduces EPCR activation -> decreases APC -> more FVa and FVIIIa -> more thrombin generated
more FVa-FXa and FVIIIa-FIXa complex formation -> more resistance to AT
What is the most important platelet activator?
thrombin
via PAR-1 and PAR-4 receptors
How doe FDPs affect coagulation in DIC
accumulation of FDPs in plasma -> compete with fibrinogen for thrombin exosite I binding site -> less fibrin formation -> less clotting
Explain the mechanisms by which unfractionated heparin works as an antithrombotic
- AT rate of thrombin inactivation low -> heparin induces** conformational change of AT** -> irreversibly binds the Active Site of thrombin
- heparin binds to the Exosite II of thrombin
- -> forming AT-thrombin-heparin complex -> increaes rate of thrombin inactivation by thousan fold
What is the difference between unfractionated and low-molecular-weight heparin?
unfractionated 3,000-30,000 Da
LMWH 1,000-10,000 Da
* LMWH can cause conformational change of AT and significantly increase FXa inhibition, but
* cannot hold Exosite II binding site as well to maintain AT-thrombin-heparin complex -> less thrombin inhibition
longer half-life of LMWH and more predictable repsonse
