Regulation of coagulation

• What are the 3 types of coagulant pathways?

  Tissue factor pathway inhibitor pathway

  Protein C pathway

  Antithrombin Heparin
• How does blood clot?

  There are clotting factors in the blood and the anticoagulant endothelium in contact with the blood. Under the endothelium, there are tissue factors which drive the clotting factor. When there is vascular injury, TF is exposed to blood and the clotting factors. Platelets will stick down and a coagulation cascade will start. Thrombin FIIa is generated rapidly which catalyses fibrin deposition. Thrombus formation will rapidly attenuate blood loss at the site of injury.
• Describe how there is a clot formation in vivo.

  Exposure of sub-endothelial proteins --> accumulation of activated platelets at site of injury --> fibrin deposition and then there is clot formation
• How does tissue factor pathway inhibitor (TFPI) work?

  Works by stopping the stimulus of blood clotting
• What does TFPI prevent?

  Prevent efficient activation of coagulation
• What do TFPI block?

  Block interaction between tissue factor, factor VIIa and factor Xa
• TFPI only works in what conditions?

  Only works where stimulus (tissue factor exposure) is low
• Essentially, what does TFPI do?

  Dampen response by essentially putting a blanket over it
• What is the mechanism of TFPI inhibition?

  TFPI binds to and inhibits FVIIa/FX -->

  Clotting stimuli blocked -->

  Slows down initiation of clot formation
• Why is activated protein C (APC) important?

  Really important to stop thrombosis
• What type of deficiencies are there of APC and what does it cause?

  Homozygous deficiency: has no protein C. Rare and usually fatal.

  Heterozygous deficiency: 50% normal protein C. Increase of 5-10 fold risk of venous thrombosis
• Excess thrombin binds to? which is on what?

  Excess thrombin binds thrombomodulin on intact endothelial cells
• How does thrombin, thrombomodulin and APC work?

  Excess thrombin that flows away will be captured by thrombomodulin on the endothelium and they stick together very tight. Thrombomodulin makes protein C go to APC which is an anticoagulant and stop clotting.
• Unbound thrombin is?

  Procoagulant
• What is the role of procoagulant serine protease (free thrombin)?

  Activates platelets

  Feedback amplification (activates FV, FVIII, FXI)

  Cleaves fibrinogen to form fibrin.

  Cell signalling (via PAR molecules)
• Thrombin bound to TM loses?

  Loses its procoagulant activity and loses its pro-coagulant properties
• Thrombin bound to TM can activate ?

  Activates protein C
• Describe how APC is activated.

  Thrombin binds thrombomodulin on EC surface. Concentrates thrombin and aligns thrombin's active site.

  Thrombin:TM complex activates PC. Thrombin cleaves protein C to release the activation peptide and generate APC.

  Factor Va is cleaved and inactivated by APC
• Describe how thrombin functions as an anticoagulant on endothelial cells.

  FIIa binds to TM on endothelial cells

  Protein C binds to EPCR on endothelial cells

  FIIa:TM complex activates protein C

  Activated proteinC:Protein S inhibit thrombin generation at the thrombus edge. APC mediated inihibition occurs on EC (and platelet membranes)
• Where is antithrombin made?

  liver mainly and endothelial cells
• Antithrombin is related to which family?

  Serpin
• What are serpin families?

  Serine protease inhibitors

  Bind to activate serine proteases and stop activity of clotting so it cant contribute to coagulation with an enzyme
• What does antithrombin function as?

  Scavenger of thrombin
• What does antithrombin bind to?

  Thrombin (active proteinase)
• Where do antithrombin and thrombin bind?

  Vessel wall heparan sulphate
• What does antithrombin form?

  A 1:1 irreversible complex with all serine proteases (XIIa, XIa, Xa, IXa, thrombin)
• Antithrombins affinity is highest with what?

  thrombin
• What is the most important direct inhibitor of thrombin?


  Antithrombin
• Antithrombin accounts for how much of the antithrombin activity in plasma?

  70%
• INactive inhibitor.protease complexes are removed by?

  Liver and spleen
• What does heparin do?

  Boosts antithrombin function
• WHat is heparin synthesised as?

  Proteoglycan in mast cells
• What on endothelium appears to act in a similar manner to heparin?

  Heparan sulphat
• What does heparin accelerate?

  Formation of AT-protease complexes x2000-10000 fold
• Analogous accelerating role operates in vivo via?

  Heparin sulphate intercalated in the surface membrane of endothelial cells
• How does heparin accelerate antithrombin action against thrombin?

  Without heparin, AT and T do not fit that well. When heparin binds to AT, it alters the conformation of its active site and allows T and AT to bind better. It aslo approximates AT to the substrate
• What is the role of TFPI?

  STops blood coagulation from starting.

  Inhibits tissue factor/FVIIa/FXa complex
• What is the role of protein C pathways?

  Detects excess thrombin to trigger anticoagulant pathway.

  Breaks down coagulation proteins important for thrombin generation.

  Negative feedback loop
• What is the role of antithrombin-heparin pathway?

  Binds to active coagulation proteases and inactivates and clears from circulation.

  Requires heparans/heparin for optimal effect