Blood coagulation Flashcards
Haemostasis, porocesses that
1) stop haemorrhage
2) prevent haemorrhage
3) maintain blood flow
Primary haemostasis includes
secondary includes
1) vasoconstriction
2) haemostatic plug formation
Activation of clotting factors to form a thrombus
Haemostasis is carried out by which mechanisms
1) cellular= platelets
2) Humoral = coagulation
3) Tissue = vessels
Disordered haemostasis
Thrombosis- excess or innappriate clotting
1) Arterial caused by platelets
Coronary thormbosis, thrombotic stroke
2) Venous caused by coagulation
DVT and PE
Haemorrhage- excess or innappropriate bleeding
Trauma or bleeding diatheses
bleeding diatheses
tendency to suffer from a particular kind of disease- in this case bleeding.
e.g. haemophilia, von Willebrand disease
Virchow’s triad
3 causative factors of thrombosis:
1) Coagulation factors- humoral
2) Platelets (cellular)
3) Blood vessels
How do blood vessels contribute to blood control?
Produce vasoactive mediators and haemostatic agents
Vasospasm (vasoconstriction)
Coagulation cascade
Intrinsic pathway
XIIa (12)
XIIIa (8) + XIa (11)
Va (5) + IXa (9)
IIa (thrombin)
Ia (fibrin)
Extrinsic pathway
Initial damage
IIIa (3)
VIIa (7)
Xa (10)
IIa (thrombin)
I (fibrin)
Why is it phsiologically beneficial to have so many steps in the coagulataion cascade?
The more steps you have, the more control you have over it.
This is particulalry important where there is positive feedback and you don’t want to trigger this process in an unijured are
Fibrinolytic drug example
Alteplase
Examples of antiplatelet drugs
- Aspririn
- Clodidogrel
- Ticagrelor
- Tirofiban
Fibrinogen
Factor 1
Soluble glycoprotein synthesised by the liver
Heterohexamer (2x a chain and 2x B and 2x Y)
Acts as a bifunctional ligand for aIIbB3 integrin (glycoprotein IIb/IIIa) on platelets
Mostly found in plasma, some found in platelet granules
Target for drugs
Cross-links platelets –> aggregation
Fibrin
Factor Ia
Insoluble protein
Formed by action of thrombin on a and B chains of fibrinogen
Fibrinopeptide A and B
Spontaneously polymerizes –> forms fibrin clot–> coagulation
Entangle platelets, building up a spongy mass that gradually hardens and contracts to form the blood clot.
Hardening process stabilized by FXIII (fibrin-stabilizing factor)
As fibrin is formed it binds to thrombin- limiting the thrombin that is active
Fibrinogen –>?
Fibrinogen –> fibrin monomer–> finrin polymer –> fibrin fibril
Glycoprotein IIb/IIIa (integrin αIIbβ3)
Integrin complex found on platelets
Receptor for fibrinogen and von Willebrand factor
Aids in platelet activation
formed via calcium-dependent association of gpIIb and gpIIIa, a required step in normal platelet aggregation
Platelet activation by ADP (blocked by clodipogrel) causes conformtational change in integrin αIIbβ3 receptors to allow FII binding
Prothrombin
Factor II
Zymogen, soluble plasma protein
monomer
Vitamin k-dependant factor
10y-carboxyglutamate residues confer a strong negative charge which binds to Ca2+
Thrombin
Factor IIa
Active enzyme
Serine protease
Formed by action of FXa enhanced by FVa
Co-translationally modified in a vitamin k dependant reaction to have a Gla (inhibited by warfarin or vit K deficiency)
Activates many other factors, most notably fibrinogen by cleaving a and B chains
Target for gatran drugs
Engages in negative feedback
Made of a light and heavy chain
What does thrombin convert?
FXI–>XIa
FVIII–>VIIIa
FV–> Fva
fibrinogen –> fibrin
XIII–> XIIIa
Actiavtes platelets via proteinase activated receptors
Negative feedback mechanisms of thrombin
Bound to thrombomodulin (endothelial membrane protein), activates protein C, an inhibitor of the coagulation cascade.
Stimulates production of antithrombin (serine protease inhibitor)
Dabigatran etxilate
Class: NOAC(novel oral anticoagulant)
Direct thrombin inhibotr
Chemistry: Prodrug, small molecule, active compound
Pharamacology:
Target- thrombin
Activity- competitive, reversible inhibitor
Physiology: Anticoagulant
Decreases fibrin formation and thrombin-induced plateletc aggregation
Hirudin
From leecehs is a peptide inhibitor of thrombin
Vitamin K dependant factors
Those that require a particular postranslational mdofication to be fully functional. This is the gamma-carboxylation of glutamate residues, enabling it to interact with Ca2+
These are factors:
II, VII, IX and X
Factor VII
Zymogen –> enzyme (serine protease)
Activated to VIIa with TF and thrombin, X, IX and VIIa/Tf itself
With TF in complex, it activates F VII and IX respectively. Part of the extrinsic pathway.
action of the factor is impeded by tissue factor pathway inhibitor (TFPI), which is released almost immediately after initiation of coagulation
Factor III
Tissue factor
Co-factor, not an enzyme
Transmembrane glycoprotein found on sub-endothelial tissues and in leukocytes.
Forms a complex with FVII to greatly increase its function
Seperated from FVII by endothelium
Factor X
- AKA stuart-prower factor
- Zymogen
- plasma glycoprotein
- RKR tripeptide excised to form 2 chains (light + heavy)
- Vitamin K dependant factor
- Convergence point for both the intrinsic and extrinsic pathways
- Once in a complex with FVa with Ca2+ and phosphopilkids on its surface can form the prothrombinase complex.
Factor Xa
- Active enzyme (serine endopeptidase)
- Activated by actino of F VIIa or F IXa
- Forms complex with FVa (+Ca2+ and PL) cleaving prothrombin to thrombin
- Target for -xaban drugs
Extrinsic pathway of coagulation
Activated by external trauma that causes blood to escape from the vascular system. This pathway is quicker than the intrinsic pathway. It involves factors VII, III, II, I and X.
Clinically measrued as prothrombin time
Shorter, activates the common pathway intially
Factor Va
(proaccelerin)
Active co-factor
Plasma glycoprotein
Heavy chain and light chain helf together by Ca2+
Forms the prothrominase complex with FXa (+Ca2+ and PL)
Rivaroxaban
Class; NOAC, direct Xa inhibitor
Chemistry; small molecule
Pharmacology; Target- FXa (prothrombinase complex) Activity-competitive inhibitor
Clinical; treatment/ prophylaxis of DVT and PE
Common coagulation pathway
X–> Xa via the tenase complex
Tenase has two forms: extrinsic (F VII + tissue factor) and Ca2+
Or intrinsic- cofactor F VIII, F IXa, a phospholipid, and Ca2+.
Once activated to factor Xa, it goes on to activate prothrombin to thrombin.
F Xa requires FV as a cofactor to cleave prothrombin –> thrombin.
Thrombin goes on to activate fibrinogen –> fibrin.
Thrombin also goes on to activate other factors in the intrinsic pathway (factor XI) as well as cofactors V and VIII and factor XIII.
Fibrin subunits come together to form fibrin strands, and factor XIII acts on fibrin strands to form a fibrin mesh. This mesh helps to stabilize the platelet plug.
Factor IXa
Intrinsic/ christmas factor
Prodcued as a zymogen –> Serine protease after cleavage by F XIa (of the contact pathway) or F VIIa (of TF pathway)
Deficiency causes haemophilia type B
In the presence of Ca2+, membrane phospholipids and co-factor FVIII, activates FX
Inhibited by antithrombin
Intrinsic pathway
1) Exposed, damaged tissue (inc. collagen) activates F XII
2) Phosphatidylserine and F XIIa actiavte F XI
3) F XIa actiavtes F IX in the presence of Ca
4) F IXa and F VIII in the presence of Ca2+ and a phospholipid surface
actiavte F X
In vitro anticoagulants
Blood samples clot easily
Chelatorsdecrease the amount of free Ca, disrupting F V, VIII, XII and vitamin K dependant factors
Examples of in vitro coagulants
Citrate
EDTA
Heparin + unfractionated heparin
Heparin aka unfractionated heparin
Glycosaminoglycan produced by the liver
Polymerised by disaccharide units
Highly sulphated polysaccharides
Strong negative charge
MoA: Binds to antithrombin, thereby activating it
Low molecular weight heparin
Shorter strands of heparin that also act as an anticoagulant
e.g. dalteprin, tinzaprin
Fewer side effects
Derived from animals, not ok for vegans?
Antithrombin- III
Serine protease inhibitor
Plasma glycoprotein
Inhibits thrombin, IXa, Xa, XIa
Activity greatly enhanced by heparin
bound to fondaparinux in complex with Xa
Factor XIII
AKA fibrin-stabilizing factor
Activated by thrombin
Transglutaminase crosslinking glutamate and lysine in fibrin, polymerizing it and stabilizing it further. Clot retracts and hardens, closing the injury further
Ca and thrombin activate it
Intrinsic tenase
made of IXa and its regulatory cofactor VIIIa
Fibrinolysis
Fibrinogen–> firbin monomer–> fibrin polymer –>cross-linked fibrin polymer –> dissolution into FDPs
Carried out by plasmin (precursor plaminogen)
Plasminogen
Zymogen –> plasmin
Activated by urokinase or tissue plasminogen actiavtor
How are clotting factors removed?
When normal blood flow is restored, clotting factors are washed away
Alteplase
Class: fibrinolytic
Chemistry: recombinant human tPA
Pharmacology: target- plasminogen activity- activator
Physiology- promotes endogenous fibrinolytic system and thrombus dissolution
Clinical: acute MI, PE
Targets?
Dabigatran
Rivaroxaban
Heparin
Alteplase
F2
F10
antithrombin
synthetic tpa- fibrin
Warfarin
MoA: Inhibits vitamin K metabolism and thereby the synthesis of several clotting factors, including prothrombin
Disrupts the posttranslatinal modification of all vit k dependant factors:
IX (intrinsich pathway)
VII (extrinsic)
X (common)
and F II (final)
Citrates
Binds Ca 2+
EDTA
Chelataes Ca2+ for in vitro coag
Aspirin
Irreversibly inhibits the COX 1 and 2 enzymes so blocks the production of prostaglandins and thromboxanes, inc thromboxane A2 in platelets
Thus attentuates platelet aggregation and reduces the risk of arterial thrombosis.
Haemophilia A, B and C
Haemophilia A- deficiency of VIII, X linked
B- IX, X linked
C-XI, autosomal recessive
Intrinsic tenase
Fcator VIIIa +VIIIIa + Ca2+
What does antithrombin inhibit
2a 9a 10a and 11a
Fondaparineux
Synthetic type of heparin that binds and activates antithrombin
