Cardio L17 Blood Clotting Flashcards
Blood Clotting
The need →
to avoid blood loss when blood vessels damaged (excessive blood loss produces shock and death).
Problems associated with blood clotting →
Inadequate e.g. haemophilia and some venom gives excessive bruising and haemorrhaging.
Inappropriate e.g. deep vein thrombosis, coronary thrombosis, stroke and other venoms.
A blood clot consists of a
plug of platelets enmeshed in a network of insoluble fibrin molecules:
2 components:
1. Platelets → are cell fragments made from megakaryocyte (no nucleus).
2. Fibrin
Process:
1. Tissue damage activates
a. Conversion of Prothrombin to thrombin via a complex proteolytic cascade
i. Proteolytic cleavage of fibrinogen to fibrin which polymerises to form clot.
b. Activation and aggregation of platelets
Platelets aggregation:
Von Willebrand factor
Serotonin
Von Willebrand factor
links collagen exposed on damaged blood vessels to platelets.
• The activated platelets release factors such as
as ADP and thromboxane to activate more platelets causing aggregation that forms a plug of platelets.
Serotonin
is also released causing local vasoconstriction as are other factors that enhance thrombin cleavage.
Aggregation of activated platelets: (3)
- Platelet activation causes changes in the shape of platelets and conformation changes in glycoprotein IIb/IIIa receptors
- ADP and thromboxane binding lads to a conformation change in IIB/IIIa receptors = active receptors.
- The activated receptors can be cross-linked by fibrinogen to form bridges between adjacent platelets and facilitate platelet aggregation.
Fibrinogen (precursor molecules) structure
3 component chains with globular units and alpha helices with cleavage sites:
- Gamma
- Beta → cleaved by thrombin
- Alpha → cleaved by thrombin
Process: of fibrinogen
- Thrombin cleaves of peptides from alpha and beta subunits.
- Produces a truncated fibrin which can polymerise as the ends can bind into groves → this polymerisation is the formation of a clot (soft – rapid formation).
Hard clot →
Clot is stabilised by cross-linking glutamines with lysines using a transglutaminase (Factor XIIIa)
How do we activate the Thrombin:
thrombin is a proteolytic reaction catalysed by a serine protease factor Xa together with a stimulatory protein factor Va.
Cleavage → 2 places (see below) by proteases factor Xa and Va
Cleavage two sites:
Kringle domain
Gla domain
Kringle domain →
a triple loop structure stabilised by 3 dulsphide bonds → involved in protein interactions between blood clotting factors and mediators.
- Gla domain →
contains 10 gamma-carboxyglutamte.
Gla domain cleavage requires
a. This cleavage requires calcium to bind the gamma-carboxyglutamate residues of the Gla region.
b. The calcium binds the prothrombin to the phospholpds of the platelet cell membranes at the damage site.
c. Gamma-carboxyglutamate is formed form glutamate by a vitamin K dependent carboxylation reaction.
Enzyme inhibited fibrin Gla domain cleavage
d. This enzyme is inhibited by dicoumarol (found in spoiled sweet clover that cause haemorrhagic disease in cattle) and warfarin (rat poison) used to prevent blood clots in patients prone to DVT.
Gamma-carboxyglutamate information:
Found
Gamma-carboxyglutamate information: During platelet activation prothrombin is cleaved to thrombin how:
Cleavage requires calcium to bind to the gamma-carboxyglutamate residues in this region.
Gamma-carboxyglutamate information: Calcium recruitment
Calcium binds to prothrombin to allow binding to phospholipids on the platelet membrane (damamge site)
Gamma-carboxyglutamate information: Gamma-carboxyglutamate formed from
From glutamate by a vitamin K dependent carboxylation reaction.
Gamma-carboxyglutamate information: Inhibited by
By dicoumarol which inhibits vit K reductase
Thrombin =
Blood clotting therefore control is upstream
Thrombin produced by
TF Xa and Va
Tissue factors
Serum proteases that cleave
Reason for cascade of proteases
To amplify the process
Extrinsic pathway: factors involved
(3,7,10)
Extrinsic pathway process
- Trauma causes the release of TF III.
- TF VIIa is a proteases that causes TF 10 to 10a
- TF is another serum protease
TF III location
Exposed on damaged tissue
TF III type
Trans-membrane protein
TF III function
Binds and activates factor VII to VIIa
Intrinsic pathway: factors involved
(12,11,9,8,10)
Intrinsic pathway process
Release Kinogens and Kalikreins
- Once activated stimulated factor XII to XIIa (12) activated
- XIIa proteolytic cleaves Xi → Xia
- Xia converts IX to IXa
- IXa in conjunction with VIIa cleaves
- X to Xa
Factor IXa interaction with VIIIa →
clinically haemophilia A (X linked).
• Actor 8 stimulates the conversion of factor 10 to 10a by factor 9a
• Factor 10 a activates factor 8 (need a small amount to accelerate the factor 9 function)
Haemophilia A →
factor 8a missing.
Von Willebrand factor → interacts with
Factor VIII as well as binding platelets to collagen on the damaged blood vessel wall.
Controlling the blood clotting cascade:
Clot formation is a balancing act → too much and clost form where they should Not; too little and you bleed to death. Hence an anti-clotting cascade starts almost as soon as vclotting begins.
The extrinsic pathway blocked by
- Tissue Factor Pathway Inhibitor (TFPI)
- Antithrombin III (AT3)
- Protein C and S
TFPI structure and function
Has three tandem protease inhibitory domains which inhibit Factor Xa and the factor VIIa-Tissue Factor Complex.
TFPI circulates in the blood predominantly bound to
To lipproteins (LDL, HDL, lipoprotein a). In addition, platelets carry about 10% of the TFPI.
Antithrombin III: Structure
Is a protein synthesized by hepatocytes and endothelial cells.
Antithrombin III: Functions
By inhibiting the serine proteases activity of the clotting factors – especially thrombin and Factor Xa
Antithrombin III: Heparin
A highly sulphated oligosaccharide, enhance the inhibitory effect of AT£ by 10000 times! Hence its anticoagulant properties. It is secreted by stimulated mast cells.
Proteins C and S: Description
Are Vitamin K-dependent serine proteases synthesized in the liver and circulating in the blood as an inactive form (zymogen)
Activation and function of Protein C
Protein C is activated by thrombin complexed to thrombomodulin and its protease activity inhibits the activity of factors Va and VIIIa
Function Protein s
Protein S is a cofactor in this process and significantly potentiates the action of Protein C.
Thrombomodulin
Is a receptor expressed on the surface of endothelila cells which converts hrombin into a conformation that activates Protein C and hence enhancing its anticoagulant activity.
Dissolving blood clots: Involves
Tissue plasminogen Activator
TPA Action
Converts plasminogen to Plasmin
Plasmin
Capable of breaking down fibrin clots producing fibrin split products which are soluble.
TPA
Serine protease normally found on the surface of endothelial cells of veins, capillaries, the pulmonary artery, heart and uterus
Secreted after vascular injury
Urokinase
Also aids dissolving the clot by breaking down plasminogen to plasmin
TPA clinically
Clot busting – older treatment.
Plasminogen Activator-inhibitors
There to control the breakdown
