HIS27 Biochemistry Of The Blood Coagulation System Flashcards
Relevance of coagulation to medicine
Functions of blood
- Transport (gases, nutrients, regulatory molecules e.g. hormones)
- Defence against invading pathogens (immune cells, Ab, inflammation)
Blood can considered as a “formless organ”
- Components can be lost as a “fluid” due to injuries
- Haemostasis is achieved by a very special protective process known as “coagulation”
Haemostasis is not a perfect process
- Same protective function can be lethal as illustrated in diseases with ***abnormal coagulation activities
- Proper treatment of bleeding disorder can only be formulated by a thorough understanding of entire coagulation process
Biochemistry of blood coagulation
Molecular species involved:
1. Proteins: Enzymes / Cofactors
2. Ca ions (heavily required, regarded as a coagulation factor)
3. Cell membranes (surface for assembly of coagulation factors)
4. Cellular component (Platelets)
5. Non-cellular component (Fibrin: soft protein polymer) —> fibrin clot: fibrin meshwork with platelets reinforcing
—> Intermolecular interactions play important role in blood coagulation
Enzymes:
- proteases (cleavage of specific peptide bond)
- takes place on membrane surface (in form of complexes)
Coagulation:
- ***Rapid
- ***Localised
2 biochemical pathways involved in blood clot formation:
Intrinsic + Extrinsic
Structure of coagulation ***factors
記: 7, 9, 10, Prothrombin
Proteins (i.e. large molecules) —> All are enzymes (Serine proteases)
- ***Factor 7, 9, 10, Prothrombin: blood clot formation
- ***Protein C: inhibit blood coagulation
Common structural layout:
- Gla domain (important for functioning of blood coagulation)
- EGF domain (high degree of analogy with Epidermal Growth Factor protein)
- ***Serine protease (catalytic apparatus —> catalytic activity of enzyme dependent on presence of serine residue in catalytic site)
- Disulphide bond (maintains conformation of proteins)
- ***Specific peptide bond at a particular site (cleaved when coagulation needed)
Latent vs Active state:
- Latent: coagulation not needed —> circulating in blood
- Active: coagulation needed —> specific peptide bond cleaved —> activation
Structure of coagulation ***cofactors
記: 5, 8
Factor 5, Factor 8: assist other factor enzymes (i.e. Factor 7, 9, 10, Prothrombin)
- also possess ***Specific peptide bonds at a particular site (cleaved when coagulation needed)
Latent vs Active state:
- Latent: coagulation not needed —> circulating in blood
- Active: coagulation needed —> specific peptide bond cleaved —> activation
Haemostasis depends on formation of a blood clot
Blood vessel injury —>
- Humoral pathway (composed of soluble coagulation factors): Formation of Fibrin meshwork
—> Activation of coagulation pathways (Extrinsic + Intrinsic) by Activated platelets
—> **Prothrombin (latent form) to Thrombin (activated enzyme) (Thrombin —> activate platelets)
—> **Thrombin convert Fibrinogen to Fibrin (monomer capable of polymerisation)
—> **Fibrin cross-linking (by Factor 13 (enzyme))
—> **Fibrin meshwork (undergo degradation readily)
—> TAIF inhibit fibrinolysis (Thrombin activated inhibitor of fibrinolysis; mediated by Thrombin)
—> Net production of fibrin meshwork at site of vessel injury - Cellular pathway:
—> Activation of platelets by Thrombin
—> platelets placed into fibrin meshwork
—> complete blood clot
Complete blood clot: mechanical strength to withstand pressure of blood vessels
***Intrinsic pathway
***簡單而言: Factor 12a —> Factor 11a —> Factor 9a —> Factor 10a —> Thrombin —> Fibrin
A cascade
Factor 12 —(unknown mechanism)—> Factor 12a (activated enzyme)
Factor 11 —(11a / **Thrombin)—> Factor 11a
Factor 9 —(11a)—> Factor 9a
Factor 10 —(9a, membranes, Ca, **co-Factor 8a)—> Factor 10a
Prothrombin —(10a, membranes, Ca, ***co-Factor 5a)—> Thrombin
(Factor 8 —(9a / **10a / **Thrombin)—> co-Factor 8a)
(Factor 5 —(10a / ***Thrombin)—> co-Factor 5a)
Fibrinogen —(Thrombin)—> Fibrin monomer —(Linear polymerisation + Cross-linking)—> Fibrin meshwork
***Thrombin: Amplification of cascade by producing more Factor 5a, 8a, 11a —> producing more Thrombin in return (+ve feedback)
Extrinsic pathway
Tissue factor
- transmembrane protein (vs Factor 12: soluble factor)
—> Extracellular domain (interact with Factor 7 in blood when cell is exposed after vascular injury)
—> Cytosolic domain (intracellular signal)
- located in tissue adventitia (underneath endothelium)
- found in endothelium, smooth muscle cells, fibroblasts
- expressed in most cells except resting endothelial cells (except in inflammation)
- only **exposed to intravascular space after vascular injury
- does **not require proteolytic activation (have intrinsic activity)
Factor 7
- single chain zymogen (always in blood)
- low intrinsic enzyme activity
- highly active when bound to Tissue factor
***Activation of extrinsic pathway
Extrinsic pathway:
- Responsible for ***initiation of coagulation pathway
- Takes place on ***cell surface (e.g. fibroblast surface)
Vascular damage
—> Tissue factor exposed to blood (+ Factor 7) (Factor 7 —(unknown protease in plasma)—> Factor 7a (slightly active))
—> Tissue factor/Factor 7a complex (strongly active, short-lived)
—> **activate Factor 9, Factor 10
—> **activation of Prothrombin —> Thrombin
—> Fibrinogen —> Fibrin
***Thrombin: Amplification of cascade by producing more Factor 5a, 8a, 11a —> producing more Thrombin in return (+ve feedback)
***Interaction of Extrinsic and Intrinsic pathways
***簡單而言: TF/7a —> Factor 10a —> Factor 9a —> Factor 10a —> Thrombin
Initiation phase: TF-bearing cell event
Extrinsic pathway: formation of TF/7a —>
Factor 10 —(TF/7a: short-lived)—> Factor 10a (small amount) (This process will disappear soon after initiation phase due to inhibition by TFPI: Tissue factor pathway inhibitor)
Factor 9 —(10a)—> Factor 9α
Factor 9α —(TF/7a, cannot be inhibited by TFPI)—> Factor 9a
Factor 10 —(9a)—> Factor 10a —>
1. +ve feedback loop to produce more Factor 9α
(Self-sustained loop to produce more Factor 10a —> sustained input to form Fibrin)
- Factor 10a —> Intrinsic pathway —> activate Prothrombin
(TFPI: tissue factor pathway inhibitor)
***Intrinsic and Extrinsic humoral pathways - protein components involved
Intrinsic pathway:
- Factor 12 (Hageman factor)
- Factor 11
Extrinsic pathway:
- Factor 7
- Tissue factor (cofactor)
Common components:
- Factor 9
- Factor 10
- Factor 5 (cofactor)
- Factor 8 (cofactor)
Why are there 2 pathways in haemostasis?
Factor 12 (initiator protein) deficiency does not cause clotting defects (not really needed) —> But lack of Factor 11 sometimes results in bleeding episodes (∵ Factor 11 —(Thrombin)—> Factor 11a —> a positive feedback loop by Thrombin)
Lack of Factor 8: **Haemophilia A
Lack of Factor 9: **Haemophilia B
—> Factor 8, Factor 9 are essential for blood coagulation
How can coagulation be fast
***Positive feedback loop
Provided by:
- Thrombin (helping to produce more Factor 5a, 8a, 11a —> 再造多d Thrombin)
- Factor 10a (helping to produce more Factor 8a, 9α —> 再造多d 10a)
- Factor 9, TF/7 complex
***Role of platelets
Resting platelet (circulating in blood)
—> Activated/Recruited by **Collagen, **von Willebrand factor at site of injury (extracellular matrix molecules)
—> Activated platelets adhered to injury site
—>
1. Release reactions: **Coagulation factors, ADP (important for blood coagulation)
2. **Membrane vesiculation (forming small membrane vesicles)
3. ***Thromboxane A2 —> promote coagulation
***Spatial organisation of coagulation pathways
簡單而言: Thrombin —> activate platelets —> activate coagulation pathway —> more Thrombin
- Initiation of coagulation pathway: TF-bearing cell events
- occur on cell surface (e.g. fibroblast surface) at injury site
—> TF/7a, Factor 9a, Factor 10a
—> Thrombin formation - Platelet recruited to injury site by **Collagen, **von Willebrand factor
—> Platelet adhere to ECM molecules at injury site (mediated by receptors for ECM molecules)
—> ***Thrombin diffuse and bind to thrombin receptor on Platelet surface
—> Thrombin + ECM molecules serve as stimulus to activate platelet
—> Activated platelet - Activated platelet surface
- ***Release reactions: Coagulation factors, ADP (important for blood coagulation)
—> increase substrate availability for Thrombin
—> produce more active Factors
—> produce more Thrombin (+ve feedback)
—> enormous magnification of Thrombin formation within short time
—> Fibrinogen —> Fibrin
- Membrane vesiculation (forming small membrane vesicles) —> provide more membrane surface for coagulation complex to form
- Thromboxane A2 —> promote coagulation
Location of formation of coagulation complex
- Platelet membrane surface
- Surface of membrane vesicles budding off from activated platelet
- Cell surface (e.g. fibroblast)
