Coagulation Cascade

Ahmad Shihada Silmi Msc,FIBMSIUG

Faculty of Science Medical Technology Dept

A set of reactions in which blood is transformed from a liquid to a gel

Coagulation follows intrinsic and extrinsic pathways

The final three steps of this series of reactions are: Prothrombin activator is formed Prothrombin is converted into thrombin Thrombin catalyzes the joining of

fibrinogen into a fibrin mesh

Coagulation

Coagulation

Detailed Events of Coagulation

May be initiated by either the intrinsic or extrinsic pathway Triggered by tissue-damaging events Involves a series of procoagulants Each pathway cascades toward factor X

Once factor X has been activated, it complexes with calcium ions, PF3, and factor V to form prothrombin activator

Coagulation Phase 1: Two Pathways to Prothrombin Activator

Prothrombin activator catalyzes the transformation of prothrombin to the active enzyme thrombin

Coagulation Phase 2: Pathway to Thrombin

Thrombin catalyzes the polymerization of fibrinogen into fibrin

Insoluble fibrin strands form the structural basis of a clot

Fibrin causes plasma to become a gel-like trap Fibrin in the presence of calcium ions activates

factor XIII that: Cross-links fibrin Strengthens and stabilizes the clot

Coagulation Phase 3: Common Pathways to the Fibrin Mesh

Fibrinogen Fibrin

Fibrinogen FibrinThrombin

Fibrinogen FibrinThrombin

Prothrombin

XaVa

Fibrinogen FibrinThrombin

Prothrombin

XaVa

VIIa

TF

Extrinsic Pathway

Fibrinogen FibrinThrombin

Prothrombin

XaVa

VIIa

TF

Extrinsic Pathway

IXa

VIIIa

XIa

XIIa

Intrinsic pathway

Fibrinogen FibrinThrombin

Prothrombin

XaVa

VIIa

TF

Extrinsic Pathway

IXa

VIIIa

XIa

XIIa

Intrinsic pathway

XIIIa

Soft clot

FibrinHard clot

Fibrinogen FibrinThrombin

Prothrombin

XaVa

VIIa

TF

Extrinsic Pathway

IXa

VIIIa

XIa

XIIa

Intrinsic pathway

XIIIa

Soft clot

FibrinHard clot

VVIII

The intrinsic pathway begins with the contact factors, factor XII, HMWK, and prekallikrein, which activates factor XI.

Activated factor XI can then activate factor IX, which then acts with its cofactor,factor VIII,to form tenase complex on a phospholipid surface to activate factor X.

Activated factor X then combines with its cofactor, factor V, to form the prothrombinase complex on a phospholipid surface, converting prothrombin to thrombin.

Thrombin (IIa)

Prothrombin (II)

Xa

VIIa

TF

IXa

Extrinsic pathway (Tissue Factor Pathway)

IX

NB: production of IXaInteraction of intrinsic and extrinsic pathways

Tissue Factor Pathway Inhibitor

• Kunitz-type protease inhibitor (kringles)• 34 and 41 kD forms in plasma (C-term truncation)• Directly inhibits Xa• Inhibits VIIa-TF complex in a [Xa]-dependent

manner• Bound to LDL, HDL and Lp (a)• ~10% present in platelets (endothelium also)

Thrombin (IIa)

Prothrombin (II)

Xa

VIIa

TF

IXa

Tissue Factor Pathway Inhibitor

IX

TFITFPI

NB: Inhibition of Xa and VIIa

Production of IXa

Production of small amounts of thrombin (IIa)

Tissue Factor Pathway

Net result:

NB: no/little fibrin formed!

Tissue Factor Pathway

• VIIa forms as usual via binding of VII to TF• VIIa activates some XXa• Xa converts a small amt of prothrombin to thrombin;

this thrombin is used to produce small amts of VIIIa and Va

• As the conc of TF-VIIa-Xa-thrombin increases, Tissue Factor Pathway Inhibitor inactivates this complex stopping further production of thrombin.

• New: VIIa also activates IXIXa (Key to new scheme) • IXa, with VIIIa (produced as above), produces Xa;

this Xa with Va (produced as above) produces new thrombin; this thrombin produces more VIIIa and Va and now we get lots of thrombin and fibrin!

Thrombin (IIa)

VIII

VIIIa

V

Va

Fibrinogen FibrinThrombin

Prothrombin

XaVa

VIIa

TF

IXa

VIIIa

XIIIa

Soft clot

FibrinHard clot

VVIII

Tissue Factor Pathway NB: activation of IX by VIIa

IX

However, The coagulation “cascade” doesn’t explain how blood clots in vivo.

Several evidence support that coagulation cascade goes in different way

Cerebal matter injection led to large clot in the heart (Dupuy experement).

Deficiency of factor X, HMWK,PK are without haemorrhagic symptoms.

Prothrombin time reagent, this brought into question that patient lacking factor VIII & IX showed normal clotting time.

All these have led to

Current Concept of coagulation

Summary

This coagulation cascade suggests that there are “intrinsic” and “extrinsic” pathways in the coagulation process, but we must modify our definition of these pathways a little.

The “extrinsic” or tissue factor pathway consists of the FVIIa/TF complex and the FXa/Va complex. It operates on the TF-bearing cell to initiate the coagulation process. Its components are shown in aqua in the slide.

The “intrinsic” pathway does not include FXII or its cofactors PK and HMWK, which do not appear to be necessary for hemostasis.

Thus, we can consider the “intrinsic” pathway to consist of FXI(a), the FIXa/VIIIa complex, and the FXa/Va complex. It operates on the platelet surface during the propagation phase to generate a burst of thrombin. Its components are shown in hot pink in the slide.

Thus, both of these pathways are needed for hemostasis, because they operate on different surfaces and play distinct roles.

This described above makes it clear what goes wrong in Hemophilia:-

Platelet adhesion at a site of injury occurs normally in hemophilia,as does production of FXa and small amounts of thrombin on TF-bearing cellsduring the initiation stage of coagulation.

However, platelet surface FX activation by FIXa/FVIIIa is abolished and, therefore, platelet surface thrombin generation fails.

The FVIIa/TF complex cannot effectively substitute for the FIXa/VIIIa complex,because it produces FXa on the “wrong” surface.

Naturally Occurring Inhibitors

Ahmad Sh. Silmi Msc, FIBMSIUG

Faculty of ScienceMedical Technology Dept

Coagulation InhibitorsThe activity of coagulation system must be attenuated.

– Numerous inhibitors are found in blood.

Coagulation is controlled by three types of actions.– Inhibition of converting enzymes (e.g., AT III, C1 esterase inhibitor, 2-

macroglobulin, 2-antiplasmin, 1-antitrypsin, HC-II).• Act on one or more of the converting enzymes (activated factors).• AT III-heparin pathway: major system - 80% of the thrombin inhibitory action in

plasma.

– Destruction of protein cofactors (e.g., TM-PC-PS system).• TM-PC-PS system degrades cofactors V & VIII:C, inhibiting prothrombinase and

tenase complexes, respectively.

– Blocking receptor availability needed for complex formation (e.g., Tissue factor pathway inhibitor (TFPI) and annexin V).

Antithrombin III (AT III)

• naturally-occuring anticoagulant

• binds to factors IXa, Xa, XIa, XIIa (slow)

• accelerated by heparin manyfold

Implication:

Heparin has almost NO anticoagulant

action without AT III

Proposed Mechanism of AT III-Heparin System

HeparinThrombin Antithrombin III

Lysine sites

Serine site

Argininesite

H

Th

H

AT III

AT III

Th

Proposed Mechanism of Thrombomodulin, Protein C and Protein S (TM-PC-PS) System

Thrombin

Prothrombin

Protein C

Thrombomodulin

Thrombin

F-Xa

Activatedplatelet

PS

F-Va

x

Ca++

Ca++

ActivatedProtein C

Proposed Mechanism of Tissue Factor Pathway Inhibitor (TFPI) Activity

F-Xa

Endothelium

Tissue factorF-VIIa

TFPI

F-Xa

TFPITFPI

F-Xa

Anticoagulant Factors

INHIBITORS (cont.)