Coagulation
Physiology
Platelet activation
Damage to blood vessel walls exposes subendothelium proteins, most notably
ollagen,
present under the
endothelium. Circulating platelets bind collagen with surface
collagen-specific
glycoprotein Ia/IIa receptors. The adhesion is strengthened further by the
large, multimeric circulating proteins
von Willebrand factor (vWF), which forms links between the platelets
glycoprotein Ib/IX/V and the collagen fibrils. This adhesion activates the
platelets.
Activated platelets release the contents of stored granules into the blood
plasma. The granules include
ADP,
serotonin,
platelet activating factor (PAF),
vWF,
platelet factor 4 and
thromboxane A2 (TXA2) which in turn activate
additional platelets. The granules contents activate a
Gq-linked protein receptor cascade resulting in increased calcium
concentration in the platelets' cytosol. The calcium activates
protein kinase C which in turn activates
phospholipase A2 (PLA2). PLA2 then modifies
the ntegrin
membrane glycoprotein IIb/IIIa, increasing its affinity to bind fibrinogen. The
activated platelets changed shape from spherical to stellate and the fibrinogen
cross-links with glycoprotein IIb/IIIa aid in aggregation of adjacent platelets.
The coagulation cascade
The coagulation cascade of secondary hemostasis has two pathways, the
contact activation pathway (formerly known as the intrinsic pathway) and the
tissue factor pathway (formerly known as the extrinsic pathway) that lead
to fibrin formation. It was previously thought that the coagulation
cascade consisted of two pathways of equal importance joined to a common
pathway. It is now known that the primary pathway for the initiation of blood
coagulation is the tissue factor pathway. The pathways are a series of
reactions, in which a
ymogen
(inactive enzyme precursor) of a
serine protease and its
glycoprotein co-factor are activated to become active components that then
catalyze the next reaction in the cascade, ultimately resulting in cross-linked
fibrin. Coagulation factors are generally indicated by
Roman numerals, with a lowercase a appended to indicate an active
form.
The coagulation factors are generally
serine proteases (nzymes).
There are some exceptions. For example, FVIII and FV are glycoproteins and
Factor XIII is a
transglutaminase. Serine proteases act by cleaving other proteins at
specific sites. The coagulation factors circulate as inactive
zymogens.
The coagulation cascade is classically divided into three pathways. The
tissue factor and contact activation pathways both activate the
"final common pathway" of factor X, thrombin and fibrin.
Tissue factor pathway
The main role of the tissue factor pathway is to generate a "thrombin burst",
a process by which
hrombin,
the most important constituent of the coagulation cascade in terms of its
feedback activation roles, is released instantaneously. FVIIa circulates in a
higher amount than any other activated coagulation factor.
- Following damage to the blood vessel, endothelium Tissue Factor (TF) is
released, forming a complex with FVII and in so doing, activating it (TF-FVIIa).
- TF-FVIIa activates FIX and FX.
- FVII is itself activated by thrombin, FXIa,
lasmin,
FXII and FXa.
- The activation of FXa by TF-FVIIa is almost immediately inhibited by
tissue factor pathway inhibitor (TFPI).
- FXa and its co-factor FVa form the
prothrombinase complex which activates
prothrombin to thrombin.
- Thrombin then activates other components of the coagulation cascade,
including FV and FVIII (which activates FXI, which in turn activates FIX),
and activates and releases FVIII from being bound to vWF.
- FVIIIa is the co-factor of FIXa and together they form the "enase"
complex which activates FX and so the cycle continues. ("Tenase" is a
contraction of "ten" and the suffix "-ase" used for enzymes.)
Contact activation pathway
The contact activation pathway begins with formation of the primary complex
on ollagen
by
high-molecular weight kininogen (HMWK),
prekallikrein, and FXII (Hageman factor).
Prekallikrein is converted to
allikrein
and FXII becomes FXIIa. FXIIa converts FXI into FXIa. Factor XIa activates FIX,
which with its co-factor FVIIIa form the
enase complex,
which activates FX to FXa. The minor role that the contact activation pathway
has in initiating
clot formation can be illustrated by the fact that patients with severe
deficiencies of FXII, HMWK, and
prekallikrein do not have a bleeding disorder.
Final common pathway
Thrombin has a large array of functions. Its primary role is the
conversion of
fibrinogen to fibrin, the building block of a hemostatic plug. In addition,
it activates Factors VIII and V and their inhibitor
rotein C
(in the presence of
thrombomodulin), and it activates Factor XIII, which forms
covalent bonds that crosslink the fibrin polymers that form from activated
monomers.
Following activation by the contact factor or tissue factor pathways the
coagulation cascade is maintained in a prothrombotic state by the continued
activation of FVIII and FIX to form the
enase complex,
until it is down-regulated by the anticoagulant pathways.
Cofactors
Various substances are required for the proper functioning of the coagulation
cascade:
- alcium
and
phospholipid (a
latelet
membrane constituent) are required for the tenase and prothrombinase
complexes to function. Calcium mediates the binding of the complexes via the
terminal gamma-carboxy residues on FXa and FIXa to the phospholipid surfaces
expressed by platelets as well as procoagulant microparticles or
microvesicles shedded from them. Calcium is also required at other
points in the coagulation cascade.
-
Vitamin K is an essential factor to a hepatic
gamma-glutamyl carboxylase that adds a
carboxyl group to
glutamic acid residues on factors II, VII, IX and X, as well as
rotein S,
Protein C and
rotein Z.
In adding the gamma-carboxyl group to glutamate residues on the immature
clotting factors Vitamin K is itself oxidized. Another enzyme,
Vitamin K epoxide reductase, (VKORC) reduces vitamin K back to its
active form. Vitamin K epoxide reductase is pharmacologically important as a
target for anticoagulant drugs
arfarin
and related
coumarins such as
acenocoumarol,
phenprocoumon and
icumarol.
These drugs create a deficiency of reduced vitamin K by blocking VKORC,
thereby inhibiting maturation of clotting factors. Other deficiencies of
vitamin K (e.g. in
malabsorption), or disease (epatocellular
carcinoma) impairs the function of the enzyme and leads to the formation
of PIVKAs (proteins formed in vitamin K absence) this causes partial or non
gamma carboxylation and affects the coagulation factors ability to bind to
expressed phospholipid.
Regulators
Five mechanisms keep platelet activation and the coagulation cascade in
check. Abnormalities can lead to an increased tendency toward thrombosis:
-
Protein C is a major physiological anticoagulant. It is a vitamin
K-dependent serine protease enzyme that is activated by thrombin into
activated protein C (APC). Protein C is activated in a sequence that starts
with Protein C and thrombin binding to a cell surface protein
thrombomodulin. Thrombomodulin binds these proteins in such a way that
it activates Protein C. The activated form, along with protein S and a
phospholipid as cofactors, degrades FVa and FVIIIa. Quantitative or
qualitative deficiency of either may lead to
thrombophilia (a tendency to develop thrombosis). Impaired action of
Protein C (activated Protein C resistance), for example by
having the "Leiden" variant of Factor V or high levels of FVIII also may
lead to a thrombotic tendency.
-
Antithrombin is a
serine protease inhibitor (erpin)
that degrades the serine proteases; thrombin, FIXa, FXa, FXIa and FXIIa. It
is constantly active, but its adhesion to these factors is increased by the
presence of
heparan sulfate (a
glycosaminoglycan) or the administration of
eparins
(different heparinoids increase affinity to FXa, thrombin, or both).
Quantitative or qualitative deficiency of antithrombin (inborn or acquired,
e.g. in
proteinuria) leads to thrombophilia.
-
Tissue factor pathway inhibitor (TFPI) limits the action of tissue
factor (TF). It also inhibits excessive TF-mediated activation of FIX and
FX.
- lasmin
is generated by proteolytic cleavage of plasminogen, a plasma protein
synthesized in the liver. This cleavage is catalyzed by
tissue plasminogen activator (t-PA) which is synthesized and secreted by
endothelium. Plasmin proteolytically cleaves fibrin into fibrin degradation
products which inhibits excessive fibrin formation.
-
Prostacyclin (PGI2) is released by endothelium and activates
platelet Gs protein linked receptors. This in turn activates
adenylyl cyclase which synthesizes cAMP. cAMP inhibits platelet
activation by counteracting the actions that result from increased cytosolic
levels of calcium and by doing so inhibits the release of granules that
would lead to activation of additional platelets and the coagulation
cascade.
Fibrinolysis
-
Eventually, all blood clots are reorganised and resorbed by a process termed
fibrinolysis. The main enzyme responsible for this process (lasmin)
is regulated by various activators and inhibitors.
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