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1. PLATELET ACTIVATION
- blood vessel damage  clot formation; initial events:
1. binding of platelets to collagen in damaged vessel wall
2. release of glycoprotein Tissue Factor; activation and aggregation of platelets 
constriction of vessel and formation of platelet plug
3. plug provides microenvironmental surface upon which tissue factor-initiated
coagulation cascade occurs
- platelet activation has three steps
1. platelet adhesion
2. release of bioactive constituents into blood
3. platelet aggregation
- immediately after injury  platelets accumulate at site of vascular damage and begin to
adhere to adhesive proteins (collagen) in subendothelial tissue that has become exposed to
blood
- platelets attach to adhesive proteins by cell surface receptors (glycoproteins) in a reaction
involving von Willebrand factor (vWF) and factor VIII; vWF first adheres to collagen fibers,
then binds platelets via plasma membrane receptors
Platelet adhesion – triggers platelets to activate
1.) activation of cell surface receptors for thrombin, collagen, prostaglandins, and ADP
2) triggering of second messenger pathways including increased cAMP and cytoplasmic Ca2+
 activation of PKC
3.) tertiary events, such as change in cell shape and exocytosis of platelet granules; change in
platelet shape from usual disk form to spheres with pseudopodia enhances ability of platelets
to interact with each other and with subendothelial surface; platelet granules contain
prostaglandin precursors  induce vessel constriction, coagulation (factor V, fibrinogen,
HMWK, Ca2+), and immunoglobulins
- coagulation cascade initiated Tissue Factor occurs on platelet surfaces and produces
crosslinked fibrin that mixes with platelet plug  form blood clot; this clotting mechanism
balanced by limiting reaction; improper hemostasis  hemorrhage or hyperactive clotting
2.BLOOD COAGULATION CASCADES
- intrinsic pathway of blood clotting involves a series of proteolytic cleavages of specific blood
clotting proteins
- tissue adds Tissue Factor which also allows blood to clot  extrinsic pathway of blood
coagulation
- blood coagulation involves sequential proteolytic cleavage of zymogens that are proteases,
themselves; zymogens in uncleaved form  low proteolytic activity; zymogens activated by
proteolysis (remove inhibition)  cascade produces an amplified response
- coagulation cascade starts with activation of a protease which hydrolyzes and activates
subsequent proteases  so on and so on
- blood clotting proteins called factors (Roman numerals, table on 21-3)
- no factor VI; factor IV is calcium (not a protein)
- seven of blood coagulation factors are serine proteases in activated forms: kallikrein,
thrombin, factor VIIa, factor IXa, factor Xa, factor XIa and factor XIIa
- four factors are receptor cofactors to enhance or facilitate specific proteases: Tissue Factor,
factor Va, factor VIIIa, and high molecular weight kininogen (HMWK)
- Tissue Factor and HMWK activated by wounding and actually initiate clotting cascades
- Prothrombin, factor VII, factor IX, and factor X converted to activatable forms via
vitamin K-dependent processes
Intrinsic pathway of blood coagulation
- initiated when a negatively charged subendothelial surface (collagen) is exposed (wound)
- at this site  complex forms between docking molecule, high molecular weight kininogen
(HMWK), and protease zymogens, factor XII, and prekallikrein; factor XII undergoes
conformational change  partially active to cleave prekallikrein  active kallikrein
- reciprocal activation reaction  active kallikrein proteolitically cleaves factor XII  factor
XIIa; reciprocal reactions amplify activation sequence
- factor XIIa hydrolyzes HMWK  release bradykinin (vasodilator); this step initiated in vivo
by exposure of blood to negatively charged subendothelial collagen; net result is amplified
production of Factor XIIa
- Factor XIIa begins zymogen activation cascade by proteolitically cleaving factor XI 
factor XIa which cleaves factor IX  factor IXa
- factor IXa with factor VIIIa, Ca2+, and phospholipid surface (platelets) cleaves factor X 
factor Xa; thrombin cleaves factor VIII  factor VIIIa; common pathway joined
- factor Xa, with cofactor/receptor Va (anchor) converts prothrombin  thrombin;
thrombin cleaves factor V  factor Va
- thrombin (serine protease) converts fibrinogen  fibrin monomer with crosslinks into a clot
Extrinsic pathway of blood coagulation
- initiated by release of Tissue Factor (TF) by injured cells in response to injury; TF is a
glycoprotein present in membranes of most mesenchymal cells; usually widely available to
trigger coagulation
- released TF forms a complex with factor VIIa
- factor VII  factor VIIa conversion is unclear, although it is activatable by factor Xa and
thrombin
- major reaction catalyzed by factor VIIa-TF complex is conversion of factor X  factor Xa;
complex also interacts with intrinsic pathway by converting factor IX  factor IXa
- TF is analogous to factor V in that it is a receptor that anchors an active serine protease; TF
anchors factor VIIa in proper conformation to cleave factor X  factor Xa; this condition only
met when tissues have been damaged and platelet surfaces available
- for full activity  TF/VIIa complex requires binding of trace amount of factor Xa
- extrinsic system caused more rapid, less extensive clot formation than intrinsic pathway
Common pathway
- intrinsic and extrinsic pathway converge at activation of factor X  factor Xa; now common
pathway continues
- factor Xa, with cofactor/receptor Va (anchor) converts prothrombin  thrombin;
thrombin cleaves factor V  factor Va
- thrombin (serine protease) converts fibrinogen  fibrin monomer  fibrin polymer with
crosslinks into a clot
- thrombin cleaves factor XIII  factor XIIIa which assembles fibrin polymer  clot
3. Vitamin K-dependent reactions in blood coagulation
- proteolytic activation of factors VII, IX, X and prothrombin all require:
1. presence of Ca2+
2. phospholipid surface (platelet)
3. a protease
- substrates for these reactions a pre-activated in a vitamin K dependent manner, which
increases negative charge density  higher affinity for platelet cell surface
- proteases that cleave factor X and prothrombin are localized to the platelet cells surface via
cofactor receptor proteins; these are: TF (for factor VIIa), factor VIIIa (for factor IXa), and
factor Va (for factor Xa)
- Vitamin K is a lipophilic vitamin required for the post-translational carboxylation of
glutamic acid (glu) residues to form gamma-carboxyglutamic acid (gla) in prothrombin and
factors VII, IX, and X
- instead of one –CO2- functional group in glu, gla residues have two –CO2- functional groups
 higher affinity for calcium
- vitamin K-dependent glutamate carboxylation of these proteins results in multiple gla
residues in close apposition
- factor X: platelet membrane also serves as an anchor for the receptor VIIIa, which positions
protease IXa in the proper orientation to cleave the tethered factor X  factor Xa (directing
site of cleavage); similar reaction during cleavage of prothrombin  thrombin releases high
concentration of active thrombin, localized precisely at wound site to promote healing without
generalized intravascular clotting
- Vitamin K required for glutamate carboxylase activity; the carboxylation reaction results in
incorporation of one atom of oxygen into vitamin Kred  Vitamin K epoxide; vitamin K
epoxide  dietary form of vitamin K (vitamin Kox) by an epoxide reductase that is inhibited
by coumarins; vitamin Kox  vitamin Kred via vitamin K reductase
- coumarins are vitamin K analogs that competitively inhibit Vitamin K-dependent enzymes;
coumarin therapy (coumadin, warfarin) used to retard clot formation in individuals susceptible
to thrombosis (heart disease/stroke); must be monitored because coumarins inhibit synthesis of
all vitamin K-dependent proteins in blood coagulation cascade (susceptible to hemorrhage)
- vitamin K deficiency in only breast-fed babies is a concern if vitamin K not administered
because:
1. infants born with low vitamin K stores
2. neonatal liver is immature with respect to prothrombin synthesis
3. breast milk is low in vitamin K
Formation of Fibrin Clot
- fibrin formed from soluble plasma protein, fibrinogen, through a proteolytic cleavage by
thrombin
- fibrinogen contains three pairs of polypeptide chains; amino terminal regions of these peptides
have a high content of negatively charged amino acids that contribute to solubility of
fibrinogen in plasma and help prevent aggregation by causing electrostatic repulsion between
fibrinogen molecules
- thrombin cleaves these peptides on fibrinogen to expose binding sites on adjacent monomers
 allows for spontaneous aggregation of fibrin monomer in staggered array  forms
insoluble clot
- insoluble fibrin polymer traps platelets, RBCs, and other components to form initial soft
fibrin clot
- thrombin next activated factor XIII  factor XIIIa (transglutamidase)  stabilizes fibrin
clot by covalently cross-linking fibrin molecules via interpeptide bond
- blood consists of RBCs, WBCs, and platelets suspended in plasma; cell trapped in clot, leaving
a liquid called serum; serum lacks clotting factors present in plasma
Importance of intrinsic and extrinsic pathways
- deficiencies in factors VIIa, IXa, Xa, VIIIa, thrombin and Va result in clinical disease 
essential for proper hemostasis
- four factors: factor XIIa, prekallikrein, HMWK, and XIa considered unimportant;
deficiencies do not cause clinical disease, but do have long clotting times
- factor IX  factor IXa by factor VIIa:TF complex (physiologically important ancillary
pathway)
- large amount of factor VII circulates in activated factor VIIa state; factor VIIa most likely
activates X indirectly through IX (directly) because of sever clinical consequences of VIII
(Hemophilia A)or IX deficiency (Hemophilia B)
- majority of clotting events initiated by release of TF