Chapt. 45
Ch. 45 Blood Plasma proteins,
Coagulation and Fibrinolysis
Student Learning Outcomes:
• Describe basic components of plasma
• Explain the formation of a blot clot (thrombus) – the
cascade necessary for hemostasis and thrombosis:
• the role of platelets, many factors, enzymes
• Explain the effects of genetic defects of factors
• Describe the regulatory mechanisms for the
coagulation cascade, and antifibrinolysis
Inheritance of
X-linked gene
for Factor VIII
hemophilia A
Pedigree of hemophilia transmission
among Queen Victoria's descendants
Hartl & Jones, Genetics
Plasma components
1. Plasma proteins maintain proper distribution of
water between blood and tissues
• Plasma contains water, nutrients, metabolites,
hormones, electrolytes, proteins
• Same electrolyte composition as extracellular fluids
• Many disease states alter plasma proteins –
observe by electrophoresis
• Osmotic pressure or plasma proteins is critical for
bringing water back into blood from tissues
Albumin and other proteins
Albumin is major serum protein:
• 69-kDa glycoprotein, 60% of total plasma protein
• Carries free fatty acids, zinc, Ca2+, steroid
hormones, many drugs
Table 1 other serum proteins made in liver:
Ceruloplasmin
binds copper, storage
Corticosteroid-binding globulin
binds cortisol
Haptoglobulin
binds extracellular heme
Lipoproteins
bind cholesterol and fatty acids
Retinol-binding protein
binds vitamin A
Sex hormone-binding globulin
binds estradiol, testosterone
Transferrin
transports iron
II. Other plasma proteins
Immune response involves plasma proteins:
• Immunoglobulins (antibodies) made by B
lymphocytes – bind specific antigens
• Complement proteins aid in clearance of Ag-Ab;
also bind bacterial cell wall and lyse through C3b
• Activation of complement is a proteolytic cascade
Protease inhibitors in plasma
control inflammatory response
III. Plasma proteins maintain integrity of circulatory system
Platelets are critical for clotting:
If damage to endothelial lining of blood
vessels exposes subendothelial layer:
(Extracell matrix, smooth muscle cells)
• Barrier (hemostatic plug, clot) forms
• Platelets initiate, bind damage area
• Clot formation is limited by regulation
• Vessel heals, clot is degraded
Platelets from megakaryocyte (~8 nuclei)
→ 4000 non-nucleated platelets
Adhesion of platelets to subendothelium
Platelets adhere to site of blood vessel injury:
•
Exposes collagen, vWF and other components
(vWF (von Willebrand factor) is glycoprotein, is also in
plasma, binds factor VIII to protect it; disorder → bleeding)
• Then other GPs (glycoproteins) bind to fibrinogen, vWF
• Platelet activation (release granules), brings more platelets
(ADP is attractant); platelets release vasoconstrictors
Fig. 1
Fibrinogen, fibrin
Fibrinogen is cleaved to
fibrin to form ‘soft clot’
• Fibrinogen in blood and in platelet
granules
• Assists platelet aggregation
(of activated platelets)
• Cleavage by thrombin protease
releases fibrin monomers which
aggregate
• Thrombin also binds platelets
through receptor, activates them
Fig. 2
B. Blood coagulation cascade
Thrombus (clot) is enhanced by thrombin activation
• Proteins in plasma are
proproteins (zymogens)
• Activated by proteolytic
cleavages, sequential
• Highly regulated
• Proenzymes:
serine proteases:
VII, XI, IX, X, prothrombin
• Cofactor proteins bind:
Tissue factor, V, VIII
Fig. 3 PL, Ca are cofactors
bound to phospholipids on
platelet, vessel wall in Ca2+
complex
Blood coagulation process
Blood coagulation is complex cascade
• Platelets adhere, plasma proteins react to subendothelium
• Common path at Xa
• Platelet membrane is
scaffold for formation
• g-carboxylated clotting
proteins chelate to
membrane surface
(- PL, Ca2+)
• Thrombin is critical:
• Positive feedback
• Accelerate process
• Cleaves XIII → XIIIa
Fig. 3 positive
feedback by
thrombin
Table 45.2 Proteins of blood coagulation
Factor
descriptive name
function/active form
Coagulation factors
I
II
III
IV
V
VII
VIII
IX
X
XI
XIII
Fibrinogen
Fibrin
Prothrombin
Serine protease
Tissue factor
Receptor, cofactor
Ca2+
Cofactor
Proaccelerin, labile factor
Cofactor
Proconvertin
Serine protease
Antihemophilia factor A
Cofactor
Antihemophilia factor B
Serine protease
Stuart-Prower factor
Serine protease
Plasma thromboplastin antecedent
Serine protease
Fibrin-stabilizing factor
Ca2+-dep transglutaminase
Regulatory factors
Thrombomodulin
endothelial cell receptor, binds thrombin
Protein C
activated by thrombomodulin-bound thrombin; serine protease
Protein S
cofactor; binds activated protein C
Crosslink fibrin monomers
1. Fibrin molecules cross link to form hard clot:
•
•
•
•
Thrombin forms Factor XIIIa
Factor XIIIa catalyzes transamidation reaction (Gln-Lys)
3D mesh of fibrin fibers traps platelets, cells
Factor XIIIa is only enzyme that is not a serine protease
Fig. 4
g-carboxyglutamate in clotting factors
Vitamin K is critical for clotting:
• Coenzyme for g-carboxyglutamate
• Modified residues on clotting proteases, done in liver
• Proteases bind platelets through the Ca2+ and PLAssembly of factors, enzymes on
complexes speeds up cascade
Warfarin, rat poison, inhibits
carboxylase, resembles VitK
Fig. 5
Regulation of blood clotting
Thrombin has prothrombotic role: (Fig. 3)
• Feedback amplification
• Stimulates its own formation by activating factors V, VII, XI
• Activates platelet aggregation, release factor VIII from vWF,
Thrombin has antithrombotic role:
• Binds thrombomodulin, abolishes function of thrombin
• protein C and cofactor protein S form activated protein C (APC)
• Destroy factors Va, VIIIa
Fig. 6
Serpins
Serpins: Serine protease inhibitors:
•
•
•
•
•
Natural products ~10% of plasma proteins
Each has reactive site like substrate, binds enzyme
Antithrombin III (AT III) inactivates Thrombin
Heparin (glycosaminoglycan) binds ATIII, allosteric enhance
ATIII-heparin also inactivates XIIIa, Xa, IXa, Xa proteases
ATT III + heparin
Plasmin
Plasmin degrades clot – highly regulated process:
•
•
•
•
•
Plasmin is serine protease, from plasminogen
APC releases tPA from tissues (plasminogen activator)
APC inactivates inhibitor of t-PA (PAI-I)
Scu-PA is single-chain urokinase, can degrade fibrin
Streptokinase binds plasminogen, allosteric modifier so that
plasmin is formed
• Circulating a-antiplasmin
blocks soluble plasmin in blood
T-PA and streptokinase treat
Myocardial infarction
Fig. 7
Key concepts
Key concepts:
• Plasma contains water, nutrients, hormones,
electrolytes, proteins
• Plasma proteins provide osmotic pressure for fluid
balance
• Plasma proteins and platelets maintain integrity of
circulatory system
• Clot formation is carefully regulated process to
prevent overclotting (thrombosis) or underclotting
(bleeding)
• Hemophilia A is lack of Factor VIII
• heparin, warfarin inhibit clotting; tPA dissolves
Review question
Review question:
3. A recent surgery patient receiving warfarin therapy was
found to be bleeding internally. The clotting process is
impaired in this patient because of which of the following?
A. Inability of the liver to synthesize clotting factors
B. Specific inhibition of factor XIII activation
C. Inability to form clotting factor complexes on membranes.
D. Reduction of plasma calcium levels
E. Enhancement of protein C activity