Chapt. 45

Ch. 45 Blood Plasma proteins, Coagulation and FibrinolysisStudent 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 VIIIhemophilia A

Pedigree of hemophilia transmission among Queen Victoria's descendantsHartl & 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 cascadeProtease 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

Fig. 3 positive feedback by thrombin

Blood coagulation is complex cascade • Platelets adhere, plasma proteins react to subendothelium• Common path at Xa • Platelet membrane is scaffold for formation• -carboxylated clotting proteins chelate to membrane surface (- PL, Ca2+)

• Thrombin is critical:• Positive feedback• Accelerate process• Cleaves XIII → XIIIa

Table 45.2 Proteins of blood coagulationFactor descriptive name function/active formCoagulation factorsI Fibrinogen FibrinII Prothrombin Serine proteaseIII Tissue factor Receptor, cofactorIV Ca2+ CofactorV Proaccelerin, labile factor CofactorVII Proconvertin Serine proteaseVIII Antihemophilia factor A CofactorIX Antihemophilia factor B Serine proteaseX Stuart-Prower factor Serine proteaseXI Plasma thromboplastin antecedent Serine proteaseXIII Fibrin-stabilizing factor Ca2+-dep transglutaminase

Regulatory factorsThrombomodulin endothelial cell receptor, binds thrombinProtein C activated by thrombomodulin-bound thrombin; serine proteaseProtein 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

carboxyglutamate in clotting factors

Fig. 5

Vitamin K is critical for clotting: • Coenzyme for -carboxyglutamate• Modified residues on clotting proteases, done in liver• Proteases bind platelets through the Ca2+ and PL-

Assembly of factors, enzymes on complexes speeds up cascade

Warfarin, rat poison, inhibits carboxylase, resembles VitK

Regulation of blood clotting

Fig. 6

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

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 -antiplasmin blocks soluble plasmin in blood

T-PA and streptokinase treatMyocardial 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