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Chapt. 45

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  1. 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

  2. Inheritance of X-linked gene for Factor VIIIhemophilia A Pedigree of hemophilia transmission among Queen Victoria's descendants Hartl & Jones, Genetics

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. Table 45.2 Proteins of blood coagulation • Factor descriptive name function/active form • Coagulation factors • I Fibrinogen Fibrin • II Prothrombin Serine protease • III Tissue factor Receptor, cofactor • IV Ca2+ Cofactor • V Proaccelerin, labile factor Cofactor • VII Proconvertin Serine protease • VIII Antihemophilia factor A Cofactor • IXAntihemophilia factor BSerine protease • X Stuart-Prower factor Serine protease • XI Plasma thromboplastin antecedent Serine protease • XIII 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

  12. 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

  13. 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 PL- • Assembly of factors, enzymes on • complexes speeds up cascade • Warfarin, rat poison, inhibits • carboxylase, resembles VitK Fig. 5

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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? • Inability of the liver to synthesize clotting factors • Specific inhibition of factor XIII activation • Inability to form clotting factor complexes on membranes. • Reduction of plasma calcium levels • Enhancement of protein C activity