Hemostasis

• Constriction of vessel• Aggregation of platelets• Clotting: web of fibrin polymers

Platelet Activation

• Exposed extracellular matrix at injury binds receptors on platelets

• Granule release activates additional platelets

Aggregation Of Platelets

• Activated platelets adhere strongly to ECM and to each other • Von Willebrandt factor associates platelets with ECM• Fibrinogen cross-links platelets using GPIIb/IIIa receptors• GPIIb/IIIa inhibitors as drugs

Signaling Using Prostaglandins And Thromboxanes

• Cyclooxygenase converts arachidonic acid to PGG2

• PGG2 converted to:– Thromboxane A2 (platelets); promotes platelet aggregation– PGI2 (endothelial cells); inhibits platelet aggregation

Membrane Phospholipids

Arachidonic Acid

Phospholipase A2

PGG2; PGH2

Cyclooxygenase

TxA2

Thromboxanesynthase

Promote platelet aggregation and vasoconstriction

PlateletsProstacyclinsynthase

Adenylate cyclase

PGI2 released(+)

Inhibit platelet aggregation and vasoconstriction

(-)

EndothelialCells

Inhibiting Platelet Aggregation With Aspirin

• Aspirin is cyclooxygenase inhibitor• Inhibits platelet aggregation by

lowering thromboxane A2 levels

Formation Of Fibrin Network

• Conversion of soluble fibrinogen to insoluble fibrin fibers

Structure And Cleavage Of Fibrinogen

• Fibrinogen molecule has 2,2,2 arrangement

• Thrombin cleavage of N-terminal peptides results in fibrin monomer

Formation Of Fibrin Fibers

• Monomers associate as half-staggered arrays• Forms “soft clot”

Cross-linking Of Fibrin Monomers

• Isopeptide bonds form between side chains at C-termini• Forms “hard clot”

(factor XIII)

Pathways For Stimulating Fibrinogen Cleavage

• Cascade of proteases that are activated by cleavage• Two pathways activated by different stimuli feed

into common pathway

Intrinsic Pathway

• Everything needed for pathway contained in blood• Stimulated by contact with negatively charged surface

Extrinsic Pathway

• Stimulated by tissue factor (thromboplastin) that is normally buried below endothelium

Common Pathway

• Activated factor X cleaves cleaves prothrombin into thrombin• Thrombin cleaves fibrinogen

Structure And Cleavage Of Prothrombin

• Two factor X cleavage sites• Thrombin composed of peptides A and B• N terminal region released

Role Of Vitamin K In Clotting

• Carboxylation of glutamic acid into Gla required for some clotting factors

• Vitamin K is cofactor for modifying enzyme

• Dicoumarol and warfarin inhibit clotting through vitamin K cycle

Function Of Gla Residues In Prothrombin Cleavage

• Cleavage requires association with phospholipid membrane through Ca2+ bridges

• Gla strongly attracts Ca2+

Blood Clotting Disorders

• Hemophilia A: inherited deficiency of factor VIII• Von Willebrandt disease: inherited deficiency of vWf

Controlling Blood Clotting

• Proteolysis of Factors V and VIII by activated protein c-protein s complex

• Inherited disorders protein c, protein s, Factor V

Controlling Blood Clotting

Antithrombin Blood clotting proteases

Heparin• Heparin increases

activity of antithrombin

Plasminogen Plasmin

Plasminogenactivator

Fibrinclot

Peptides

• Plasminogen activator protease produces active plasmin