3. Chemical structure Prothrombin, the natural substrate of FXa, is cleaved by FXa at two positions, each proceeded by the same four amino acid sequence. FXa activity can be determined by the chromogenic substrate S-2222 which is composed of the same amino acids coupled to a chromophore
26. Antithrombin monitoring chromogenic activity assays Chromogenic heparin cofactor activity assays: - The sample is incubated with heparin and an excess amount of thrombin or FXa. The residual thrombin or FXa then cleaves a chromogenic substrate AT + Heparin [AT*Heparin] [AT*Heparin] + FXa (excess) [AT-FXa- Heparin] + FXa(residual) Chromogenic substrate FXa(residual) Peptide + pNA
27. Antithrombin: anti-FXa assay Sample/Standard dilution: 25 l sample+ 3000 l saline Procedure Volumes Diluted sample/standard 50 l Factor Xa (2.9 nkat/ml in Hep Buffer) 50 l Incubate at 37°C 90 sec S-2765 (0.8 mg/ml) 50 l Read A/min at 405 nm for rate method or add 50 l Acetic acid after 90 sec incubation for end-point method.
36. Protein C in the coagulation system Extrinsic pathway Intrinsic pathway TF FVIIa FIXa FVIIIa PL, Ca2+ FXa FX FVa PL, Ca2+ Prothrombin Thrombin Fibrinogen Fibrin Protein C APC Thrombo-modulin Protein S FV
37. Protein C - an anticoagulant Vitamin K dependent proteins: Anticoagulants: protein C and protein S Procoagulants: Factor II, VII, IX and X Synthesised in the liver Glu Gla (glutamic acid residues are converted to gamma-carboxyglutamic acid) The Gla domain binds calcium ions which form a bridge to the phospholipid surfaces on platelets and endothelial cells.
39. Protein C- the function Protein C inhibits coagulation through inactivation of FVIIIa and FVa Protein C potentiates the fibrinolytic system by inhibiting PAI-1, the major inhibitor of fibrinolysis.
40. Protein C activation Protein C is activated (to APC) by thrombin. A small peptide is removed. Activation by thrombin alone is slow. The complex thrombin-thrombomodulin activates protein C 20 000 times faster. PC APC TM
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44. Coamatic Protein C - the principle Protein C APC Protac S-2366 Peptide + pNA APC
48. Substrate selectivity The Chromogenix catalogue includes a section which shows the cross-reactivity of the substrates with the different enzymes tested
49. Kinetic data The Chromogenix catalogue includes a section which shows the kinetic data of the substrates toward the different enzymes tested
Where do we find protein C in the coagulation system? Protein C is an anticoagulant which when activated cleaves FVIIIa and FVa. To do this the two cofactors protein S and FV are needed. Protien C is activiated by thrombin in a complex with thrombomodulin.
Protein C belongs to a class of proteins wich are dependent on vitamin K to become functional. In this class of proteins there are two proteins with anticoagulant properties: protein C and protein S. Also proteins with procoagulant properties belong to this class, namely thrombin, FVII, FIX and FX. All the vitamin K dependent proteins are syntesized in the liver. Vitamin K participates in a reaction in which glutamin acid residues in these proteins are -carboxylated to -carboxyglutamic acid. The region containing the -carboxyglutamic acid residues then binds calcium ions which form a bridge to phospholipids on platelets and endothelial cells. So these proteins need to be bound to phospholipid surfaces to be functional.
So protein C consists of three different regions: -the Gla-domain which interacts with the negatively charged phospholipids in the presence of calcium ions - the EGF-domain which interacts with protein S - the protease domain which contains the active site which cleaves FVIIIa and FVa
The function of protein C is to inhibit coagulation through inactivation of FVIIIa and FVa. The result of this inactivation is decreased thrombin generation. Protein C does not cleave the non-activated forms of FVIII and FV. Protein C also potentiates the fibrinolytic system by inhibiting PAI-1 the major inhibitor of fibrinolysis.
Protein C is a zymogen that is converted to an active serine protease, activated protein C (APC), by thrombin. When activated a small peptide is removed. Thrombin alone can activate protein C but this activation is slow. When thrombin is bound to a protein called thrombomodulin the activiation is 20 000 times faster.
Thrombomodulin is a membrane-protein that is present on the endothelium. The highest concnetration of TM is found in the capillaries, because of the high ratio of endothelial surface to volume of circulating blood. Thrombin that enters the microcirculation will therefore quickly be bound to thrombomodulin and start to activate protein C. TM has three different effects on thrombin: 1. It increases the rate by which thrombin activates protein C 2. It removes the procoagulant properties of thrombin. When bound to TM thrombin does not act on fibrinogen to produce fibrin. 3. TM aslo accelerates the thrombin-antithrombin reaction. So thrombomodulin has got its name becuase it modulates the activity of thrombin from a procoagulant to a anticoagulant.
APC has two known cofactors: protein S and Factor V. Protein S functions as a cofactor for APC by helping APC to bind to platelets or endothelial surfaces via calcium ion bridges. However, protein S in only available for APC when protein S is free of the complement cofactor C4BP to which it is bound while in the circulation. Approximately 60% of protein S in plasma is bound to C4BP. That Factor V is a cofactor to APC was recently discovered. It has been shown that APC degrades FVIII and FVa more effectively when FV is present compared to if only APC and protein S are present.
The functional protein C assay is either a chromogenic assay or a clotting assay. The chromogenic assay is performed in two steps. In the first step the specific protein C activator, Protac, is used to activate the protein C in the plasma. Protac is a serine protease purified from the venom of a snake. It activates protein C without interfering with other coagulation factors. In the next step the activated protein C cleaves a chromogenic substrate.
Coamtic Protein C is performed in two steps In the first step protein C is activated by Protac. The activated protein C then cleaves the chromogenic substrate S-2366.