Complexation with antithrombin

Answer B. Heparin forms aril complex with antithrombin III and enhances its activity from 100- to 1,000-fold. The peak effect of heparin is not reached for several hours, and continued use over several days has no effect on thrombin levels. Prostacyclin (PGI2) is a platelet inhibitor, and its levels are not affected by heparin,... 

Small amounts of thrombin are formed all the time but are normally inactivated by formation of a complex with antithrombin III in the presence of heparin. 

Both drugs have an indirect action on the clotting cascade—heparin must form a complex with antithrombin III in order to produce an effect, and warfarin acts through depletion of vitamin K, which affects specific clotting factors. 

CAS 60800-63-7 EINECS/ELINCS 232-681-7 Synonyms Heparin ammonium salt Uses Anticoagulant laboratory reagent forms complexes with antithrombin stimulates cell division of cultured mammalian cells Manuf/Distrib. CarboMer http //www.carbomer.com] Fluka http //www.sigma-aidrich.com] Sigma Trade Name Synonyms SPL Heparin Ammonium f[Scientific Protein Labs http //www.spi-pharma. com]... 

Fig. 5 Structure of dodecasaccharide studied as complex with antithrombin protein.

Four naturally occurring thrombin inhibitors exist in normal plasma. The most important is antithrombin III (often called simply antithrombin), which contributes approximately 75% of the antithrombin activity. Antithrombin III can also inhibit the activities of factors IXa, Xa, XIa, Xlla, and Vila complexed with tissue factor. a2-Macroglobulin contributes most of the remainder of the antithrombin activity, with heparin cofactor II and aj-antitrypsin acting as minor inhibitors under physiologic conditions. 

Complexation of antithrombin with thrombin (and probably also with other proteinases) occurs by formation of a covalent bond which, by subsequent splitting, produces unaltered proteinase and irreversibly modified antithrombin.405,406 The strength of heparin-antithrombin complexes, as determined by u.v.-difference,405 fluorescence,406,407 af-... 

Although the order of affinity of PF-4 for different glycosaminogly-cans, and dissociation of their complexes with salts, are typical of nonspecific, electrostatic interactions, PF-4 is not strictly a cationic protein.452 It is probable that heparin binds to clusters of basic amino acids (two lysine pairs) near the carboxyl terminal of a polypeptide chain that has an overall preponderance of acidic amino acid residues.457 High-molecular-weight heparin species can bind two PF-4 molecules, with formation of complexes 10 to 100 times as strong as those with antithrombin.217... 

Like antithrombin, heparin cofactor II inhibits proteases by forming a I I stoichiometric complex with the enzyme. The protease attacks the reactive site of heparin cofactor II located on the C-terminus, resulting in the formation of a covalent bond. Heparin cofactor II has higher protease specificity than antithrombin. Of the coagulation enzymes, heparin cofactor II is known only to inhibit thrombin (92). Additionally heparin cofactor II has been shown to inhibit chymotrypsin (93) and leukocyte cathepsin G (94), This protease specificity appears to be due to the active site bond present in heparin cofactor II. Whereas antithrombin contains an Arg-Ser bond as its active site, heparin cofactor II is unique in containing a Leu-Ser bond. This suggests than another portion of the heparin cofactor II molecular may be required for protease binding,... 

UFH binds to exosite 2, located on antithrombin, forming a ternary complex. This ternary complex is necessary for the inhibition of thrombin by antithrombin (Fig. I A, left). Conversely to thrombin inhibition, inactivation of factor Xa does not require the formation of the ternary complex. UFH inhibits thrombin and factor Xa in the same proportion (the ratio of anti-Xa/lla activity equals I) (Fig, I A, right). The interaction of the heparins (UFH and LMWH) with antithrombin is mediated by a unique pentasaccharide sequence, which is present in approximately one-third of the UFH chains (2). 

Fondaparinux is a chemically synthesized pentasaccharide that mimics the antithrombin-binding site of heparin and LMWH. Its molecular size (1728Da) is too small to bind to thrombin molecules while it is bound to antithrombin, Therefore, it is a pure anti-Xa inhibitor. It binds very little to platelets, proteins, or endothelium and is excreted in the urine, It does not form a complex with PF4 or other positively charged molecules. It is not neutralizable by protamine sulfate, Recent clinical trials have resulted in FDA approval for prophylaxis of deep vein thrombosis in orthopedic surgery, It has been shown to be effective and safe for the treatment of pulmonary embolism (20,21) and ACS (non-ST-elevation Ml) (OASIS 5—Michelangelo Trial) (17). 

The complex consisting of heparin and antithrombin III neutralizes the protease activity of activated factors unlike unfrac-tionated heparin, low-molecular-weight heparin fragments or the minimal molecular subunit of heparin, fortdaparinux, inhibit only activated factor Xa when com-plexed with antithrombin III. 

A discerning inhibitor. Antithrombin III forms an irreversible complex with thrombin but not with prothrombin. What is the most likely reason for this difference in reactivity ... 

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