Yy

Binds to active site (Gly)4 Binds to fibrinogen-binding region of thrombin region of thrombin

(exosite 1)

Bivalirudin binds reversibly to thrombin

FIGURE 1 The structure of bivalirudin. (Top) Bivalirudin is comprised of 20 amino acids, with an N-(amino-)terminal d-Phe-Pro-Arg-Pro (F-P-R-P) region that binds with high affinity to the active site region of thrombin; a (gly)4 (G4) "spacer" region; and a C-(carboxy-)terminal Asn-Gly-Asp-Phe-Glu-Glu-Ile-Pro-Glu-Glu-Tyr-Leu dodecapeptide (N-G-D-F-E-E-I-P-E-E-Y-L) that binds to the fibri-nogen-binding region (exosite 1) of thrombin. The 11 C-terminal amino acids (shaded circles) correspond exactly to the 53- to 64-amino-acid sequence of lepirudin. Highly-specific, non-competitive binding between bivalirudin and thrombin results. (Not shown is the heparin-binding region [exosite 2] of thrombin.) However, proteases (including other thrombin molecules [not shown]) can cleave the Arg3-Pro4 of bivalirudin, leading to loss of antithrombin activity. (Bottom) Initially, there is bivalent binding of bivalirudin to thrombin, as shown. Following cleavage at Arg3-Pro4, the N-terminal sequence of bivalirudin no longer binds to thrombin, leaving the residual C-terminal dodecapeptide with greatly reduced binding affinity for exosite 1 of thrombin. Thus, the bivalirudin remnant transforms to a competitive inhibitor of thrombin. Other substrates, e.g., fibrinogen, can compete with, and displace, bivalirudin, thus allowing thrombin to resume its prohemostatic functions. Abbreviations: Arg (R), arginine; Asn (N), asparagine; Asp (D), aspartic acid; Glu (E), glutamic acid; Gly (G), glycine; Ile (I), isoleucine; Leu (L), leucine; Phe (F), phenylalanine; Pro (P), proline; Tyr (Y), tyrosine.

dual blockage with complete inhibition of thrombin's multiple activities (Sciulli and Mauro, 2002). Once the amino-terminal moiety of bivalirudin is cleaved, however, the carboxy-terminal region acquires low-affinity, weakly competitive binding properties. Fibrinogen can now displace the bivalirudin remnant from thrombin and align itself over the active site to be converted to fibrin (Parry et al., 1994).

Bivalirudin is not inactivated by platelet factor 4 (PF4), nor does it require any cofactor for its activity. It does not bind to red blood cells or proteins other than thrombin.

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