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are given. Inhibitors designed to make more extended interactions with thrombin are also presented.<br />

II. <strong>Structure</strong> of Thrombin<br />

Page 248<br />

Thrombin consists of two polypeptides, an A chain of 36 residues and a 259-residue B chain, linked <strong>by</strong> a<br />

disulfide bond. The crystallographic structure of thrombin reveals a globular protein organized about<br />

two β barrels with the overall folding pattern of the chymotrypsin serine protease family [6,7]. The<br />

catalytic triad and near<strong>by</strong> oxyanion hole are located roughly between the β barrels and adopt the<br />

geometric arrangement required for serine-protease-assisted, peptide bond cleavage (Figure 1).<br />

Thrombin's multifunctionality and regulation of activity are achieved <strong>by</strong> specialized subsites on the<br />

enzyme's surface (Figure 2). Fibrinogen cleavage, for example, involves interactions at the primary<br />

specificity pocket, the extended fibrinogen recognition exosite, and an additional specificity pocket.<br />

Subsite interactions differ for cleavage of other thrombin substrates including the thrombin receptor and<br />

protein C. Additional and overlapping subsites exist for thrombin effector molecules including heparin,<br />

antithrombin III, and heparin cofactor II [8,9].<br />

Figure 1<br />

Stereoscopic view of the crystallographic structure of thrombin complexed with<br />

N-acetyl-(D-Phe)-Pro-boroArg-OH. Helical regions are represented in the standard<br />

way and arrows indicate regions of β sheet. Solid lines show the thrombin<br />

bound conformation of N-acetyl-(D-Phe)-Pro-boroArg-OH (taken from Reference 10).<br />

Active-site residues, His57 and Ser195, are shown with a ball-and-stick representation.<br />

The authors thank Dr. C. L. Strickland for the drawing.<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_248.html [4/5/2004 5:10:15 PM]

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