THE THROMBIN E192Q-BPTI COMPLEX REVEALS GROSS STRUCTURAL REARRANGEMENTS - IMPLICATIONS FOR THE INTERACTION WITH ANTITHROMBIN AND THROMBOMODULIN

Previous crystal structures of thrombin indicate that the 60-insertion loop is a rigid moiety that partially occludes the active site, sugge sting that this structural feature plays a decisive role in restrictin g thrombin's specificity. This restricted specificity is typified by t he experimental observation that thrombin is not inhibited by micromol ar concentrations of basic pancreatic trypsin inhibitor (BPTI). Surpri singly, a single atom mutation in thrombin (E192Q) results in a 10(-8) M affinity for BPTI. The crystal structure of human thrombin mutant E 192Q has been solved in complex with BPTI at 2.3 (A) over circle resol ution, Binding of the Kunitz inhibitor is accompanied by gross structu ral rearrangements in thrombin, In particular, thrombin's 60-loop is f ound in a significantly different conformation, Concomitant reorganiza tion of other surface loops that surround the active site, i,e. the 37 -loop, the 148-loop and the 99-loop, is observed, Thrombin can therefo re undergo major structural reorganization upon strong ligand binding, Implications for the interaction of thrombin with antithrombin and th rombomodulin are discussed.