X-Ray Analysis of a Thrombin Inhibitor-Trypsin Complex1

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Abstract
The three-dimensional structure of a thrombin inhibitor-trypsin complex has been determined by an X-ray analysis at 2.5 Å resolution. The result has given experimental support to the mechanisms previously proposed by the authors for the selective inhibition of trypsin, thrombin, factor Xa, and plasmin by inhibitors with an arginine or lysine backbone. The differences in the amino acid sequences at the positions corresponding to Ile63, Leu99, and Ser190 of trypsin give each enzyme different binding affinities toward inhibitors and result in the selective inhibition. Furthermore, the X-ray analysis has revealed a novel type of interaction between the inhibitor and trypsin. The hydrogen bonds between the inhibitor main chain and trypsin G1y216 play an essential role in the complex formation.