Structural basis of the thrombin selectivity of a ligand that contains theconstrained arginine mimic (2S)-2-amino-(3S)-3-(1-carbamimidoyl-piperidin-3-yl)-propanoic acid at P1

We have studied the thrombin and trypsin complexed structures of a pair of peptidomimetic thrombin inhibitors, containing different P1 fragments. The first has arginine as its P1 fragment, and the second contains the constrai ned arginine mimic (2S)-2-amino-(3S)-3-(1-carbamimidoyl-piperidin-3-yl)-pro panoic acid (SAPA), a fragment known to enhance thrombin/trypsin selectivit y of inhibitors. On the basis of an analysis of the nonbonded interactions present in the structures of the trypsin and thrombin complexes of the two inhibitors, the calculated accessible surfaces of the enzymes and inhibitor s in the four complexes, data on known structures of trypsin complexes of i nhibitors, and factor Xa inhibitory potency of these compounds, we conclude that the ability of this arginine mimic to increase thrombin selectivity o f an inhibitor is mediated by its differential interaction with the residue at position 192 (chymotrypsinogen numbering). Thrombin has a glutamic acid at residue 192, and trypsin has a glutamine. The analysis also suggests th at this constrained arginine mimic, when present in an inhibitor, might enh ance selectivity against other trypsin-like enzymes that have a glutamine a t residue position 192.