Crystal structures of the recombinant kringle 1 domain of human plasminogen in complexes with the ligands epsilon-aminocaproic acid and trans-4-(aminomethyl)cyclohexane-1-carboxylic acid

The X-ray crystal structures of the complexes of the recombinant kringle 1 domain of human plasminogen (Kl(pg)) with the Ligands epsilon-aminocaproic acid (EACA) and trans-4-(aminomethyl)cyclohexane-1-carboxylic acid (AMCHA), which are representative of a class of in vivo antifibrinolytic agents, have been determined at 2.1 Angstrom resolution. Each Kl(pg)/ligand unit cell contained a dimer of the complexes, and some small differences were noted in the kringle/ligand interatomic distances within the monomeric components of the dimers. The structures obtained allowed predictions to be made of the amino acid residues of Kl(pg) that are likely important to ligand binding. In the crystal structure, the anionic center of Kl(pg) responsible for coordinating the amino group of the ligands is composed of both Asp54 and Asp56, and the cationic center that stabilizes binding of the carboxylate moiety of the ligands is Arg70, with a possible contribution from Arg34. A hydrogen bond between the carboxylate of the ligand to the hydroxyl group of Tyr63 also appears to contribute to the kringle/ligand binding energies. The methylene groups of the ligand are stabilized in the binding pocket by van der Waals contacts with side-chain atoms of Trp61 and Tyr71. These conclusions are in general agreement with site-directed mutagenesis results that implicate many of the same amino acid residues in this binding process, thus showing that the crystal and solution structures are in basic accord with each other. Further comparisons of the X-ray crystal structures of the Kl(pg)/ligand complexes with each other and with apo-K1(pg) show that while small differences in K1(pg) side-chain geometries may exist in the three structures, the binding pocket can be considered to be preformed in the apokringle and not substantially altered by the nature of the w-amino acid ligand that is inserted into the site. From the similar geometries of the binding of EACA and AMCHA, it appears that the k(on) is an important component to the tighter binding of AMCHA to K1(pg), as compared to EACA. Ordered solvation effects of the bound AMCHA may contribute to its longer lifetime on Kl(pg), thereby retarding k(off), both effects thus accounting for the higher binding energy of AMCHA as compared to EACA.