Regions of the human plasminogen (Pg) cDNA containing its kringle 4 (K4) and K5 domains have been expressed in Escherichia coli, and binding constants of omega-amino acid ligands for recombinant (r)-[K4Pg] and r-[K5Pg] have been obtained. In each case, the results showed that of a series of aliphatic alpha, omega-amino acid analogues, 6-aminohexanoic acid showed maximal affinity for these modules, and all ligands interacted more strongly with r-[K4Pg] than with r-[K5Pg]. Site-directed mutagenesis investigations demonstrated that the major amino acid side chain contributors to ligand binding were similar for each of these kringles. Ligand binding was stabilized by charged groups at Asp56 of r-[K4Pg] and Asp57 of r-[K5Pg] as well as by Arg69 of both r-[K4Pg] and r-[K5Pg]. Some hydrophobic amino acids that contributed significantly to the binding strength of the ligands were identified. These were provided by homologous residues in each of the domains, viz. Trp60 and Trp70 of r-[K4Pg] and Trp62 and Tyr72 of r-[K5Pg]. Tyr74 of r-[K5Pg] also substantially contributed to its ligand binding energy.
