The kringle 4 domain of equine plasminogen (ePgn/K4), a close variant of the human homolog (hPgn/K4), contains residues, such as Trp 32, which also appear in human apolipoprotein(a) kringle 4-type modules. The ePgn/K4 was investigated as a complex with e-aminocaproic acid, an antifibrinolytic drug, by two-dimensional IH-NMR spectroscopy at 500 MHz. Secondary structure elements were recognized from sequential medium and long-range dipolar (proton Overhauser) interactions, as well as from the identification of resonances originating from backbone amide protons with slow IH-ZH exchange in ZHzO. Antiparallel/3-sheets, consisting of strands 52-53, 61-65 and 71-75, were identified. Additionally, the segments 14-16 and 20-22 were found to assume characteristic interstrand antiparallel (0-sheet-like) H-bond pairing. Four type I turns could be identified in strands 6-9, 16-19, 24-27 and 67-70. Ten structures were generated using distance geometry methods, followed by dynamic simulated annealing calculations. The root mean squares deviation of the distances was 2.79 A for all atoms and 1.81 A for backbone atoms only. Hydrogen bridges, involving side chain hydroxyl groups, were identified for Thr 16 and Thr 65. As observed for the hPgn/K4, the three-dimensional structure of the ePgn/K4 is mainly defined by two antiparallel /3-sheets, 14-16/20-22 and 62-66/71-75, which are oriented perpendicular to each other. Adjacent to these is a hydrophobic pocket, formed by Trp 62, Tyr 64, Trp 72 and Phe 74, whose side chains contribute a lipophilic component to the exposed lysine binding site surface. In contrast to the Trp 25, Trp 62 and Trp 72 residues, conserved in the human and equine homologs, the spectrum of the Trp 32 side chain reveals an unrestrained, solvent-exposed indole ring.