Proton magnetic resonance study of kringle 1 from human plasminogen. Insights into the domain structure

The aromatic H NMR spectrum of the kringle 1 domain from human plasminogen has been investigated by proton Overhauser experiments, acid-base titration, and two-dimensional chemical shift correlated spectroscopy. Spin-echo and pH response experiments lead to the identification of the N-terminal Tyr-3 phenol ring signals. The connectivities among the tryptophanyl aromatic protons have been established and sets of singlet-doublet-triplet resonances stemming from each of the two indole groups sorted according to their common side chain origin. Similarly, the four histidyl singlets have been identified and paired per imidazole group. From their pH responses, it is indicated that a histidyl (His31) and a tryptophanyl (Trp-II) residue are placed in the neighborhood of carboxyl groups. The high-field chemical shifts observed for proton resonances of the ligand epsilon-aminocaproic acid upon binding to kringle 1 indicate that the ligand-binding site is rich in aromatic components. Overhauser experiments reveal that Leu46 is surrounded by a cluster of interacting aromatic side chains, which includes Trp25, Phe36, His41, Trp62, and Tyr64, and define a hydrophobic region contiguous to the kringle lysine-binding site. Relative internuclear distances have been estimated for aromatic H-atoms in the vicinity of Leu46 by reference to one of the latter's CH3 sigma, sigma' groups. Some of the connectives have previously been found for Leu46 in kringle 4 which further supports the idea of a common structure for the homologous domains.