Proton magnetic resonance study of lysine-binding to the kringle 4 domain of human plasminogen: The structure of the binding site

The binding of l-Lys, d-Lys and ε-aminocaproic acid (εACA) to the kringle 4 domain of human plasminogen has been investigated via one and two-dimensional 1H-nuclear magnetic resonance spectroscopy at 300 and 600 MHz. Ligand-kringle association constants ( K a) were determined assuming single site binding. At 295 K, pH ∗ 7.2, d-Lys binds to kringle 4 much more weakly (K a = 1.2m m −1) than does l-Lys (K a = 24.4 m m −1). l-Lys binding to kringle 4 causes the appearance of ring current-shifted high-field resonances within the −1 ≲ δ ≲ 0 parts per million range. The ligand origin of these signals has been confirmed by examining the spectra of kringle 4 titrated with deuterated l-Lys. A systematic analysis of ligand-induced shifts on the aromatic resonances of kringle 4 has been carried out on the basis of 300 MHz two-dimensional chemical shift correlated (COSY) and double quantum correlated spectroscopies. Significant differences in the effect of l-Lys and d-Lys binding to kringle 4 have been observed in the aromatic COSY spectrum. In particular, the His31 H4 and Trp72 H2 singlets and the Phe64 multiplets appear to be the most sensitive to the particular enantiomers, indicating that these residues are in proximity to the ligand C α center. In contrast, the rest of the indole spectrum of Trp72 and the aromatic resonances of Trp62 and Tyr74, which are affected by ligand presence, are insensitive to the optical nature of the ligand isomer. These results, together with two-dimensional proton Overhauser studies and ligand-kringle saturation transfer experiments reported previously, enabled us to generate a model of the kringle 4 ligand-binding site from the crystallographic co-ordinates of the prothrombin kringle 1. The latter, although lacking recognizable lysine-binding capability, is otherwise structurally homologous to the plasminogen kringles.