Analysis of ligand-binding to the kringle 4 fragment from human plasminogen

The interaction of the isolated human plasminogen kringle 4 with the four -amino acid ligands -aminocaproic acid (ACA), N-acetyl-l-lysine (AcLys), trans-aminomethyl(cyclohexane)carboxylic acid (AMCHA) and p-benzylaminesulfonic acid (BASA) has been further characterized by ¹H-NMR spectroscopy at 300 and 600 MHz. Pronounced high-field shifts, reaching 3 ppm, are observed for AMCHA resonances upon binding to kringle 4, which underscores the relevance of ligand lipophilic interactions with aromatic side chains at the binding site. Ligand titration curves for the nine His and Trp singlets found in the kringle 4 aromatic spectrum reveal a striking uniformity in the kringle response to the various ligands. The average binding curves exhibit a clear Langmuir absorption isotherm saturation profile and the data were analyzed under the assumption of one (high affinity) binding site per kringle. Equilibrium association constants (K_a) and first order dissociation rate constants (k_off) were derived from linearized expressions of the Langmuir isotherm and of the spectral line-shapes, respectively. The results for the four ligands, at 295 K, pH^* 7.2, indicate that: (a) AMCHA exhibits the strongest binding (K_a=159 mM^-1) and ACA the weakest (K_a=21 mM^–1) with AcLys and BASA falling in between; (b) ACA dissociates readily (k_off = 5.3 × 10³ s^–1) and AMCHA associates the fastest (k_off = 2.0 × 10⁸M^–1 s^–1) while the kinetics for BASA exchange is relatively slow (k_off = 0.8 × 10³ s^–1, k_on = 0.6 × 10⁸M^–1s^–1); (c) the ligand-binding kinetics is close to diffussion-controlled.Abbreviations ACA -aminocaproic acid - AcLys N-acetyl-l-lysine - AMCHA t-aminomethyl(cyclohexane)carboxylic acid - BASA p-benzylaminesulfonic acid - K4 kringle 4 - NOE nuclear Overhauser effect - ppm parts-per-million - pH^* glass electrode pH reading uncorrected for deuterium isotope effects - K_a ligand-kringle 4 equilibrium association constant - k_off ligand-kringle 4 dissociation rate constant - k_on ligand-kringle 4 association rate constant