Solution Structure and Dynamics of the Plasminogen Kringle 2−AMCHA Complex:  3<sub>1</sub>-Helix in Homologous Domains<sup>†</sup><sup>,</sup><sup>‡</sup>

The kringle 2 (K2) module of human plasminogen (Pgn) binds l-lysine and analogous zwitterionic compounds, such as the antifibrinolytic agent <i>trans</i>-(aminomethyl)cyclohexanecarboxylic acid (AMCHA). Far-UV CD and NMR spectra reveal little conformational change in K2 upon ligand binding. However, retarded <sup>1</sup>H−<sup>2</sup>H isotope exchange kinetics induced by AMCHA indicate stabilization of the K2 conformation by the ligand. Assessment of secondary structure content from CD spectra yields ∼26% <i>β</i>-strand, ∼13% <i>β</i>-turn, ∼15% 3<sub>1</sub>-helix, and ∼6% 3<sub>10</sub>-helix. The NMR solution conformation of the K2 domain complexed to AMCHA has been determined [heavy atom rmsd = 0.49 ± 0.09 Å (backbone) and 1.02 ± 0.08 Å (all)]. The K2 molecule has overall dimensions of ∼34.5 Å × ∼33.4 Å × ∼22.7 Å. Analogous with the polypeptide outline of homologous domains, K2 contains three short antiparallel <i>β</i>-sheets (paired strands 15−16/20−21, 24−25/48−49, and 62−64/72−74) and four defined <i>β</i>-turns (residues 6−9, 16−19, 53−56, and 67−70). Consistent with the CD analysis, albeit novel in the context of kringle folding, the NMR structure reveals an unpaired <i>β</i>-strand structured by residues 30−32, a turn of 3<sub>10</sub>-helix comprising residues 38−41, and a 3<sub>1</sub>-helix for residues 21−24 and 74−79. We also identify alignable 3<sub>1</sub>-helices in previously reported homologous kringle structures. Rather high order parameter <i>S</i><sup>2</sup> values (〈<i>S</i><sup>2</sup>〉 ∼ 0.85 ± 0.04) characterize the K2 backbone dynamics. The lowest flexibility is observed for the two inner loop segments of residues 51−63 and 63−75 (〈<i>S</i><sup>2</sup>〉 ∼ 0.86−0.87 ± 0.03). Overhauser connectivities reveal close hydrophobic contacts of the ligand ring with side chains of Tyr<sup>36</sup>, Trp<sup>62</sup>, Phe<sup>64</sup>, Trp<sup>72</sup>, and Leu<sup>74</sup>. In most K2 structures, the N atom of AMCHA places itself ∼3.9 and ∼4.4 Å from the anionic groups of Glu<sup>57</sup> and Asp<sup>55</sup>, respectively, while its carboxylate group, H-bonded to the Tyr<sup>36</sup> side chain OH<sup>η</sup>, ion-pairs the Arg<sup>71</sup> guanidinium group. Consistent with the preference of K2 for binding 5-aminopentanoic acid over 6-aminohexanoic acid, the positions of the ionic centers within the K2 binding site approach each other ∼1 Å closer relative to what is observed in lysine binding sites of homologous Pgn modules.