The aromatic 1H-NMR spectrum of plasminogen kringle 4. A comparative study of human, porcine and bovine homologs.

The isolated kringle 4 domain of human plasminogen has been compared with homologous structures from bovine and porcine sources, both free and in the presence of the ligand 6-aminohexanoic acid, by two-dimensional 1H-NMR spectroscopies at 300 MHz and 600 MHz. The chemical-shift-correlated, spin-echo-correlated, and double-quantum-correlated aromatic spectra of the three proteins reveal that the globular conformation of the fourth kringle is closely maintained throughout the set of homologs. Direct comparison shows that the three conserved Trp residues (at sites 25, 62 and 72) which exhibit highly non-degenerate subspectra, find themselves in similar intramolecular environments. In particular, proton Overhauser experiments reveal that the close steric interaction between the Trp-II (Trp62 or Trp25) indole group and the aromatic ring at site 74 (Tyr74 or Phe74) is strictly preserved. This feature forces the kringle inner loop, closed by the Cys51-Cys75 link, to fold back onto itself so as to place the site 74 residue proximal to the Cys22-Cys63 bridge. Single-residue substitutions enable unambiguous assignments of His-I to His3, Tyr-III to Tyr41 and Tyr-IV to Tyr74. From this direct evidence, comparison with the kringle 1 spectrum, and the previously reported chemical modification of Tyr-II (Tyr50) [Trexler M., Bányai L., Patthy L., Pluck N. D. & Williams R. J. P. (1985) Eur. J. Biochem. 152, 439-446], Tyr-I and Tyr-V (the latter, an immobile ring on the 600-MHz time scale) could be assigned to Tyr2 and Tyr9, respectively. Since Trp-III has previously been assigned to Trp72 at the lysine-binding site, the present study completes the assignment of 10 out of 12 aromatic spin systems in the kringle 4 1H-NMR spectrum; the only ambiguity which remains concerns the Trp-I and Trp-II indole spin systems, which are totally identified but as yet only tentatively assigned to Trp25 and Trp62, respectively.