Studies of the solution structure of the bleomycin-A2-zinc complex by means of two-dimensional NMR spectroscopy and distance geometry calculations.

Application of various two-dimensional NMR techniques (SECSY, COSY and NOESY) enabled the complete assignment of the 1H-NMR spectrum of the bleomycin-A2-zinc complex in H2O and D2O at pH 6.7. The spectra were interpreted at 277 K as well as at 300 K. Identification of the resonances permitted a vicinal coupling constant analysis which revealed that the conformation around the C alpha and C beta bond of the beta-aminoalanine and beta-hydroxyhistidine residues is fixed. From this finding it was concluded that both amino functions of the beta-aminoalanine fragment and the amide and imidazole groups of the beta-hydroxyhistidine moiety are involved in zinc coordination. Also, for the mannose carbamoyl group and the pyrimidine ring active participation in zinc coordination could be established. NOE data together with the six coordination sites proposed above were used as interpoint distance constraints in distance geometry calculations for the bleomycin-A2-zinc complex in H2O. Sets of ten structures, randomly chosen within the distance constraints, were calculated (with and without the zinc ion). The calculated structures were very similar but in case that the zinc ion was omitted some flexibility was observed, within the distance constraints, in the pyrimidine-aminoalanine region. Because of the great overall similarity between the structures, a reliable representation of the solution conformation of the bleomycin-zinc complex was reached. Surprisingly, no regular symmetry around the zinc ion was found to be present in the generated structures.