The binding of zinc(II) to bleomycin: an investigation using 1H NMR spectroscopy

Contributions of NMR to the understanding of the coordination chemistry and DNA interactions of metallo-bleomycins

Bleomycins are a family of glycopeptide antibiotics that have the ability to bind and degrade DNA when bound to key metal ions, which is believed to be responsible for their antitumor activity. Knowledge of the structures of metallo-bleomycins is vital to further characterize their mechanism of action. To this end, numerous structural studies on metallo-bleomycins have been conducted. NMR spectroscopy has had a key role in most of these studies, and has led to very important findings involving the coordination chemistry of metallo-bleomycins, and the details of many metallo-bleomycin-DNA spatial correlations for this important drug. This paper reviews the most important contributions of NMR to the bleomycin field.

Copper(I)-bleomycin: structurally unique complex that mediates oxidative DNA strand scission

Copper(I)-bleomycin [Cu(I) X BLM] was characterized in detail by 13C and 1H NMR. Unequivocal chemical shift assignments for Cu(I) X BLM and Cu(I) X BLM X CO were made by two-dimensional 1H-13C correlated spectroscopy and by utilizing the observation that Cu(I) X BLM was in rapid equilibrium with Cu(I) and metal-free bleomycin, such that individual resonances in the spectra of BLM and Cu(I) X BLM could be correlated. The binding of Cu(I) by bleomycin involves the beta-aminoalaninamide and pyrimidinyl moieties, and possibly the imidazole, but not N alpha of beta-hydroxyhistidine. Although no DNA strand scission by Cu(II) X BLM could be demonstrated in the absence of dithiothreitol, in the presence of this reducing agent substantial degradation of [3H]DNA was observed, as was strand scission of cccDNA. DNA degradation by Cu(I) X BLM was shown not to depend on contaminating Fe(II) and not to result in the formation of thymine propenal; the probable reason(s) for the lack of observed DNA degradation in earlier studies employing Cu(II) X BLM and dithiothreitol was (were) also identified. DNA strand scission was also noted under anaerobic conditions when Cu(II) X BLM and iodosobenzene were employed. If it is assumed that the mechanism of DNA degradation in this case is the same as that under aerobic conditions (i.e., with Cu(I) X BLM + O2 in the presence of dithiothreitol), then Cu X BLM must be capable of functioning as a monooxygenase in its degradation of DNA.

Distance geometry analysis of the N.M.R. evidence on the solution conformation of bleomycin

Conformational constraints derived from n.m.r. experiments, X-ray data and the known stereochemistry have been used to investigate by the distance geometry method the range of allowed solution conformations for Cu(II):P-3A (a biosynthetic precursor of bleomycin), Fe(II):bleomycin:carbon monoxide, and Zn(II):bleomycin. The experimental data have been found to be self-consistent and lead to the following observations. 1) Designation of the ligands and the dihedral angles available from vicinal coupling constants are not sufficient to define uniquely the geometry around the metal. 2) When only five bleomycin ligands are invoked (e.g. Cu(II):P-3A or Fe(II):bleomycin:carbon monoxide) there is considerable freedom in the allowed coordination scheme around the metal, but some regions of the molecule have well determined conformation. 3) Introduction of a sixth bleomycin ligand, as in Zn(II):bleomycin, considerably constrains the conformational freedom of the groups coordinated to the zinc. The utility of the distance geometry approach for analysis of data and design of experiments is discussed.

Carbon-13 N.M.R. study of bleomycin-A2 protonation

The acid-base titration of bleomycin-A2 in D2O solution at 35 +/- 5 degrees has been monitored by 13C n.m.r. spectroscopy at 67.89 MHz. The following pKDa values were obtained: 3.68 +/- 0.05 (secondary amine), 5.29 +/- 0.03 (imidazole), and 8.23 +/- 0.19 (primary amine), where KDa is the dissociation constant in D2O solution. The equilibrium isotope effects (pKDa--pKa in H2O) are: 0.70 +/- 0.06 (secondary amine), 0.28 +/- 0.04 (imidazole), and 0.85 +/- 0.19 (primary amine). Titration of the imidazole group of Bleo-A2 occurs at Npi, i.e. only Ntau is protonated in basic solution. Significant protonation shifts are almost completely limited to carbons of the N-terminal tetrapeptide, suggesting that the C-terminal tripeptide extends into the solvent and interacts to a minimal extent with the rest of the molecule. Long range protonation shifts associated with titration of the imidazole and secondary amine groups indicate that protonation of one or both of these sites is probably accompanied by significant conformational changes. The observed protonation shifts generally fail to correlate with Zn(II) complexation shifts reported by Dabrowiak et al. (1973, Biochemistry 17., 4090) indicating that ligation sites cannot unambiguously be determined from these complexation shifts. The complexation shifts previously attributed to coordination of the imidazole and carbamoyl groups probably result from conformational changes.