A New Arrangement for the Anticancer Antibiotics Tallysomycin and Bleomycin When Bound to Zinc: An Assessment of Metal and Ligand Chirality by NMR and Molecular Dynamics

Abstract

Bleomycin A2 (BLMA2, a clinically used drug) and tallysomycin A (TLMA) are two closely related anticancer antibiotics activated by O2 reaction with their Fe(II) complexes. Fe(II) can be modeled by Zn(II). Evidence obtained that the disaccharide and metal-binding domains of ZnTLMA and ZnBLMA2 are superimposable includes the following very similar NMR features: the 1H and 13C NMR chemical shifts, the 1H and 13C chemical shift changes upon Zn(II) binding, and the NOESY spectra. We evaluated several ZnTLMA structural models with four and five ligating donor atoms from TLMA by using 2D NMR, NOESY back-calculation methods, and restrained molecular mechanics/molecular dynamics calculations. Our results are most consistent with ligation by five N donors, the β-aminoalanine (ALA) amines (NC2 and NC3), the pyrimidinylpropionamide (PRO) pyrimidine (NC10), and the β-hydroxyhistidine amide (NC12) and imidazole (NC29). Metal complexation to TLMA or BLMA2 creates newly stable chiral centers (the metal and the ALA secondary amine, NC3); for the first time, an extensive analysis of the chirality of both centers has been performed. A cross-peak between a PRO H and a disaccharide mannose H is clearly present in the low mixing time NOESY spectrum of ZnTLMA and in the published spectrum of ZnBLMA2. This cross-peak has led us to discover a novel square pyramid (sp) basket arrangement of the drug donor atoms, with PRO NC10 at the apex and SS chirality. A close variant, with donors adopting a trigonal bipyramidal (tbp) arrangement, gave results almost as satisfactory. Our findings raise interesting aspects relevant to drug activation. The literature suggests that the activated form is HO2Fe(III)BLMA2; the five N donors are in an SS-sp I arrangement, with the ALA primary amine (NC2) at the apex. If the Fe(II) form of the drugs had the SS-sp basket or SS-tbp arrangement, addition of O2 could yield products with the drug in an SS-sp I arrangement. Models with RR chirality, such as proposed previously for ZnBLMA2, are energetically unfavorable, cannot account for the NMR results, and cannot readily convert to the SS-sp I geometry. Unlike in RR models, the carbamoyl group of the mannose cannot bind to the metal in SS models. Instead, in our model the disaccharide covers the sixth binding site.