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

Bleomycin A(2) (BLMA(2), a clinically used drug) and tallysomycin A (T LMA) an two closely related anticancer antibiotics activated by Oz rea ction with their Fe(II) complexes. Fe(II) can be modeled by Zn(II). Ev idence obtained that the disaccharide and metal-binding domains of ZnT LMA and ZnBLMA(2) are superimposable includes the following very simil ar NMR features: the H-1 and C-13 NMR chemical shifts, the H-1 and C-1 3 chemical shift changes upon Zn(II) binding, and the NOESY spectra. W e evaluated several ZnTLMA structural models with four and five Ligati ng donor atoms from TLMA by using 2D NMR, NOESY back-calculation metho ds, and restrained molecular mechanics/molecular dynamics calculations . Our results are most consistent with ligation by five N donors, the beta-aminoalanine (ALA) amines (NC2 and NC3), the pyrimidinylpropionam ide (PRO) pyrimidine (NC10), and the beta-hydroxyhistidine amide (NC12 ) and imidazole (NC29). Metal complexation to TLMA or BLMA(2) creates newly stable chiral centers (the metal and the ALA secondary amine, NC 3); 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 disacc haride mannose H is clearly present in the low mixing lime NOESY spect rum of ZnTLMA and in the published spectrum of ZnBLMA(2). This cross-p eak has led us to discover a novel square pyramid (sp) basket arrangem ent of the drug donor atoms, with PRO NC10 at the apex and SS chiralit y. A close variant, with donors adopting a trigonal bipyramidal (tbp) arrangement, gave results almost as satisfactory. Our findings raise i nteresting aspects relevant to drug activation. The literature suggest s that the activated form is HO2Fe(III)BLMA(2); the five N donors are in ail SS-sp I arrangement, with the AEA primary amine (NC2) at the ap ex. if the Fe(LI) form of the drugs had the SS-sp basket or SS-tbp arr angement, addition of O-2 could yield products with the drug in an SS- sp I arrangement. Models with RR chirality, such as proposed previousl y for ZnBLMA(3). are energetically unfavorable, cannot account for the NMR results, and cannot readily convert to the SS-sp I geometry. Unli ke 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.