W0121

Structural Impact of the Leukemia Drug Ara-C on the Covalent Human Topoisomerase I DNA Complex. Jill Chrencik, Univ. of North Carolina at Chapel Hill, B-5 Venable Hall, Dept. of Chemistry, Chapel Hill, NC 27599.

1-B-D-Arabinofuranosylcytosine (Ara-C) is a potent antineoplastic drug used in the treatment of acute leukemia. Previous biochemical studies indicated the incorporation of Ara-C into DNA reduced the catalytic activity of human topoisomerase I by decreasing the rate of single DNA strand religation by the enzyme 2- to 3-fold. We present the 3.1 Å crystal structure of human topoisomerase I in covalent complex with an oligonucleotide containing Ara-C at the +1 position of the non-scissile DNA strand. The structure reveals that a hydrogen bond formed between the 2-hydroxyl of Ara-C and the O4 of the adjacent -1 base 5 to the damage site stabilizes a C3-endo pucker in the Ara-C arabinose ring. The structural distortions at the site of damage are translated across the DNA double-helix to the active site of human topoisomerase I. The free sulfhydryl at the 5-end of the nicked DNA strand in this trapped covalent complex is shifted out of alignment with the 3 - phosphotyrosine linkage at the catalytic tyrosine-723 residue, producing a geometry not optimal for religation. The subtle structural changes caused by the presence of Ara-C in the DNA duplex may contribute to the cytotoxicity of this leukemia drug by prolonging the lifetime of the covalent human topoisomerase I-DNA complex.