Structure of the DNA repair enzyme endonuclease IV and its DNA complex: Double-nucleotide flipping at abasic sites and three-metal-ion catalysis

Endonuclease IV is the archetype for a conserved apurinic/apyrimidinic (AP) endonuclease family that primes DNA repair synthesis by cleaving the DNA b ackbone 5' of AP sites. The crystal structures of Endonuclease IV and its A P-DNA complex at 1.02 and 1.55 Angstrom resolution reveal how an alpha(8)be ta(8) TIM barrel fold can bind dsDNA. Enzyme loops intercalate side chains at the abasic site, compress the DNA backbone, bend the DNA similar to 90 d egrees, and promote double-nucleotide flipping to sequester the extrahelica l AP site in an enzyme pocket that excludes undamaged nucleotides. These st ructures suggest three Zn2+ ions directly participate in phosphodiester bon d cleavage and prompt hypotheses that double-nucleotide flipping and sharp bending by AP endonucleases provide exquisite damage specificity while aidi ng subsequent base excision repair pathway progression.