AP endonuclease, family 2, zinc binding site <p>Cellular DNA is spontaneously and continuously damaged by environmental andinternal factors such as X-rays, UV light and agents such as the antitumordrugs bleomycin and neocarzinostatin or those that generate oxygen radicals.Apurinic/apyrimidinic (AP) sites can form spontaneously or as highlycytotoxic intermediates in the removal of the damaged base by the baseexcision repair (BER) pathway. DNA repair at the AP sites is initiated byspecific endonuclease cleavage of the phosphodiester backbone. Suchendonucleases are also generally capable of removing blocking groups from the3'terminus of DNA strand breaks.</p><p>AP endonucleases can be classified into two families based on sequence similarity. Family 2 groups the enzymes listed below [<cite idref="PUB00004710"/>, <cite idref="PUB00000730"/>].<ul><li>Bacterial endonuclease IV (<db_xref db="EC" dbkey="3.1.21.2"/>).</li><li> <taxon tax_id="1769">Mycobacterium leprae</taxon> probable endonuclease [<cite idref="PUB00003837"/>].</li><li> <taxon tax_id="4932">Saccharomyces cerevisiae</taxon> (Baker's yeast) apurinic endonuclease APN1 (<db_xref db="EC" dbkey="4.2.99.18"/>).</li><li> <taxon tax_id="6239">Caenorhabditis elegans</taxon> hypothetical protein APN-1 or T05H10.2.</li></ul> </p><p>APN1 and endonuclease IV have been shown to be transition metalloproteins that bind three zinc ions [<cite idref="PUB00002671"/>, <cite idref="PUB00017986"/>]. The metal-binding sites have been determined from the 3D-structure of <taxon tax_id="562">Escherichia coli</taxon> endonuclease IV [<cite idref="PUB00017986"/>, <cite idref="PUB00052615"/>, <cite idref="PUB00048372"/>], which shows an alpha/beta-barrel fold similar to that of other divalent metal-dependent TIM barrel enzymes, such as xylose isomerase. </p>