AP Endonucleases

AP Endonucleases

Apurinic/apyrimidinic (AP) endonuclease (BRENDA = 4.2.99.18) is an enzyme that is involved in the DNA base excision repair pathway (BER). Its main role in the repair of damaged or mismatched nucleotides in DNA is to create a nick in the phosphodiester backbone of the AP site created when DNA glycosylase removes the damaged base. There are four types of AP endonucleases that have been classified according to their sites of incision. Class I and class II AP endonucleases incise DNA at the phosphate groups 3´ and 5´ to the baseless site leaving 3´-OH and 5´-phosphate termini. Class III and class IV AP endonucleases also cleave DNA at the phosphate groups 3´ and 5´to the baseless site, but they generate a 3´-phosphate and a 5´-OH. Human AP Endonuclease (APE1), like most AP endonucleases, is of class II and requires an Mg2+ in its active site in order to carry out its role in base excision repair.