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Apurinic/apyrimidinic (AP) endonuclease 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.
The activity of AP endonuclease in the repair of AP sites in the frontal/parietal cortex, cerebellum, brain stem, midbrain and hypothalamus declines with age in rats on an ad libitum diet. [5] In calorie restricted rats, by comparison, AP endonuclease activity in these brain regions remains higher with age. [5]
AP endonucleases are divided into two families based on their homology to the ancestral bacterial AP endonucleases endonuclease IV and exonuclease III. [6] Many eukaryotes have members of both families, including the yeast Saccharomyces cerevisiae , in which Apn1 is the EndoIV homolog and Apn2 is related to ExoIII.
The AP endonuclease recognizes this sugar and essentially cuts the DNA at this site and then allows for DNA repair to continue. [10] E. coli cells contain two AP endonucleases: endonuclease IV (endoIV) and exonuclease III (exoIII) while in eukaryotes, there is only one AP endonuclease. [11]
All cells, from simple prokaryotes to humans, have evolved systems to identify and repair such sites. Class II AP endonucleases cleave the phosphodiester backbone 5' to the AP site, thereby initiating a process known as base excision repair (BER). The APEX gene (alternatively named APE1, HAP1, APEN) encodes the major AP endonuclease in human cells.
The enzyme DNA-(apurinic or apyrimidinic site) lyase, also referred to as DNA-(apurinic or apyrimidinic site) 5'-phosphomonoester-lyase (systematic name) or DNA AP lyase (EC 4.2.99.18) catalyzes the cleavage of the C-O-P bond 3' from the apurinic or apyrimidinic site in DNA via β-elimination reaction, leaving a 3'-terminal unsaturated sugar and a product with a terminal 5'-phosphate. [1]
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Monofunctional glycosylases have only glycosylase activity, whereas bifunctional glycosylases also possess AP lyase activity that permits them to cut the phosphodiester bond of DNA, creating a single-strand break without the need for an AP endonuclease. β-Elimination of an AP site by a glycosylase-lyase yields a 3' α,β-unsaturated aldehyde ...