Human Ape2-like subfamily of the ExoIII family purinic/apyrimidinic (AP) endonucleases
This subfamily includes human APE2, Saccharomyces cerevisiae Apn2/Eth1, and related proteins. These are Escherichia coli exonuclease III (ExoIII)-like AP endonucleases and they belong to the large EEP (exonuclease/endonuclease/phosphatase) superfamily that contains functionally diverse enzymes that share a common catalytic mechanism of cleaving phosphodiester bonds. AP endonucleases participate in the DNA base excision repair (BER) pathway. AP sites are one of the most common lesions in cellular DNA. During BER, the damaged DNA is first recognized by DNA glycosylase. AP endonucleases then catalyze the hydrolytic cleavage of the phosphodiester bond 5' to the AP site, and this is followed by the coordinated actions of DNA polymerase, deoxyribose phosphatase, and DNA ligase. If left unrepaired, AP sites block DNA replication, and have both mutagenic and cytotoxic effects. AP endonucleases can carry out a variety of excision and incision reactions on DNA, including 3'-5' exonuclease, 3'-deoxyribose phosphodiesterase, 3'-phosphatase, and occasionally, nonspecific DNase activities. Different AP endonuclease enzymes catalyze the different reactions with different efficiences. Many organisms have two AP endonucleases, usually one is the dominant AP endonuclease, the other has weak AP endonuclease activity. For examples, Ape1 and Ape2 in humans, and Apn1 and Apn2 in bakers yeast. Ape2 and Apn2/Eth1 are both found in this subfamily, and have the weaker AP endonuclease activity. Ape2 shows strong 3'-5' exonuclease and 3'-phosphodiesterase activities; it can reduce the mutagenic consequences of attack by reactive oxygen species by removing 3'-end adenine opposite from 8-oxoG, in addition to repairing 3'-damaged termini. Apn2/Eth1 exhibits AP endonuclease activity, but has 30-40 fold more active 3'-phosphodiesterase and 3'-5' exonuclease activities. Class II AP endonucleases have been classified into two families, designated ExoIII and EndoIV, based on their homology to the Escherichia coli enzymes exonuclease III (ExoIII) and endonuclease IV (EndoIV). This subfamily belongs to the ExoIII family; the EndoIV family belongs to a different superfamily.
Comment:based on similarity to ExoIII-like AP endonuclease and other EEP superfamily members; compared with other EEP superfamily members, members of this family have a conserved Tyr residue is in place of a conserved His
Jiyao W et al.(2023). "The conserved domain database in 2023.",