Human Ape1-like subfamily of the ExoIII family apurinic/apyrimidinic (AP) endonucleases
This subfamily includes human Ape1 (also known as Apex, Hap1, or Ref-1) 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 example, Ape1 and Ape2 in humans. Ape1 is found in this subfamily, it exhibits strong AP-endonuclease activity but shows weak 3'-5' exonuclease and 3'-phosphodiesterase 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
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