Escherichia coli endonuclease III (EC 4.2.99.18) [1] is a DNA repair enzyme that acts both as a DNA N-glycosylase, removing oxidised pyrimidines from DNA, and as an apurinic/apyrimidinic (AP) endonuclease, introducing a single-strand nick at the site ...
Escherichia coli endonuclease III (EC 4.2.99.18) [1] is a DNA repair enzyme that acts both as a DNA N-glycosylase, removing oxidised pyrimidines from DNA, and as an apurinic/apyrimidinic (AP) endonuclease, introducing a single-strand nick at the site from which the damaged base was removed. Endonuclease III is an iron-sulfur protein that binds a single 4Fe-4S cluster. The 4Fe-4S cluster does not seem to be important for catalytic activity, but is probably involved in the proper positioning of the enzyme along the DNA strand [2]. The 4Fe-4S cluster is bound by four cysteines which are all located in a 17 amino acid region at the C-terminal end of endonuclease III. A similar region is also present in the central section of mutY and in the C-terminus of ORF-10 and of the Micro-coccus UV endonuclease [4].
HhH-GPD superfamily base excision DNA repair protein
This family contains a diverse range of structurally related DNA repair proteins. The superfamily is called the HhH-GPD family after its hallmark Helix-hairpin-helix and Gly/Pro rich loop followed by a conserved aspartate [2]. This includes endonucle ...
This family contains a diverse range of structurally related DNA repair proteins. The superfamily is called the HhH-GPD family after its hallmark Helix-hairpin-helix and Gly/Pro rich loop followed by a conserved aspartate [2]. This includes endonuclease III, EC:4.2.99.18 and MutY an A/G-specific adenine glycosylase, both have a C terminal 4Fe-4S cluster. The family also includes 8-oxoguanine DNA glycosylases such as Swiss:P53397. The methyl-CPG binding protein MBD4 Swiss:Q9Z2D7 also contains a related domain [1] that is a thymine DNA glycosylase. The family also includes DNA-3-methyladenine glycosylase II EC:3.2.2.21 and other members of the AlkA family.
Oxidative DNA damage is caused by reactive oxygen species that are generated during aerobic respiration and immune response. The oxidative lesions in DNA caused are repaired by the base excision repair pathway. Hydroxyl radicals rapidly react with guanine C8 producing a steady state level of mutagenic 8-oxoguanines in cells. It also creates 8-oxo-GTP that, like 8-oxoguanines in DNA template, is often mispaired with adenine by replicative polymerases, creating A-T to C-G and G-C to T-A transversion mutations.
DNA glycosylase MutY recognises the mutational intermediate 8-oxoguanine-adenine mispair and excises adenine from the mispair. This creates an abasic site that is then processed by the multi-enzyme base excision repair pathway. The sequence of MutY is conserved from bacteria to humans suggesting its fundamental importance in DNA repair. Mutations of MutY increases transversion frequencies and hence cancer susceptibility in human. MutY belongs to the base excision repair glycosylases superfamily which includes both pure DNA glycosylases and DNA glycosylase/lyases. The most conserved structural element in this family is the Helix-hairpin-Helix (HhH) motif. A revised MutY reaction mechanism was published in 2016, which involves a covalent intermediate and two oxocarbenium ion-like transition states. There is sufficient structural, mutagenesis and biochemical date to support the proposed mechanism described.
