Exonuclease III

Exonuclease III is a highly important enzyme that plays a crucial role in DNA repair and degradation processes. It is a type of exonuclease that specifically acts on the 3' end of DNA molecules. In this article, we will explore the structure, function, and significance of Exonuclease III in cellular processes.

Structure

Exonuclease III is a protein enzyme that is encoded by the xthA gene in Escherichia coli (E. coli). It consists of a single polypeptide chain with a molecular weight of approximately 30 kDa. The enzyme is composed of two distinct domains: the N-terminal domain and the C-terminal domain. The N-terminal domain contains the catalytic site responsible for the exonuclease activity, while the C-terminal domain is involved in DNA binding.

Function

Exonuclease III primarily functions in DNA repair mechanisms, specifically in the removal of damaged or mismatched nucleotides from the 3' end of DNA strands. It is involved in the base excision repair pathway, which is responsible for repairing DNA damage caused by oxidative stress, alkylating agents, and other mutagens. The enzyme recognizes and removes damaged nucleotides, creating a gap in the DNA strand that can be subsequently repaired by DNA polymerases and ligases.

Mechanism of Action

Exonuclease III exhibits a processive exonuclease activity, meaning it can continuously degrade DNA molecules by sequentially removing nucleotides from the 3' end. The enzyme hydrolyzes the phosphodiester bond between the last nucleotide and the adjacent nucleotide, releasing a mononucleotide and exposing the next nucleotide for further degradation. This process continues until the enzyme encounters a DNA lesion or a mismatched nucleotide, which it can efficiently remove.

Significance

Exonuclease III plays a vital role in maintaining the integrity of the genome by repairing damaged DNA. Its ability to remove damaged nucleotides ensures the fidelity of DNA replication and prevents the accumulation of mutations. Additionally, Exonuclease III is involved in the processing of DNA intermediates during recombination and DNA repair pathways, contributing to the overall stability of the genome.

References

1. DNA repair 2. Base excision repair 3. Escherichia coli 4. DNA polymerase 5. DNA ligase

See Also

Exonuclease DNA repair enzymes Nucleotide excision repair DNA damage response