3'-exonuclease

3'-Exonuclease

3'-Exonucleases are enzymes that cleave nucleotides one at a time from the 3' end (the end with a free hydroxyl group) of a polynucleotide chain. These enzymes play crucial roles in various biological processes, including DNA replication, repair, and RNA processing.

Function

3'-Exonucleases are involved in the degradation of nucleic acids by removing nucleotides from the 3' end. This activity is essential for the proofreading function of DNA polymerases, which ensures the fidelity of DNA replication. During DNA synthesis, if an incorrect nucleotide is incorporated, the 3'-exonuclease activity of the DNA polymerase can remove the mismatched nucleotide, allowing the polymerase to insert the correct one.

Mechanism

The mechanism of 3'-exonuclease activity involves the hydrolysis of the phosphodiester bond between nucleotides. This process requires the presence of divalent metal ions, such as Mg²⁺ or Mn²⁺, which are essential cofactors for the catalytic activity of the enzyme. The enzyme binds to the 3' end of the nucleic acid and catalyzes the removal of nucleotides in a sequential manner.

Biological Importance

3'-Exonucleases are critical for maintaining genomic stability. They are involved in:

• DNA Repair: By removing damaged or mismatched nucleotides, 3'-exonucleases help repair DNA lesions and prevent mutations.
• RNA Processing: In RNA processing, 3'-exonucleases are involved in the maturation and degradation of RNA molecules, including the trimming of precursor tRNA and rRNA.
• Replication Fidelity: The proofreading function of DNA polymerases, which is dependent on 3'-exonuclease activity, is vital for reducing the error rate during DNA replication.

Examples

Several enzymes exhibit 3'-exonuclease activity, including:

• DNA Polymerase I: In prokaryotes, DNA Polymerase I has a 3'-5' exonuclease activity that is crucial for proofreading during DNA replication.
• Exonuclease III: This enzyme is involved in DNA repair and recombination in bacteria.
• TREX1: A human 3'-exonuclease that degrades single-stranded DNA and is involved in preventing autoimmune diseases.

Also see

• DNA Polymerase
• DNA Repair
• RNA Processing
• Exonuclease