helicaseexonuclease

Helicaseexonuclease is a descriptive term for enzymes or protein complexes that combine helicase activity with exonuclease activity. Helicases use ATP hydrolysis to unwind double-stranded nucleic acids, while exonucleases cleave nucleotides from the ends of nucleic acids. In many organisms these two activities are contained within a single polypeptide that carries both functional domains, whereas in others they are performed by separate subunits within a larger complex.

Mechanism and organization: The helicase domain translocates on DNA and separates the strands, powered by ATP hydrolysis. The exonuclease domain degrades one strand from an end, providing processed DNA suitable for repair or recombination. The two activities are typically coordinated so that unwinding exposes a substrate for nuclease cleavage, shaping the DNA ends for downstream pathways such as homologous recombination or replication fork processing. Directionality and substrate preference vary among systems, with some enzymes acting on 5' ends and others on 3' ends.

Biological roles: Helicaseexonucleases participate in DNA damage response, including processing double-strand breaks and stalled replication forks, resection to enable homologous recombination, and Okazaki fragment processing during lagging-strand synthesis. They may also contribute to telomere maintenance and to the resolution of difficult DNA structures such as R-loops.

Examples: In bacteria, the RecBCD complex contains a DNA-unwinding helicase with nuclease activity, notably RecB, which contributes to end processing. In eukaryotes, DNA2 is a well-characterized enzyme with both nuclease activity and a helicase domain, cooperating with other factors during Okazaki fragment processing and DSB resection. Other helicase-nuclease combinations occur across different taxa as part of repair and maintenance pathways.