ligase

Ligases belong to EC class 6 and catalyze the formation of new chemical bonds (C-C, C-N, C-O, C-S) between two molecules, coupled with the hydrolysis of a high-energy phosphate bond (usually ATP). The most clinically relevant example for boards is DNA ligase, which seals the phosphodiester backbone by joining a 3'-OH end to a 5'-phosphate end.

Key roles of DNA ligase: - Lagging strand synthesis: joins Okazaki fragments after RNA primer removal and gap filling by DNA polymerase I (in prokaryotes) or FEN1/Pol δ (eukaryotes) - DNA repair: essential final step in base excision repair (BER), nucleotide excision repair (NER), and mismatch repair (MMR) - Recombinant DNA technology: T4 DNA ligase is used in molecular cloning to join DNA fragments with complementary sticky or blunt ends

Eukaryotic DNA ligase I is the primary ligase for replication and long-patch BER; DNA ligase III works with XRCC1 in short-patch BER and single-strand break repair; DNA ligase IV is critical for non-homologous end joining (NHEJ) of double-strand breaks, working with XRCC4.

Clinical significance: Mutations in DNA ligase IV cause LIG4 syndrome, a rare autosomal recessive disorder resembling a combined immunodeficiency with microcephaly, growth retardation, and radiosensitivity, due to impaired NHEJ affecting V(D)J recombination in lymphocytes.

Other ligases include RNA ligase (repairs RNA breaks, important in tRNA splicing) and ubiquitin ligases (E3 ligases), which are crucial for tagging proteins for proteasomal degradation and are dysregulated in various cancers.

High-yield connection: Understanding DNA ligase mechanism helps distinguish it from DNA polymerase (which extends strands by adding nucleotides) and helicase (which unwinds DNA) - together these enzymes are frequently tested in the context of DNA replication and repair pathways.