ATP-dependent polynucleotide ligases catalyze phosphodiester bond formation using nicked nucleic acid substrates with the high energy nucleotide of ATP as a cofactor in a three step reaction mechanism. DNA ligases play a vital role in the diverse processes of DNA replication, recombination and repair. ATP-dependent ligases are present in many organisms such as viruses, bacteriophages, eukarya, archaea and bacteria. There are three classes of ATP-dependent DNA ligase in eukaryotic cells (I, III and IV). DNA ligase IV is required for DNA non-homologous end joining pathways, including recombination of the V(D)J immunoglobulin gene segments in cells of the mammalian immune system. DNA ligase IV is stabilized by forming a complex with XRCC4, a nuclear phosphoprotein, which is phosphorylated by DNA-dependent protein kinase. DNA ligases have a highly modular architecture consisting of a unique arrangement of two or more discrete domains. The adenylation and C-terminal oligonucleotide/oligosaccharide binding (OB)-fold domains comprise a catalytic core unit that is common to all members of the ATP-dependent DNA ligase family. The adenylation domain binds ATP and contains many of the active-site residues. The common catalytic unit comprises six conserved sequence motifs (I, III, IIIa, IV, V and VI) that define this family of related nucleotidyltransferases.
Comment:based on similarity to other members of the family
Comment:Sealing a nick in DNA requires three catalytic steps: first, enzyme adenylation is accomplished using ATP resulting in an AMP-linkage to the enzyme; second, the AMP is transferred to the 5' phosphate at the site of a nick; and third, phosphoryl transfer and sealing the nick DNA.
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