Mammalian Ligases
In mammals, there are four specific types of ligase.
- DNA ligase I: ligates the nascent DNA of the lagging strand after the Ribonuclease H has removed the RNA primer from the Okazaki fragments.
- DNA ligase III: complexes with DNA repair protein XRCC1 to aid in sealing DNA during the process of nucleotide excision repair and recombinant fragments.
- DNA ligase IV: complexes with XRCC4. It catalyzes the final step in the non-homologous end joining DNA double-strand break repair pathway. It is also required for V(D)J recombination, the process that generates diversity in immunoglobulin and T-cell receptor loci during immune system development.
DNA ligase in E. coli uses energy gained by cleaving nicotinamide adenine dinucleotide (NAD) to create the phosphodiester bond. DNA ligase from eukaryotes and some other microbes uses adenosine triphosphate (ATP) rather than NAD. Also, a number of other structures present in the DNA ligase are the AMP and lysine, both of which are important in the ligation process since they create an intermediate enzyme.
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