Ligaasit

Ligaasit are enzymes that catalyze ligation, the joining of two substrates by forming a covalent bond. They typically require an energy source, such as ATP or NAD+, to activate one substrate and drive bond formation. By enabling the formation of phosphodiester, peptide, or other covalent linkages, ligases play essential roles in cells and in a range of biotechnological processes.

Major families include DNA ligases, RNA ligases, and protein ligases. In cells, DNA ligases seal nicks in the sugar-phosphate backbone during replication and repair; bacteria use NAD+-dependent DNA ligases, whereas eukaryotes predominantly rely on ATP-dependent ligases (for example, DNA ligase I, III, and IV in humans). In molecular biology, T4 DNA ligase (ATP-dependent) and related enzymes are used to join DNA fragments in cloning. RNA ligases participate in RNA processing and splicing, and are used in certain sequencing and library preparation workflows. Protein ligases include ubiquitin ligases (E3s) that attach ubiquitin to substrate proteins, marking them for degradation or altering activity, as well as other ligases that join peptide or protein segments in post-translational modifications.

Mechanistically, most ligases operate via an activated intermediate: the enzyme forms a covalent enzyme-adenylate, which is transferred to the substrate to create an activated intermediate that then reacts to form the final covalent bond, releasing a leaving group such as AMP or NMN. The exact donor and byproducts vary with the ligase type.