Practice question

DNA ligases catalyze phosphodiester bond formation via an enzyme-AMP intermediate requiring adenylation. The source of AMP differentiates ligase families. Bacteriophage T4 ligase and most eukaryotic ligases are ATP-dependent, cleaving ATP to AMP and pyrophosphate to activate the active-site lysine. In contrast, Escherichia coli DNA ligase and several other bacterial ligases are NAD+-dependent, using oxidized nicotinamide adenine dinucleotide as adenyl donor and releasing NMN. Both classes require divalent magnesium for catalysis. Recognizing cofactor specificity is essential for choosing buffers and understanding bacterial DNA replication, repair, and cloning enzymology.