The synthesis of a phosphodiester bond involves condensation of a5′- phosphoryl group on one nucleotide with a 3′ hydroxyl group onan adjacent nucleotide. DNA Ligases catalyze this reaction bycoupling condensation to cleavage of a phosphoanhydride bond fromeither NAD+ or ATP.

The overall reaction catalyzed by T4 DNA ligase is seen inthis equation: ATP + nicked bond -> AMP + PPi +phosphodiester bond

This is actually the sum of two half reactions. The sum of a hydrolysis reaction coupled to a condensation reaction is thermodynamically equivalent to the ligase reaction represented by the overall reaction (above).

What are the two half reactions?