Scheme 11-2

Because the kinetics is ping pong, the adenylyltransfer steps can be isolated and their rates measured, and because each step requires Mg2+, the rate of phosphodiester bond formation can be measured as well. The rates of all of the steps can be compared with the overall rate of strand joining. Some of the results for two bacterial DNA ligases are summarized in table 11-1; one ligase is NAD-dependent and the other is ATP dependent. The rate constants are typical of those for bacteriophage T4, chlorella virus, and archaeal DNA ligases (Hall and Lehman, 1969; Ho et al., 1997; Nakatani et al., 2002). Those for the NAD-dependent ligase are much higher in the presence of ammonium ion. As a rule in enzymology, potassium and ammonium ions are essentially interchangeable in activating enzymes that depend on monovalent cations. The ligase activities measured in vitro are sufficient to meet cellular needs for DNA replication and repair (Lehman, 1974).

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