The addition of a new deoxynucleotide to a growing DNA chain requires more energy than can be obtained by the hydrolysis of ATP to ADP + Pi
What alternative series of reactions is used, and how does this help overcome the energy barrier for DNA synthesis?
The hydrolysis of ATP to ADP is favorable, with a ?G between –11 and –13 kcal/mole. However, this is not sufficient to drive the addition of a nucleotide to the end of a growing DNA strand. Instead, two reactions are used. The first reaction converts ATP to a DNA-linked AMP residue when a phosphodiester bond is formed during DNA synthesis; simultaneously, a pyrophosphate molecule (PPi) is released. In the second reaction, the PPi is hydrolyzed to form two molecules of Pi. This second reaction is also favorable, providing roughly another –13 kcal/mole. Adding up the ?G for the entire process, there will be about –26 kcal/mole to drive the addition of the nucleotide to the growing DNA chain, which is sufficient to drive the reaction strongly in one direction.
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