Adenosine triphosphate, the second most energy rich of the four currencies, is shown in Figure 12.2. In earlier chapters we met many chemical processes in the cell that are driven by ATP hydrolysis. When one mole of ATP is hydrolyzed, 30 kJ of energy are released. The y phosphate is easily transferred between nucleotides in reactions such as this one:
So as far as energy is concerned, we can regard GTP, CTP, TTP, and UTP as equivalent to the most commonly used nucleotide energy currency, ATP. Another easy, reversible reaction is this one:
in which one ADP transfers its i phosphate to another, so that it itself is left as AMP while converting the other ADP to ATP. This conversion is at equilibrium in cells and is important in maintaining the supply of ATP when ATP is used in reactions.
NADH and ATP take part in so many reactions within the cell that they are often called coenzymes, meaning molecules that act as second substrates for many enzymes as they do
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