Adenosine triphosphate, ATP, is used as a free-energy source by biological cells. (See the essay on page 624.) ATP hydrolyzes in the presence of enzymes to give ADP:
Consider a hypothetical biochemical reaction of molecule A to give molecule B:
Calculate the ratio [B]/[A] at 25°C at equilibrium.
Now consider this reaction “coupled” to the reaction for the hydrolysis of ATP:
If a cell maintains a high ratio of ATP to ADP and H2PO4− by continuously making ATP, the conversion of A to B can be made highly spontaneous. A characteristic value of this ratio is
Calculate the ratio [B][A] in this case and compare it with the uncoupled reaction. Compared with the uncoupled reaction, how much larger is this ratio when coupled to the hydrolysis of ATP?
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