Many of the endergonic reactions in a cell are driven by being coupled to the reaction in which ATP is hydrolyzed to ADP and Pi
ATP → ADP + Pi
for which ΔG'o is -30.5 kJ/mol.
The amount of work that can be accomplished by ATP hydrolysis under cellular conditions (the free energy change for ATP hydrolysis, ΔGp), can be calculated with this living equation. Let's explore the effects of the concentrations of ATP, ADP, and Pi on ΔGp. We read in Box 13-2 that in red blood cells, the relevant concentrations are: ATP, 2.25 mM; ADP, 0.25 mM; and Pi, 1.65 mM. Substitute these values into the equation below and calculate ΔGp. Now, see what happens when a cell has used up 90% of its ATP, converting it to ADP and Pi. What is ΔGp now? What if the cell had used 99% of its ATP? Would ATP still be useful as an energy source for endergonic processes under these conditions? What would happen to ΔGp if [ATP] and [ADP] were held at the levels given above, but [Pi] became 10 higher?
If the reaction ATP → ADP + Pi were allowed to go to equilibrium within a cell, what would ΔGp be?
Original material from Lehninger Principles of Biochemistry, 5a edición, de D. Nelson and M. Cox; 2009. ISBN: 0-7167-7108-X.
Dr. José Antonio Encinar. (IBMC-UMH)