When ATP is hydrolyzed to ADP and Pi under the conditions that normally exist in a cell, the free-energy change is roughly –46 to –54 kJ/mole (–11 to –13 kcal/mole). This extremely favorable ∆G depends on maintaining a high concentration of ATP compared with the concentrations of ADP and Pi. When ATP, ADP, and Pi are all present at the same concentration of 1 mole/liter (so-called standard conditions), the ∆G for ATP hydrolysis drops to the standard free-energy change (∆G°), which is only –30.5 kJ/mole (–7.3 kcal/mole). At much lower concentrations of ATP relative to ADP and Pi, ∆G becomes zero. At this point, the rate at
AT EQUILIBRIUM: synthesis conc. of × conc. of hydrolysis conc. of + synthesis rate constant ×conc. of phosphate ×conc. of ADP synthesis synthesis rate = 2 4 ATPADPPi conc. of conc. of hydrolysis
ADP phosphate rate constant thus, = = equilibrium constant K concentration synthesis of ATP rate constant or abbreviated,
At equilibrium the reaction has no net effect on the disorder of For the reaction the universe, so °G = 0. Therefore, at equilibrium, [ATP]the following equation applies:
But the concentrations of reactants at equilibrium must satisfy the equilibrium equation:
Therefore, at equilibrium, where °G and °Go are in Joules per mole, R is the gas constant (8.3 J/mole K), T is the absolute temperature (K), and all the concentrations are in moles per liter. We thus see that whereas °Go indicates the equilibrium point for a
When the concentrations of all reactants are at 1 M, °G = °Go reaction, °G reveals how far the reaction is from equilibrium. °G is (since RT ln 1 = 0). °Go is thus a constant defned as the a measure of the “driving force” for any chemical reaction, just as the standard free-energy change for the reaction. proton-motive force is the driving force for the translocation of protons.
which ADP and Pi will join to form ATP will be equal to the rate at which ATP hydrolyzes to form ADP and Pi. In other words, when ∆G = 0, the reaction is at equilibrium (see Figure 14–29).
It is ∆G, not ∆G°, that indicates how far a reaction is from equilibrium and determines whether it can drive other reactions. Because the efficient conversion of ADP to ATP in mitochondria maintains such a high concentration of ATP relative to ADP and Pi, the ATP hydrolysis reaction in cells is kept very far from equilibrium and ∆G is correspondingly very negative. Without this large disequilibrium, ATP hydrolysis could not be used to drive the reactions of the cell. At low ATP concentrations, many biosynthetic reactions would run backward and the cell would die.
the atp Synthase Is a Nanomachine that produces atp by rotary Catalysis
The ATP synthase is a finely tuned nanomachine composed of 23 or more separate protein subunits, with a total mass of about 600,000 daltons. The ATP synthase can work both in the forward direction, producing ATP from ADP and phosphate in response to an electrochemical gradient, or in reverse, generating an electrochemical gradient by ATP hydrolysis. To distinguish it from other enzymes that hydrolyze ATP, it is also called an F1Fo ATP synthase or F-type ATPase.
Resembling a turbine, ATP synthase is composed of both a rotor and a stator (Figure 14–30). To prevent the catalytic head from rotating, a stalk at the periphery of the complex (the stator stalk) connects the head to stator subunits embedded in the membrane. A second stalk in the center of the assembly (the rotor stalk) is connected to the rotor ring in the membrane that turns as protons flow through it, driven by the electrochemical gradient across the membrane. As a result, proton