Physical Chemistry Third Edition

346 7 Chemical Equilibrium

by being distorted into a conformation resembling a transition state. The important

aspect of the proposed mechanism is that the phosphate is not simply released into the

solution; it is held in the active site of the enzyme until it reacts with an ADP molecule.

Since the first step is not repeated until the second step occurs, the hydrolysis of PEP

does not occur without the regeneration of ATP and the two reactions are combined

into a single reaction.

EXAMPLE7.24

Find the equilibrium constant at 298.15 K for the combined reaction:

ADP+PEP
AT P+Py

Solution

The equilibrium constant for the combined reaction is

Ke−∆G

◦′/RT

exp

(

24300 J mol−^1

(8.3145 J K−^1 mol−^1 )(298.15 K)

)

e^9.^803  1. 81 × 104

Exercise 7.23

Find the equilibrium ATP concentration for the initial concentrations of Example 7.23, treating

the combined reaction as a single reaction.

EXAMPLE7.25

The coupling of the spontaneous hydrolysis of ATP to drive other reactions is similar to the

coupling that regenerates ATP. Write a possible mechanism for the coupling of the sponta-

neous hydrolysis of ATP to drive the phosphorylation of glucose, Eq. (7.7-3).

Solution

(1) E+AT P+H 2 O
Ep+ADP

(2) Ep+glucose
glucose 6-phosphate+E+H 2 O

AT P+glucose→ADP+glucose 6-phosphate

where Ep stands for a phosphorylated enzyme molecule.

Exercise 7.24

For the phosphorylation of glucose,∆G◦′ 19 .4kJmol−^1. Find∆G◦′and the equilibrium

constant for the combined reaction of the previous example.