Organic Chemistry

Section 27.2 ATP: The Carrier of Chemical Energy 037

3-D Molecules:

Adenosine -triphosphate

(ATP)

5 ¿

(^1) The prime in indicates that two additional parameters have been added to the defined in
Section 3.7—the reaction occurs in aqueous solution at and the concentration of water is
assumed to be constant.
pH= 7
¢G°¿ ¢G°
D-glucose with hydrogen phosphate to form D-glucose-6-phosphate is endergonic
or.^1 The hydrolysis of ATP, on the other
hand, is highly exergonic or When the two
reactions are added together (the species occurring on both sides of the reaction arrow
cancel), the net reaction is exergonic or
Thus, the energy released from the hydrolysis of ATP is more than enough to drive the
phosphorylation of D-glucose. Two reactions in which the energy of one is used to
drive the other are known as coupled reactions.
This nonmechanistic description of ATP’s power makes ATP look like a magical
source of energy. Let’s look at the mechanism of the reaction to see what really hap-
pens. The reaction is a simple one-step nucleophilic substitution reaction. The 6-OH
group of glucose attacks the terminal phosphate of ATP, breaking a phosphoanhydride
bondwithout forming an intermediate. Essentially it is an reaction with an
adenosine pyrophosphate leaving group.
Now we have a chemical understanding of why the phosphorylation of glucose requires
ATP. Without ATP, the 6-OH group of D-glucose would have to displace a very basic
group from hydrogen phosphate. With ATP, the 6-OH group of D-glucose displaces
the weakly basic ADP.
too basic to be displaced
−O OH
O
O−
P

• H
  H H
  HO
  HO
  H
  O
  H
  CH 2 OH
  OH
  OH
  hydrogen
  phosphate

• OH

O O

O

P

O−

−O

O

P

O−

O

O

P

O−

adenosine

ATP

H

OH

O

+

a phosphoanhydride bond

O

O

P

O−

−O

−O

O

O

O

P

P

O−

O−

adenosine

ADP

+

D-glucose

D-glucose-6-phosphate

HO

OH

CH 2 OH

HO

HO

H

OH

O

HO

OH

CH 2 O

SN 2

(¢G°¿=-4.0 kcal>mol -16.7 kJ>mol).

(¢G°¿=-7.3 kcal>mol -30.5 kJ>mol).

(¢G°¿=+3.3 kcal>mol +13.8 kJ>mol)

D-glucose + hydrogen phosphate D-glucose-6-phosphate H 2 O 3.3 kcal/mol

∆G°′

+ +

D-glucose + ATP D-glucose-6-phosphate + ADP −4.0 kcal/mol

ATP + H 2 O ADP + hydrogen phosphate −7.3 kcal/mol

+13.8 kJ/mol

−16.7 kJ/mol

−30.5 kJ/mol

or

or

or