Page 365 - Physical Chemistry

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Chapter 11
Reaction Equilibrium
in Nonideal Systems

Figure 11.8
Biochemical coupling.

Figure 11.9
H O ATP B C
2
G°
for species involved in the
coupled reactions (11.38).
ATP B∆ ADP BP
G°′
H O ADP BP C
2
ATP H O ∆ ADP P i BP C ∆ D P i
2

ADP P D H O
2
i
B C ∆ D H O
2
B C
ADP P i
of large biochemicals (for example, amino acids, proteins, RNA, and DNA) from
small molecules, the transport of chemical species from regions of low to regions of
high chemical potential (active transport—Sec. 12.4), and muscle contraction to per-
form mechanical work. The thermodynamically unfavored resynthesis of ATP from
ADP is made to occur by being coupled with the oxidation of glucose, for which G°
0. See Fig. 11.8.
As an example of a biochemical coupling scheme, the thermodynamically unfa-
vored synthesis B C ∆ D H O can be brought about by coupling it with ATP
2
hydrolysis according to the enzyme-catalyzed reactions
B ATP ∆ BP ADP
(11.38)
BP C ∆ D P i
where the intermediate species BP (phosphorylated B) is common to the two reac-
tions. The net reaction for the sequence (11.38) is

B C ATP ∆ D ADP P i
which is the sum of B C ∆ D H O and ATP H O ∆ ADP P . In this
2

i
2
scheme, G° of BP H O is substantially greater than G° of B P , so the equilib-
2 i
rium constant of BP C ∆ D P is much greater than that of the uncoupled
i
reaction B C ∆ D H O. See Fig. 11.9. (All species in these reactions are in
2
aqueous solution.)
Reaction equations like ATP H O ∆ ADP P or those in (11.38) are called
2 i
biochemical reaction equations. In a biochemical reaction equation, the hydrogen
atoms and the charges are not balanced, and symbols like ATP and P stand for the sum
i
of all forms in which ATP and inorganic phosphate exist. Thus, P includes such
i

3
2
species as PO , HPO , H PO , and MgHPO , which exist in equilibrium with one
4 4 2 4 4
4
2
2
3
another; ATP includes such species as ATP , HATP , H ATP , MgATP , etc. To
2