Physical Chemistry , 1st ed.

This is the reverse of the reaction for glucose oxidation/metabolism. By Hess’s
law, the enthalpy change of this reaction is the negative of the enthalpy change
for the original process:rxnH+2799 kJ per mole of glucose produced. For
both processes, the individual steps in the overall, complex biochemical reac-
tion are ignored. Only the overall reaction is needed to determine the enthalpy
change.
One very important biochemical reaction is the conversion of adenosine
triphosphate (ATP) to adenosine diphosphate (ADP) and vice versa (Figure
2.13). We can summarize this process as

ATP + H 2 O ADP + phosphate (2.61)

Here, “phosphate” refers to any of several inorganic phosphate ions (H 2 PO 4 ,
HPO 42 , or PO 43 ), depending on the ambient conditions. This conversion is
a major energy storage/utilization process at the subcellular level.
These reactions occur in cells, not in the gas phase, so the specification of
the conditions of the reaction are different. A biochemical standard statein-
cludes the requirement that an aqueous solution be neutral (that is, neither
acidic nor basic), with a pH of 7.* We use the prime symbol on a state func-
tion to imply that it refers to a reaction at the biochemical standard state. For
the ATP →ADP reaction in equation 2.61, the rxnH°is 24.3 kJ per mole
of ATP reacted.
This is not a large enthalpy change. However, it is enough energy to fuel
other biochemically important chemical reactions. Details can be found in
most biochemistry textbooks.

JQPJ

2.12 Biochemical Reactions 61

NH 2

 H 2 O

N

N

O

OO

HO

HO

N

N

P

O

O

P O

O

O

P O

O

O

NH 2

N

N

O

OO

HO

HO

N

N

P

O

O

P O

O

O

OOHP

OH

O



Figure 2.13 Hydrolysis of adenosine triphosphate (ATP) to make adenosine diphosphate
(ADP) and inorganic phosphate.

*A more detailed discussion of pH is in Chapter 8.