Page 13 - Thermodynamics of Biochemical Reactions

6 Introduction to Apparent Equilibrium Constants

Adenine

D -Ribose

OH OH
Adenosine

Adenosine inonophosphate (AMP)

Adenosine diphosphate (ADP)

Adenosine triphosphate (An)

Figure 1.1 Structure of adenosine triphosphate

adenine group with pK about 4. The other three pKs are in the neighborhood of
2 or below and can be ignored in treating biochemical reactions. The anions of
ATP bind metal ions as well as hydrogen ions. The dissociation constants for the
complex ions that are formed can be determined by use of acid titrations because
the binding of a metal ion reduces the apparent pK for the phosphate group
(Alberty, Smith, and Bock, 1951; Smith and Alberty, 1956; Silbey and Alberty,
2001). The apparent pK of the phosphate group is the midpoint of the titration of
H,POi- in the presence of magnesium ions at the desired concentration of free
metal ions. Because of the importance of ATP in energy metabolism, a great deal
of data on the acid dissociation constants and the dissociation constants of
complex ions of ATP, ADP, AMP, and Pi are available. Goldberg and Tewari
(1991) and Larson, Tewari, and Goldberg (1993) critically evaluated these data
including that on glucose 6-phosphate (G6P). The values for acid dissociation
constants and magnesium complex ion dissociation constants involved in the
ATP series are given in Table 1.2.
Since ATP is made up of three species in the physiological pH range in the
absence of metal ions that are bound, its concentration is given by

[ATP] = [ATP4-] + [HATP3-] + [H,ATP2-] (1.3-1)
Substituting the expressions for the two acid dissociation constants yields

[ATP] = [ATP4-] (1.3-2)
KIATPK2ATP
The mole fraction r of the ATP in the ATP4- form at a specified concentration of
hydrogen ions is given by
1
r(ATPP4) = (1.3-3)
1+- CH'1 + W+I2
KIATP KlATPK2ATP
The mole fractions of ATP in the other two forms are readily derived:

CH'I
KIATP
r(HATP3 -) = (1.3-4)
CH'1 CH'I'
1 I-I