Page 12 - Thermodynamics of Biochemical Reactions

1.3 Binding of Hydrogen Ions and Magnesium Ions by Adenosine Triphosphate 5

Table 1.1 pH,-pH, as a Function of Ionic
Strength and Temperature

IIM 10°C 25°C 40°C

0 0 0 0
0.05 0.082 0.084 0.086
0.1 0.105 0.107 0.110
0.15 0.119 0.122 0.125
0.2 0.130 0.133 0.137
0.25 0.138 0.142 0.146

Source: R. A. Alberty, J. Phys. Chenz. B 105, 7865 (2001).
Copyright 2001 American Chemical Society.

where pMg = -log[Mg2+] and K,,, is a function of the ionic strength, as well
as temperature.
Strictly speaking, equations 1.2-7 and 1.2-9 should have co in the denomina-
tor, where cn = 1 M is the standard concentration, to make the equilibrium
constant dimensionless (Mills et al., 1993). However, the co is omitted in this book
in order to simplify expressions for equilibrium constants. Nevertheless, equilib-
rium constants are still considered to be dimensionless, so their logarithm can be
taken.
In using acid dissociation constants and the dissociation constants of complex
ions, it is convenient to take the base 10 logarithms of equations 1.2-7 and 1.2-9
to obtain

(1.2-10)

(1.2-1 1)

where pK,, = -log KH, and pKMgA = -log KMgA are functions of ionic strength
at constant temperature. Table 1.3 in the last section of this chapter gives the pKs
of some weak acids of interest in biochemistry as a function of ionic strength.
Note that the effect of ionic strength is larger for acids with larger charges. For
polyprotic acids pK, applies to the weakest acid group, pK, to the second
weakest, and so on, in the pH range considered (usually 5 to 9). The calculation of
Table 1.3 is based on the extended Debye-Huckel equation.

W 1.3 BINDING OF HYDROGEN IONS AND MAGNESIUM
IONS BY ADENOSINE TRIPHOSPHATE

Acid dissociation constants and dissociation constants of complex ions determine
the concentrations of species that are present in a solution at equilibrium under
specified conditions. Ionic dissociation reactions occur rapidly and tend to remain
at equilibrium during an enzyme-catalyzed reaction. Since ATP (see Fig. 1.1) is
the primary carrier of energy in biochemical systems and since a good deal is
known about its binding properties, these properties are considered here in some
detail.
An ATP ion with four negative charges can bind five hydrogen ions in
strongly acidic solutions, but biochemistry is primarily concerned with the neutral
region. We will consider only the hydrogen ion bindings that affect equilibrium in
this region, namely the terminal phosphate group with a pK about 7 and the