Page 27 - Color Atlas of Biochemistry

18 Basics
Equilibriums
A. Group transfer reactions
Every chemical reaction reaches after a time a
state of equilibrium in which the forward and
back reactions proceed at the same speed. The
law of mass action describes the concentra-
tions of the educts (A, B) and products (C, D) in when ∆E>0, i. e. ∆G<0.
equilibrium. The equilibrium constant K is di-
0
rectly related to ∆G ,the change in free
enthalpy G involved in the reaction (see
0
p.16) under standard conditions (∆G =–R
T ln K). For any given concentrations, the
lower equation applies. At ∆G< 0, the reac-
tion proceeds spontaneously for as long as it
takes for equilibrium to be reached (i. e., until electron transfer from lactate to NAD is not
∆G=0). At ∆G>0, a spontaneous reaction is
no longer possible (endergonic case; see
p.16). In biochemistry, ∆G is usually related
to pH 7, andthisisindicated by the“prime”
0
symbol (∆G or ∆G ).
As examples, we can look at two group
transfer reactions (on the right). In ATP (see
p.122), the terminal phosphate residue is at a
high chemical potential. Its transfer to water
(reaction a, below) is therefore strongly exer- involved in proton exchange reactions (see
gonic. The equilibrium of the reaction
(∆G = 0; see p.122) is only reached when
more than 99.9% of the originally available
ATP has been hydrolyzed. ATP and similar
compounds have a high group transfer
potential for phosphate residues. Quantita-
tively, thisis expressedasthe 'Gofhydrolysis
0
–1
(∆G = –32 kJ mol ; see p.122).
In contrast, the endergonic transfer of am-
monia (NH 3 ) to glutamate (Glu, reaction b,
0
–1
∆G =+14 kJ mol ) reaches equilibrium so
quickly that only minimal amounts of the
product glutamine (Gln) can be formed in
this way. The synthesis of glutamine from
these preliminary stages is only possible
through energetic coupling (see pp.16, 124).
B. Redox reactions
The course of electron transfer reactions (re-
dox reactions, see p.14) also follows the law of
mass action. For a single redox system (see
p. 32), the Nernst equation applies (top). The
electron transfer potential of a redox system
(i. e., its tendency to give off or take up elec-
trons) is given by its redox potential E (in
0
0
standard conditions, E or E ). The lower the
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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redox potential of a system is, the higher the
chemical potential of the transferred elec-
trons. To describe reactions between two re-
dox systems, ∆Ε—the difference between the
two systems’ redox potentials—is usually
used instead of ∆G. ∆Gand ∆Ehavea simple
relationship, but opposite signs (below). A
redox reaction proceeds spontaneously
Theright side of theillustration shows the
way in which the redox potential E is depen-
dent on the composition (the proportion of
the reduced form as a %) in two biochemically
important redox systems (pyruvate/lactate
+
+
and NAD /NADH+H ;see pp. 98, 104).In the
standard state (both systems reduced to 50%),
+
possible, because ∆Eis negative (∆E= –0.13 V,
red arrow). By contrast, transfer can proceed
successfully if the pyruvate/lactate system is
+
reduced to 98% and NAD /NADH is 98% oxi-
dized (green arrow, ∆E=+0.08 V).
C. Acid–base reactions
Pairs of conjugated acids and bases are always
p. 30). The dissociation state of an acid–base
+
pair depends on the H concentration. Usu-
ally, it is not this concentration itself that is
expressed, but its negative decadic logarithm,
the pH value. The connection between the pH
value and the dissociation state is described
by the Henderson–Hasselbalch equation (be-
low). As a measure of the proton transfer
potential of an acid–base pair, its pK a value
is used—the negative logarithm of the acid
constant K a (where “a” stands for acid).
The stronger an acid is, the lower its pK a
value. The acid of the pair with the lower pK a
value (the stronger acid—in this case acetic
acid, CH 3 COOH) can protonate (green arrow)
the base of the pair with the higher pK a (in
this case NH 3 ), while ammonium acetate
+
–
(NH 4 and CH 3 COO )only forms very little
CH 3 COOH and NH 3 .