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32 Basics
–
Redox processes other (see below). Only the combination 2 e /
+
1H ,the hydride ion, is transferred as a unit.
A. Redox reactions
C. Biological redox systems
Redox reactions are chemical changes in
which electrons are transferred from one re- In thecell, redox reactions arecatalyzed by
action partner to another (1;see also p.18). enzymes, which work together with soluble
Like acid–base reactions (see p. 30), redox re- or bound redox cofactors.
actionsalwaysinvolve pairs of compounds. A Some of these factors contain metal ions as
pair of this type is referred to as a redox redox-active components. In these cases, it is
system (2). Theessentialdifferencebetween usually single electrons that are transferred,
the two components of a redox system is the with the metal ion changing its valency. Un-
number of electrons they contain. The more paired electrons often occur in this process,
electronrich component is called the reduced but these are located in d orbitals (see p. 2)
form of the compound concerned, while the and are therefore less dangerous than single
other one is referred to as the oxidized form. electrons in non-metal atoms (“free radicals”;
The reduced form of one system (the reducing see below).
agent) donates electrons to the oxidized form We can only show here a few examples
of another one (the oxidizing agent). In the from the many organic redox systems that
process, the reducing agent becomes oxidized are found. In the complete reduction of the
and the oxidizing agent is reduced (3). Any flavin coenzymes FMN and FAD (see p.104),
+
–
given reducing agent can reduce only certain 2e and 2 H are transferred. This occurs in
other redox systems. On the basis of this type two separate steps, with a semiquinone radi-
of observation, redox systems can be ar- cal appearing as an intermediate. Since or-
ranged to form what are known as redox ganic radicals of this type can cause damage
series (4). to biomolecules, flavin coenzymes never oc-
The position of a system within one of cur freely in solution, but remain firmly
these series is established by its redox bound in the interior of proteins.
potential E (see p.18). The redox potential In the reduction or oxidation of quinone/
has a sign; it can be more negative or more quinol systems,freeradicals also appear as
positive than a reference potential arbitrarily intermediate steps, but these are less reactive
setatzero (thenormal potential of thesystem than flavin radicals. Vitamin E, another qui-
+
[2 H /H 2 ]). In addition, E depends on the con- none-type redox system (see p.104), even
centrations of the reactants and on the reac- functions as a radical scavenger, by delocaliz-
tion conditions (see p.18). In redox series (4), ing unpaired electrons so effectively that they
the systems are arranged according to their can no longer react with other molecules.
+
increasing redox potentials. Spontaneous The pyridine nucleotides NAD and NADP +
electron transfers are only possible if the re- always function in unbound form. The oxi-
dox potential of the donor is more negative dized forms contain an aromatic nicotinamide
than that of the acceptor (see p.18). ring in which the positive charge is delocal-
ized. The right-hand example of the two res-
onance structures showncontains anelec-
B. Reduction equivalents
tron-poor, positively charged C atom at the
+
In redox reactions, protons (H )are often para position to nitrogen. If a hydride ion is
–
transferred along with electrons (e ), or pro- added at this point (see above), the reduced
tons may be released. The combinations of forms NADH or NADPH arise. No radical inter-
electrons and protons that occur in redox mediate steps occur. Because a proton is re-
processes are summed up in the term reduc- leased at thesametime, thereduced pyridine
tion equivalents. For example, the combina- nucleotide coenzymes are correctly expressed
+
+
–
tion 1 e /1 H corresponds to a hydrogen as NAD(P)H+H .
–
+
atom, while 2 e and 2 H together produce
ahydrogen molecule. However, this does not
mean that atomic or molecular hydrogen is
actually transferred from one molecule to the
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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