Biochemical Redox Reactions By Khair Ullah, Jr. Research.Fellow International Center for Chemical and Biological Sciences (ICCBS) University of Karachi

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By
Khair Ullah
Jr. Research. Fellow
International Center for Chemical and
Biological Sciences (ICCBS) University
of Karachi

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Oxidation-Reduction
Oxidation is the removal of electrons.
Reduction is the gain of electrons.
Redox reaction is an oxidation reaction paired with a
reduction reaction.

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Oxidation-Reduction
 In biological systems, the electrons are often associated with
hydrogen atoms.
 Transfer of electrons or hydrogen atoms from one
molecule (hydrogen or electron donor) to another (the
acceptor)
 Biological oxidations are often dehydrogenations.

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Oxidation-Reduction Reactions Carbon atoms can exist
in five oxidation states, depending on the elements with
which they share electrons.
.

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FIGURE 13-9
FIGURE 13–9 The oxidation states of carbon in biomolecules.

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FIGURE 13–10
FIGURE 13–10 An oxidation-reduction reaction.

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Biological Oxidation-Reduction
Reactions
The Flow of Electrons Can Do Biological Work
Because the two chemical species differ in their affinity for
electrons, electrons flow spontaneously through the
circuit, driven by a force proportional to the difference in
electron affinity, the electromotive force, emf.

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 Living cells have an analogous biological “circuit,” with a
relatively reduced compound such as glucose as the
source of electrons

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Oxidation-Reductions Can Be Described as Half-Reactions
 The oxidation of ferrous ion by cupric ion,
can be described in terms of two half-reactions:
Fe2+
and Fe3+
constitute a conjugate redox pair.

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FIGURE 13-22
FIGURE 13–22 Oxidation states
of carbon in the biosphere.

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 Electrons are transferred from one molecule to another in
one of four ways:
1. Directly as electrons.
2. As hydrogen atoms.
3. As a hydride ion (:H–
), which has two electrons. This
occurs in the case of NAD-linked dehydrogenases.

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4. Through direct combination with oxygen.
All four types of electron transfer occur in cells. The neutral
term reducing equivalent is commonly used to designate
a single electron equivalent participating in an oxidation-
reduction reaction.

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Reduction Potentials Measure Affinity for Electrons
The standard reduction potential, E°, a measure
(in volts) of this affinity.
Measurement of the
standard reduction
potential (E'°)
of a redox pair.

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Cellular Oxidation of Glucose to Carbon Dioxide Requires
Specialized Electron Carriers
The complete oxidation of glucose:
C6H12O6 + 6O2 → 6CO2 + 6H2O

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A Few Types of Coenzymes and Proteins Serve as Universal
Electron Carriers
 The multitude of enzymes that catalyze cellular
oxidations channel electrons from their hundreds of
different substrates into just a few types of universal
electron carriers.
 NAD, NADP, FMN, and FAD are water-soluble
coenzymes that undergo reversible oxidation and
reduction in many of the electron-transfer reactions of
metabolism.

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NADH and NADPH Act with Dehydrogenases as Soluble
Electron Carriers

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 More than 200 enzymes are known to catalyze reactions
in which NAD+
(or NADP+
) accepts a hydride ion from a
reduced substrate, or NADPH (or NADH) donates a
hydride ion to an oxidized substrate.
AH2 + NAD+
→ A + NADH + H+
A + NADPH + H+
→ AH2 + NADP+
where AH2 is the reduced substrate and A the oxidized
substrate. The general name for an enzyme of this type is
oxidoreductase; they are also commonly called
dehydrogenases.

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Flavin Nucleotides Are Tightly Bound in Flavoproteins
 Flavoproteins (Table 13–9) are enzymes that catalyze
oxidation-reduction reactions using either flavin
mononucleotide (FMN) or flavin adenine dinucleotide
(FAD) as coenzyme (Fig. 13–27).
 These coenzymes, the flavin nucleotides, are derived
from the vitamin riboflavin.

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TABLE 13-9

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FIGURE 13–27 Oxidized
and reduced FAD and
FMN.

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