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By Khair UllahJr. 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 ElectronsThe 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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