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Coenzymes in Biologically Important Oxidation–Reduction Reactions
The description of redox reactions in terms of oxidation numbers, which is widely used with inorganic compounds, can be used to deal with the oxidation of carbon-containing molecules. However, our discussion will be more pictorial and easier to follow if we write equations for the half reactions and then concentrate on the functional groups of the reactants and products and on the number of electrons transferred. An example is the oxidation half reaction for the conversion of ethanol to acetaldehyde.
Writing the Lewis electron-dot structures for the functional groups involved in the reaction helps us keep track of the electrons being transferred. In the oxidation of ethanol, there are 12 electrons in the part of the ethanol molecule involved in the reaction and 10 electrons in the corresponding part of the acetaldehyde molecule; two electrons are transferred to an electron acceptor (an oxidizing agent). This type of “bookkeeping” is useful for dealing with biochemical reactions. Many biological oxidation reactions, like this example, are accompanied by the transfer of a proton (H+). The oxidation half reaction has been written as a reversible reaction because the occurrence of oxidation or reduction depends on the other reagents present.
Another example of an oxidation half reaction is that for the conversion of NADH, the reduced form of nicotinamide adenine dinucleotide, to the oxidized form, NAD+. This substance is an important coenzyme in many reactions.
Figure 15.3 shows the structure of NAD+ and NADH; the nicotinamide por-tion, the functional group involved in the reaction, is indicated in red and blue. Nicotinamide is a derivative of nicotinic acid (also called niacin), one of the B-complex vitamins. A similar compound is NADPH (for which the oxidized form is NADP+). It differs from NADH by having an addi-tional phosphate group; the site of attachment of this phosphate group to ribose is also indicated in Figure 15.3. To simplify writing the equation for the oxidation of NADH, only the nicotinamide ring is shown explicitly, with the rest of the molecule designated as R. The two electrons that are lost when NADH is converted to NAD+ can be considered to come from the bond between carbon and the lost hydrogen, with the nitrogen lone-pair electrons becoming involved in a bond. Note that the loss of a hydrogen and two electrons can be considered as the loss of a hydride ion (H:-) by NADH and is sometimes written that way.
The equations for both the reaction of NADH to NAD+ and that of ethanol to acetaldehyde have been written as oxidation half reactions. If ethanol and NADH were mixed in a test tube, no reaction could take place because there would be no electron acceptor. If, however, NADH were mixed with acetal-dehyde, which is an oxidized species, a transfer of electrons could take place, producing ethanol and NAD+. (This reaction would take place very slowly in the absence of an enzyme to catalyze it. Here we have an excellent example of the difference between the thermodynamic and kinetic aspects of reactions. The reaction is spontaneous in the thermodynamic sense but very slow in the kinetic sense.)
Such a reaction does take place in some organisms as the last step of alcoholic fermentation. The NADH is oxidized while the acetaldehyde is reduced.
Another important electron acceptor is FAD (flavin adenine dinucleotide) (Figure 15.4), which is the oxidized form of FADH2. The symbol FADH2 explic-itly recognizes that protons (hydrogen ions) as well as electrons are accepted by FAD. The structures shown in this equation again point out the electrons that are transferred in the reaction. Several other coenzymes contain the flavin group; they are derived from the vitamin riboflavin (vitamin B2).
Oxidation of nutrients to provide energy for an organism cannot take place without reduction of some electron acceptor. The ultimate electron acceptor in aerobic oxidation is oxygen; we shall encounter intermediate electron accep-tors as we discuss metabolic processes. Reduction of metabolites plays a signifi-cant role in living organisms in anabolic processes. Important biomolecules are synthesized in organisms by many reactions in which a metabolite is reduced while the reduced form of a coenzyme is oxidized.
Two coenzymes, NADH and FADH2, play a crucial role in biological oxidation-reduction reactions. Hydrogen ions are transferred in addition to electrons.
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