Biological Oxidation-Reduction Reactions:- Biological Oxidations Often Involve Dehydrogenation
The carbon in living cells exists in a range of oxidation states (Fig. 13–13). When a carbon atom shares an electron pair with another atom (typically H, C, S, N, or O), the sharing is unequal in favor of the more electronegative atom. The order of increasing electronegativity is H<C<S<N<O.
In oversimplified but useful terms, the more electronegative atom “owns” the bonding electrons it shares with another atom. For example, in methane (CH4), carbon is more electronegative than the four hydrogens bonded to it, and the C atom therefore “owns” all eight bonding electrons (Fig. 13–13). In ethane, the electrons in the C-C bond are shared equally, so each C atom owns only seven of its eight bonding electrons. In ethanol, C-1 is less electronegative than the oxygen to which it is bonded, and the O atom therefore “owns” both electrons of the C-O bond, leaving C-1 with only five bonding electrons. With each formal loss of electrons, the carbon atom has undergone oxidation—even when no oxygen is involved, as in the conversion of an alkane (-CH2=CH2-) to an alkene (-CH=CH-). In this case, oxidation (loss of electrons) is coincident with the loss of hydrogen. In bio logical systems, oxidation is often synonymous with de hydrogenation, and many enzymes that catalyze oxidation reactions are dehydrogenases. Notice that the more reduced compounds in Figure 13–13 (top) are richer in hydrogen than in oxygen, whereas the more oxidized compounds (bottom) have more oxygen and less hydrogen.
Not all biological oxidation-reduction reactions in volve carbon. For example, in the conversion of molecular nitrogen to ammonia, 6H++6e-+ N2 → 2NH3, the nitrogen atoms are reduced. Electrons are transferred from one molecule (electron donor) to another (electron acceptor) in one of four different ways:
- Directly as electrons. For example, the Fe2+ /Fe3+ redox pair can transfer an electron to the Cu+/Cu2+ redox pair:
- As hydrogen atoms. Recall that a hydrogen atom consists of a proton (H+) and a single electron (e-). In this case we can write the general equation
3. As a hydride ion(:H-), which has two electrons. This occurs in the case of NAD-linked dehydrogenases, described below.
4. Through direct combinationwithoxygen. In this case, oxygen combines with an organic reductant and is covalently incorporated in the product, as in the oxidation of a hydrocarbon to an alcohol:
The hydrocarbon is the electron donor and the oxygen atom is the electron acceptor.
All four types of electron transfer occur in cells. The neutral term reducing equivalentis commonly used to designate a single electron equivalent participating in an oxidation-reduction reaction, no matter whether this equivalent is an electron per se, a hydrogen atom, or a hydride ion, or whether the electron transfer takes place in a reaction with oxygen to yield an oxygenated product. Because biological fuel molecules are usually enzymatically dehydrogenated to lose two reducing equivalents at a time, and because each oxygen atom can accept two reducing equivalents, biochemists by convention regard the unit of biological oxidations as two reducing equivalents passing from substrate to oxygen.
