Part Bioenergetics and Metabolism 485 Haneaytie-(H=C+H alyze these oxidations are generally called oxidases r,if the oxygen atom is derived directly from molecu Carbon h atom lar oxygen(02), oxygenases radical Every oxidation must be accompanied by a reduc tion, in which an electron acceptor acquires the electrons 一C-C--C+"C removed by oxidation. Oxidation reactions generally release energy(think of camp fires: the compounds in Carbon radicals wood are oxidized by oxygen molecules in the air). Most living cells obtain the energy needed for cellular work by oxidizing metabolic fuels such as carbohydrates or fat; photosynthetic organisms can also trap and use the en ergy of sunlight. The catabolic(energy-yielding) path- Carbanion Proton ways described in Chapters 14 through 19 are oxidative reaction sequences that result in the transfer of electrons from fuel molecules, through a series of electron carri- ers, to oxygen. The high affinity of O2 for electrons makes the overall electron-transfer process highly exergonic. Carbocation Hydride providing the energy that drives ATP synthesis-the central goal of catabolism. 2. Reactions that make or break carbon-carbon bonds het- Carbanion Carbocation erolytic cleavage of a C-C bond yields a carbanion and a carbocation(Fig. 5). Conversely, the formation of a FIGURE 5 Two mechanisms for cleavage of a C-C or C-H bond. C-C bond involves the combination of a nucleophilic In homolytic cleavages, each atom keeps one of the bonding elt carbanion and an electrophilic carbocation. Groups with trons, resulting in the formation of carbon radicals (carbons having electronegative atoms play key roles in these reactions unpaired electrons)or uncharged hydrogen atoms. In heterolytic cleav- Carbonyl groups are particularly important in the chem- ages, one of the atoms retains both bonding electrons. This can result ical transformations of metabolic pathways. As noted in the formation of carbanions, carbocations, protons, or hydride ions. above, the carbon of a carbonyl group has a partial pos- itive charge due to the electron-withdrawing nature of 1. Oxidation-reduction reactions Carbon atoms encoun- the adjacent bonded oxygen, and thus is an electrophilic tered in biochemistry can exist in five oxidation states, carbon. The presence of a carbonyl group depending on the elements with which carbon shares facilitate the formation of a carbanion on an adjoining electrons( Fig. 6). In many biological oxidations, a com- carbon, because the carbonyl group can delocalize elec- pound loses two electrons and two hydrogen ions(that trons through resonance(Fig. 8a, b). The importance is, two hydrogen atoms); these reactions are commonly of a carbonyl group is evident in three major classes of called dehydrogenations and the enzymes that catalyze reactions in which C-C bonds are formed or broken them are called dehydrogenases(Fig. 7). In some, but(Fig 8c): aldol condensations(such as the aldolase not all, biological oxidations, a carbon atom becomes co- reaction; see Fig. 14-5), Claisen condensations(as valently bonded to an oxygen atom. The enzymes that in the citrate synthase reaction; see Fig. 16-9), and 2H++2e- -CH2-CH Alkane -CH2-CH2OH Alcohol CH3-CH一C、 一cH3-C-C 2H++2e- dehyde(ketone) Lactate lactate An oxidation-reduction reaction shown here is the ox -CH2-C Carboxylic acid of lactate to pyruvate. In this dehydrogenation, two electrons and two hydrogen ions(the equivalent of two hydrogen atoms) are re- O=C=0 Carbon dioxide moved from C-2 of lactate, an alcohol, to form pyruvate, a ketone. In cells the reaction is catalyzed by lactate dehydrogenase and the elec FIGURE 6 The oxidation states of carbon in biomolecules. Each com. trons are transferred to a cofactor called nicotinamide adenine dinu. pound is formed by oxidation of the red carbon in the compound cleotide. This reaction is fully reversible: pyruvate can be reduced by isted above it. Carbon dioxide is the most highly oxidized form of electrons from the cofactor. In Chapter 13 we discuss the factors that carbon found in living systems determine the direction of a reaction1. Oxidation-reduction reactions Carbon atoms encoun￾tered in biochemistry can exist in five oxidation states, depending on the elements with which carbon shares electrons (Fig. 6). In many biological oxidations, a com￾pound loses two electrons and two hydrogen ions (that is, two hydrogen atoms); these reactions are commonly called dehydrogenations and the enzymes that catalyze them are called dehydrogenases (Fig. 7). In some, but not all, biological oxidations, a carbon atom becomes co￾valently bonded to an oxygen atom. The enzymes that catalyze these oxidations are generally called oxidases or, if the oxygen atom is derived directly from molecu￾lar oxygen (O2), oxygenases. Every oxidation must be accompanied by a reduc￾tion, in which an electron acceptor acquires the electrons removed by oxidation. Oxidation reactions generally release energy (think of camp fires: the compounds in wood are oxidized by oxygen molecules in the air). Most living cells obtain the energy needed for cellular work by oxidizing metabolic fuels such as carbohydrates or fat; photosynthetic organisms can also trap and use the en￾ergy of sunlight. The catabolic (energy-yielding) path￾ways described in Chapters 14 through 19 are oxidative reaction sequences that result in the transfer of electrons from fuel molecules, through a series of electron carri￾ers, to oxygen. The high affinity of O2 for electrons makes the overall electron-transfer process highly exergonic, providing the energy that drives ATP synthesis—the central goal of catabolism. 2. Reactions that make or break carbon–carbon bonds Het￾erolytic cleavage of a COC bond yields a carbanion and a carbocation (Fig. 5). Conversely, the formation of a COC bond involves the combination of a nucleophilic carbanion and an electrophilic carbocation. Groups with electronegative atoms play key roles in these reactions. Carbonyl groups are particularly important in the chem￾ical transformations of metabolic pathways. As noted above, the carbon of a carbonyl group has a partial pos￾itive charge due to the electron-withdrawing nature of the adjacent bonded oxygen, and thus is an electrophilic carbon. The presence of a carbonyl group can also facilitate the formation of a carbanion on an adjoining carbon, because the carbonyl group can delocalize elec￾trons through resonance (Fig. 8a, b). The importance of a carbonyl group is evident in three major classes of reactions in which COC bonds are formed or broken (Fig 8c): aldol condensations (such as the aldolase reaction; see Fig. 14–5), Claisen condensations (as in the citrate synthase reaction; see Fig. 16–9), and Part II Bioenergetics and Metabolism 485 C C Carbon radicals C C C H  Carbanion Proton Heterolytic C H cleavage C H Carbon radical C H Homolytic cleavage C H Carbocation Hydride C C C  Carbanion Carbocation C C H atom H  FIGURE 5 Two mechanisms for cleavage of a COC or COH bond. In homolytic cleavages, each atom keeps one of the bonding elec￾trons, resulting in the formation of carbon radicals (carbons having unpaired electrons) or uncharged hydrogen atoms. In heterolytic cleav￾ages, one of the atoms retains both bonding electrons. This can result in the formation of carbanions, carbocations, protons, or hydride ions. CH2 CH3 Alkane CH2 CH2 Alcohol Aldehyde (ketone) Carboxylic acid Carbon dioxide CH2OH O H(R) C CH2 O O O OH C C FIGURE 6 The oxidation states of carbon in biomolecules. Each com￾pound is formed by oxidation of the red carbon in the compound listed above it. Carbon dioxide is the most highly oxidized form of carbon found in living systems. FIGURE 7 An oxidation-reduction reaction. Shown here is the oxi￾dation of lactate to pyruvate. In this dehydrogenation, two electrons and two hydrogen ions (the equivalent of two hydrogen atoms) are re￾moved from C-2 of lactate, an alcohol, to form pyruvate, a ketone. In cells the reaction is catalyzed by lactate dehydrogenase and the elec￾trons are transferred to a cofactor called nicotinamide adenine dinu￾cleotide. This reaction is fully reversible; pyruvate can be reduced by electrons from the cofactor. In Chapter 13 we discuss the factors that determine the direction of a reaction. CH3 Lactate Pyruvate lactate dehydrogenase CH CH3 OH C C C O O O 2H 2e  2H 2e  O O