| CONSERVATION OF ENERGY BY COUPLING WITH ATP
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| Living systems must transfer energy from one molecule to another without losing all of it as heat. Some of the energy must be conserved in a chemical form in order to drive nonspontaneous biosynthetic reactions. In fact, nearly half of the energy obtained from the oxidation of metabolic fuels is channeled into the synthesis of ATP, a universal energy transducer in living systems. ATP is often referred to as the common currency of metabolic energy, because it is used to drive so many energy-requiring reactions. ATP consists of the purine base, adenine, the five-carbon sugar, ribose, and α, β, and γ phosphate groups (Fig. 8.2). The two anhydride linkages are said to be high-energy bonds, because their hydrolysis yields a large negative change in free energy. When ATP is used for metabolic work, these high-energy linkages are broken and it is converted to ADP or to AMP.
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Figure 8.2 Structures of high-energy phosphates. ATP is shown, together with its hydrolysis products, adenosine diphosphate (ADP) and adenosine monophosphate (AMP). |
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| ATP is commonly used to drive biosynthetic reactions
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The free energy of a high-energy bond, such as the phosphate anhydride bonds in ATP, can be used to drive or push forward reactions that would otherwise be unfavorable. In fact, nearly all biosynthetic pathways are thermodynamically unfavorable, but are made favorable by coupling various reactions with hydrolysis of high-energy compounds. For example, the first step in the metabolism of glucose is the synthesis of Glc-6-P. As shown in Table 8.2, this is not a favorable reaction: the hydrolysis (ΔG°' = -13.8 kJ/mol or -3.3 kcal/mol), rather than synthesis (ΔG°' = +13.8 kJ/mol or +3.3 kcal/mol) of Glc-6-P is the favored reaction. However, as shown below, the synthesis of Glc-6-P (reaction I) can be energetically coupled to the hydrolysis of ATP (reaction II), yielding a 'net reaction' III that is favorable for synthesis of Glc-6-P:
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| This is possible because of the high free energy or 'group transfer potential' of ATP. The physical transfer of the phosphate from ATP to glucose occurs in the active site of a kinase enzyme, such as glucokinase. This motif, in which ATP is used to drive biosynthetic reactions, transport processes, or muscle activity, occurs commonly in metabolic pathways.
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| NAD+, FAD, and FMN are the major redox coenzymes
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| The major redox coenzymes involved in transduction of energy from fuels to ATP are nicotinamide adenine dinucleotide (NAD+), flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) (Fig. 8.3). During energy metabolism, electrons are transferred from carbohydrates and fats to these coenzymes, reducing them to NADH, FADH2 and FMNH2. In each case, two electrons are transferred, but the number of protons transferred differs. NAD+ accepts a hydride ion (H-) that consists of one proton and two electrons; the remaining proton is released into solution. FAD and FMN accept two electrons and two protons.
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