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Coupling of Production and Use of Energy
Another important question about metabolism is: ÒHow is the energy released by the oxidation of nutrients trapped and used?Ó This energy cannot be used directly; it must be shunted into an easily accessible form of chemical energy.
We saw that several phosphorus-containing compounds, such as ATP, can be hydrolyzed easily, and that the reaction releases energy. Formation of ATP is intimately linked with the release of energy from oxidation of nutrients. The coupling of energy-producing reactions and energy-requiring reactions is a central feature in the metabolism of all organisms.
The phosphorylation of ADP (adenosine diphosphate) to produce ATP (adenosine triphosphate) requires energy, which can be supplied by the oxidation of nutrients. Conversely, the hydrolysis of ATP to ADP releases energy (Figure 15.5).
The forms of ADP and ATP shown are in their ionization states for pH 7. The symbol Pi for phosphate ion comes from its name in biochemi-cal jargon, Òinorganic phosphate.Ó Note that there are four negative charges on ATP and three on ADP; electrostatic repulsion makes ATP less stable than ADP. Energy must be expended to put an additional negatively charged phos-phate group on ADP by forming a covalent bond to the phosphate group being added. In addition, there is an entropy loss when ADP is phosphorylated to ATP. Inorganic phosphate can adopt multiple resonance structures, and the loss of these potential structures results in a decrease in entropy when the phos-phate is attached to ADP (Figure 15.6). The ∆G° for the reaction refers to the usual biochemical convention of pH 7 as the standard state for hydrogen ion. Note, however, that there is a marked decrease in electrostatic repulsion on phosphorylation of ADP to ATP (Figure 15.7).
The reverse reaction, the hydrolysis of ATP to ADP and phosphate ion, releases 30.5 kJ mol-1 (7.3 kcal mol-1) when energy is needed:
ATP + H2O - > ADP + Pi + H+
∆G° = -30.5 kJ mol-1 = -7.3 kcal mol-1
The bond that is hydrolyzed when this reaction takes place is sometimes called a Òhigh-energy bond,Ó which is shorthand terminology for a reaction in which hydrolysis of a speciÞc bond releases a useful amount of energy. Another way of indicating such a bond is ~P. Numerous organophosphate compounds with high-energy bonds play roles in metabolism, but ATP is by far the most important (Table 15.1). In some cases, the free energy of hydrolysis of organophosphates is higher than that of ATP and is thus able to drive the phosphorylation of ADP to ATP.
Phosphoenolpyruvate (PEP), a molecule we shall encounter when we look at glycolysis, tops the list. It is a very high-energy compound because of the resonance stabilization of the liberated phosphate when it is hydrolyzed (the same effect as that seen with ATP) and because keto-enol tautomerization of pyruvate is a possibility. Both effects increase the entropy upon hydrolysis (Figure 15.8).
The energy of hydrolysis of ATP is not stored energy, just as an electric cur-rent does not represent stored energy. Both ATP and electric current must be produced when they are neededÑby organisms or by a power plant, as the case may be. The cycling of ATP and ADP in metabolic processes is a way of shunting energy from its production (by oxidation of nutrients) to its uses (in processes such as biosynthesis of essential compounds or muscle contraction) when it is needed. The oxidation processes take place when the organism needs the energy that can be generated by the hydrolysis of ATP. When chemical energy is stored, it is usually in the form of fats and carbohydrates, which are metabo-lized as needed. Certain small biomolecules, such as creatine phosphate, can also serve as vehicles for storing chemical energy. The energy that must be sup-plied for the many endergonic reactions in life processes comes directly from the hydrolysis of ATP and indirectly from the oxidation of nutrients. The latter produces the energy needed to phosphorylate ADP to ATP (Figure 15.9).
Let us examine some biological reactions that release energy and see how some of that energy is used to phosphorylate ADP to ATP. The multistep conversion of glucose to lactate ions is an exergonic and anaerobic process. Two molecules of ADP are phosphorylated to ATP for each molecule of glu-cose metabolized. The basic reactions are the production of lactate, which is exergonic,
Glucose - > 2 Lactate ions ∆G° = -184.5 kJ mol−1= -44.1 kcal mol−1
and the phosphorylation of two moles of ADP for each mole of glucose, which is endergonic.
2ADP + 2Pi - > 2ATP
∆G°' = 61.0 kJ mol−1= 14.6 kcal mol−1
(In the interest of simplicity, we shall write the equation for phosphorylation of ADP in terms of ADP, Pi, and ATP only.) The overall reaction is
Glucose + 2ADP + 2Pi - > 2 Lactate ions + 2ATP
∆G°' overall = -184.5 + 61.0 = -123.5 kJ mol−1= -29.5 kcal mol−1
Not only can we add the two chemical reactions to obtain an equation for the overall reaction, we can also add the free-energy changes for the two reactions to Þnd the overall free-energy change. We can do this because the free-energy change is a state function; it depends only on the initial and Þnal states of the system under consideration, not on the path between those states. The exergonic reaction provides energy, which drives the endergonic reaction. This phenomenon is called coupling. The percentage of the released energy that is used to phosphorylate ADP is the efÞciency of energy use in anaerobic metabolism; it is (61.0/184.5) 100, or about 33%. The number 61.0 comes from the number of kilojoules required to phosphorylate 2 moles of ADP to ATP, and the number 184.5 is the number of kilojoules released when 1 mole of glucose is converted to 2 moles of lactate.
The breakdown of glucose goes further under aerobic conditions than under anaerobic conditions. The end products of aerobic oxidation are 6 molecules of carbon dioxide and 6 molecules of water for each molecule of glucose. Up to 32 molecules of ADP can be phosphorylated to ATP when 1 molecule of glucose is broken down completely to carbon dioxide and water.
The exergonic reaction for the complete oxidation of glucose is
Glucose + 6O2 - > 6CO2 + 6H2O
∆G° = -2867 kJ mol−1= -685.9 kcal mol−1
The endergonic reaction for phosphorylation is
32ADP + 32Pi - > 32ATP
∆G° = 976 kJ = 233.5 kcal
The net reaction is
Glucose + 6O2 + 32ADP + 32Pi - > 6CO2 + 6H2O + 32ATP
∆G° = -2867 + 976 = -1891 kJ mol−1= -452.4 kcal mol−1
Note that, once again, we add the two reactions and their respective free-energy changes to obtain the overall reaction and its free-energy change. The efÞciency of aerobic oxidation of glucose is (976/2867) 100, about 34%. (We performed this calculation in the same way that we did with the example of anaerobic oxidation of glucose.) More ATP is produced by the coupling process in aerobic oxidation of glucose than by the coupling process in anaerobic oxidation. The hydrolysis of ATP produced by breakdown (aerobic or anaerobic) of glucose can be coupled to endergonic processes, such as muscle contraction in exercise. As any jogger or long-distance swimmer knows, aerobic metabolism involves large quantities of energy, processed in a highly efÞcient fashion. We have now seen two examples of coupling of exergonic and endergonic processesÑ aerobic oxidation of glucose and anaerobic fermentation of glucoseÑinvolving different amounts of energy.
Hydrolysis of ATP to ADP releases energy.
In the coupling of biochemical reactions, the energy released by one reac-tion, such as ATP hydrolysis, provides energy for another.
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