Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
ATP Cycle and Cellular Bioenergetics
Chapter Summary

Chemical Reactions proceed in such a direction that, at equilibrium, the total Entropy $S$ of the System and Its surroundings is maximized, while the Free energy $G$ of the reacting molecules is minimized. Each chemical reaction is characterized by a specific change in Standard Free Energy, $\Delta G^0'$; the standard conditions are defined as: Temperature 25°C, pressure 1 atm, concentrations of all reactants and products 1 M, and pH 7.0. $\Delta G^0'$ can be calculated using the equation $\Delta G^0' = -2.30 RT\lg K'_{eq}$ if the Equilibrium Constant $K'_{eq}$ of the given reaction is known. The value of $\Delta G^0'$ for the Hydrolysis of ATP to ADP and phosphate is $-7.3$ kcal/mol. For certain phosphorylated compounds, such as 3-phosphoglyceroyl phosphate and phosphoenolpyruvate (two intermediates in The pathway of glucose degradation to lactate), the $\Delta G^0'$ of hydrolysis is significantly more negative than that of ATP; they can therefore be classified as super-high-energy compounds. Certain other phosphorylated compounds, such as glucose-6-phosphate, are characterized by smaller absolute values of $\Delta G^0'$ of hydrolysis compared to ATP; these are termed low-energy compounds. Through the action of specific Kinases, phosphate groups from super-high-energy phosphates generated during Catabolism can be transferred to ADP, resulting in the synthesis of ATP. Other specific kinases catalyze The transfer of the terminal phosphate group of ATP to acceptor molecules, converting them into phosphorylated (low-energy) compounds that thereby become activated, enabling them to participate in biosynthetic reactions. Consequently, ATP plays The Role of a universal intermediate—a carrier of phosphate groups—in cellular METABOLISM. ATP also supplies the energy required for the contraction of Actin and Myosin filaments in Skeletal Muscle. This energy drives the sliding of the filaments past one another, resulting in Muscle contraction, during which ATP is hydrolyzed to ADP and phosphate. Creatine phosphate serves as a reservoir of high-energy phosphate groups in muscle and Nerve Cells. Mediated by creatine kinase, it can transfer its phosphate groups to ADP molecules. Chemical energy for membrane ATPases is likewise supplied by ATP, enabling them to transport $H^+$ ions and certain other cations across membranes against a concentration gradient.

ATP utilized in biosynthetic reactions can donate either an orthophosphate or a pyrophosphate group; the former yields ADP, while the latter yields AMP. The product of pyrophosphate Cleavage, AMP, is re-phosphorylated to ADP in a reaction catalyzed by adenylate kinase: $\text{ATP} + \text{AMP} \rightleftharpoons 2\text{ADP}$. Other nucleoside $5'$-triphosphates—such as GTP, UTP, CTP, dATP, and dTTP—also act as carriers of high-energy phosphate groups directed into various biosynthetic reactions; these same triphosphates serve as precursors in the Biosynthesis of Nucleic acids. In intact, respiring cells, the terminal phosphate group of ATP is continuously and very rapidly replaced at the expense of the inorganic phosphate pool; they maintain a steady-state condition in which the consumption of ATP, associated with the cleavage of its terminal phosphate group, is exactly counterbalanced by the resynthesis of ATP from ADP and phosphate.

References

Books

Atkinson D. E. Cellular Energy Metabolism and Its Regulation, Academic Press, New York, 1977. A valuable book emphasizing Structure/19.html">The Importance of the cellular energy charge in the Regulation of cellular metabolism.

Becker W. M. Energy and the Living Cell, Harper and Row, New York, 1977. A book that raises numerous problems and proposes approaches to their solution.

Blum H. F. Time’s Arrow and Evolution, 3d ed., Princeton University Press, Princeton, N.J., 1968. Essays and reflections on the role of entropy in biology.

Broda E. The Evolution of Bioenergetic Processes, Pergamon, Oxford, 1975. Comparative and evolutionary aspects of Bioenergetics.

Krebs H. A., Kornberg H. L. Energy Transformations in Living Matter, Springer, New York, 1967. A classical Analysis of the energetics of Glycolysis and Respiration.

Lehninger A. L. Bioenergetics, 2d ed., Benjamin, Menlo Park, Calif., 1971. An introductory course covering various aspects of bioenergetics.

Lipmann F. Wanderings of a Biochemist, Wiley, New York, 1971. A new edition of classic papers by one of the pioneers in bioenergetics, accompanied by his memoirs.

Miller G. T., Jr. Energetics, Kinetics and Life, Academic, New York, 1970. Bioenergetics and ecology.

Wood W. B., Wilson J. H., Benbow R. M., Hood L. E. Biochemistry: A Problems Approach, 2d ed., Benjamin, Inc., Menlo Park, Calif., 1981.

Articles

Erecinska M., Wilson D. F. Homeostatic Regulation of Cellular Energy Metabolism, Trends Biochem. Sci., 3, 221-223 (1978).

Gates D. M. The Flow of Energy in the Biosphere, Sci. Am., 224, 88-100, September 1971.

Ingraham L. L., Pardee A. B. Free Energy and Entropy in Metabolism, pp. 1-46. In: D. M. Greenberg (ed.), Metabolic Pathways, 3d ed., vol. 1, Academic, New York, 1967.

Satir P. How Cilia Move, Sci. Am., 231, 44-52, October 1974.

Tribus M., McIrvine K. C. Energy and Information, Sci. Am., 225, 179-188, September 1971.

Questions and Problems

1. Calculation of $\Delta G^0'$ from the equilibrium constant. Calculate the standard free energy change for the following metabolically important enzymatic reactions at 25°C, using the given equilibrium constant values (pH 7.0):

2. Calculation of the equilibrium constant from $\Delta G^0'$. Calculate the equilibrium constants for the following reactions at pH 7.0 and 25°C using the data given in Table 14-3:

3. The value of ∆G^0' for coupled reactions. Glucose-1-phosphate is converted into fructose-6-phosphate via two sequential reactions:

Class="center">Glucose-1-phosphate → Glucose-6-phosphate,

Glucose-6-phosphate → Fructose-6-phosphate.

Using the ∆G⁰' values given in Table 14-3, calculate the equilibrium constant K^'_eq for the overall reaction at 25 °C: Glucose-1-phosphate → Fructose-6-phosphate.

4. Strategy for overcoming unfavorable steps: ATP-dependent chemical coupling. The first step in Glucose Catabolism is the phosphorylation of glucose to yield glucose-6-phosphate. Direct phosphorylation of glucose by inorganic phosphate is described by the equation

a) Calculate the equilibrium constant for the reaction above. In rat Liver cells, the physiological concentrations of glucose and phosphate are maintained at approximately 4.8 mM. What will be the equilibrium concentration of glucose-6-phosphate in the direct phosphorylation of glucose by inorganic phosphate? Would such a metabolic pathway be considered viable for glucose catabolism? Explain your reasoning.

b) Theoretically, the concentration of glucose-6-phosphate can be increased by driving the reaction equilibrium to the right through an increase in the intracellular concentrations of glucose and phosphate. Assume that the phosphate concentration remains constant at 4.8 mM. By how much would the intracellular glucose concentration need to be increased so that the equilibrium concentration of glucose-6-phosphate reaches 250 µM (its normal physiological concentration)? Would this pathway be considered physiologically feasible, given that the maximum solubility of glucose is less than 1 M?

c) In this chapter, we discussed that glucose phosphorylation is coupled in The Cell to ATP hydrolysis; thus, a portion of the Free energy of ATP hydrolysis is used to drive the energetically unfavorable phosphorylation of glucose.

Calculate The values of ∆G⁰' and K'_eq for the overall reaction. For this ATP-dependent phosphorylation of glucose, what must the glucose concentration be to yield an intracellular glucose-6-phosphate concentration of 250 µM, if the concentrations of ATP and ADP are 3.38 and 1.32 mM, respectively? Can this coupled process be considered even a theoretically viable pathway for glucose phosphorylation in the cell? Why?

d) From a thermodynamic standpoint, coupling ATP hydrolysis with glucose phosphorylation is not impossible, but the exact mechanism remains unclear. Since coupling requires a common intermediate, one possibility is that ATP hydrolysis increases the intracellular concentration of inorganic phosphate, which in turn facilitates the thermodynamically unfavorable phosphorylation of glucose by inorganic phosphate. Does this pathway seem plausible to you? Justify your answer.

e) In liver cells, ATP-coupled glucose phosphorylation is catalyzed by the enzyme glucokinase. This enzyme binds ATP and glucose to form a glucose-ATP-enzyme complex, and the phosphoryl group is transferred directly from ATP to glucose. What are the advantages of such a pathway?

5. Calculation of ∆G⁰' values for ATP-coupled reactions. Based on the data in Table 14-5, calculate the ∆G⁰' values for the reactions:

a) Creatine phosphate + ADP →

→ Creatine + ATP;

б) ATP + Fructose →

→ ADP + Fructose-6-phosphate.

6. Calculation of ∆G⁰' under physiological conditions. Calculate the free energy change ∆G' (not ∆G⁰') under physiological conditions for the reaction

Creatine phosphate + ADP →

→ Creatine + ATP,

proceeding in the Cytosol of Brain cells at 25 °C with the following component concentrations: creatine phosphate, 4.7 mM; creatine, 1.0 mM; ADP, 0.20 mM; and ATP, 2.6 mM.

7. Free energy requirement for ATP synthesis under physiological conditions. In the rat liver cytosol, the mass-action ratio is

Calculate The amount of free energy required for ATP Synthesis in the rat liver.

8. Daily ATP turnover in The Human Body.

a) The amount of free energy required to synthesize ATP from ADP and inorganic phosphate (P_i) at a reactant and product concentration of 1 M (standard state) is 7.3 kcal/mol. Because the actual physiological concentrations of ADP, P_i, and ATP in cells differ from 1 M, the amount of free energy required for ATP synthesis under physiological conditions differs from ∆G⁰'. Calculate the amount of free energy required for ATP synthesis in a human liver cell at physiological concentrations of ATP, ADP, and P_i equal to 3.5, 1.50, and 5.0 mM, respectively.

b) A healthy adult weighing approximately 70 kg requires an energy intake of 2,000 kcal per day. Dietary nutrients are broken down during metabolism, and the released free energy is used to synthesize ATP, which is subsequently consumed to power the body's daily chemical and mechanical work. Calculate (in units of weight) the amount of ATP utilized daily by an adult human body, assuming an efficiency of 50% for The conversion of dietary energy into ATP energy. What percentage of body weight does this amount of ATP represent?

c) Although significant amounts of ATP are produced daily in the human body, body weight, structure, and composition do not change significantly over this period. How can this be explained?

9. ATP reserves in Muscle tissue. The concentration of ATP in muscle tissue (which is about 70% Water) is approximately 8.0 mM. During periods of intense muscular activity, ATP is consumed to support muscle contraction at a rate of 300 μmol/min per gram of muscle tissue.

a) How long will this ATP reserve last a sprinter running a 100-meter dash?

b) The concentration of Creatine phosphate in muscle tissue is approximately 40.0 mM. How much longer will creatine phosphate be able to sustain the muscle ATP reserve?

c) What makes a marathon run possible given such limited ATP reserves?

10. Metabolic cleavage of ATP to AMP and PP_i. The formation of the activated form of acetate (acetyl-CoA) is an ATP-dependent process

Acetate + CoA + ATP →

→ Acetyl-CoA + AMP + PP_i.

a) The ∆G⁰' values for the hydrolysis of acetyl-CoA to acetate and CoA, and of ATP to AMP and PP_i, are -7.5 and -7.3 kcal, respectively. Calculate the value of ∆G⁰' for the ATP-dependent synthesis of acetyl-CoA.

b) Almost all cells contain an enzyme (inorganic pyrophosphatase) that catalyzes the hydrolysis of PP_i to inorganic phosphate (P_i). How does the presence of this enzyme affect the synthesis of acetyl-CoA? Provide a comprehensive explanation.