Biochemistry
Biochemistry
9th Edition
ISBN: 9781305961135
Author: Mary K. Campbell, Shawn O. Farrell, Owen M. McDougal
Publisher: Cengage Learning
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Chapter 19, Problem 40RE
Interpretation Introduction

Interpretation:

The overall exergonic reaction of the citric cycle is to be shown despite the fact that some of the reactions in the citric acid cycle are endergonic in nature.

Concept introduction:

 The citric acid cycle, also known as the tricarboxylic acid cycle, is an important part of the aerobic respiration that takes place inside the mitochondria.

 Gibbs free energy change or standard free energy change is the amount of energy released after a chemical reaction, which is available to do work. The change in standard energy is represented by the symbol Δ, and hence, the Gibbs free energy is represented by ΔG0.

 An endergonic chemical reaction is one that absorbs energy in the form of heat and an exergonic reaction is one that releases energy.

Expert Solution & Answer
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Answer to Problem 40RE

Solution: Though one reaction in the citric acid cycle is endergonic in nature, the overall citric acid cycle is exergonic, because the overall standard free change is 44.3kJ/mol.

Explanation of Solution

Given information:

The table for individual reactions of the citric acid cycle is as follows:

StepReaction ΔG° (kJ/mol)Pyruvate+CoA-SH+NAD+Acetyl-CoA+NADH+CO233.41Acetyl-CoA+Oxaloacetate+H2OCitrate+CoA-SH+H+32.22CitrateIsocitrate+6.33Isocitrate+NAD+α Ketoglutarate+NADH+CO2+H+7.14α Ketoglutarate+NAD++CoA-SHSuccinyl CoA+NADH +CO2+H+33.45Succinyl CoA+GDP+PiSuccinate+GTP+CoA-SH3.36Succinate+FADFumarate+FADH207Fumarate+H2OL-Malate3.88L-Malate+NAD+Oxaloacetate+NADH+H++29.2OverallPyruvate+4NAD++FAD+GDP+Pi+2H2OCO2+4NADH+FADH2+GTP+H+77.7

Any given metabolic reaction is a favorable reaction only when the value of Gibbs free energy (ΔGo) is negative. A negative value of ΔGo indicates an exergonic process. If a process is endergonic or has a positive ΔGo, then it is immediately followed by another set of reactions in which the value ΔGo is more negative. For example, in the citric acid cycle, there is only one reaction in which the energy is absorbed as a result of which the standard free energy change is positive. The endergonic reaction that takes place during the citric acid cycle is the oxidation of malate to OAA (oxaloacetate).

As observed from the table, the value of standard free energy change for the oxidation of malate into OAA is +29.2 kilojoules per mole (kJ/mol). This endergonic reaction is immediately coupled to the exergonic reaction in which citrate and CoA are produced as a result of condensation between OAA and acetyl CoA. The overall citric acid cycle is exergonic, even though the conversion of malate to OAA in the cycle is endergonic. The citric acid cycle is exergonic with an overall standard free energy change value of 44.3kJ/mol.

The overall value of standard free energy change for a citric acid cycle is obtained by subtracting the ΔGo for pyruvate as follows:

ΔG0=77.7kJ/mol(33.4)kJ/mol=(77.7+33.4) kJ/mol=44.3 kJ/mol

Conclusion

There is an endergonic reaction in the citric acid cycle in which malate is converted into OAA; however, the overall citric acid cycle is exergonic in nature because the overall free energy change of the cycle is 44.3kJ/mol. A negative value of standard free energy change indicates a spontaneous and favorable reaction.

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Chapter 19 Solutions

Biochemistry

Ch. 19 - Prob. 11RECh. 19 - Prob. 12RECh. 19 - Prob. 13RECh. 19 - Prob. 14RECh. 19 - RECALL Why is the reaction catalyzed by citrate...Ch. 19 - RECALL What does it mean when an enzyme has the...Ch. 19 - Prob. 17RECh. 19 - RECALL With respect to stereochemistry, what is...Ch. 19 - Prob. 19RECh. 19 - Prob. 20RECh. 19 - Prob. 21RECh. 19 - RECALL What are the similarities and differences...Ch. 19 - Prob. 23RECh. 19 - Prob. 24RECh. 19 - Prob. 25RECh. 19 - RECALL Why can we say that production of a GTP is...Ch. 19 - Prob. 27RECh. 19 - RECALL ATP is a competitive inhibitor of NADH...Ch. 19 - RECALL Is the conversion of fumarate to malate a...Ch. 19 - REFLECT AND APPLY We have seen one of the four...Ch. 19 - Prob. 31RECh. 19 - Prob. 32RECh. 19 - Prob. 33RECh. 19 - Prob. 34RECh. 19 - REFLECT AND APPLY How does an increase in the...Ch. 19 - REFLECT AND APPLY How does an increase in the...Ch. 19 - Prob. 37RECh. 19 - Prob. 38RECh. 19 - Prob. 39RECh. 19 - Prob. 40RECh. 19 - REFLECT AND APPLY How could the expression milking...Ch. 19 - Prob. 42RECh. 19 - Prob. 43RECh. 19 - Prob. 44RECh. 19 - BIOCHEMICAL CONNECTIONS Why is it possible for...Ch. 19 - RECALL Describe the various purposes of the citric...Ch. 19 - REFLECT AND APPLY The intermediates of glycolysis...Ch. 19 - Prob. 48RECh. 19 - REFLECT AND APPLY Many soft drinks contain citric...Ch. 19 - RECALL NADH is an important coenzyme in catabolic...Ch. 19 - BIOCHEMICAL CONNECTIONS What are the anaplerotic...Ch. 19 - REFLECT AND APPLY Why is acetyl-CoA considered the...Ch. 19 - Prob. 53RE
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