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Chapter 19: Problem 17

Glycolysis, the pyruvate dehydrogenase reaction, and the TCA cycle result in complete oxidation of a molecule of glucose to \(\mathrm{CO}_{2}\). Review the calculation of oxidation numbers for individual atoms in any molecule, and then calculate the oxidation numbers of the carbons of glucose, pyruvate, the acetyl carbons of acetyl-CoA, and the metabolites of the TCA cycle to convince yourself that complete oxidation of glucose involves removal of 24 electrons and that each acetyl-CoA through the TCA cycle gives up 8 electrons.

Short Answer

Expert verified
The total oxidation state of carbons in glucose is -6. In the case of pyruvate, it is +5 and for acetyl-CoA, it is +2. The TCA cycle metabolites are fully oxidized to form \(\mathrm{CO}_{2}\) with a carbon oxidation state of +4. Thus, during the complete oxidation of glucose, 24 electrons are removed while each acetyl-CoA gives up 8 electrons in the TCA cycle.

Step by step solution

01

Determine oxidation state of carbons in glucose

The oxidation state of any atom in its own elemental state is 0. In glucose (\(C_{6}H_{12}O_{6}\)), each carbon atom is -1 since it's bonded to more electronegative Oxygen and less electronegative Hydrogen. So, total oxidation number of carbons in glucose is -6.
02

Determine oxidation state of carbons in pyruvate

In pyruvate (\(C_{3}H_{4}O_{3}\)), pyruvate's carboxylic \(C\) carbons have an oxidation state of +3 while the one bonded to \(H\) is -1. Thus, in each pyruvate, the total oxidation state of carbons is (+3)+(+3)+(-1)= +5.
03

Determine oxidation state of acetyl carbons in acetyl-CoA

In acetyl-CoA (\(C_{2}H_{4}O_{2}S_{2}N_{7}P_{3}Co\)), the two carbons are part of an acetyl group (\(C_{2}H_{3}O\)) and have an oxidation state of -1 and +3 respectively. Thus, the total oxidation state of carbons in acetyl-CoA is +2.
04

Determine oxidation state of carbons in TCA cycle metabolites

TCA cycle metabolites have oxidized to \(\mathrm{CO}_{2}\), where the carbon has an oxidation state of +4.
05

Confirm electron removal

The difference between total oxidation state of carbons in glucose (-6) and in \(\mathrm{CO}_{2}\) (+4) is 10. As glucose is oxidized into two molecules of pyruvate first before being fully oxidized, and each glucose's oxidation to pyruvate gives out 2 electrons, thus total removal of electrons during glucose oxidation is 10x2 + 2 = 24. Similarly, the difference between total oxidation state of carbons in acetyl-CoA (+2) and in \(\mathrm{CO}_{2}\) (+4) is 2. Each acetyl-CoA molecule gives up 2x4 = 8 electrons in TCA cycle.

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Most popular questions from this chapter

Describe the labeling pattern that would result from the introduction into the TCA cycle of glutamate labeled at \(\mathrm{C}_{\gamma}\) with \(^{14} \mathrm{C}\)Describe the labeling pattern that would result from the introduction into the TCA cycle of glutamate labeled at \(\mathrm{C}_{\gamma}\) with \(^{14} \mathrm{C}\)

In most cells, fatty acids are synthesized from acetate units in the cytosol. However, the primary source of acetate units is the TCA cycle in mitochondria, and acetate cannot be transported directly from the mitochondria to the cytosol. Cells solve this problem by exporting citrate from the mitochondria and then converting citrate to acetate and oxaloacetate. Then, because cells cannot transport oxaloacetate into mitochondria directly, they must convert it to malate or pyruvate, both of which can be taken up by mitochondria. Draw a complete pathway for citrate export, conversion of citrate to malate and pyruvate, and import of malate and pyruvate by mitochondria. a. Which of the reactions in this cycle might require energy input? b. What would be the most likely source of this energy? c. Do you recognize any of the enzyme reactions in this cycle? d. What coenzymes might be required to run this cycle?

Describe the effect on the TCA cycle of (a) increasing the concentration of \(\mathrm{NAD}^{+},\) (b) reducing the concentration of \(\mathrm{ATP}\), and (c) increasing the concentration of isocitrate.

(Integrates with Chapter \(15 .\) ) The serine residue of isocitrate dehydrogenase that is phosphorylated by protein kinase lies within the active site of the enzyme. This situation contrasts with most other examples of covalent modification by protein phosphorylation, where the phosphorylation occurs at a site remote from the active site. What direct effect do you think such active-site phosphorylation might have on the catalytic activity of isocitrate dehydrogenase? (See Barford, D., 1991. Molecular mechanisms for the control of enzymic activity by protein phosphorylation. Biochimica et Biophysica Acta \(1133: 55-62 .\)

Describe the labeling pattern that would result if \(^{14} \mathrm{CO}_{2}\) were incorporated into the TCA cycle via the pyruvate carboxylase reaction.
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