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Chapter 18: Problem 4

Discuss the cycling of NADH and NAD \(^{+}\) in glycolysis and the related fermentation reactions.

Short Answer

Expert verified
NADH and NAD+ cycle in glycolysis and fermentation by acting as electron carriers. In glycolysis, NAD+ is reduced to NADH as it accepts high energy electrons. In fermentation, NADH donates those electrons back, regenerating NAD+ and allowing glycolysis to continue.

Step by step solution

01

Understanding Glycolysis and Fermentation

Glycolysis is the first stage of cellular respiration. It does not require oxygen and happens in the cytosol of the cell. It involves breaking down a glucose molecule into two pyruvate molecules (3 carbons each). NADH is produced in this process. Fermentation follows glycolysis when oxygen is not available. Remember that it allows for the recovery of NAD+ from NADH, letting glycolysis continue. There are two types of fermentation: lactic acid and alcohol fermentation.
02

Role of NADH and NAD+ in Glycolysis

During glycolysis, a total of 2 NAD+ molecules are reduced to NADH by gaining electrons. This happens during the fourth step of glycolysis, where each GA3P (Glyceraldehyde 3-Phosphate) molecule is oxidized, and consequently, NAD+ is reduced to NADH. Here, NAD+ acts as an electron acceptor. The NADH molecules, however, hold onto these high-energy electrons until they can be transferred to awaiting molecules in the next stage of cellular respiration, which is the Citric Acid Cycle if enough oxygen is present.
03

Role of NADH and NAD+ in Fermentation

In the absence of oxygen, fermentation takes place. Here, the NADH molecules created during glycolysis will donate their electrons back to the pyruvate molecules, which regenerates NAD+. This is a critical step because without the recycling of NAD+, glycolysis would stop, and no ATP would be produced. So, in lactic acid fermentation, NADH reduces pyruvate to form lactic acid, and NAD+ is replenished. In alcohol fermentation, pyruvate is converted to ethanol and carbon dioxide with the aid of NADH, which is also oxidized back to NAD+ in the process.
04

Understanding the cycling of NADH and NAD+

Therefore, NADH and NAD+ are cycled in glycolysis and fermentation. In glycolysis, NAD+ accepts high energy electrons producing NADH, effectively moving energy to the next stage of cellular respiration if oxygen is available. In fermentation, NADH donates electrons back to the products of glycolysis, regenerating NAD+ and allowing glycolysis (and therefore, ATP production) to continue in situations where oxygen is not available.

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Most popular questions from this chapter

(Integrates with Chapter 3 .) Enolase catalyzes the conversion of 2-phosphoglycerate to phosphoenolpyruvate \(+\mathrm{H}_{2} \mathrm{O}\). The standard free energy change, \(\Delta G^{\circ},\) for this reaction is \(+1.8 \mathrm{kJ} / \mathrm{mol}\). If the concentration of 2 -phosphoglycerate is \(0.045 \mathrm{m} M\) and the concentration of phosphoenolpyruvate is \(0.034 \mathrm{m} M\), what is \(\Delta G\), the free energy change for the enolase reaction, under these conditions?

For each of the following reactions, name the enzyme that carries out this reaction in glycolysis and write a suitable mechanism for the reaction.

Genetic defects in glycolytic enzymes can have serious consequences for humans. For example, defects in the gene for pyruvate kinase can result in a condition known as hemolytic anemia. Consult a reference to learn about hemolytic anemia, and discuss why such genetic defects lead to this condition.

Fructose bisphosphate aldolase in animal muscle is a class I aldolase, which forms a Schiff base intermediate between substrate (for example, fructose- 1,6 -bisphosphate or dihydroxyacetone phosphate and a lysine at the active site (see Figure 18.12 ). The chemical evidence for this intermediate comes from studies with aldolase and the reducing agent sodium borohydride, \(\mathrm{NaBH}_{4}\). Incubation of the enzyme with dihydroxyacetone phosphate and \(\mathrm{NaBH}_{4}\) inactivates the enzyme. Interestingly, no inactivation is observed if \(\mathrm{NaBH}_{4}\) is added to the enzyme in the absence of substrate. Write a mechanism that explains these observations and provides evidence for the formation of a Schiff base intermediate in the aldolase reaction.

If \(^{32}\) P-labeled inorganic phosphate were introduced to erythrocytes undergoing glycolysis, would you expect to detect \(^{32} \mathrm{P}\) in glycolytic intermediates? If so, describe the relevant reactions and the \(^{32} \mathrm{P}\) incorporation you would observe.
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