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Ch.22 Carbohydrate Metabolism
McMurry - Fundamentals of General, Organic, and Biological Chemistry 8th Edition
McMurry8th EditionFundamentals of General, Organic, and Biological ChemistryISBN: 9780134015187Non è quello che usi tu?Cambia libro di testo
Tutti i libri di testoMcMurry 8th EditionCh.22 Carbohydrate MetabolismProblema 11
Capitolo 22, Problema 11

Pyruvate has three different fates. What are the three different molecules pyruvate is converted into? What conditions exist for the formation of each product?

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Pyruvate is a key intermediate in cellular metabolism, and its fate depends on the cellular conditions. The three main molecules pyruvate can be converted into are lactate, acetyl-CoA, and ethanol. Let's explore each pathway and the conditions under which it occurs.
1. **Conversion to Lactate**: Under anaerobic conditions (lack of oxygen), pyruvate is reduced to lactate by the enzyme lactate dehydrogenase. This process regenerates NAD⁺, which is essential for glycolysis to continue. This occurs in muscle cells during intense exercise or in certain microorganisms.
2. **Conversion to Acetyl-CoA**: Under aerobic conditions (presence of oxygen), pyruvate is transported into the mitochondria and converted into acetyl-CoA by the enzyme pyruvate dehydrogenase. Acetyl-CoA then enters the citric acid cycle (Krebs cycle) for further energy production.
3. **Conversion to Ethanol**: In some microorganisms, such as yeast, pyruvate is converted into ethanol and carbon dioxide under anaerobic conditions. This process, known as alcoholic fermentation, involves two steps: (a) pyruvate is decarboxylated to acetaldehyde by the enzyme pyruvate decarboxylase, and (b) acetaldehyde is reduced to ethanol by alcohol dehydrogenase.
These three pathways highlight how pyruvate serves as a metabolic crossroads, with its fate determined by the availability of oxygen and the type of organism or cell involved.

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Pyruvate Decarboxylation

Pyruvate decarboxylation is the process by which pyruvate, produced from glycolysis, is converted into acetyl-CoA. This reaction occurs in the mitochondria and is catalyzed by the pyruvate dehydrogenase complex. Acetyl-CoA is a crucial substrate for the Krebs cycle, linking glycolysis to aerobic respiration.
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Pyruvate Oxidation Concept 2

Lactic Acid Fermentation

Lactic acid fermentation is an anaerobic process where pyruvate is converted into lactic acid, primarily in muscle cells and certain bacteria. This occurs when oxygen is scarce, allowing for ATP production without the Krebs cycle. It helps regenerate NAD+, which is essential for glycolysis to continue.
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Naming Acids

Alcoholic Fermentation

Alcoholic fermentation is another anaerobic process where pyruvate is converted into ethanol and carbon dioxide, primarily by yeast. This process occurs in the absence of oxygen and is utilized in brewing and baking. It also regenerates NAD+, allowing glycolysis to proceed and produce ATP.
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Alcohol Classification Concept 2
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In alcoholic fermentation, each mole of pyruvate is converted to one mole of carbon dioxide and one mole of ethanol. In the process, about 50 kcal/mol (209 kJ/mol) of energy is produced. Under the most favorable conditions, more than one-half of this energy is stored as ATP.

a. What happens to the remaining energy produced in alcoholic fermentation?

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What is the purpose of the Cori cycle?

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Glycolysis of one molecule of glucose produces 8 ATP molecules. How many ATP molecules are produced from glycolysis of 10 glucose molecules?

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What two types of reactions convert glycerol to dihydroxyacetone phosphate?

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In Figure 22.3, compare the starting compound (glucose) and the final product (pyruvate).

a. Which is oxidized to a greater extent?

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Use curved arrows (like those in Figure 22.3) to write an equation for the conversion of fructose to fructose 6-phosphate by ATP. At what step does fructose 6-phosphate enter glycolysis?

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