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UPSC MainsZOOLOGY-PAPER-II201120 Marks
Q23.

Draw a labelled flow chart of β-oxidation of fats to display the types of enzymes that catalyze the formation of intermediates and step-wise release of ATP.

How to Approach

This question requires a detailed understanding of beta-oxidation. The approach should be to first define beta-oxidation and its location within the cell. Then, a step-by-step flowchart illustrating the process, including the enzymes involved at each stage and the ATP yield, needs to be drawn. Emphasis should be placed on accurately depicting the cyclical nature of the process and the intermediates formed. The answer should be presented in a clear, labelled manner, suitable for a zoology student.

Model Answer

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Introduction

Beta-oxidation is the catabolic process by which fatty acid molecules are broken down in the mitochondria and peroxisomes to generate acetyl-CoA, which can then enter the citric acid cycle and ultimately lead to ATP production. This process is crucial for energy production, particularly during periods of fasting or prolonged exercise when glucose availability is limited. It’s a highly regulated pathway, ensuring efficient energy extraction from stored fats. The process involves a series of four repeating steps, each catalyzed by specific enzymes, resulting in the stepwise removal of two-carbon units from the fatty acid chain.

β-Oxidation of Fatty Acids: A Step-by-Step Flowchart

The following flowchart details the process of β-oxidation, highlighting the enzymes involved and the ATP yield at each stage. The example used will be Palmitic acid (a 16-carbon fatty acid).

Beta-Oxidation Flowchart

Detailed Steps & Enzymes Involved:

  1. Activation: Fatty acids are first activated in the cytosol by attaching to Coenzyme A (CoA) to form Fatty Acyl-CoA. This reaction is catalyzed by Acyl-CoA synthetase and requires ATP. ( -2 ATP equivalent)
  2. Transport into Mitochondria: Fatty Acyl-CoA is transported across the inner mitochondrial membrane via the Carnitine shuttle. This involves Carnitine palmitoyltransferase I (CPT I) on the outer mitochondrial membrane and Carnitine acyltransferase II (CAT II) on the inner mitochondrial membrane.
  3. Step 1: Acyl-CoA Dehydrogenase: Fatty Acyl-CoA is oxidized by Acyl-CoA dehydrogenase, forming trans-Δ2-enoyl-CoA. This reaction generates FADH2. Different isoforms of Acyl-CoA dehydrogenase exist for short, medium, and long-chain fatty acids. (1 FADH2 = 1.5 ATP)
  4. Step 2: Enoyl-CoA Hydratase: trans-Δ2-enoyl-CoA is hydrated by Enoyl-CoA hydratase, forming L-β-hydroxyacyl-CoA.
  5. Step 3: β-Hydroxyacyl-CoA Dehydrogenase: L-β-hydroxyacyl-CoA is oxidized by β-hydroxyacyl-CoA dehydrogenase, forming β-ketoacyl-CoA. This reaction generates NADH. (1 NADH = 2.5 ATP)
  6. Step 4: β-Ketothiolase: β-ketoacyl-CoA is cleaved by β-ketothiolase (also known as acyl-CoA acetyltransferase), releasing acetyl-CoA and a fatty acyl-CoA molecule that is two carbons shorter.
  7. Repetition: The shortened fatty acyl-CoA molecule then re-enters the cycle, repeating steps 3-6 until the fatty acid is completely broken down into acetyl-CoA molecules.

ATP Yield from Palmitic Acid (16-carbon fatty acid)

  • Activation: -2 ATP
  • 7 cycles of β-oxidation:
    • 7 FADH2 = 7 x 1.5 ATP = 10.5 ATP
    • 7 NADH = 7 x 2.5 ATP = 17.5 ATP
  • 8 Acetyl-CoA molecules: Each acetyl-CoA enters the citric acid cycle, yielding approximately 10 ATP (2.5 ATP per NADH, 1.5 ATP per FADH2, and 1 ATP per GTP). 8 x 10 ATP = 80 ATP
  • Total ATP yield: -2 + 10.5 + 17.5 + 80 = 106 ATP

It's important to note that the actual ATP yield can vary slightly depending on the efficiency of the electron transport chain and proton gradient.

Conclusion

Beta-oxidation is a vital metabolic pathway for energy production from fats. The cyclical process, involving four key enzymatic steps, efficiently breaks down fatty acids into acetyl-CoA, which fuels the citric acid cycle and oxidative phosphorylation. Understanding the enzymes involved and the ATP yield is crucial for comprehending energy metabolism and its regulation. Dysregulation of beta-oxidation can lead to various metabolic disorders, highlighting its importance in maintaining overall health.

Answer Length

This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.

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Additional Resources

Key Definitions

Acyl-CoA
A molecule composed of a fatty acid chain linked to Coenzyme A, serving as the activated form of fatty acids for metabolism.
Ketogenesis
The metabolic pathway that results in the formation of ketone bodies (acetoacetate, β-hydroxybutyrate, and acetone) from acetyl-CoA, often occurring when carbohydrate availability is limited and fatty acid oxidation is high.

Key Statistics

Approximately 9 calories per gram of fat are available to the body, compared to 4 calories per gram of carbohydrates or protein. This is due to the higher energy density of fatty acids.

Source: National Institutes of Health (NIH), 2023 (Knowledge Cutoff)

Approximately 30-60% of energy in a typical Western diet comes from fats, highlighting the significant contribution of beta-oxidation to overall energy production.

Source: World Health Organization (WHO), 2022 (Knowledge Cutoff)

Examples

Carnitine Deficiency

Carnitine deficiency, a genetic disorder, impairs the transport of fatty acids into the mitochondria, leading to impaired beta-oxidation and energy production. This can cause muscle weakness, cardiomyopathy, and other serious health problems.

Frequently Asked Questions

▶What is the role of carnitine in beta-oxidation?

Carnitine acts as a shuttle to transport long-chain fatty acids across the inner mitochondrial membrane, where beta-oxidation occurs. Without carnitine, fatty acids cannot enter the mitochondria for breakdown.

Topics Covered

BiologyBiochemistryMetabolismLipid MetabolismEnergy Production
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