4.2.10 Cellular Respiration of Fats and Proteins

Introduction

Carbohydrates are not the only molecules that can be used for cellular respiration. Fats and proteins can also be broken down to release energy. When fats are used as an energy source, they are broken down into glycerol and fatty acids. When proteins are used, they are broken down into amino acids. These products then enter the respiratory pathways at specific points to produce ATP.

Definition

Cellular Respiration of Fats and Proteins: The breakdown of fats and proteins to release energy through cellular respiration.

In simple words: Fats and proteins can also be used to produce energy through cellular respiration.

Core Concept

• When a fat is used as an energy source, it breaks down to glycerol and three fatty acids.
• Glycerol is converted to G3P, a metabolite in glycolysis.
• Fatty acids are converted to acetyl-CoA, which enters the Krebs cycle.
• An 18-carbon fatty acid results in nine acetyl-CoA molecules.
• The hydrolysis of proteins results in amino acids.
• The R-group size determines whether the carbon chain is oxidized in glycolysis or the Krebs cycle.
• The carbon chain is produced in the liver when an amino acid undergoes deamination, i.e., the removal of the amino group.
• The amino group becomes ammonia (NH₃), which enters the urea cycle and becomes part of urea.

Detailed Explanation

Cellular Respiration of Fats

• Fats (triglycerides) are broken down into:
• Glycerol
• Three fatty acids
• This process is called lipolysis.

• Glycerol is converted to G3P (glyceraldehyde-3-phosphate).
• G3P is a metabolite in glycolysis.
• G3P enters the glycolysis pathway.
• It can be converted to pyruvate.
• Pyruvate enters the Krebs cycle.

• Fatty acids are converted to acetyl-CoA through a process called beta-oxidation.
• Acetyl-CoA enters the Krebs cycle.
• An 18-carbon fatty acid results in nine acetyl-CoA molecules.
• Each acetyl-CoA enters the Krebs cycle.

• Fatty acids are broken down into two-carbon units.
• Each two-carbon unit is converted to acetyl-CoA.
• This process occurs in the mitochondrial matrix.
• It produces NADH and FADH₂.

• Fats are more reduced than carbohydrates.
• They contain more hydrogen and less oxygen.
• Therefore, they produce more ATP per gram than carbohydrates.
• 1 gram of fat produces about 9 kcal of energy.
• 1 gram of carbohydrate produces about 4 kcal of energy.

Cellular Respiration of Proteins

• Proteins are broken down into amino acids.
• This process is called proteolysis.

• Amino acids undergo deamination in the liver.
• Deamination is the removal of the amino group (-NH₂).
• The amino group becomes ammonia (NH₃).
• Ammonia enters the urea cycle and becomes part of urea.
• Urea is excreted in the urine.

• The remaining carbon skeleton (without the amino group) is oxidized.
• The carbon chain is converted to:
• Pyruvate
• Acetyl-CoA
• Krebs cycle intermediates
• The R-group size determines the entry point:
• Small R-group → Pyruvate → Acetyl-CoA
• Medium R-group → Acetyl-CoA
• Large R-group → Krebs cycle intermediates

Entry Points of Amino Acids:

The Metabolic Pool Concept

The metabolic pool concept explains how different biomolecules can be interconverted through common intermediates.

• When carbohydrates, fats, and proteins are used as energy sources, they enter degradative pathways at specific points.
• Degradation produces metabolites that can be used for the synthesis of other biomolecules or may release ATPs.
• G3P, pyruvate, and acetyl-CoA are key metabolites in this pool.

Diagrammatic Representation:

Comparison of Energy Yield

Key Point: Fats produce more than twice the energy of carbohydrates or proteins per gram.

Summary of Fats and Proteins Respiration

Intermediate Level

Cellular Respiration of Fats in Detail

• Occurs in the mitochondrial matrix.
• Fatty acids are broken into two-carbon units.
• Each unit becomes acetyl-CoA.
• Produces NADH and FADH₂.

Cellular Respiration of Proteins in Detail

• Occurs in the liver.
• Removal of the amino group (-NH₂).
• Produces ammonia (NH₃).
• Ammonia is toxic and must be converted to urea.

Entry Points of Amino Acids

Comparison of Energy Yield

The Metabolic Pool Concept

Important Points

• 1. When a fat is used as an energy source, it breaks down to glycerol and three fatty acids.
• 2. Glycerol is converted to G3P, a metabolite in glycolysis.
• 3. The fatty acids are converted to acetyl-CoA, which enters the Krebs cycle.
• 4. An 18-carbon fatty acid results in nine acetyl-CoA molecules.
• 5. The hydrolysis of proteins results in amino acids whose R-group size determines whether the carbon chain is oxidized in glycolysis or the Krebs cycle.
• 6. The carbon chain is produced in the liver when an amino acid undergoes deamination, i.e., the removal of the amino group.
• 7. The amino group becomes ammonia (NH₃), which enters the urea cycle and becomes part of urea.

Remember

• - Fats = glycerol + fatty acids
• - Glycerol → G3P
• - Fatty acids → Acetyl-CoA
• - Proteins = amino acids
• - Amino acids → Deamination
• - Ammonia → Urea
• - **Fats** = 9 kcal/g
• - **Carbohydrates** = 4 kcal/g
• - **Proteins** = 4 kcal/g
• - Beta-oxidation = fatty acid breakdown
• - Deamination = amino acid breakdown
• - Metabolic pool = interconversion of biomolecules

Common Mistakes

• 1. Thinking fats are broken down into glucose directly – Fats are broken down into glycerol and fatty acids; glycerol can be converted to G3P, but fatty acids cannot be converted to glucose in humans.
• 2. Thinking proteins are the primary energy source – Proteins are used for energy only when carbohydrates and fats are depleted.
• 3. Confusing deamination and transamination – Deamination = removal of amino group; Transamination = transfer of amino group.
• 4. Thinking ammonia is excreted directly – Ammonia is converted to urea in the liver before excretion.
• 5. Forgetting the energy yield of fats – Fats produce 9 kcal/g, more than carbohydrates or proteins.
• 6. Confusing beta-oxidation and deamination – Beta-oxidation = fatty acid breakdown; Deamination = amino acid breakdown.
• 7. Thinking all amino acids enter at the same point – Different amino acids enter at different points.
• 8. Forgetting the role of the liver – The liver is the main site of deamination and urea cycle.
• 9. Thinking fats cannot be used for energy – Fats are an important energy source.
• 10. Confusing the metabolic pool concept – The metabolic pool explains how different biomolecules are interconverted.
• 11. Thinking proteins are stored in the body – Proteins are not stored; excess amino acids are deaminated and excreted.
• 12. Forgetting the role of acetyl-CoA – Acetyl-CoA is a central metabolite in fat and protein metabolism.

Quick Revision

• When a fat is used as an energy source, it breaks down to glycerol and three fatty acids.
• As figure 4.20 indicates, glycerol is converted to G3P, a metabolite in glycolysis.
• The fatty acids are converted to acetyl-CoA, which enters the Krebs cycle.
• An 18-carbon fatty acid results in nine acetyl-CoA molecules.
• The hydrolysis of proteins results in amino acids whose R-group size determines whether the carbon chain is oxidized in glycolysis or the Krebs cycle.
• The carbon chain is produced in the liver when an amino acid undergoes deamination, i.e., the removal of the amino group.
• The amino group becomes ammonia (NH₃), which enters the urea cycle and becomes part of urea.

60-Second Revision

• Fats = glycerol + fatty acids
• Glycerol → G3P
• Fatty acids → Acetyl-CoA
• Proteins = amino acids
• Amino acids → Deamination
• Ammonia → Urea
• **Fats** = 9 kcal/g
• **Carbohydrates** = 4 kcal/g
• **Proteins** = 4 kcal/g
• Beta-oxidation = fatty acid breakdown
• Deamination = amino acid breakdown
• Metabolic pool = interconversion of biomolecules
• Warning: Fats are broken down into glycerol and fatty acids. Glycerol enters glycolysis; fatty acids enter the Krebs cycle. Proteins are broken down into amino acids. Deamination removes the amino group. Ammonia is converted to urea. Fats produce more energy per gram than carbohydrates or proteins. The liver is the main site of deamination. Different amino acids enter at different points. Proteins are not stored in the body.