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Chapter 4-4. Protein Degradation

Recommended Article: 【Biology】 Chapter 4. Cells and Energy Metabolism

1. Overview

2. 1^st. Hydrolysis

3. 2^nd. Deamination Reaction

4. 3^rd. Ornithine Cycle

5. 4^th. Metabolism of Alpha-Keto Acids

a. Protein Synthesis

1. Overview

⑴ Proteins are used as energy when carbohydrates are absent.

2. 1^st. Hydrolysis

⑴ Hydrolysis: Proteins are broken down into amino acids.

⑵ Example: Muscle protein breakdown during starvation.

3. 2^nd. Deamination Reaction

⑴ Overall Reaction: α-amino acid + NAD⁺ + H₂O ⇄ α-keto acid + NH₄⁺ + NADH + H⁺

⑵ 2^nd - 1^st. Action of aminotransferase (transaminase)

① α-amino acid + α-ketoglutarate → α-keto acid + glutamate

② PLP (pyridoxal phosphate) acts as a coenzyme.

⑶ 2^nd - 2^nd. Action of glutamate dehydrogenase

① Reaction: glutamate + NAD⁺ + H₂O ⇄ NH₄⁺ + α-ketoglutarate + NADH + H⁺

② Step 2-1. glutamate + NAD⁺ → NADH + H⁺ + Schiff base (intermediate)

③ Step 2-2. Schiff base + H₂O → NH₄⁺ + α-ketoglutarate

④ GPT, GOT

⑷ Types of alpha-keto acids

① Glutamate: Becomes α-Ketoglutaric acid.

② Alanine: Becomes pyruvate.

③ Aspartate: Becomes oxaloacetate.

④ Leucine, Lysine: Become acetyl CoA. Only these two amino acids do not lead to gluconeogenesis from amino acids.

⑤ Serine: Becomes pyruvate + NH₄⁺.

⑥ Threonine: Becomes α-ketobutyrate + NH₄⁺.

4. 3^rd. Ornithine Cycle

⑴ Also known as the glucose-alanine cycle.

Figure 1. Glucose-Alanine Cycle

⑵ 3^rd - 1^st. Reaction of pyruvate + NH₄⁺ → alanine occurs in muscle cells, etc.

⑶ 3^rd - 2^nd. Alanine travels to the liver through the blood.

⑷ 3^rd - 3^rd. Alanine → glutamate + pyruvate

① Glutamate → NH₄⁺ → urea

② Pyruvate → glucose

5. 4^th. Metabolism of alpha-keto acids

⑴ Non-acetyl CoA: Participates in the TCA cycle.

⑵ Acetyl CoA

① Indicates a state of starvation if protein is being decomposed.

② Oxaloacetate in the body is already used for gluconeogenesis for the brain.

③ Busy creating energy, hence no additional gluconeogenesis.

④ Excess acetyl CoA leads to ketone body formation since it is not actively used in the TCA cycle.

○ Ketone bodies: Acetoacetic acid, D-β-hydroxybutyric acid, acetone

○ Ketone bodies are used as an energy source for the heart, kidneys, muscles, and brain.

○ Excessive ketone body production lowers the pH of the blood.

Figure 2. Process of ketone body formation

Input: 2019.01.16 16:54