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Chapter 4-4. Protein Catabolism

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 Note : 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 degraded

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