Amino Acid Catabolism

Question 1

How are amino acids catabolized in different tissues?

Answer 1

Skeletal muscle → Uses amino acids as a fuel particularly branched chain amino acids like Ile, Leu, Val.

Liver → Uses amino acids as a fuel, especially as it is the only organ that can synthesise urea.

Small intestine → Uses amino acids from digestion and from the blood as a fuel.

Question 2

How can the amino group be removed from useful amino acids?

Answer 2

The amino group is transferred onto a small carbon compound using the enzyme aminotransferase. This forms a simple amino acid (eg. Ala, Glu, Gln) plus an alpha-ketoacid that can be used in the citric acid cycle.

Alanine + alpha-ketoglutarate → Pyruvate + Glutamate

Glutamate → alpha-ketoglutarate (NH2 not transferred to another compound)

Leucine → 3 x acetyl-CoA

Question 3

What happens to amino acid catabolism in the fed state?

Answer 3

Skeletal muscle → Takes up branched amino acids and uses them for fuel

Brain → only uses glucose for fuel

Liver → uses amino acids for fuel, but may also use them to synthesise glucose or fats.

Small intestine → uses amino acids from digestion and circulation as fuel, especially Glutamine

Question 4

What happens to amino acid catabolism during starvation?

Answer 4

Brain → uses glucose and keto acids as fuel

Liver → May use amino acids to synthesise glucose or keto acids, and the ones that can’t be converted are used as fuel

Small intestine → Uses amino acids from circulation, especially Glutamine

Skeletal muscle → Oxidises amino acids instead of glucose, and releases amino acids into the blood.