2.5. Metabolism of individual amino acids

1. Metabolism of glycine, serine and threonine occurs with the participation of enzymes, containing derivatives of folic acid. Tetrahydrofolate is involved in the reaction of interconversion of glycine and serine:

The main pathway of glycine catabolism is it’s splitting into CO₂, NH₃, and N₅, N₁₀-methylentetrahydrofolate according to the equation:

This catabolic pathway provides the formation of an active one-carbon fragment (N⁵,N¹⁰-CH₂-THF), which is used in synthesis of nucleotides, heme, creatine, glutathione formation, etc.

One-carbon groups metabolism disorders were observed at insufficiency of folic acid and vitamin B₁₂. Clinical manifestation is megaloblastic (macrocytic) anemia. It is characterized by an increase in the size of erythrocytes, decreased erythrocytes count, decreased hemoglobin concentration in the blood. Causes of folate deficiency are poor diet, intestine disease, malabsorption.

2. Metabolism of sulfur amino acids.

Methionine is a universal donor of methyl groups in transmethylation reactions. The immediate donor of the methyl group is a derivative of methionine S-adenosylmethionine.

Methionine + АТP  S- adenosylmethionine + РР_i + Р_i

Further, a methyl group from S-adenosylmethionine is transferred to compound which undergoes methylation.

An example of transmethylation reactions is synthesis of creatine from arginine and glycine. Creatine plays an important role in ensuring the working muscle with ATP.

Exception of methionine from the diet leads to fatty degeneration of the liver and kidneys, which is due the lack of methyl groups required for the synthesis of phosphatidylcholines.

Redox processes can easily occur in sulfur-containing radicals of cysteine and cystine. Therefore, these two amino acids transform into each other under the action of the enzyme cysteine reductase.

Homocysteine is formed from methionine. It interacts with serine to form cysteine and homoserine. Therefore, the oxidation of all sulfur amino acids comes down to the oxidation of cysteine.

The product of cysteine oxidation is taurine NH₂-CH₂-CH₂-SO₃H. Taurine conjugates with bile acids and is involved in the digestion and absorption of fats.

Homocysteine is accumulated in tissues and blood at violations of the homocysteine conversion to methionine and cysteine. Homocystinuria is the excretion of homocysteine in the urine. It is observed in hereditary deficiency of enzymes and insufficiency of folic acid, vitamins B₁₂ and B₆.

Cystinuria is urinary excretion of cystine (up to 50 times above the norm). The level of cystine in the blood is usually not higher than normal. There is a tendency to form stones in the body (cystinosis).

3. Metabolism of branched chain amino acids.

Catabolism of leucine, isoleucine and valine is not primarily in the liver but in muscle and adipose tissues, kidney and brain tissue. As a result of transamination and then oxidative decarboxylation they are converted into acyl-CoA derivatives :