Functions of amino acids in the body

 Functions of amino acids in the body

The metabolism of protein is the metabolism of amino acids. These serve their functions as intact amino acids (synthesis) and while breaking down (lysis).

Functions served by intact amino acids

1. Synthesis of cell protoplasm- Proteins are essential constituents of all living cells.

2. For the repair of wear and tear- Tissue proteins break down during metabolism. These damaged parts are repaired with the help of amino acids.

3. Protein storage- In adult people, where nitrogen equilibrium is established, proteins cannot be stored. But they can be stored in infants, children, convalescent patients, pregnant women, athletes, etc., where active growth takes place and protein synthesis exceeds protein breakdown. In old age the protein breakdown exceeds protein synthesis so mass of protein decreases.

4. Synthesis of bile acids- The two bile acids-Taurocholic acid and glycocholic acid, are obtained by conjugation with specific amino acids with cholic acid. Glycine and cholic acid give glycocholic acid; and taurine, which is derived from metabolism of sulphur-containing amino acid combines with cholic acid producing Taurocholic acid.

5. Synthesis of plasma proteins-Synthesis of prothrombin, fibrinogen, other plasma proteins and antibody products are carried out by liver, with the help of amino acids from the blood stream.

6. Essential amino acids- There are a number of amino acids which cannot be synthesized in the body, but are essential for growth and maintenance of life 

7. Synthesis of hemoglobin. Glycine and succinyl CoA go to form aminolevulinic acid which polymerises to form porphyrine. Then porphyrine with the incorporation of Fe forms haeme which with globin (protein) goes to form haemoglobin.

8. Synthesis of hormones- Hormones of the thyroid gland and adrenal medulla are derivatives of tyrosine. Insulin is also protein in nature. Hence intact amino acids are required for their synthesis.

9. Synthesis of enzymes. The enzymes are proteinous in nature so amino acids are required for their synthesis.

10. Synthesis of milk proteins- Milk proteins in lactating mothers are formed from amino acids by the mammary gland.

11. Synthesis of glutathione and cytochrome- The former is a tripeptide and the latter is a haemochromogen. They take part in tissue oxidation and are synthesised from amino acids.

12. Synthesis of purine and pyrimidine- Glycine, glutamic acid, aspartic acid, etc., help in the synthesis of purine and pyrimidine bases and thereby there is endogenous nucleoprotein synthesis.

13. Synthesis of antibodies- Antibodies are globulin in nature.

14. Synthesis of melanin- Melanin, which is the pigment of the skin, choroid, hair, substantia nigra, etc., is derived from the amino acid tyrosine.

15. Formation of Rhodopsin- Rhodopsin, also known as visual purple, is made up of vitamin A and another protein component. Necessarily, amino acids are required for its synthesis.

16. Role of Arginine in urea formation- The amino acid, arginine, helps in the formation of urea.

Functions of amino acids while breaking down

1. Supply energy- Amino acids break down and liberate energy. One gram of protein is equivalent to 4 Calories approximately.

2. High specific dynamic action. While breaking down, amino acids exert a specific-stimulating action on tissue metabolism. The specific dynamic action of protein is about 30%.

3. Deamination. Deamination is the process by which the amino radical (-NH₂) is taken away from the amino acid. It is carried out chiefly in the liver with the help of an enzyme-deaminase. Deamination may also take place by the enzyme transaminase, which transfers the amino group of the amino acid to a keto acid, converting the latter into an amino acid, and the former into a keto acid. It is also possible to some extent in other tissues, such as kidneys. About 5% energy of the protein molecule is lost in the process.

By the process of Deamination the amino acid molecule is broken down into two parts-

(A) The nitrogenous part (ammonia)

(B) The non-nitrogenous part (an α-ketonic acid or an aldehyde or a hydroxy acid).

The fate and functions of these two parts are described below.

A. Fate and functions of the nitrogenous part

The nitrogenous part, viz., ammonia, undergoes the following fate-

1. Formation of urea- Most of the ammonia, under normal conditions, is converted into urea because about 80% of total urinary nitrogen is found in the form of urea. Urea is formed mainly in the liver.

2 . Formation of ammonium salts. A small part of ammonia combines with acids, other than carbonic acid, and appears in the urine in the form of ammonium salts, such as the ammonium phosphate, sulphate, urate, etc. It is obvious therefore that, the amount of ammonium salts formed will not depend upon the amount of ammonia formation but on the relative proportion of acids and bases in the body. In acidosis more ammonia is needed to neutralize the excess acids, and so there will be proportionate increase in the amount of ammonium salts. In alkalosis opposite changes occur. The function served by the formation of ammonium salts is to neutralize acid of the body, so that the blood reaction may remain constant.

3. Synthesis of simple amino acids- Ammonia may be utilized for the synthesis of simple amino acids, such as glycine, alanine, etc., and also for amination of glutamic acid to give glutamine.

4. Synthesis of various nitrogenous substances- It may also be used for the synthesis of various nitrogenous substances, such as creatine, purine, uric acid, pyrimidine, lecithin and such others.

Fate and functions of the non-nitrogenous part

The non-nitrogenous part has the following fate:

1. Some will have the fate like carbohydrates- The non-nitrogenous residues of some amino acids are utilized in the body as carbohydrates. These amino acids increase a corresponding amount of sugar in the urine. For this reason, in a diabetic subject about 60% of food protein is converted into sugar. Glycine, alanine, aspartic acid, glutamic acid, serine, cysteine, arginine, proline, hydroxyproline, histidine, lysine, isoleucine, citrulline, methionine, valine, threonine, partially phenylalanine and tyrosine belong to this class. These amino acids are called antiketogenic amino acids because they act as carbohydrates in the body and prevent the formation of ketone bodies. They are also called glucogenic.

2. Some undergo the fate like fats. There are certain amino acids e.g. leucine and in part isoleucine, phenylalanine and tyrosine, which yield no glucose, but give rise to ketone bodies when administered to a diabetic animal. It is obvious that the non-nitrogenous part of these amino acids is broken down in the body as fatty acids, from which the ketone bodies are formed. It is known that in a diabetic subject about 40% of proteins are converted into ketone bodies. These amino acids are therefore known as ketogenic amino acids.

3. The sulphur and phosphorus, derived from the non-nitrogenous part of the amino acids, are converted into various sulphur and phosphorus compounds and are excreted as such.