Amino Acids

Amino Acids: Know About the function & facts of Amino Acids

What is Amino Acids?

Amino acids are the essential constituent of all living cells. They are the building blocks of proteins. Chemically, amino acids are nitrogenous compounds having both an acidic carboxyl (-COOH) group and a basic amino (-NH²) group

The general structural formula of the amino acid is:

                                common to all

                                a-amino acids

side chain R CH-COOH

                       NH2

In the above formula one carbon is the part of the carboxyl group and it is the second carbon which is animated (i.e., contains amino group)

This carbon is known as O-carbon and it is joined to four different groups by covalent bonds. Amino acids differ in their side chain, abbreviated as ‘R’ group. Glycine, the simplest amino acid, for example, has a hydrogen atom as its R group (or side chain) and alanine has a methyl-CH3) group.

So far more than 200 amino acids have been isolated and identified from different sources but only 25 of them have been found to enter into the composition of proteins. Out of these 25, only 20 amino acids are found in almost all proteins.

Some amino acids found in plant cells are listed in below Of these, only those marked with an asterisk (*) occur in plant proteins.

Some amino acids contain aromatic side chains, (e.g., phenylalanine, tyrosine). tyrosine  phenylalanine  figure 

Classification of Amino Acids

Amino acids have been classified on the basis of

(i) their synthesis in the body, 

(ii) their reaction in solution (i.e., acidic, basic or neutral)

(iii) composition of the side chain.

1. Classification on the basis of their synthesis in the body. On this basis, amino acids have been classified into following 3 groups:

(a) Essential amino acids.

These amino acids but are synthesized in the body are not indispensable and therefore, have to be provided diet. Examples Leucine, isoleucine in the threonine, valine, methionine, phenylalanine, lysine and tryptophan are essential amino acids and must form a part of human diet.

(b) Non-essential amino acids.

These amino acids are synthesized in our body from simple starting materials containing carbon, hydrogen, Oxygen and nitrogen. Examples-alanine, arginine, aspartic acid, asparagine, cystine, cysteine, glutamic acid, glycine, etc.

(C) Semi-essential amino acids.

These are the amino acids which are synthesized in the body but are not enough to meet the requirements OF the body. Examples are arginine, histidine.

2. Classification of amino acids on the basis of their reaction. On this basis amino acids have been classified into acidic, basic and neutral groups.

(a) Acidic amino acids.

These amino acids have two carboxylic groups and one amino group in each molecule, hence they are acidic in nature. They are also known as mono-amino-dicarboxylic amino acids; e.g., aspartic acid, asparagine, glutamic acid and glutamine

(b) Basic amin0 acids.

These amino acids contain two amino groups and one carboxyl group, thus are basic. They are also known as diamino mono-carboxylic amino acids, e.g., lysine, arginine.

(c) Neutral amino acids.

These are mono- amino mono-carbOxylic acids, e.g., alanine, glycine, valine, etc.

3. Classification on the basis of the composition of functional group in the side

1. Glycine* Gly

2. Alanine* Ala

3. Serine* Ser

4. Phenylalanine* Phe

5. Tyrosine* Tyr

6. Dihydroxyphenylalanine

7. Tryptophan Trp

8. Histidine* His

9. Cysteine* Cys

10. Cystine*

11. a-aminobutyric acid

12. Methionine* Met

13. Ethionine

14. Homoserine

15. Threonine* Thr

16. Aspartic acid Asp

17. Asparagine* Asn

18. Azetidine carboxylic acid

19. Glutamic acid* Glu

20. Glutamine* Gin

21. y-amino butyric acid

22. Proline* Pro

23. Hydroxyproline*

24. Valine* Val

25. Leucine* Leu

26. Norvaline

27. Ornithine

28. Arginine* Arg

29. Citrulline

30. Isoleucine* Ile

31. Norleucine

32. Lysine* Lys

33. Pipecolic acid

chain (R). On this basis the amino acids can be classified into following 8 categories

(a) Simple amino acids.

These amino acids have no functional group in the side chain; e.g., glycine, alanine, valine, leucine, etc.

(b) Hydroxy amino acids.

As the name indicates, these amino acids contain a hydroxyl -OH) group in the side chain, e.g. serine, threonine.

(c) Sulphur-containing amino acids.

Side chains of these amino acids possess sulphur, e.g cysteine, methionine.

(d) Acidic amino acids.

The side chains contain acidic (carboxyl) groups; e.g., aspartic acid, glutamic acid.

(e) Amino acid amides.

These are derived from acidic amino acids. In such amino acids one carboxyl group of acidic amino acids gets transformed into an amide group (-CO.NH2); e.g., aspartate, glutamate.

(f)Basic amino acids.

These include amin0 acids with side chains containing basic groups; e.g., arginine, lysine, hydroxylysine, etc.

(g) Aromatic amino acids.

The side chains of these amino acids have aromatic rings, e.g., phenylalanine, tyrosine, tryptophan.

(h) Heterocyclic amino acids.

Strictly speaking these are not true amino acids, They contain amino group NH) instead of an o (NH+ group; e.g.. proline, hydroxyproline 

A few other amino acids are found only in some specialızed types of proteins. All such aming acids are derivatives of some standard am Amino

example,4-hydroxy acids. For proline, derivative of proline, is found in the fibrous protein – collagen and in some plant nro to Similarly, hydroxylysine, a hydroxy derivative of lysine, also occurs in collagen.

Non-protein Amino Acids In addition to the 20 common amino acids anglu several rare amino acids (which occur only in specialized proteins), there are over 15U OU known amino acids. They occur as natural products or play metabolic roles, but are neve part of proteins. For example, L-ornithine

L-citrulline, which occurs in animal tissues free state, are metabolic intermediates in the cycle. r-aminobutyric acid, another non-prolel amino acid, functions as chemical agent forth transmission of nerve impulses and D-glutamic acid is found in substantial amounts in the cell walls of many bacteria,

Properties of Amino Acids

Amino acids are usually crystalline colourless solids with high melting point. Most of them are soluble in water at least to some extent and are solids soluble in common organic solvents.

All amino acids behave both as acids and bases because the amino group is a strong base and carbOxyl group Is a strong acid. Some amino acids (e.g. arginine, lysine) are more groups while others (e.g., aspartic acid and strongly basic because they contain extra amino glutamic acid) are more strongly acidic because of the carboxyl group.

Peptides

Amino acids combine chemically with one another by bonding the carboxyl carbon of one molecule to the amino nitrogen of another. This covalent bond linking two amino acids is referred to as a peptide bond and two or more amino acids joined Through peptide bonds are called peptides. Peptides Containing two, three or more amino acids are called dipeptides, tri peptides or polypeptides.

A protein may contain hundreds of amino acids joined in a specific linear order and as such an almost infinite variety of protein molecules is possible. Various types of proteins differ from one another in the number, types and arrangement of amino acids that they contain.

In human beings many small peptides with less than ten amino acids perform important biochemical and physiological functions.

Functions of Amino Acids

The principal functions of amino acids are listed Below.

(1) These are the basic units of proteins.

(2) Amino acids alsc serve as sources of carbon and nitrogen, when required.

(3) Some amino acids are deaminated (amino group is removed) to produce acids which are broken down to pyruvate and then converted to carbohydrate. For example, alanine is deaminated to pyruvate which can be utilized for the synthesis of glucose or glycogen.

(4) Tyrosine is converted into hormones like thyroxine and adrenaline and also the skin pigment melanin.

(5) Tryptophan is involved in the formation of vitamin nicotinamide and indole-3-acetic acid (IAA), a plant hormone.

(6) Lysine, present in corn and wheat, is nutritionally important as it is not synthesized by higher animals.

(7) Histidine, which occurs in limited amounts in most of the proteins, plays an important

role in the catalytic activities of some enzymes.

(8) B-alanine, a non-protein amino acid, is an important constituent of pantothenic acid, (a vitamin) and coenzyme A.

(9) Creatine, a derivative of glycine, plays an important role in the energy storage process in vertebrates. 

(10) A few small  peptides called peptidoglycans, are associated with certain structural units of the prokaryotic cell wall.

(11) Amino acids like citrulline and ornithine  are actively involved in the urea cycle in the liver and thus help in maintaining the ammonia concentration below toxic levels.

(12) Many biologically active peptides play important physiological roles. For example, vasopressin secreted by posterior pituitary gland causes kidney to retain water from urine; methionine enkephalin, an opiate- like peptide found in brain, inhibits sense of pain; glucagon serves like a pancreatic hormone and is involved in regulating glucose metabolism and little gastrin, a hormone secreted by mucosal cells in the stomach, causes parietal cells of the stomach to secrete acid.

FAQs About Amino acids