Amino Acids

AMINO ACIDS

Amino acids are small molecules made of carbon, hydrogen, oxygen and nitrogen and in some cases also sulphur.

Each amino acid has a free amino group, a free carboxyl group and 'R' as side chain as same substituents on same carbon atom.

Amino group leads basic character while carboxylic group leads acidic character to the molecule.

Lysine and arginine are Basic Amino Acids because they carry two amino groups and one carboxylic group.

Glutamic acid (glutamate) and aspartic acid (aspartate) contain one amino and two carboxyl groups each and are classified as Acidic Amino Acids.

Alanine, glycine, valine are Neutral Amino Acids as these contain one amino and one carboxyl group each.

These are 20 different amino acids coded by our DNA that differ in the side chain.

Side chain of a basic and an acidic amino acid

Examples of polar and nonpolar amino acids

Most amino acids are laevo-rotatory while glycine is optically inactive.

There are three important non-protein amino acids.

They are ornithine, citrulline (both are involved in ornithine cycle to synthesise urea) and diaminopimelic acid.

A particular property of amino acids is the ionizable nature of -NH₂ and -COOH groups.

Hence, in solutions of different pH, the structure of amino acids changes.

 

B is called Zwitter ionic form

There are two types of amino acids viz. essential and non-essential amino acids.

Essential amino acids cannot be synthesized by animals whereas non-essential amino acids can be synthesised in animal's body.

There are seven essential amino acids in animals whereas 8 essential amino acids in man.

These are leucine, isoleucine, valine, tryptophan, phenylalanine, lysine and methionine.

Threonine is an additional essential amino acid in human beings.

Two amino acids viz. arginine and histidine are semi-indispensable amino acids as they can be synthesised by human beings but very slowly.

Differences between Essential and Nonessential Amino Acids

Concept Builder

Amino acids are classified into following groups :

(i) Neutral amino acid: With one -NH₂ and one -COOH group e.g. glycine, alanine (non-polar).

(ii) Acidic amino acid: Have an extra COOH group (monoamino dicarboxylic), e.g. glutamic and aspartic acid.

(iii) Basic amino acid: Have additional NH₂ group (diamino monocarboxylic) e.g. arginine, lysine.

(iv) Sulphur containing amino acid: Have sulphur e.g. cysteine, cystine and methionine.

(v) Alcoholic amino acid: Have -OH group e.g. serine, threonine.

(vi) Aromatic amino acid : Have cyclic structure having a side chain with -COOH and NH₂ groups e.g. phenyl alanine, tryptophan, tyrosine.

(vii) Heterocyclic amino acid: N is present in the ring e.g. proline, histidine, hydroxyproline.

(viii) Semi essential amino acid : Arginine and histidine are semi-essential amino acids required by children.

Protein amino acids are laevorotatory and -type except glycine. (Glycine: Simplest amino acid, involved in the formation of heme).

Functions of Amino Acids

Besides their principal function as building blocks for proteins, specific amino acids are also converted into different types of biologically active compounds.

For example, tyrosine is converted into the hormones thyroxine and adrenaline, as well as the skin pigment melanin, glycine is involved in the formation of heme and tryptophan in the formation of the vitamin nicotinamide as well as the plant hormone indole-3-acetic acid.

After the removal of the amino group the carbon chain of many amino acids is converted into glucose.

On losing the carboxyl groups as carbon dioxide, amino acids form biologically active amines such as histamine. Histamine is required for the functioning of muscles, blood capillaries and gastric juices.

Ornithine and citrulline are components of urea cycle.

Antibiotics contain non protein amino acids.

Amino acids form organic acids which form glucose by gluconeogenesis.

Lysine is an essential amino acid because it is not formed in the body and has to be provided through diet.