Amino Acids

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What are the Amino Acids?

Amino acids are organic acids in which one or more hydrogen atoms attached to the hydrocarbon skeleton are replaced by equal number ofamino(-NH₂) groups. Each amino acid contains at least one acidic carboxyl (-COOH) group and one basic amino (-NH2) group. Some amino acids may have an additional amino and/or a carboxyl group. All amino acids are made up of C₂H2O and N₂ while some of them contain Sulphur(S) in addition.

Types of Amino Acids

As constituent of proteins most of the natural amino acids are found. Several natural amino acids are now known that are not found in proteins but remain in free or bound form. So many types of amino acids are known to be present in protein. Those are:
1) Lysine
2) Valine
3) leucine
4) Isoleucine
5) Threonine
6) Methionine
7) phenyl alanine
8) Tryptophan
9) Glycine
10) Alanine
11) Serine
12) Aspartic acid
13) Glutamic acid
14) Arginine
15) Cysteine
16) Cystine
17) Tyrosine
18) Histidine
19) Proline
20) Hydroxyproline
21) Hydroxylysine
22) Aspargine
23) Glutamine.

Essential Amino Acids and Non-essential Amino Acids

From nutritional point of view, amongst the abovementioned amino acids, the first eight are called essential or indispensable amino acids as they are not synthesized in our body and their presence in diet is essential.
The remaining ones are referred to as non-essential or dispensable amino acids because they can be synthesized in the body and are not necessarily to be taken through diet. In addition to the eight essential amino acids, histidine and arginine are considered as semi-essential amino acids because these two amino acids are essential for infants but not for adults.

Classification of amino acids

Amino acids may also be classified into three groups according to the number of amino and carboxyl groups present in the molecule or according to the reaction in solution as follows: —

Neutral amino acids

Neutral amino acids have equal number of basic amino and acidic carboxyl groups. These may again be of two types—(a) mono amino mono carboxylic acid e.g., glycine, alanine, serine, valine etc., and (b) diamino dicarboxylic acid e.g., cystine.

Acidic amino acids

Mono-amino dicarboxylic acids are Acidic amino acids because they contain an extra acidic carboxylic group. Example: — aspartic acid and glutamic acid.

Basic amino acids

Basic amino acids are diamino monocarboxylic acids e.g., lysine, hydroxylysine and arginine.

According to their metabolic fates, amino acids are of three types, those are: —

Glycogenic amino acids

Amino acids which can enter into the neoglucogenic pathway to produce glucose and glycogen are called glyogenic aminoacids. They comprise glycine, alanine, serine, cysteine, valine, methionine, glutamine, aspartic acid, histidine, arginine, proline and hydroxy proline.

Ketogenic amino acids

Amino acids whose carbon skeletons are converted to ketone bodies and not to glucose are called ketogenic amino acids. They comprise leucine and lysine.

Glycogenic-ketogenic amino acids

Amino acids whose carbon skeletons are converted partly to glucose and partly to ketone bodies belong to this group. They include phenyl alanine, tyrosine, tryptophan, isoleucine and threonine.
And the amino acids contain sulphur: -

Sulphur containing amino acids

There are three amino acids namely methionine, cysteine and cystine belong to this group because their molecules contain sulphur (S).