• Amino acids are carboxylic acid in which α-carbon is attached to the amino group as well as it is attached to R- (alkyl) group. They are the basic unit of proteins.

• There are 20 types of amino acids which are coded by the gene of vertebrate but large numbers of modified (non-standard) amino acids are found in proteins.

Characteristics of amino acids:

• Amino acids are basic units of protein.
• All amino acids have at least one acidic carboxylic acid (-COOH) group and one basic amino (-NH2) group.
• Amino acids are colorless, crystalline solid
• They are soluble in water and insoluble in organic solvent
• Only L- form of amino acids are found in Proteins in human body. If D-form is present, it is converted into l-form by enzymes in liver.
• More than 300 amino acids are found in nature but only 20 amino acids are standard and present in protein because they are coded by gene.
• Other non-standard amino acids are modified aminoacids and called non-protein amino acids.

Some examples of Modified amino acids:

1. 4-hydroxyproline: synthesized from proline
2. 5-hydroxylysine: synthesized from lysine
3. Gamma-aminobutyric acid (GABA): synthesized from glutamic acid
4. Gamm-carboxylglutamate: synthesized from glutamic acid
5. Desmosine: synthesized from lysine

Classification of amino acids

I. Classification on the basis of R-group

II. Classification on the basis of nutrition

III. Classification on the basis of Catabolism

I. Classification of amino acids on the basis of R-group

1. Group A : Hydrophobic amino acids

• Amino acids in which R-group is non-polar
• (GAVLIMPPT); Glycine, Alanine, Valine, leucine, Isoleucine, Methionine, Phenylalanine, Proline, Tryptophan

2. Group B: Hydrophilic, uncharged amino acids

• Amino acids in which R-group is polar, hydrophilic but uncharged
• CSTGAT; Cystein, Serine, Threonine, Glutamine, Asparagine, Tyrosine

3. Group C: acidic amino acids

• Amino acids in which R-group is acidic or negatively charged
• GA; Glutamic acid and Aspartic acid

4. Group D: basic amino acids

• Amino acids in which R-group is basic or positively charged
• LAH; Lysine, Arginine, Histidine

II. Classification of amino acids on the basis of Nutrition:

1. Essential amino acids:

• These amino acids are not synthesized in cells of human beings, so these should be essentially present in diet.
• PVTTIMHALL; Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine*, Leucine, Lysine
• Arginine is conditional amino acids (Essential for infants, non essential for adults)

2. Non essential amino acids:

• These aminoacids can be synthesized in body, so need not be included in diet.
• (GASCAGAGTP); Glycine, Alanine, Serine, Cysteine, Asparagine, Glutamine, Aspartic acid, Glutamic acid, Tyrosine, Proline

III. Classification of amino acids on the basis of Catabolism

1. Glucogenic amino acids:

• These aminoacids serves as precursors gluconeogenesis for glucose formation
• GAMD (Glycine, Alanine, methionine, Aspartic acid).

2. Ketogenic amino acids:

• These aminoacids breakdown to form ketone bodies.
• Leucine and Lysine

3. Both glucogenic and ketogenic amino acids:

• These amino acids breakdown to form precursors for both ketone bodies and glucose.
• Isoleucine, Phenylalanine, Tryptophan and tyrosine

Amino acids: Characteristics and Classification of amino acids

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