Amino acids are an important group of organic acids having both an acidic carboxylic group (-COOH) and a basic amino (-NH2) group.
The first carbon atom of an amino acid is the part of carboxyl group and The second carbon atom is known as a-carbon (alpha-carbon) to which the four different groups are attached :
1) One Hydrogen atom
2) One -COOH group with a potential negative charge (-COO–) the carboxylic group is responsible for the acidic property.
3) One NH2 group with a potential positive charge (NH3). Proline is an amino acid where the amino group is not free but a part of a ring structure, hence it is referred to as amino acid.
4) A variable side group (R) that is different in different amino acids. In glycine R is only a hydrogen atom (H), and in alanine the R is a methyl group (-CH3). Hence glycine having the simplest R-group is the simplest amino acid. In other amino acids the R-group may be a straight or branched chain or a cyclic group.
The side chain further, may be polar (as in serine, glutamic acid) or non-polar (as in alanine).
Except glycine in all other amino acids four different groups are attached to the a-carbon atom making the a-carbon asymmetric. Because of the symmetric a-carbon all amino acids except glycine exists in two optically active froms: those having -NH2 group to the right are designated as D-forms and those having -NH2 group to left are l-forms.
Out of the many kinds of naturally occurring amino acids only 20 different types of amino acids and amides are found as constituents of proteins. Certain proteins also contain some modified amino acids derived from the 20 standard amino acids.
These derived amino acids are known as non-standard amino acids. Hydroxyproline (derived from proline) and hydroxylysine (derived from lysine) are non-standard amino acids found in high percentage in collagen (an important structural protein of animals).
The standard twenty amino acids found in proteins are grouped into seven different categories.
1. Aliphatic Amino acids: The hydrocarbon chain (R-group) is neutral, non polar as in Glycine, Alanine, Valine, Leucine, Isoleucine.
2. Hydroxy (Alcoholic) Amino acids: The side chain having alcoholic (-OH) group as in Serine, Threonine.
3. Sulphur containing Amino acids: The side chain having sulphur group as in Cystine, Methionine.
4. Dicarboxylic amino acids and amides: The side chain has extra carboxylic group as in glutamic acid and aspartic acid. They are also known as acidic amino acids. These amino acids also exist in their respective amide forms such as glutamine and asparagines.
5. Basic Amino acids: They have an additional amino group without forming amides. These amino acids are daimio-mono carboxylic acids. Examples of such amino acid are Lysine, Arginine and Histidine.
6. Aromatic amino acids: They have a cyclic structure with a straight chain bearing carboxylic and amino groups. Examples of such amino acids are Phenylalanine, Tyrosine, and Tryptophan.
7. Heterocyclic amino acid: Having nitrogen group in the ring structure. Ex- proline.
There are about 300 additional natural amino acids which are not found as constituents of proteins but have some other roles in metabolism. These are known as non-protein amino acids. Some examples of non-protein amino acids are:
L- Ornithine and L-citrulline are metabolic intermediates in urea-cycle.
b- alanine, an isomer of alanine, occurs free in nature and also as a constituent of an important vitamin pantothenic acid and of co-enzyme A.
1. They are building blocks of proteins (both structural proteins and enzymes).
2. They serve as storage of nitrogen in the form of amides.
3. Amino acids like p-alanine take part in the formation of vitamin pantothenic acid and co-enzyme A.
4. Glycine, cysteine and glutamate form the co-enzyme glutathione.
5. Surplus amino acids are delaminated in liver to organic acids, which in turn are changed to glucose (gluconeogeneiss) or used in metabolism.