Peptides and proteins

Peptids and proteins
Proteins are of paramount importance in biological systems. All the major structural and functional aspects of the body are carried out by protein molecules. All proteins are polymers of amino acids. Proteins are composed of a number of amino acids linked by peptide bonds.20Although about 300 amino acids occur in nature, only of them are seen in human body. Most of the amino acids (except proline) are alpha amino acids, which means that the amino group is attached to the same carbon atom to which the carboxyl group is attached .
CLASSIFICATION OF AMINO ACIDS
Based on Structure
A. Aliphatic amino acids
a. Monoamino monocarboxylic acids:
• Simple amino acids: Glycine, Alanine
• Branched chain amino acids: Valine, Leucine, Isoleucine
• Hydroxyamino acids: Serine, Threonine
• Sulfur-containing amino acids: Cysteine, Methionine
• Amino acids with amide group: Asparagine, Glutamine
b. Monoamino dicarboxylic acids: Aspartic acid, Glutamic acid
c. Dibasic monocarboxylic acids: Lysine, Arginine.
B. Aromatic amino acids: Phenylalanine, Tyrosine
C. Heterocyclic amino acids:
Tryptophan , Histidine
D. Imino acid: Proline .
E. Derived amino acids:
i. Derived amino acids found in proteins: After the synthesis of proteins, some of the amino acids are modified, e.g. hydroxy proline and hydroxy lysine are important components
of collagen. Gamma carboxylation of glutamic acid residues of proteins is important for clotting process . In ribosomal proteins and in histones, amino acids are extensively methylated and acetylated.
ii. Derived amino acids not seen in proteins (Non-protein amino acids): Some derived amino acids are seen free in cells, e.g. Ornithine Citrulline, Homocysteine. These are produced during the metabolism of amino acids. Thyroxine may be considered as derived from tyrosine.
iii. Non-alpha amino acids: Gamma amino butyric acid (GABA) is derived from glutamic acid. Beta alanine, where amino group is in beta position, is a constituent of pantothenic acid (vitamin) and co-enzyme A. Each amino acid will have three-letter and one-letter abbreviations . Sometimes asparagine/aspartic acid may not be separately identified, for which 3-letter abbreviation is Asx and 1-letter abbreviation is B. Similarly Glx or Z stands for glutamine/glutamic acid.