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Amino Acids are the basic structural units (monomers) of proteins. Although over 300 different amino acids are known to occur in cells and tissues, only 20 of them universally occur in the proteins of all forms of life.
These 20 amino acids are called protein amino acids or standard amino acids or magic 20. The others are called non-standard or non protein amino acids.
Out of 20 standard amino acids nineteen are a-amino acids and proline is the exceptional α-imino acid having α- imino group (-NH-).
Each a-amino acid consists of an amino group (-NH2), a carboxyl group, (-COOH), a hydrogen atom and a variable side chain or R- group bonded to a central carbon atom called α-Carbon atom. In proline, the R-group is bonded to both the nitrogen and a-carbon atom, resulting a cyclic structure. Thus, proline has a secondary rather than primary amino group.
Nomenclature:
The standard amino acids have been assigned three letter abbreviations and one letter symbols, e.g., for Glycine, Gly is the abbreviation and G is the symbol. Asparagine was the 1st amino acid discovered and isolated from protein of Asparagus in 1806 and the last of the 20 amino acids discovered was threonine.
Classification of Amino Acids:
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On the basis of their synthesis in animal body, amino acids are classified into 3 groups:
(a) Essential amino acids:
These amino acids can’t be synthesized in animal body, and therefore must be present in their diet. For example, in human being essential amino acids are valine, isoleucine, phenylalanine, methionine, leucine, lysine, tryptophan and threonine. (Remember as VIP, MLL, TT).
(b) Non-essential amino acids:
These amino acids can be synthesized in animal body and may not be supplied in the diet. For example, alanine, arginine, tyrosine, asparagin, glutamine, proline, etc. In plants all the amino acids are non essential.
(c) Semi-essential Amino acid:
These amino acids are synthesized at a slower rate than their requirement in the body, e.g. arginine and histidine.
On the basis of the nature of their side chain, amino acid classified into following categories,
(i) Amino acid aliphatic, hydrophobic side chains: e.g. Gly, Ala, Val, lie, Pro.
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(ii) Amino acid with aromatic hydrophobic side chains: e.g. Phe, Tyr, Trp. Trypophan has indole ring and benzene ring. The tyrosine is less hydrophobic than the other two, because it contains a – OH group.
(iii) Amino acid with Sulfur containing hydrophobic side chains, e.g. cysteine (cys or C) and methionine (met or M).
(iv) Amino acid with aliphatic hydroxyl side chains, e.g. Serine (Ser or S), Threonine (Throt T).
(v) Amino acid with basic side chains, e.g. Lysine (K), arginine (R), histidine (H).
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(vi) Amino acid with acidic side chains, e.g. Aspartic acid (asp or D) and Glutamic acid (Glu rr E). Asparagine (asu. N) and Glutamine (glu or Q) are uncharged derivatives a terminal amide group.
Derivatives of standard amino acids:
Some polypeptides chain unusual or rare amino acid residues which are derived by the post transcriptional modification from one of the 20 amino acids. The rare amino acids are not coded by DNA.
Some of them are as follows:
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1. N-formylmethionine. It is the 1st amino acid of prokaryotic protein.
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2. 4-hydroxyproline and 5-hydroxylysine found in collagen fibers in connective tissue.
3. carboxyglutamic acid found as a constituent of blood clotting factor prothrombin.
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4. Cystine. The sulfhydryl groups (- SH) of two cysteine resides oxidized to form a cystine residue having disulfide bond (-S -S-). Cystine residues mostly found in extracellular proteins.
5. Acetylation: The N- termini when acetylated, makes the proteins more resistance to degradation.
6. Phosphorylation: Phsophoserine, phosphothreonine and phosphotyrosine are formed by the phosphorylation of their – OH group.
Non protein amino acids:
Over 150 modified amino acids are known that are not components of proteins.
A few examples are as follows:
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(a) D-amino acids present in the peptides of bacterial cell wall are defensive in nature as they are not hydrolyzed by peptidases.
(b) Ornithine and citrulline, participate in Urea cycle.
(c) λ-aminobutyric acid (GABA) is a neurotransmitter derived from decarboxylation of glutamate.
{d) Dopamine is also a neurotransmitter derived from tyrosine.
(e) Thyroxin, a thyroid hormone, is a product of tyrosine.
(f) Histamine is a decarboxylation product of histidine. It is a vasodilator, causes allergic response, powerful stimulant of gastric secretion and constrict bronchial smooth muscles.
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(g) Azaserine, valinomycin, atinomycin D, gramicidin S and tyrocidine are the useful components of antibiotics.
