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Amino acid

Amino acids are organic compounds that contain an amino group, a carboxyl group, a central carbon atom, and a side chain. There are 20 standard amino acids that are the building blocks of proteins. Amino acids can be classified based on their structure, polarity, nutritional requirements, and metabolic fate. They perform important functions including serving as monomers for protein synthesis, participating in cellular processes, and acting as precursors for other compounds.

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BY SRIDARSHINI CHANDRA AMINO ACID
CONTENTS Introduction Classification of amino acid Physical properties of amino acids Chemical properties of amino acids Functions of amino acids
What is amino acid? Amino acids are the molecules having one amino group , one carboxyl group, one H atom and one specific group (R group) attached to the central C atom. R group varies in structure, size , electric charge and influence the solubility of amino acid in water. The key elements of amino acids are C,N,O,H. Amino acids are basic structural building blocks of protein.
Basic structure of amino acid
Classification of amino acid According to the structure of side chain According to polarity of the side chain (R group) According to the nutritional requirements According to the metabolic fate
Amino acids with aliphatic side chains Name of Amino acid Structure of the amino acid Special group present Symbol ( 3 letters ) Symbol (1 letter) Glycine Hydrogen atom Gly G Alanine Methyl group Ala A Valine Isopropyl group Val V Leucine Isobutyl group Leu L Isoleucine Sec-butyl group IIe I
Amino acids containing Hydroxyl(-OH) group Name of amino acid Structure of amino acid Special group present Symbol (3 letter) Symbol (1 letter) Serine Hydroxymethyl group Ser S Threonine 1-Hydroxyethyl group Thr T Tyrosine 4-Hydroxybenzyl group Tyr Y
Amino acids containing Sulfur Name of amino acid Structure of amino acid Special group present Symbol (3 letter) Symbol (1 letter) Cysteine Sulfhydryl Cys C Cystine Disulfide _ _ Methionine Thioether Met M
Acidic Amino acids and their derivatives Name of amino acid Structure of amino acid Special group present Symbol (3 letter) Symbol (1 letter) Aspartic acid 2-Carboxyl group Asp D Asparagine Amide Asn N Glutamic acid 3-carboxyl group Glu E Glutamate amaide Gln Q
Basic amino acids and their derivatives Name of amino acid Structure of amino acid Special group present Symbol (3 letter) Symbol (1 letter) Lysine amino Lys K Arginine Guanidino Arg R Histidine Imidazole His H
Aromatic amino acids Name of amino acid Structure of amino acid Special group present Symbol (3 letter) Symb ol (1 letter ) Phenylalanine Benzylyl group Phe F Tyrosine 4-hydroxybenzyl group Tyr Y tryptophan Indole Trp W
Imino Acid Name of amino acid Structure of amino acid Special group present Symbol (3 letter) Symbol (1 letter) Proline Pyrrolidine Pro P
Nutritional classification of amino acid Essential or indispensable amino acid Non-essential or dispensable amino acid amino acid
Essential amino acid  Essential amino acids are not synthesized by the body.  Need to be supplied through diet.  Required for proper growth and maintainance of individual. Arginine,Valine,Histidine,Isoleucine,Leucine, Lysine, Methionine, Threonine, Tryptophan, Phenylalanine
Semi-essential amino acid  Arginine and Histidine can be synthesized by adults but not by growing children, hence these are considered as semi-essential amino acids.
Non-essential amino acid  These can be synthesized by the body to meet the biological needs.  need not to be consumed through the diet. Glycine, Alanine, Serine, Cysteine, Aspartate, Asparagine, Glutamate, Glutamine, Proline, Tyrosine.
Classification of amino acid based on polarity Non-Polar, aliphatic amino acid Aromatic amino acid Polar uncharged( R group) amino acid Polar positively charged amino acid Polar negatively charged amino acid
Non-polar, aliphatic amino acid These are Hydrophobic amino acids . Glycine Alanine Valine Leucine Isoleucine Methionine Proline
Aromatic amino acid Contains aromatic side chain. Less hydrophobic as compared to aliphatic non-polar amino acids. Phenylalanine, Tyrosine, Tryptophan. Tyrosine and tryptophan are more polar than phenylalanine because of the hydroxyl group of tryosine and the nitrogen atom of tryptophan.
Polar, uncharged R group amino acids  Hydrophilic in nature.  Contains functional groups that form hydrogen bond with water. Serine Threonine Cysteine Asparagine Glutamine
Polar, charged (R group) Amino acids Polar positively charged amino acids  More hydrophilic as compared to non-polar amino acids and polar uncharged amino acids.  These are basic in nature. Lysine Arginine Histidine Polar negatively charged Amino acids  More hydrophilic as compared to non-polar amino acids and polar uncharged amino acids.  These are acidic in nature. Aspartate glutamate
Classification of amino acid according to metabolic fate  Both Glycogenic and Ketogenic amino acid: Isoleucine, Tyrosine, Phenylalanine, Tryptophan  Purely Ketogenic amino acids: Leucine, Lysine  Purely Glycogenic amino acids: Alanine, Valine, Serine, Threonine, Glycine, Methionine, Asparagine, Glutamine, Cysteine, Cystine, Aspartic acid, Glutamic acid, Histidine, Arginine.
Non- standard amino acid  In addition to 20 common amino acids proteins may contain residues created by modification of the common residues already incorporated into polypeptide chain. eg. 4-Hydroxyproline 5-Hydroxylysine 6-N-Methyllysine
Desmosine Derivative of four lysine residues
Selenocysteine  It is present during protein synthesis rather than created through post synthetic modification of amino acid residues.  Known as 21st amino acid.
Amino acids as AMPHOLYTES: Amino acids contain both acidic group (-COOH) and basic (-NH2) group. They can donate or accept a proton and hence known as ampholytes.
 In strongly acidic pH amino acid is positively charged.  In strongly alkaline pH amino acid is negatively charged.
Isoelectric pH Isoelectric pH may be defined as a pH at which a molecule exist as a zwitter ion or dipolar ion and carries no net charge. Molecule is electrically neutral at isoelectric pH.
Isoelectric point 7/5/2012 29 Biochemistry For Medics
Physical properties of amino acids Solubility : most of the amino acids are soluble in water but insoluble in organic solvents. Melting point: Amino acids generally melt at high temperature , often above 2000 c. Taste: Amino acids may be sweet(Gly , Ala, Val); tasteless (Arg, Ile); Monosodium glutamate(MSG ; ajinamoto) is used as flavoring agent in food industry. In some individuals intolerant to MSG Chinese restaurant syndrome (flu like) is observed.
Stereochemistry of Amino acids All AMINO ACIDS ARE OPTICALLY ACTIVE(EXCEPT GLYCINE) All amino acids (except Glycine) have one asymmetrical carbon or chiral carbon, to which four different groups are attached(carboxyl group, amino group, hydrogen atom, R group)  The mirror images of a molecule of amino acid are non-superimposable to each other. Amino acids do not have plane of symmetry.
Nonsuperimposable mirror image of Amino acid
 All amino acids rotate the plane of polarized light. These are nonsuperimposable mirror image of each other and known as Enantiomer of each other. If the carboxyl group is written at the top , the D form refers to the isomer having –NH2 at on the right; the L form refers to the amino acid having -NH2 group on the left.
Reactions of amino acids 1) Reactions due to amino group. 2) Reactions due to carboxyl group. 3) Reactions due to side chain. 4) Reaction due to both amino and carboxyl groups.
REACTIONS DUE TO BOTH AMINO & CARBOXYL GROUPS FORMATION OF PEPTIDE BOND 7/5/2012Biochemistry For Medics
Functions of amino acids 1) Provide the monomer units from which the long polypeptide chain of proteins are synthesized. (Condensation of two molecules of amino acids forms a peptide bond) 2) L-amino acids and their derivatives participate in cellular functions as diverse as nerve transmission and the biosynthesis of porphyrins, purines, pyrimidines, and urea. 3) Short polymers of amino acids called polypeptide perform prominent roles in the neuroendocrine system as hormones, hormone-releasing factors, neurotransmitters
Functions of amino acids 4) Niacin, Serotonin and melatonin are synthesized from Tryptophan 5) Melanin, thyroid hormone, catecholamines are synthesized from Tyrosine 6) GABA (neurotransmitter) is synthesized from Glutamic acid 7) Nitric oxide, a smooth muscle relaxant is synthesized from Arginine. 8) Act as precursors for haem, creatine , Porphyrins, purines and pyrimidines.
Reference  Principle of Biochemistry Albert L. Lehninger, David L. Nelson, Michael M.Cox 6th edition,chapter-3,amino acid and protein, page no- 75-85.  Biochemistry, U.Satyanarayana, U.Chakrapani 3rd edition, chapter- 3, proteins and amino acids , page no-43-57  Outlines of Biochemistry, Eric E. Conn & P.K.Stumpf 3rd edition, chapter3,amino acid and proteins ,page no-55-67  Harper’s Illustrated Biochemistry, Murry,Bender,Botham,Kennelly,Rodwell,Weil, 28th edition, amino acids, page no-14-25.
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