chemistry of aminoacids

Information about chemistry of aminoacids

Published on October 14, 2014

Author: drmustansar

Source: authorstream.com

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PowerPoint Presentation: 7/5/2012 Biochemistry For Medics 1 Chemistry of amino acids: Chemistry of amino acids 7/5/2012 Biochemistry For Medics 2 Amino acids: Amino acids Provide the monomer units from which the long polypeptide chains of proteins are synthesized L-amino acids and their derivatives participate in cellular functions as diverse as nerve transmission and the biosynthesis of porphyrins, purines, pyrimidines, and urea . Short polymers of amino acids called peptides perform prominent roles in the neuroendocrine system as hormones, hormone-releasing factors, neuromodulators, or neurotransmitters. 7/5/2012 Biochemistry For Medics 3 General structure of an amino acid: General structure of an amino acid Each amino acid (except proline) has a carboxyl group , an amino group and a distinctive side chain bonded to the alpha carbon atom. At physiological pH the carboxyl group is dissociated forming the negatively charged carboxylate ion(-COO - ), and the amino group is protonated(-NH 3 + ) 7/5/2012 Biochemistry For Medics 4 Classification of amino acids: Classification of amino acids Amino acids can be classified in 4 ways: Based on structure Based on the side chain characters Based on nutritional requirements Based on metabolic fate 7/5/2012 Biochemistry For Medics 5 1)Classification based on structure: They are classified in three broad categories: Mono amino mono carboxylic acid It is further subdivided in 5 groups: Simple amino acids-example: glycine, alanine Branched chain amino acids- eg : valine,leucine,isoleucine Hydroxyl group containing amino acids- eg : serine, threonine Sulphur containing amino acids- eg : cysteine,cystine,methionine Amide group containing amino acids-e.g.- asparagine,glutamine Mono amino dicarboxylic acid Example :aspartic acid, glutamic acid Di /poly amino mono carboxylic acid Example : lysine,arginine 7/5/2012 Biochemistry For Medics 6 1)Classification based on structure 1)Classification based on structure: 1)Classification based on structure Aliphatic Amino Acids: a) Mono-amino mono-carboxylic acids: Simple amino acids: Glycine , Alanine 7/5/2012 Biochemistry For Medics 7 1)Classification based on structure: 1)Classification based on structure Aliphatic Amino Acids: a) Mono-amino mono-carboxylic acids : Branched chain amino acids: Valine, Leucine and Isoleucine 7/5/2012 Biochemistry For Medics 8 1)Classification based on structure: 1)Classification based on structure Aliphatic Amino Acids: a) Mono-amino mono-carboxylic acids: - OH group-containing amino acids: Serine and Threonine 7/5/2012 Biochemistry For Medics 9 1)Classification based on structure: 1)Classification based on structure Aliphatic Amino Acids: a) Mono-amino mono-carboxylic acids: Sulfur-containing amino acids: Cysteine, Cystine(Formed by linking of two cysteine residues) and Methionine. 7/5/2012 Biochemistry For Medics 10 1)Classification based on structure: 1)Classification based on structure Aliphatic Amino Acids: a) Mono-amino mono-carboxylic acids: Amide group-containing amino acids: Glutamine and Asparagine 7/5/2012 Biochemistry For Medics 11 1)Classification based on structure: 1)Classification based on structure Aliphatic Amino Acids: a) Mono-amino di-carboxylic acids: Aspartic acid and Glutamic acid 7/5/2012 Biochemistry For Medics 12 1)Classification based on structure: 1)Classification based on structure Aliphatic Amino Acids: a) Di- basic mono-carboxylic acids: Arginine and Lysine 7/5/2012 Biochemistry For Medics 13 1) Classification based on structure: 1) Classification based on structure ii ) Aromatic amino acids- Phenyl alanine and tyrosine 7/5/2012 Biochemistry For Medics 14 1)Classification based on structure: 1)Classification based on structure iii) Heterocyclic Amino Acids: Tryptophan and Histidine 7/5/2012 Biochemistry For Medics 15 1)Classification based on structure: 1)Classification based on structure iv) Imino acid- Proline 7/5/2012 Biochemistry For Medics 16 1)Classification based on structure: 1)Classification based on structure Derived Amino Acids : Non- α -amino acids e.g.: β -alanine, γ -amino butyric acid (GABA), δ -amino Levulinic acid Derived and Incorporated in tissue proteins : e.g.: Hydroxy-proline, hydroxy-lysine Derived but not incorporated in tissue proteins: e.g.: Ornithine, Citrulline, Homocysteine, Argino succinic acid 7/5/2012 Biochemistry For Medics 17 2) Classification based on side chain characters: 2) Classification based on side chain characters Amino acids with a non-polar side-chain: e.g.: Alanine, Valine, Leucine, Isoleucine, Phenylalanine, Tryptophan, Proline Each of these amino acids has a side chain that does not bind or give off protons or participates in hydrogen or ionic bonds. Side chains of these amino acids can be thought of as “Oily” or lipid like , a property that promotes hydrophobic interactions . 7/5/2012 Biochemistry For Medics 18 Amino acids with a non-polar side-chain: : 7/5/2012 Biochemistry For Medics 19 Amino acids with a non-polar side-chain: 2) Classification based on side chain characters: 2) Classification based on side chain characters B) Amino acids with a polar but uncharged side-chain: e.g. Serine, Threonine, Tyrosine, Cysteine, Asparagine and Glutamine. These amino acids are uncharged at neutral pH, although the side chains of cysteine and Tyrosine can lose a proton at an alkaline pH. Serine , Threonine and Tyrosine each contains a polar hydroxyl group that can participate in hydrogen bond formation. Side chains of Asparagine and Glutamine contain a carbonyl group and amide group, they can also participate in hydrogen bond formation. 7/5/2012 Biochemistry For Medics 20 Amino acids with a polar but uncharged side-chain: 7/5/2012 Biochemistry For Medics 21 Amino acids with a polar but uncharged side-chain 2) Classification based on side chain characters: 2) Classification based on side chain characters C) Amino acids with a charged side-chain Amino acids with a positively charged side-chain: The basic amino acids- Lysine, Arginine and Histidine Amino acids with a negatively charged side-chain: The acidic amino acids- Glutamic acid and Aspartic acid They are hydrophilic in nature. 7/5/2012 Biochemistry For Medics 22 Amino acids with a charged side-chain: 7/5/2012 Biochemistry For Medics 23 Amino acids with a charged side-chain 3)- Classification based on nutritional requirements: 3)- Classification based on nutritional requirements Essential amino acids: These amino acids cannot be synthesized in the body and have to be present essentially in the diet. Examples- Valine, Isoleucine, Leucine, Lysine, Methionine, Threonine, Tryptophan and Phenylalanine . Semi-essential amino acids : These amino acids can be synthesized in the body but the rate of synthesis is lesser than the requirement(e.g. during growth, repair or pregnancy) Examples- Arginine and Histidine. Non-essential amino acids: These amino acids are synthesized in the body, thus their absence in the diet does not adversely affect the growth. Examples- Glycine, Alanine, and the other remaining amino acids . 7/5/2012 Biochemistry For Medics 24 4)-Classification based on metabolic fate: 4)-Classification based on metabolic fate The carbon skeleton of amino acids can be used either for glucose production or for the production of ketone bodies, Based on that Both glucogenic and ketogenic amino acids: Isoleucine, Tyrosine, Phenylalanine and Tryptophan Purely Ketogenic amino acids: Leucine and Lysine Purely Glucogenic amino acids: The remaining 14 amino acids are glucogenic. Alanine, valine ,serine, threonine, glycine, methionine, asparagine , glutamine, cysteine, cystine, aspartic acid, glutamic acid, histidine and arginine. 7/5/2012 Biochemistry For Medics 25 Non standard amino acids: Non standard amino acids Of the over 300 naturally occurring amino acids, 20 constitute the monomer units of proteins. These 20 amino acids are called the Primary or Standard amino acids . Seleno cysteine is the 21 st Amino Acid The other are Pyroglutamate and Pyrolysine. 7/5/2012 Biochemistry For Medics 26 Naming of Amino acids: Naming of Amino acids Each amino acid has three letter (code) and one letter (Symbol) abbreviations- Examples-1) Unique first letter Cysteine- Cys- C Histidine- His- H 2) Priority of commonly occurring amino acids Alanine- Ala- A (Preference over Aspartate) Glycine- Gly-G (Preference over Glutamate) 7/5/2012 Biochemistry For Medics 27 Naming of Amino acids: Naming of Amino acids 3) Similar sounding names- Some one letter symbols sound like the amino acids they represent- Example Tryptophan – W (Twyptophan) Phenyl alanine – F 4) Letters close to initial letter Aspartate- Asx- B( near A) Lysine Lys- K(near L) 7/5/2012 Biochemistry For Medics 28 Amino acid abbreviations: 7/5/2012 Biochemistry For Medics 29 Amino acid abbreviations Special groups in amino acids: Special groups in amino acids Arginine - Guanidinium group Phenyl Alanine- Benzene group Tyrosine - Phenol group Tryptophan - Indole group Histidine - Imidazole group Proline - Pyrrolidine Proline has a secondary amino group, hence it is an imino acid. 7/5/2012 Biochemistry For Medics 30 Properties of amino acids: Properties of amino acids Physical properties- Colorless Crystalline May be sweet(Glycine, Alanine, Valine), tasteless(Leucine) or bitter(Arginine, Isoleucine). Aspartame- An artificial sweetener contains Aspartic acid and Phenyl alanine. Soluble in water, acids, alkalis but insoluble in organic solvents High melting point(More than 200 0 c) 7/5/2012 Biochemistry For Medics 31 Isoelectric point: Isoelectric point Amino acids can exist as ampholytes or zwitterions in solution, depending upon pH of the medium. The pH at which the amino acids exist as zwitterions, with no net charge on them is called Isoelectric pH or Isoelectric point. In acidic medium, the amino acids exist as cations In alkaline medium , they exist as anions. 7/5/2012 Biochemistry For Medics 32 Due to no net charge, there is no electrophoretic mobility at Isoelectric pH. Solubility and buffering capacity are also minimum at Isoelectric pH Isoelectric point: Isoelectric point 7/5/2012 Biochemistry For Medics 33 Titration of amino acids: Titration of amino acids If HCl is added drop wise to am amino acid solution, at a particular p H, 50 % of the molecules are in the cationic form and 50% are in the zwitterion form. This pH is pK1(with regard to COOH) If the titration is done from the Isoelectric point with NaOH, molecules acquire the anionic form. When 50 % of the molecules are in the anionic form and 50% are in the zwitterion form. This pH is pK2(with regard to NH 2 ) 7/5/2012 Biochemistry For Medics 34 Titration of Glycine: Titration of Glycine For mono amino mono carboxylic amino acids- pI = pK1+pK2 ------------- 2 The buffering action is maximum in and around pK1or at pK2 but is minimum at pI 7/5/2012 Biochemistry For Medics 35 pKa of some amino acids: 7/5/2012 Biochemistry For Medics 36 pK a of some amino acids Optical properties of amino acids: Optical properties of amino acids The α carbon of each amino acid is attached to four different groups and is thus a chiral or optically active carbon atom. Glycine is exceptional because there are two hydrogen substituents at the α carbon, thus it is optically inactive. Amino acids with asymmetric centre at the α carbon can exist in two forms, D and L forms that are mirror images of each other and are called Enantiomers . All amino acids found in proteins are of L- configuration D- amino acids are found in some antibiotics and in bacterial cell walls. 7/5/2012 Biochemistry For Medics 37 L & D isomers of amino acids: 7/5/2012 Biochemistry For Medics 38 L & D isomers of amino acids Reactions of amino acids: Reactions of amino acids Reactions due to amino group Reactions due to carboxyl group Reactions due to side chain Reaction due to both amino and carboxyl groups 7/5/2012 Biochemistry For Medics 39 Reactions due to amino group: Reactions due to amino group Oxidative deamination- α amino group is removed and corresponding α -keto acid is formed. α -keto acid produced is either converted to glucose or ketone bodies or is completely oxidized. Transamination- Transfer of an α amino group from an amino acid to an α keto acid to form a new amino acid and a corresponding keto acid. 7/5/2012 Biochemistry For Medics 40 Reactions due to amino group: Reactions due to amino group Formation of carbamino compound CO 2 binds to α amino acid on the globin chain of hemoglobin to form carbamino hemoglobin The reaction takes place at alkaline pH and serves as a mechanism for the transfer of Carbon dioxide from the tissues to the lungs by hemoglobin. 7/5/2012 Biochemistry For Medics 41 Reactions due to carboxyl group: Reactions due to carboxyl group Decarboxylation- Amino acids undergo alpha decarboxylation to form corresponding amines. Examples- Glutamic acid GABA Histidine Histamine Tyrosine Tyramine Formation of amide linkage Non α carboxyl group of an acidic amino acid reacts with ammonia by condensation reaction to form corresponding amides Aspartic acid Asparagine Glutamic acid Glutamine 7/5/2012 Biochemistry For Medics 42 Reactions due to side chains : Reactions due to side chains 1) Ester formation OH containing amino acids e.g. serine, threonine can form esters with phosphoric acid in the formation of phosphoproteins. OH group containing amino acid can also form: Glycosides – by forming O- glycosidic bond with carbohydrate residues. 7/5/2012 Biochemistry For Medics 43 Reactions due to side chains : Reactions due to side chains 2) Reactions due to SH group (Formation of disulphide bonds) Cysteine has a sulfhydryl group( SH) group and can form a disulphide (S-S) bond with another cysteine residue. The dimer is called Cystine Two cysteine residues can connect two polypeptide chains by the formation of interchain disulphide chains. 7/5/2012 Biochemistry For Medics 44 Formation of disulphide bond: 7/5/2012 Biochemistry For Medics 45 Formation of disulphide bond Reactions due to side chains : Reactions due to side chains 3)Transmethylation The methyl group of Methionine can be transferred after activation to an acceptor for the formation of important biological compounds. 7/5/2012 Biochemistry For Medics 46 : 4)Reactions due to both amino & carboxyl groups Formation of peptide bond 7/5/2012 Biochemistry For Medics 47 Reactions due to side chains Special functions of Amino acids: Special functions of Amino acids Incorporated in to tissue proteins Niacin, Serotonin and melatonin are synthesized from Tryptophan Melanin, thyroid hormone, catecholamines are synthesized from Tyrosine GABA (neurotransmitter) is synthesized from Glutamic acid Nitric oxide , a smooth muscle relaxant is synthesized from Arginine. Act as precursors for haem, creatine and glutathione, Porphyrins, purines and pyrimidines. 7/5/2012 Biochemistry For Medics 48 Colour reactions of amino acids: Colour reactions of amino acids 7/5/2012 Biochemistry For Medics 49 S.No . Test Significance 1) Ninhydrin reaction Given by all Alpha amino acids 2) Xanthoproteic test Given by aromatic amino acids 3) Millon’s test Confirmatory test for Tyrosine 4) Biuret test Not given by free amino acids 5) Sakaguchi test Given by Arginine 6) Hopkins Cole reaction Confirmatory test for Tryptophan 7) Lead acetate test Given by cysteine and cystine but not given by Methionine 8) Nitroprusside reaction Given by SH group containing amino acids

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