Dr Nazia Khan Assistant professor College of medicine

Amino Acids Amino acids are a group of organic compounds containing two functional groups – amino (basic) and carboxylic group (acidic). Amino acids are the building blocks of proteins. The general structure of amino acids includes an α-amino group, an α- carboxylate group and a variable side chain (R).

Properties of AA Physical properties- Colorless Crystalline May be sweet(Glycine, Alanine, Valine), tasteless(Leucine) or bitter(Arginine,Isoleucine). Aspartame- An artificialsweetener contains Aspartic acid and Phenylalanine Soluble in water, acids, alkalis but insoluble in organic solvents High melting point(More than 200°c)

Optical Properties of AA The α carbon of each amino acid is attached to four different groups and is thus 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 αcarbon 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.

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. Due to no net charge, there is no electrophoretic mobility at Isoelectric pH. Solubility and buffering capacity are also minimum at Isoelectric pH

Classification of AA Amino acids can be classified in 4 ways: 1. Based on structure 2. Based on the side chain characters 3. Based on nutritional requirements 4. Based on metabolic fate

Classification based on Structure Simple amino acids: - Glycine, Alanine Branched chain amino acids: - Valine,Leucine and Isoleucine OH group-containing amino acids: - Serine and Threonine Sulfur-containing amino acids: - Cysteine, Cystine and Methionine. Amide group-containing amino acids: - Glutamine and Asparagine

Acidic Amino Acids: Aspartic acid and Glutamic acid Basic amino acids: Arginine, Lysine and Histidine Aromatic amino acids Phenyl alanine, tyrosine and tryptophan Imino acid Proline

Classification based on side chain character A. 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 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.

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 They can also participate in hydrogen bond formation. C) Amino acids with a charged side-chain a) Amino acids with a positively charged side- chain: The basic amino acids Ex: Lysine, Arginine and Histidine b) Amino acids with a negatively charged side- chain: The acidic amino acids Ex: Glutamic acid and Aspartic acid They are hydrophilic in nature. Can participate in Hydrogen bond formation

Classification based on Nutritional requirement I. 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. II. 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. III. 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.

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 I. Both glucogenic and ketogenic amino acids: Isoleucine, Tyrosine, Phenylalanine and Tryptophan II. Purely Ketogenic amino acids: Leucine and Lysine III. 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.

AMINO ACIDS NOT FOUND IN PROTEINS 1. CARNITINE 2. CITRULLINE 3. GLUTATHIONE 4. TAURINE 5. ORNITHINE Carnitine Citrulline

Functions of AA Incorporated into tissue proteins Act as precursors for haem, creatine, glutathione, Porphyrins, purines and pyrimidines synthesis 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

Reactions of AA 1) Reactions due to amino group: Oxidative deamination: Transamination: Formation of carbamino compound 2) Reactions due to carboxyl group Decarboxylation Formation of amide linkage 3) Reactions due to side chain Ester formation Formation of disulphide bonds Transmethylation 4) Reaction due to both amino and carboxyl groups Formation of peptide bond

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Extra points and detailed information about reactions of Amino acids.

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. Seleno cysteine is the 21st Amino Acid Pyrolysine is 22 nd aminoacid.

Special Groups in AA Arginine- Guanidinium group Phenyl Alanine- Benzene group Tyrosine- Phenol group Tryptophan- Indole group Histidine- Imidazole group Proline- Pyrrolidine 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. Formation of carbamino compound CO2 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.

Reactions due to carboxyl group 1) Decarboxylation- Amino acids undergo alpha decarboxylation to form corresponding amines. Examples- Glutamic acid  GABA Histidine  Histamine Tyrosine  Tyramine 2) 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

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. 2)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

3)Transmethylation The methyl group of Methionine can be transferred after activation to an acceptor for the formation of important biological compounds D)Reactions due to both amino & carboxyl groups: - Formation of peptide bond THE END