Amino acids

• Outline


  • AA and their structures
  • Optical isomers of AA
  • Classification of AA
  • Physical properties of AA
  • Acid-Base Properties of the AA
  • Chemical properties of AA
  • Amino acid derivatives of importance
• Amino acids (AAs)


  Organic compounds containing 2 functional groups - AMINO & CARBOXYL
• Amino group (-NH2)


  • Basic in nature
• Carboxyl group (-COOH)


  • Acidic in nature
• α-AAs


  AAs where both the amino & carboxyl groups are attached to the same C atom
• α-carbon atom


  • Binds to a side chain (R) which is different for each of the 20 AA found in proteins
  • The side chain dictates an AA's chemical properties
  • Each AA has a UNIQUE SIDE CHAIN
• AAs mostly exist in the ionized form in biological systems
• Optical isomers


  If a C atom is attached to 4 different groups, it is ASYMMETRIC & therefore exhibits OPTICAL ISOMERISM
• All AAs except Glycine possess 4 distinct groups (H, R, COO-, NH3+) held by the α-carbon
• Optical isomers of AAs

  • D-amino acids
  • L-amino acids
• All AAs in proteins are L-amino acids
• 1. AAs are found in some antibiotics & in bacterial cell wall
• Classification of AAs

  • AA with aliphatic side chains
  • AA containing hydroxyl groups
  • Sulfur containing AA
  • Acidic AA & their amides
  • Basic AA
  • Aromatic AA
  • Imino acid
• AA with aliphatic side chains

  • Monoamino monocarboxylic acids
  • Consist of simple AA - G,A,V,L,I
  • L, I, V contain branched aliphatic side chains
  • Branched chain AA
• AA containing hydroxyl groups

  • S, T,Y are –OH group containing AA
  • Tyrosine is also aromatic in nature
• Sulfur containing AA

  • Cysteine with sulfhydryl group
  • Methionine with thioether group
  • Cystine is another important sulfur containing AA formed by the condensation of 2 molecules of cysteine
  • Contain disulfide
• Acidic AA & their amides

  • Aspartic acid & Glutamic acid are dicarboxylic monoamino acids
  • Asparagine & Glutamine are their respective amide derivatives
• Basic AA


  • R, K, H are dibasic monocarboxylic acids
  • Highly basic in character
• Aromatic AA


  • F, Y, W (with indole rings)
  • H may be considered under this category
• Imino acid


  • P containing pyrrolidine ring is a unique AA
  • It has 1 imino group (=NH)
  • Alpha-imino acid
• Classification of AAs by Polarity

  • Hydrophobic {Non-polar}
  • Hydrophilic {Polar}
  • Positively-charged {Polar}
  • Negatively-charged {Polar}
• Hydrophobic/Non-polar AAs


  • Water hating
  • No charge on R group
• Hydrophilic/Polar AAs


  • Water loving
  • Possess groups like hydroxyl, sulfhydryl & amide
  • Participate in H+ bonding of protein structure
• Classification of AAs by Nutritional requirement

  • Essential (indispensable)
  • Non essential (dispensable)
• Essential AAs


  • Required by the body and must be provided by the diet
  • Cannot be synthesized by the body
  • Required for proper growth & maintenance of the individual
• Essential AAs


  • Phenylalanine
  • Valine
  • Threonine
  • Tryptophan
  • Isoleucine
  • Methionine
  • Histidine
  • Arginine
  • Leucine
  • Lysine
• Non-essential AAs


  • Asparagine
  • Aspartic acid
  • Cysteine
  • Glutamine
  • Glycine
  • Glutamic acid
  • Proline
  • Serine
  • Tyrosine
  • Alanine
• Glycogenic AAs


  Serve as precursor for the formation of glucose or glycogen - Ala, Gly, Met, etc.
• Ketogenic AAs


  Fats can be synthesize from these AAs - Leu & Lys are exclusively Ketogenic
• Glycogenic & Ketogenic AAs

  4 AAs (lle, Phe, Ala, Trp) are precursors for the synthesis of glucose & fat
• Physical properties of AAs

  • Solubility - Most AAs are soluble in water & insoluble on organic solvents
  • Melting points - AAs generally melt at higher temp. often above 200°C
  • Taste - AAs may be sweet (Gly, Ala, Val), tasteless (Leu) or bitter (Arg, lle)
  • Optical properties - All AAs except Gly possess optical isomers due to the presence of asymmetric C atom. Some AAs also have a 2nd asymmetric C (lle, Thr)
• AAs as AMPHOLYTES


  • AAs contain both acidic (COOH) & basic (NH2) groups
  • They can donate a proton or accept a proton ~ AMPHOLYTES
  • Zwitteron or dipolar ion - A hybrid molecule containing +ve & -ve ionic groups
  • The AAs rarely exist in a neutral form with free –COOH & free –NH2 groups
• Isoelectric point (pI)


  • pH at which a molecule exists as a Zwitteron & carries no net charge
  • i.e. the molecule is electrically neutral
  • The pI value can be calculated by taking the average pka values corresponding to the ionizible groups
• pH determines the ionic nature of AAs

  • At low pH, an AA is in its cationic form; i.e. both the amino & carboxylic groups are protonated (-NH3+, –COOH)
  • As the pH rises, the carboxylic group looses its proton & the ampholyte form appears
  • With further increases in pH, the amino group (-NH3+) is also deprotonated & the anionic form of the molecule is formed
• Reactions due to –COOH group

  1. AA form salts with bases

  2. AA form esters with alcohols

  3. Decarboxylation - AA undergoes decarboxylation to produce corresponding amines

  4. Reaction with ammonia - The carboxyl group of dicarboxylic AA reacts with NH3 to form amide
• Reactions due to NH2 group

  1. The amino group behave as bases & combine with acids (e.g. HCl) to form salts (-NH3+Cl-)

  2. Reaction with Ninhydrin - The alpha AA react with ninhydrin to form a purple, blue or pink colour complex (Ruhemann's purple)

  3. Proline & Hydroxyproline gives yellow colour with Ninhydrin
• Colour reactions of AA/Proteins

  • Biuret reaction
  • Ninhydrin reaction
  • Xanthoproteic reaction
  • Millions reactions
  • Hopkins-Cole reactions
  • Sakaguchi reactions
  • Nitroprusside reactions
  • Sulfur test
  • Pauly's Test
  • Folin-Coicalteau's Test
• Transamination


  1. Transfer of an amino group from an AA to a keto group to form a new AA

  2. Interconversion of a pair of AA & a pair of keta acids

  3. Shuffling of amino groups among the AAs
• Oxidative deamination


  1. AAs undergo oxidative deamination to liberate free ammonia

  2. Liberation of free NH3 from the amino group of AAs coupled with oxidation
• Amino acid derivatives of importance

  • Collagen
  • Histones
  • γ-Carboxyglutamic acid
  • Cystine