Amino Acids, Peptides, and Proteins by abctutor

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Amino Acids, Peptides
    and Proteins
   The Proteins speak:
  “We are the basis of structure
       and function of life;
  Composed of twenty amino acids,
        the building blocks;
Organized into primary, secondary,
     and quaternary structure;
 Classified as simple, conjugated
      and derived proteins.”
           AMINO ACIDS

- group of organic compounds
  containing two functional groups:

 amino group (-NH2)    basic
 carboxyl group (-COOH) acidic
General Structure of Amino Acids

       H                H

R      C    COOH    R   C     COOH

      NH2               NH3

General Structure   Exists as ion
⍺ - amino acids

              groups – attached to the
 carboxyl               same carbon Atom

⍺ - carbon atom  binds to a side chain
        represented by R (different for
              each of the 20 amino acids
              found in proteins)

Ionized forms  how they exist
Classification of Amino Acids
      based on polarity
        of the R group

   4 groups

   Polarity  reflects the
  functional role
  of AA in protein structure
1. Non-polar AA

      hydrophobic (water hating)
      No charge on the ‘R’ group
      Examples are:
         Alanine      Methionine
         Leucine      Phenylalanine
         Isoleucine       Tryptophan
         Valine       Proline
3. Polar AA with (+) ‘R’ group

      carries (+) charge
         Histidine         Arginine

4. Polar AA with (-) ‘R’ group

    • carries (-) charge
    • Examples:
         Glutamic Acid      Aspartic Acid
2. Polar AA with no charge
   on ‘R’ group
     no charge on the ‘R’ group
     possess groups       hydroxyl
     participate in hydrogen bonding of
     protein structure
        Asparagine       Glycine Cysteine
        Tyrosine         Serine
        Threonine        Glutamine
A.Physical Properties
 1. Solubility - soluble in water and insoluble
 in                  organic solvents

 2. Melting Points - melt at higher
 temperatures                     often 200°C

 3. Taste
       sweet (Gly, Ala, Val)
       tasteless (Leu)
       bitter (Arg, Ile)
       Sodium Glutamate
            – salt of Glutamic Acid – flavoring agent
4. Optical Properties
    - Assymetric  a carbon atom is
                   attached to 4
                   different groups

      exhibiting optical isomerism

 4 distinct groups R
    - held by an
All AA except Glycine possess
optical isomers due to
asymmetric ⍺-carbon atom

Some AA (Isoleucine, Threonine)
 2nd asymmetric carbon
 D- and L- forms of AA based on
 the structure of glyceraldehyde


     H               C         OH
OH               C         H

         CH2OH                  CH2OH

 D-Glyceraldehyde                   L-

       H              C           NH2
H 2N              C           H

           COOH                         COOH

     D-Amino Acid
L-Amino Acid

The proteins are composed of L-⍺ amino
5. Amino acids as ampholytes

     can donate a proton or accept a

      AA contain both acidic (-COOH)
     and basic (-NH2) groups
 Zwitterion or dipolar ion:

    from German word – means


Zwitter ion (or dipolar ion)
    a hybrid molecule containing
(+)         and (-) ionic groups
AA rarely exist in a neutral form with
free carboxylic (-COOH) and free Amino
(-NH2) groups

Strongly acidic pH (low pH)  AA (+)

Strongly alkaline pH (high pH)  AA (-)

Each AA has a characteristic pH (e.g.
Leucine, pH – 6.0), at which it carries
        Existence of an amino acid as Cation,
                 Anion and Zwitterion

                H໋               R          C           COOH

           H               Amino Acid

R              C       COOH
    R              C     COO ¯

        NH3໋                H                    NH2
      Cation H໋                             H໋         Anion
    (low pH)                            R              C
B. Chemical Properties
  General Reactions  mostly due to the
  2                      functional

Reactions due to - COOH group
  1. AA from salts (-COONa) with bases
     and esters (-COOR’) with alcohols

  2. Decarboxylation
     - AA undergo decarboxylation to
3. Reaction with Ammonia

    - the carboxyl group of
 dicarboxylic AA reacts       with
 NH3 to form amide

 Asparatic Acid + NH3  Asparagine
 Glutamic Acid + NH3  Glutamine
Reactions due to -NH2 group

4. The Amino groups behave as bases and
   combine with acids (e.g. HCl) to form
   salts (-NH3 + Cl¯)

5. Reaction with NINHYDRIN
     - the ⍺-AMINO ACIDS react with
  Ninhydrin to form a    purple, blue or
  pink colour complex (Ruhemann’s
Amino acid + Ninhydrin  Keto
 acid + NH3 + CO2
 + Hydrindantin

Hydrindantin + NH3 + Ninhydrin 

Ninhydrin reaction –
 quantitative determination of
6. Colour reactions of Amino Acids
      - AA can be identified by specific
     colour reactions

      Color Reactions of proteins / AA
     Reaction                          Specific
     group or AA
1.   Buiret Reaction          Two peptide linkages
2.   Ninhydrin Reaction       ⍺-Amino acids
3.   Xanthoproteic Reaction   Benzene ring of
                              aromatic AA (Phe, Tyr,
4.   Million’s reaction       Phenolic Group (Tyr)
6. Sakaguchi Reaction     Guanidino Group
7. Nitroprusside Reaction Sulfhydryl groups
8. Paulys’ test           Imidazole ring (His)
9. Sulfur test            Sulfhydryl groups
10. Folin – Coicalteau’s  Phenolic groups
    test                  (Tyr)
7. Transamination
    - important reaction in AA
    - transfer of an amino group
 from an amino acid    to a keto
 acid to form a new AA

8. Oxidative deamination
    - AA undergo oxidative
 deamination to liberate
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