Amino acids peptides and proteins


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Amino acids peptides and proteins

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  2. 2. Amino Acids, Peptides and Proteins
  3. 3. 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, tertiary and quaternary structure; Classified as simple, conjugated and derived proteins.”
  4. 4. AMINO ACIDS - group of organic compounds containing two functional groups: amino group (-NH2)  basic carboxyl group (-COOH) acidic
  5. 5. General Structure of Amino Acids H H R C COOH R C COOH NH2 NH3 General Structure Exists as ion
  6. 6. ⍺ - amino acids amino 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
  7. 7. Classification of Amino Acids based on polarity of the R group 4 groups Polarity  reflects the functional role of AA in protein structure
  8. 8. 1. Non polar AA- hydrophobic (water hating) No charge on the ‘R’ group Examples are: Alanine Methionine Leucine Phenylalanine Isoleucine Tryptophan Valine Proline
  9. 9. 3 Polar AA with R group. (+) ‘ ’ carries (+) charge Examples: Histidine Arginine Lysine 4 Polar AA with R group. (-) ‘ ’ • carries (-) charge • Examples: Glutamic Acid Aspartic Acid
  10. 10. 2 Polar AA with no charge. on R group‘ ’ no charge on the ‘R’ group possess groups  hydroxyl sulfhydryl amide participate in hydrogen bonding of protein structure Examples: Asparagine Glycine Cysteine Tyrosine Serine Threonine Glutamine
  11. 11. 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
  12. 12. 4. Optical Properties - Assymetric  a carbon atom is attached to 4 different groups exhibiting optical isomerism 4 distinct groups R H - held by an COOH -⍺ carbon
  13. 13. All AA except Glycine possess optical isomers due to asymmetric -carbon atom⍺ Some AA (Isoleucine, Threonine)  2nd asymmetric carbon
  14. 14. D- and L- forms of AA based on the structure of glyceraldehyde CHO CHO H C OH OH C H CH2OH CH2OH D-Glyceraldehyde L- Glyceraldehyde
  15. 15. R R H C NH2 H2N C H COOH COOH D-Amino Acid L- Amino Acid The proteins are composed of L- amino⍺ acids
  16. 16. 5. Amino acids as ampholytes can donate a proton or accept a proton AA contain both acidic (-COOH) and basic (-NH2) groups
  17. 17. Zwitterion or dipolar ion: Zwitter  from German word – means “hybrid” Zwitter ion (or dipolar ion)  a hybrid molecule containing (+) and (-) ionic groups
  18. 18. AA rarely exist in a neutral form with free carboxylic (-COOH) and free Amino (-NH2) groups Strongly acidic pH (low pH)  AA (+) charged (cation) Strongly alkaline pH (high pH)  AA (-) charged (anion) Each AA has a characteristic pH (e.g. Leucine, pH – 6.0), at which it carries both (+) and (-) charges and exist as zwitterion
  19. 19. Existence of an amino acid as Cation, Anion and Zwitterion H H໋ R C COOH H໋ NH2 H Ami no Aci d H R C COOH R C COO ¯ NH3໋ H NH2 Cat i on H໋ H໋ Ani on ( l ow pH) R C
  20. 20. B. Chemical Properties General Reactions  mostly due to the 2 functional groups Reactions due to - COOH group 1. AA from salts (-COONa) with bases and esters (-COOR’) with alcohols 2. Decarboxylation - AA undergo decarboxylation to produce corresponding
  21. 21. 3. Reaction with Ammonia - the carboxyl group of dicarboxylic AA reacts with NH3 to form amide Asparatic Acid + NH3  Asparagine Glutamic Acid + NH3  Glutamine
  22. 22. 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 purple)
  23. 23. Amino acid + Ninhydrin  Keto acid + NH3 + CO2 + Hydrindantin Hydrindantin + NH3 + Ninhydrin  Ruhemman’s purple Ninhydrin reaction – quantitative determination of AA
  24. 24. 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, Trp) 4. Million’s reaction Phenolic Group (Tyr)
  25. 25. 6. Sakaguchi Reaction Guanidino Group (Arg) 7. Nitroprusside Reaction Sulfhydryl groups (Cys) 8. Paulys’ test Imidazole ring (His) 9. Sulfur test Sulfhydryl groups (Cys) 10. Folin – Coicalteau’s Phenolic groups test (Tyr)
  26. 26. 7. Transamination - important reaction in AA metabolism - 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 free ammonia
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