Le 17. Peptide linkage and chemical.pptx

biochemistry

1. Peptide linkage and chemical properties of amino acids An overview 1

2. Peptide linkage Peptide linkage or peptide bond or amide bond: Proteins are linear polymers of L- α -amino acids in which the carboxyl group of one amino acid is linked to the amino group of another amino acid. The peptide bond is an amide bond formed between the -COO - group of one amino acid and the - NH 3 + group of another amino acid. The bond is covalent and rather strong which serve as cementing material between the individual amino acids (bricks) 2

3. Peptide linkage Formation of peptide bond: The linkage is formed by removal of the elements of water (dehydration) from the α -carboxyl group of one amino acid and the α -amino group of another. It is a dehydration reaction, a common class of reactions in living cells. The molecule formed by condensing two amino acids is called a dipeptide Three amino acids can be joined by two peptide bonds to form a tripeptide and so on. 3

4. Peptide linkage Both –C=O and -NH groups of peptide bonds are polar and are involved in hydrogen bond Formation. The peptide bond is planer. 4

5. Peptide linkage Writing of peptide structures: In a peptide, the amino acid residue at the end with a free α - amino group is the amino-terminal (or N-terminal) residue And the amino acid with a free -COO - group at the other end, is known as the carboxyl, or C-terminal amino acid. The peptide chains are conventionally written with their N-terminal amino acid on the left and free carboxyl end (C-terminal amino acid) to the right. Proteins biosynthesis also starts from N-terminal amino acid. 5

6. Naming of peptides: Peptides are named as derivatives of the C-terminal amino acid, which receives its entire name. For all other amino acids, the ending - ine is changed to - yl . Thus, the dipeptide alanyl -glycine has glycine as its C-terminal amino acid, as indicated by its full name, glycine : 6

7. Peptide linkage Chemical properties of amino acids: Amino acids participate in variety of reactions involving - COO – group , - NH 3 + group , SH group and R group. Reactions involving - COO – group: Reaction with alkalies (salt formation): The carboxylic group release H + ion and become carboxylate ( - COO – ) ions. 7

8. chemical properties of amino acids These may be neutralized by cation like Na + and Ca 2+ to form salt. e.g., sodium salt of glutamic acid (monosodium glutamate), which is used as flavouring agent to impart meat like flavour to soups. 8

9. chemical properties of amino acids 2. Reaction with alcohols (esterification): With alcohol corresponding ester are formed. The ester is more volatile than free amino acid. Reaction with amines: Amino acids react with amines to form amides. B. Reactions involving NH 2 group: Reaction with mineral acids: when treated with mineral acid like HCl , the acid slats are formed 9

10. chemical properties of amino acids + HO―C 2 H 5 → R―CH―COO―C 2 H 5 + H 2 O NH 2 Ethyl ester of amino acid R―CH―CO OH + H NH—R R―CH―CO―NH—R + H 2 O NH 2 NH 2 amine 10 amide

11. chemical properties of amino acids R―CH―COOH + HCL R―CH―COOH + H 2 O NH 2 NH 2 .HCL amino acid hydrochloride Reaction with nitrous acid: Reaction with HNO 2 liberate N 2 gas with formation of corresponding α - hydroxy acids. The reaction is used for the estimation of amino acid by measuring the volume of N 2 gas. The imino acid proline don not respond to this reaction. 11

12. chemical properties of amino acids Reaction with FDNB or Sanger’s reagent: FDNB (1-fluoro- 2,4-dinitrobenzene) reacts with α -amino acids to produce yellow cloured derivatives, DNB- amino acids. The reaction was first used by Sanger in determining the sequence of amino acids in insulin. 12

13. chemical properties of amino acids DNB-amino acids (yellow) 13

14. chemical properties of amino acids Reactions involving both COOH and NH 2 groups: Reaction with ninhydrin : ninhydrin or triketohydrindene hydrate (2,2-dihydroxyindane-1,3-dione ) is a powerful oxidizing agent and cause oxidative decarboxylation of α -amino acids producing CO 2 , NH 3 and aldehyde with one less carbon atom than the parent amino acid. 14

15. chemical properties of amino acids II. Ninhydrin I. amino acid VIII. Hydrindantin VII. Diketohydrindylidene-diketohydrindamine ( Ruheman’s purnple ) 15

16. chemical properties of amino acids The reduced ninhydrin ( hydrindantin ) the reacts with the liberated NH 3 and a mole of ninhydrin , forming blue- coloured Ruheman’s complex. Van Slyke used this reaction for quantitative estimation of amino acids. Reaction with phenylisothiocyanate or Edam reagent : phenylisothiocyanate reacts with N- teminal amino acid to produce thiohydantion . This reaction is useful in studies of protein structure. 16

17. chemical properties of amino acids

18. chemical properties of amino acids Reactions involving side chain or R-group: Biuret test: alkaline 0.2% copper sulphate solution (biuret reagent) reacts with peptide bonds containing compound to produce purple colour. If the test is negative, the CuSO 4 solution stays blue The reaction is called biuret reaction because it is also given by substance biuret. The test is used both in qualitative and quantitative purposes 18

19. chemical properties of amino acids Biuret

20. chemical properties of amino acids Millon’s test: Red colour develops when proteins are heated with HgNO 3 in HNO 2 . The reactions is specific for tyrosine and takes place between mercuric and mercurous nitrates and tyrosine residue of protein. Tryptophan also respond to this reaction. 20