Aminoacids

1. Aminoacids By Dr. Sumbul Fatma 29th September, 2010

2. Types of proteins Type Examples • Structural tendons, cartilage, hair, nails • Contractile muscles • Transport hemoglobin • Storage milk • Hormonal insulin, growth hormone • Enzyme catalyzes reactions in cells • Protection immune response

3. Amino acids • Building blocks of proteins • 20 are present in mammalian proteins

4. Structure of amino acids • Groups attached to α- carbon • a carboxyl group • an amino group • a side chain

5. Examples H I H2N—C —COOH I H glycine CH3 I H2N—C —COOH I H alanine

6. Zwitterions Net charge is zero on the molecule

7. Isoelectric point • The pH at which the molecule carries no net charge • In acidic solution-cationic • In alkaline solution- anionic

8. Titration curve of glycine • pK1- pH at which 50% of molecules are in cation form and 50% are in zwitterion form • pK2- pH at which 50% of molecules are in anion form and 50% are in zwitterion form • Buffering action is maximum around pK values and minimum at pI

9. What if amino acid has more than two ionizable groups?

10. Titration curve of aspartic acid

11. Learning check-1 CH3 CH3 +H3N–CH–COOH H2N–CH2–COO– (1) (2) Select from the above structures A. Alanine in base. B. Alanine in acid.

12. Solution-1 CH3 CH3 +H3N–CH–COOH H2N–CH2–COO– (1) (2) Select from the above structures A. Alanine in base- (2) B. Alanine in acid – (1)

13. Classification on the basis of side chain • Non-polar • Side chain does not bind or give off protons • Promote hydrophobic interaction • In aqueous solution- clustered in the interior • Helps giving a 3-d shape

15. Proline • imino acid • Has a secondary amino group

16. Classification on the basis of side chain • Polar • uncharged • Acids • Bases • Hydrophilic

17. Polar uncharged amino acids • Have zero net charge at neutral pH • Hydrophillic

19. Polar acidic amino acids • Have a negative charge on the R-group

20. Polar basic amino acids • Have a positive charge on the R-group

21. Learning check-2 Identify each as (1) polar or (2) nonpolar A. NH2–CH2–COOH (Glycine) CH3 | CH–OH | B. NH2–CH–COOH (Serine)

22. Solution-2 Identify each as (1) polar or (2) nonpolar A.(2) NH2–CH2–COOH (Glycine) CH3 | CH–OH | B. (1)NH2–CH–COOH (Serine)

23. Optical properties of amino acids • Amino acids having asymmetric carbon atom exhibit optical activity • The mirror image forms produced with reference to the α-C atom are called D and L isomers • L-amino acids – natural amino acids • D-amino acids are found in antibiotics (like Gramicidin-S, Actinomycin-D and Valinomycin) and in plant and bacterial cell walls • Glycine an exception- no optical activity

24. Stereoisomers/Enantiomers

25. Learning check-3 • All 20 amino acids exist in nature equally as both D and L stereoisomers True/False

26. Solution-3 • All 20 amino acids exist in nature equally as both D and L stereoisomers • False

27. Amino acid derivatives of importance • Gamma amino butyric acid (GABA, a derivative of glutamic acid) and dopamine (from tyrosine) are neurotransmitters • Histamine (Histidine) is the mediator of allergic reactions • Thyroxine (Tyrosine) is an important thyroid hormone

28. Peptide bond • Proteins are made by polymerization of amino acids through CO-NH bridge • α-carboxyl group of one amino acid reacts with α-amino group of another amino acid

29. Proteins are made by controlled polymerization of amino acids

30. Peptides • 2 aa- dipeptide • 3-? • 4- ? • Upto 10- oligo peptide • 10-50- polypeptide • More than 50 - proteins

31. Learning check-4 What are the possible tripeptides formed from one each of leucine, glycine, and alanine?

32. Solution-4 Tripeptides possible from one each of leucine, glycine, and alanine Leu-Gly-Ala Leu-Ala-Gly Ala-Leu-Gly Ala-Gly-Leu Gly-Ala-Leu Gly-Leu-Ala

33. References • Lippincott’s Illustrated reviews: Biochemistry 4th edition – unit 1