Properties of Amino Acids

Transcription

1 Biochemistry Department Date:19/9/ 2017 Properties of Amino Acids Prof.Dr./ FAYDA Elazazy Professor of Biochemistry and Molecular Biology 1

2 Intended Learning Outcomes (ILOs) By the end of this lecture, the student will be able to: 1.Describe the properties of amino acids 2.Explain amphoteric character of amino acids 3.Define isoelectric point and zwitter ion

3 Properties of Amino acids Solubility Color Chiral: optical isomers Ionization Amphoteric Buffering activity Peptide bond formation

4 1-Amino Acids Solubility *Soluble in polar solvents like water and alcohol due to presence of charged groups (COOH &NH3)

5 colorless Properties of Amino Acids Amino acids do not absorb visible light Aromatic amino acids absorb ultraviolet light (wave length 280nm)

6 Properties of Amino Acids COOH 1-Asymetric C atom (chiral carbon) Attached to 4 different groups R C H NH2 Not in glycine (R=H) Topic 12-1

7 Properties of Amino Acids Asymmetric C atom A.A. solution 1- Optical activity: ability to rotate the plane polarized light to Rt or Lt *All amino acids are optically active except Glycine Topic 12-1

8 Properties of Amino Acids Asymmetric C atom COOH COOH L NH2 C R R C NH2 D H H 2- Allow formation of Optical isomers (D&L) Stereoisomers OR enantiomers OR Mirror image Topic 12-1

9 Stereoisomerism of a-amino Acids Left hand Right hand Stereoisomerism Definition: Molecules have the same molecular formula and same structural groups but differ in orientation of the groups in space

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11 2) Ionization AA in neutral solution (aqueous solution) are ionized. Ionized carboxyl group (negative charge) and ionized amino group (positive charge). The ionized amino acid is Known as zwitterion or dipolar ion ( one positive pole and one negative pole) = net zero charge Topic 12-1

12 AA In crystalline form AA In neutral environment, in water

13 AA + Electric field (electrophoresis) Isoelectric point (PI): The specific ph at which the AA can exist in dipolar form with zero net charge so cannot migrate in electric Topic 12-1 field

14 Acidic environment Alkaline environment NH 2 H + R-C-H COO- ph ~ 2 H NH 2 R-C-H COO - 7 NH 2 R-C-H H + H + COO - ph ~ Isoelectric point

15 2) Ionization At a higher ph, the amino acid loses a proton, carries a net negative charge & migrates to the anode At pi, the amino acid carries a net charge = zero and does not move in an electric field At a lower ph, the amino acid accepts a proton, carries a net positive charge & moves to the cathode

16 4-Amphoteric Property Amphoteric property: The property of amino acid to behave as an acid (proton donor) at extreme high ph (pka of COOH=9) or as a base (proton acceptor) at extremely low ph. Both are far from blood PH 7.4. So, Amino acids have no buffering activity in plasma. Amino acids have a buffering capacity in stomach to buffer HCl (low ph ) ph ~ 2

17 Tends to accept a proton H Tends to loose a proton H 2 N C COOH Basic group R Acidic group Amino acids are amphoteric molecules

18 At physiological ph: + H So, amino acids are water soluble - H 3 N C COO Basic group R Acidic group Side group may also carry positive or negative charge

19 Isoelectric point Higher ph More basic Basic solution (Lower Hydrogen Concentration) AAs Carry Negative charge

20 Lower ph More acidic Isoelectric point Acidic solution (high hydrogen Concentration) AAs carry Positive chargre

21 Formation of Peptide Bonds by Dehydration A A are joined covalently by peptide bonds NH2 1 COOH 2 NH2 COOH 1 O NH2 C N COOH H Dehydration -H 2 O 2

22 Oligopeptides 2 Amino acids = Dipeptide 3 Amino acids = Tripeptide 10 Amino acids = Decapeptide

23 Proteins are formed of polypeptide chains Amino-terminus With free amino group (N-terminus) Carboxyl-terminus With free carboxyl group (C-terminus)

24 Peptides N terminal Dipeptide C-terminal Tripeptide Oligopeptide: 2-10 AA Polypeptide: >10 AA

25 Proteins H 2 O Hydrolysis (Proteolysis) Amino acids

26 Naming the peptide The amino acid sequences are read from the N to the C terminal end of the peptide Amino acids suffixes are changed to yl except the C terminal AA Alanylglycylphenylalanine

27 Which of the following is not an essential amino acid? A-histidine B-lysine C-tryptophan D-alanine

28 Of the following amino acids which would be least likely to be present on the outer surface of a protein? a. arginine b. glycine c. glutamate d. leucine e. serine

29 Phenyl alanine amino acid : a- is a heterocyclic amino acid b- has a negatively charged R-group c- is a non polar amino acid d- is a non essential amino acid

30 All the followings are true about Zwitter ion EXCEPT a- is formed at the isoelectric point b- is a dipolar ion c- is formed at a specific ph for each amino acid d- migrates to the cathode in an electric field

31 A peptide bond is formed by a) Carboxyl group of one aminoacid and amino group of another aminoacid. b) Electrostatic attraction between negatively charged carboxyl and positively charged amino group. c) Both (a) and (b). d) Neither (a) nor (b). Peptide bond is a covalent bond

32 A linear tripeptide is formed of a) Two amino acids and three peptide bonds. b) Three amino acids and two peptide bonds. c) Three amino acids and three peptide bonds d) Four amino acids and three peptide bonds.

33 Amino acid number 1 in a protein is a) The aminoacid at the N terminus. b) The aminoacid at the C terminus. c) The smallest aminoacid. d) The aminoacid with the biggest side chain.

34 Amino acids are amphoteric molecules, so they a) have both acidic and basic groups. b) can react with acids or alkalis. c) can act as buffers at more than one ph. d) all the above.

35 In electrophoresis: An aminoacid moves towards the anode a) At ph equal to its isoelectric point. b) At ph higher that its isoelectric point. c) At ph lower that its isoelectric point. d) At ph equal to zero.

36 Two amino acids that have different molecular formulas are called a)isomers. b)stereoisomers. c)optical isomers. d)d- and L- amino acids. e)none of the above.

37 Topic 12-1