3. AMINO ACID AND PEPTIDES

Transcription

1 3. AMINO ACID AND PEPTIDES

2 3.1 Amino Acids and Peptides General structure - Only 20 amino-acids are found in proteins - Amino group and carboxyl group - α-carbon and side chain group

3 3.1 Amino Acids and Peptides Stereoisomers - Chiral: nonsuperimposable mirror image, many important biomolecules - Achiral: Superimposable, symmertric

4 3.2 Individual Amino Acids: Their Structures and Properties Classification of the R-groups - Polar or nonpolar nature - Presence of an acidic or basic group Side-chain carbon atoms with letters of the Greek alphabet

5 3.2 Individual Amino Acids: Their Structures and Properties Group 1- Amino Acids with Nonpolar Side Chains -Aliphatic: absence of a benzene or related structure +glycine

6 3.2 Individual Amino Acids: Their Structures and Properties Group 1- Amino Acids with Nonpolar Side Chains 6/29

7 3.2 Individual Amino Acids: Their Structures and Properties Group 2 Amino Acids with Electrically Neutral Polar Side Chains - Thiol group (-SH): can form disulfide(-s-s-) bridges and lose a proton.

8 3.2 Individual Amino Acids: Their Structures and Properties Group 2 Amino Acids with Electrically Neutral Polar Side Chains

9 3.2 Individual Amino Acids: Their Structures and Properties Group 3 Amino Acids with Carboxyl Groups in Their Side Chains - Because of the presence of the carboxylate, the side chain is negatively charged at neutral ph

10 3.2 Individual Amino Acids: Their Structures and Properties Group 4 Amino Acids with Basic Side Chains - Positively charged at near neutral ph

11 3.2 Individual Amino Acids: Their Structures and Properties Amino acids by three letters or one letter abbreviations

12 Biochemical Connections: Amino acid functions other than in peptides Glycine: most water soluble Methionine: a derivative source of the methyl group in many reactions Glutamic acid: MSG (monosodium glutamate), flavor enhancer, Chinese restaurant syndrome b-alanine: important part in Coenzyme A Histidine: converted to histamine, released as immune response Arginine: involved in urea cycle Asparagine and Glutamine: detoxification of ammonia in animals, nitrogen storage in plants

13 3.2 Individual Amino Acids: Their Structures and Properties Uncommon Amino Acids Common examples of the many possibilities

14 3.3 Amino Acids Can Act as Both Acids and Bases Charge in a free amino acid - carboxylate portion: negatively - amino group: positively - A zwitterion has equal positive and negative charges in solution. Titration curve when an amino acid is titrated. - The reaction of each functional group with hydrogen ion (or ph)

15 3.3 Amino Acids Can Act as Both Acids and Bases - Alanine

16 3.3 Amino Acids Can Act as Both Acids and Bases - Histidine (with a titratable side chain)

17 3.3 Amino Acids Can Act as Both Acids and Bases - The titration curves of alanine and histidine

18 3.3 Amino Acids Can Act as Both Acids and Bases The titratable groups of each of amino acids have characteristic pk a values. -This groups can still be titrated after the amino acid is incorporated into a peptide or protein * R group ionization occurs before amino group ionization.

19 3.3 Amino Acids Can Act as Both Acids and Bases Electrophoresis - A common method for separating molecules in an electric field. - Principle: The amino acids, peptides, and proteins have different pk a values. - Isoelectric ph (pi): ph at which a molecule has no net charge. pi = pk a1 + pk a2 2

20 3.4 The Peptide Bond Peptide bond - Individual amino acids can be linked together by forming covalent bonds. - Between the α-carboxyl group of one amino acid and the α-amino group of the next one. - Water is eliminated.

21 3.4 The Peptide Bond Peptides - Compounds formed by linking small numbers of amino acids Polypeptide chain - In a protein, many amino acids are linked by peptide bonds. - Amide: A compound formed by the reaction between an amino group and a carboxyl group.

22 3.4 The Peptide Bond Resonance structures - Structures that differ from one another only in the positioning of electrons - The peptide bond can be written as a resonance hybrid. Three dimensional conformation - How can the protein backbone fold?

23 3.5 Small Peptides with Physiological Activity Dipeptide - Carnosine (antioxidant) found in muscle tissue - Used in antiaging therapy - Between the carboxyl group of the β-alanine and the amino group of the histidine

24 3.5 Small Peptides with Physiological Activity - aspartame (sweet peptide): L-aspartyl-L-phenylalanine methyl ester

25 3.5 Small Peptides with Physiological Activity Tripeptide - Glutathione, a scavenger for oxidizing agents - γ-glutamyl-l-cysteinylglycine

26 3.5 Small Peptides with Physiological Activity - The oxidized form of glutathione: generated from two molecules of the reduced peptide by forming a disulfide bond between the SH groups of the two cysteine residues

27 3.5 Small Peptides with Physiological Activity - The full structure of oxidized glutathione

28 3.5 Small Peptides with Physiological Activity Pentapeptides: enkephalins (pain reliever) Tyr Gly Gly Phe Leu (three-letter abbreviations) T G G F L (one-letter abbreviations) Leucine enkephalin Tyr Gly Gly Phe Met T G G F M Methionine enkephalin

29 3.5 Small Peptides with Physiological Activity Cyclic structures of some important peptides - Oxytocin and vasopressin: S-S bond, nine amino acid residues - oxytocin: hormone for contraction at birth and milk ejection - vasopressin: hormone for blood pressure control

30 3.5 Small Peptides with Physiological Activity Some cyclic structures are formed by the peptide bonds themselves. - Two cyclic decapeptides by the bacterium Bacillus brevis (antibiotics)

31 The end.. One person with a belief is equal to a force of ninety-nine who have only interests. - John Stuart Mill