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- 1. AMINO ACIDS AND PEPTIDES MR. R. PHIRI CH 2011 March-June 2021 SEMESTER SESSION
- 2. •Amino acids are the monomer units of proteins. •There are over 300 amino acids that occur in nature but of these only 20 amino acids repeatedly occur in proteins. The 20 amino acids are referred to as standard amino acids. •Amino acids contain 2 functional groups: amino and carboxyl group bonded to an 𝛼 −carbon. There’s an 𝑅 −group attached to the 𝛼 −carbon. The 𝑅 −group specifies the type of amino acid. •The carboxyl end of amino acids is acidic whilst the amino end is basic. •Amino acids show stereoisomerism about the 𝛼 −carbon: most amino acids are 𝛼 − L− amino acids. Proteins are synthesized from these 20 L-α-amino acids
- 3. General Structure of an Amino Acid
- 4. Three dimensional structure of amino acids
- 5. Biomedical importance of amino acids 1. Amino acids are the building blocks of proteins. 2. Amino acids and their derivatives are important in the biosynthesis of porphyrins, purines, pyrimidines, and urea. 3. Amino acids and their derivatives participate in diverse cellular functions such as nerve transmission, and as chemical messengers.
- 7. Classification of amino acids Amino acids can be classified based on: 1. Structure 2. their metabolic fate 3. Chemical nature 4. Nutritional requirement.
- 8. 1.Classification based on structure and chemical nature The 20 amino acids found in proteins can be classified into 6 major groups.
- 9. 1.Non polar Aliphatic side chains • These are amino acids that contain non polar aliphatic side chains. • These are generally simple amino acids. • They include glycine, alanine, leucine, isoleucine, valine and proline • (Leu, lle, Val) contain branched aliphatic side chains • Proline can be classified as an imino acid due to possessing a secondary amine group
- 11. 2.Amino acids containing hydroxyl (𝑂𝐻− ) groups •Serine, threonine and tyrosine •Tyrosine is also aromatic and is therefore considered under aromatic amino acids too.
- 12. 3. Sulfur containing amino acids •Cysteine and Methionine •Cystine, another important sulfur containing amino acid formed by condensation of two molecules of cysteine.
- 13. 4.Acidic amino acids and their amide Derivatives • Glutamic, Aspartic, Asparagine,Glutamine • asparagine and glutamine are their respective amide derivative of aspartic and glutamic
- 15. 5. Basic amino acids •Lysine, Histidine, Arginine •These are highly basic. Histidine forms an imidazole and it may also be considered under aromatic amino acids.
- 17. 6.Aromatic amino acids • Phenylalanine, Tyrosine, Tryptophan • Histidine may also be considered to be an aromatic amino acid.
- 19. 2. Classification based on their chemical properties 1.Non-polar amino acids - these are also said to be hydrophobic 2. Polar amino acids with no charge -These are hydrophilic amino acids that do not contain any charges on them 3.Polar with a positive charge - polar amino acids with a net positive charge on them. 4.Polar amino acids with a negative charge - carboxylic
- 20. 3. Nutritional classification of amino acids •Amino acids can be classified as essential or non – essential. •Some essential amino acids include: Arginine, Valine, Histidine, lsoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, and Tryptophan. •But Arginine and Histidine can be synthesized by adults though children are unable to synthesize it and so they are considered semi - essential amino acids. •There are about 10 amino acids that are non-essential. They include: glycine, alanine, serine,cysteine,aspartate,asparagnine, glutamate, glutamine, tyrosine and proline.
- 21. 4.Amino acid classification based on their metabolic fate • The carbon skeleton of amino acids can serve as a precursor for the synthesis of glucose( glycogenic) or fat (ketogenic). • Based on metabolic fate amino acids can be classified into 3 groups. 1. Glycogenic 2. Ketogenic 3. Both ketogenic and glycogenic
- 22. Modified amino acids •In addition to the 20 standard amino acids, there are other amino acids found in proteins which are a result of posttranslational modification of parent amino acid. •These include: Hydroxyproline, hydroxylysine and thyroxine. •Hydroxyproline and hydroxylysine differ from the parent amino acids in that they have hydroxyl groups on their side chains •Thyroxine differs from tyrosine in that it contains an iodine- containing aromatic group on the side chain. Thyroxine is a hormone.
- 23. Amino Acids Can Act as Both Acids and Bases • Amino acids have both acidic and basic properties (amphoteric). • They are generally weak acids and bases. They can donate protons and are generally polyprotic acids. • Most amino acids are diprotic. • Carboxyl (negatively charged) and amino (positively charged) are charged at neutral pH. • Amino acids can dissociate and thus donate protons to a solution
- 24. Titration of amino acids • When an amino acid is titrated, its titration curve indicates the reaction of each functional group with hydrogen ion. • At very low pH, amino acids are protonated and positively charged. • When we add a base to this solution, the amino acid starts to lose its protons and there are changes in pH. As we add more of the base, amino acids become negatively charged at very high pHs. • Titratable groups of each of the amino acids have characteristic pKa values. The pKa values of α-carboxyl groups are fairly low, around 2. The pKa values of amino groups are much higher, with values ranging from 9 to 10.5. • The pKa values of side-chain groups, including side-chain carboxyl and amino groups, depend on the groups’ chemical nature.
- 25. Titration curve of amino acids e.g Glycine
- 26. Dissociation of an amino acid
- 27. •Amino acids can act as buffers, the maximum buffering capacity of an amino acid is at at it’s pka whilst maximum buffering range is within ±1 pKa value. •Amino acids have different pKa values and that means each amino acid will have different charge at a given pH and this property can be used to separate amino acids by electrophoresis
- 28. •For example alanine and histidine, both have a -1 charge at high pH but at lower pH of about 5, alanine is a zwitterion whilst histidine is positive. Thus these two amino acids can be separated from each other at this pH in an electric field. •The pH at which a molecule has zero net charge is called the isoelectric pH (pI). At its isoelectric pH, a molecule will not migrate in an electric field. The net charge of an amino acid determines its mobility in an electric field. This property can also be used in separating amino acids.
- 29. Calculating pI • The isoelectric point (pI) value can be calculated by taking the average pKa values of corresponding ionizable groups. pI = 𝑝𝐾1 +𝑝𝐾2 2
- 30. Example • Calculate the pI of leucine with pka COOH (2.4) and pka 𝑁𝐻3 + 9.6 . 𝒑𝑰 = 𝒑𝑲𝒂𝟏 + 𝒑𝑲𝒂𝟐 𝟐 𝒑𝑰 = 𝟐. 𝟒 + 𝟗. 𝟔 𝟐 =6.0
- 31. Peptides • Peptides are compounds in which an amide bond links the amino group of one a-amino acid and the carboxyl group of another. • Peptides can be said to be compounds formed by linking small numbers of amino acids, ranging from two to several dozen. • An amide bond of this type is often referred to as a peptide bond. • The carbon–nitrogen bond formed when two amino acids are linked in a peptide bond is usually written as a single bond
- 32. Importance of peptides • In the neuroendocrine system peptides useful as hormones (vasopressin & oxytocin), hormone-releasing factors, neuromodulators, and neurotransmitters. • Some peptides are useful as analgesics Leucine and methionine enkephalin. • Some Bacteria peptides contain both D-and L-α-amino acids which possess therapeutic value such as the antibiotics Bacitracin and the antitumor agent Bleomycin. • Some peptides are useful as antioxidants in the body e.g. Glutathione • Some peptides such as aspartame are used as food sweeteners.
- 33. Formation of a peptide Glycine Alanine + Alanylglycine + 𝐻2𝑂 → The two α- amino acids are joined by a peptide bond in alanylglycine by a condensation reaction. They form a dipeptide. Ala-Gly AG
- 35. • The peptide bond can be written as a resonance hybrid of two structures, one with a single bond between the carbon and nitrogen and the other with a double bond between the carbon and nitrogen. • The peptide bond is also stronger than an ordinary single bond because of this resonance stabilization. • The peptide bond is characterized by a planar geometry, partial double bond characteristic and inability to rotate. • Which bonds have the ability to rotate on a peptide ? • There is free rotation around the bonds between the α-carbon of a given amino acid residue and the amino nitrogen (phi) and carbonyl carbon(Psi) of that residue but no rotation on the peptide bond. • This constraint plays a role in how the protein backbone folds.
- 36. Classification of peptides •Peptides are classified according to the number of amino acids linked together. • dipeptides, tripeptides, tetrapeptides, etc •Some examples of peptides include Glutathione and Oxytocin
- 37. Oxytocin • Oxytocin induces labor in pregnant women and controls contraction of uterine muscle. Oxytocin also plays a role in stimulating the flow of milk in a nursing mother • Oxytocin is a cyclic Nona peptide. Instead of having its amino acids linked in an extended chain, two cysteine residues are joined by an S—S bond
- 38. Oligopeptide •Peptides made of small numbers of amino acids are called oligopeptides. •Oligopeptides have about 2 to 20 amino acids residues and some even up to 40 amino acid residues. •Thus oligopeptides include dipeptides, tripeptides, tetrapeptides, pentapeptides and octapeptides etc.