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Chapter 18 Amino Acids, Proteins and Enzymes

Chapter 18 Amino Acids, Proteins and Enzymes. Amino Acids. 20 amino acids; all are α -amino acids:. Essential AA’s Isoleucine Leucine Lysine Methionine Phenylalanine Threonine Tryptophan Valine Arginine Histidine. Non-Essential AA’s Alanine Asparagine Aspartate Cysteine

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Chapter 18 Amino Acids, Proteins and Enzymes

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  1. Chapter 18 Amino Acids,Proteinsand Enzymes

  2. Amino Acids • 20 amino acids; all are α-amino acids:

  3. Essential AA’s Isoleucine Leucine Lysine Methionine Phenylalanine Threonine Tryptophan Valine Arginine Histidine Non-Essential AA’s Alanine Asparagine Aspartate Cysteine Glutamate Glutamine Glycine Proline Serine Tyrosine 20 Amino Acids

  4. Serotonin • Non-protein significance of AA’s • Tryptophan precursor for serotonin, melatonin and niacin Melatonin Tryptophan Niacin • Glycine is a precursor for porphyrins such as heme Glycine Porphyrin Heme B

  5. ‘Non-Protein’ Amino Acids • β-Alanine • Carnosine • β-alanyl-L-histidine • Anserine • β-alanyl-N1-methyl-L-histidine • Glutathione • γ-L-glutamyl-L-cysteinylglycine

  6. Classification of AA R groups(Pg. 549) • Nonpolar Side Chains • Proline: a secondary amine • Methionine: contains S • Tryptophan & Phenylalanine: aromatic rings • Polar, Neutral Side Chains • Cysteine: S-H group • Tyrosine: aromatic ring • Acidic Side Chains • Basic Side Chains

  7. Zwitterions Although -amino acids are commonly written in the un-ionized form, they are more properly written in the zwitterion (internal salt) form. COOH donates H to NH2 • Unionized form doesn’t exist in aqueous soln. or solid state • All have high melting pts • All are soluble in water

  8. Isoelectric Point • Isoelectric point, pI: the pH at which the majority of molecules of a compound in solution have no net charge

  9. Handedness or Chirality “Left Handed” Stereocenter All but one of the AA’s are of the L configuration, which means that their configuration around the central Carbon is similar to that of L-glyceraldehyde.

  10. Enantiomers • Enantiomers (optical isomers): the 2 mirror image forms of a chiral molecule • Same formula but different arrangement • ALL enantiomers are stereoisomers • Same formula, same chemical groups (COOH, NH3, etc.) but different spatial arrangement (like cis-trans) • Enantiomeric pairs differ in effect on polarized light.

  11. Blue represents which atom? Red? White? Grey? Which AA is this? (Note: this is not the zwitterionic form)

  12. Peptides and Polypeptides • Peptide bond - Review • Naming Conventions • Amino end (N-terminal)Carboxyl end (C-Terminal) • ‘yl’ added to name except to C-Terminal AA • Ser-gly-ala not the same as Ala-gly-ser • Primary Structure = AA sequence

  13. Primary Structure • Primary structure: the sequence of amino acids in a polypeptide chain • The number peptides derived from the 20 protein-derived amino acids is enormous • there are 20 x 20 = 400 dipeptides possible • there are 20 x 20 x 20 = 8000 tripeptides possible • the number of peptides possible for a chain of n amino acids is 20n • for a small protein of 60 amino acids, the number of proteins possible is 2060 = 1078, which is possibly greater than the number of atoms in the universe!

  14. Primary Structure • Just how important is the exact amino acid sequence? • human insulin consists of two polypeptide chains having a total of 51 amino acids; the two chains are connected by disulfide bonds • in the table are differences between four types of insulin

  15. 1o Structure Crucial to Function • Hemoglobin and Sickle Cell Anemia • Normal Hb: -Thr-Pro-Glu-Glu-Lys-Ala Position: 4 5 6 7 8 9 • Sickle Cell: -Thr-Pro-Val-Glu-Lys-Ala

  16. Primary Structure • vasopressin and oxytocin are both nonapeptides but have quite different biological functions • vasopressin is an antidiuretic hormone • oxytocin affects contractions of the uterus in childbirth and the muscles of the breast that aid in the secretion of milk

  17. Primary Structure • The ‘linear’ AA sequence • Protein ‘Backbone’ consists of the alpha C, amino N and Carboxyl C. The 6 atoms, from alpha-C to alpha-C are arranged within an imaginary plane. • Important in 2o structure

  18. Shape-Determining Interactions in Proteins • H-bonding along backbone • R-group interactions • H-bonding • Hydrophobic interactions • Salt bridges • Covalent bonds • S-S

  19. Secondary Structure • Secondary structure: conformations of amino acids in localized regions of a polypeptide chain • the most common types of secondary structure are a-helix and b-pleated sheet • a-helix: a type of secondary structure in which a section of polypeptide chain coils into a spiral, most commonly a right-handed spiral • b-pleated sheet: a type of secondary structure in which two polypeptide chains or sections of the same polypeptide chain align parallel to each other; the chains may be parallel or antiparallel Interaction: H-bonding along backbone

  20. a-Helix Grey = C Blue = N Red = O Yellow = R-group White = H Hydrogen bonds are between the C=O of peptide bond and the H-N of another peptide linkage 4 AA’s further along the chain.

  21. b-Pleated Sheet Grey = C Blue = N Red = O Yellow = R-group White = H

  22. Beta-pleated sheet can be parallel or anti-parallel

  23. Fibrous Proteins (water-insoluble) Water Insoluble Secondary Structure determines nature Ex. Alpha-keratins Hair, fingernails, wool Composition of alpha-helixes; multiple strands twist together Ex. Fibroin Silk Composed of b-sheets Fibrous Proteins

  24. Collagen Triple Helix

  25. Tertiary Structure • Overall Conformation of a Peptide chain • How it folds, interactions between various AA’s along the chain. • Determined by interactions of R-groups • Hydrophobic • Salt-bridges • H-bonds • disulfide

  26. Example of a Globular Protein

  27. Myoglobin • Conjugated Globular Protein • Contains a heme group

  28. Quaternary Structure • Quaternary structure: the arrangement of polypeptide chains into a noncovalently bonded aggregation • the individual chains are held in together by hydrogen bonds, salt bridges, and hydrophobic interactions • Hemoglobin • adult hemoglobin: two alpha chains of 141 amino acids each, and two beta chains of 146 amino acids each • each chain surrounds an iron-containing heme unit • fetal hemoglobin: two alpha chains and two gamma chains; fetal hemoglobin has a greater affinity for oxygen than does adult hemoglobin

  29. Hemoglobin

  30. Chemical Properties of Proteins • Hydrolysis – breaking of peptide bonds • By enzymes • By acid • Denaturation – disruption of non-covalent bonds • Heat • Mechanical agitation • Detergents • pH change • Disrupt salt-bridges • Solvents • Disrupt hydrophobic interactions

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