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Chapter 19: Proteins

Chapter 19: Proteins. CHEM 1152 Dr. Clower. Outline. Amino Acids Amino Acids as Acids and Bases Formation of Peptides Functions of Proteins Protein Structure: Primary, Secondary, Tertiary and Quaternary Structure Protein Hydrolysis and Denaturation. 19.1 Amino Acids. Amino acids:

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Chapter 19: Proteins

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  1. Chapter 19: Proteins CHEM 1152 Dr. Clower

  2. Outline • Amino Acids • Amino Acids as Acids and Bases • Formation of Peptides • Functions of Proteins • Protein Structure: Primary, Secondary, Tertiary and Quaternary Structure • Protein Hydrolysis and Denaturation

  3. 19.1 Amino Acids Amino acids: • The building blocks of proteins. • Two functional groups: • carboxylic acid group • amino group on the alpha () carbon. • Have different side groups (R) that give each amino acid unique characteristics.

  4. Nonpolar Amino Acids • Amino acids are classified as nonpolar when the R groups are H, alkyl, or aromatic • Note three-letter abbreviations

  5. Polar Neutral Amino Acids • Amino acids are classified as polar neutral when the R groups are alcohols, thiols, or amides

  6. Acidic and Basic Amino Acids • Amino acids are classified as acidic when the R group is a carboxylic acid • Amino acids are classified as basic when the R group is an amine

  7. Essential Amino Acids Essential amino acids: • Ten amino acids that are not synthesized by the body • Must be obtained from the diet

  8. Essential Amino Acids • Complete proteins contain all 10 essential amino acids (animal products) • Incomplete proteins are deficient in one or more (plant proteins) • Obtained by combining two or more vegetables that provide complementary proteins

  9. Learning Check • Classify each amino acid as polar or nonpolar: A. Glycine NH3+—CH2—COO- CH3 | CH—OH | B. ThreonineNH3+—CH—COO-

  10. Learning Check • Classify the following amino acids as hydrophobic (nonpolar), hydrophilic (polar, neutral), acidic, or basic: A. Lysine R = CH2CH2CH2CH2NH2 B. Aspartate R = CH2CO2H C. Leucine R = CH2CH(CH3)2 D. Serine R = CH2OH

  11. Learning Check • Give the name of the amino acid represented by each of the following three-letter abbreviations: A. Trp B. Met C. Pro D. Gly

  12. 19.2 Zwitterions • Both the –NH2 and the –COOH groups in an amino acid undergo ionization in water • A zwitterion forms that has + and – charge • Melting points of amino acids = very high

  13. Isoelectric Point • At the isoelectric point (pI), the + and – charges in the zwitterion are equal (no net charge). • At pH values above the isoelectric point, the amino acid has a net negative value. • At pH values below the isoelectric point, the amino acid has a net positive value.

  14. Amino Acids as Acids • In solutions more basic (higher pH) than the pI, the —NH3+ in the amino acid loses a proton to become —NH2 + OH− H3N—CH2—COO–H2N—CH2—COO– Zwitterion Negative ion at pI Higher pH

  15. Amino Acids as Bases • In solution more acidic (lower pH) than the pI, the —COO- in the amino acid gains a proton to become —COOH + H+ + H3N—CH2—COO– H3N—CH2—COOH Zwitterion Positive ion at pI Low pH

  16. pH and Ionization • Alanine

  17. pH and Ionization • Acidic amino acids such as aspartic acid have a second carboxyl group that can donate and accept protons • Ex: the pI for aspartic acid occurs at a pH of 2.8

  18. Learning Check CH3 CH3 +|| H3N—CH—COOH H2N—CH2—COO– (1) (2) Which structure represents: A. Alanine at a pH above its pI? B. Alanine at a pH below its pI?

  19. Learning Check • Write the zwitterion of each of the following amino acids: A. Phenylalanine B. Methionine

  20. Learning Check • Would the following ions of serine exist at a pH above, below, or at pI?

  21. 19.3 Reactions of Amino Acids • Two reactions of interest: • Oxidation of cysteine • Peptide bond formation

  22. Oxidation of Cysteine • Cys side chain = ─CH2─SH • Remember oxidation of thiols to form disulfide bonds (section 13.9)? R─SH + R─SH R─S─S─R • Two Cys amino acids can react to form a disulfide bridge [O]

  23. Disulfide Bridge/bond

  24. Review • Carboxylic acid + amine = ? • Structure of amino acid

  25. The Peptide Bond • A peptide bond is an amide bond between the carboxyl group of one amino acid and the amino group of the next amino acid O CH3 O +||+| || H3N—CH2—C—O– + H3N—CH—C—O– O H CH3 O +|||| || H3N—CH2—C—N—CH—C—O– + H2O peptide bond

  26. Peptides • Dipeptide = two amino acids; tri = 3; poly = many • Protein = polypeptide with >50 amino acid residues • Peptide is named from the N-terminal end (left; NH3+) • Use -yl endings for the names of the amino acids • The C-terminal amino acid (right; COO-) uses its amino acid name

  27. Peptides

  28. Learning Check Write the names and three-letter abbreviations of the amino acids in the tripeptides that could form from two glycine and one alanine.

  29. Learning Check Write the name of the following tetrapeptide using amino acid names and three-letter abbreviations.

  30. Learning Check • Draw the structural formula of each of the following peptides. A. Methionylaspartic acid B. Alanyltryptophan

  31. 19.5 Characteristics of Proteins • Size: • Varies from 6000 to several million g/mol • Hemoglobin = C2952H4664O832S8Fe4

  32. Characteristics of Proteins • Function:

  33. Characteristics of Proteins • Function:

  34. Example • Classify each of the following proteins according to its function: A. Insulin, a hormone needed for glucose utilization B. Antibodies, proteins that disable foreign proteins C. Casein, milk protein D. Lipases that hydrolyze lipids

  35. Characteristics of Proteins • Classification: • Fibrous vs. globular (based on structural shape) • Fibrous proteins: • Long rods or strings; fibers • Water-insoluble • Connective tissue, hair, skin • Globular proteins: • Spherical • Water-soluble • Synthesis, transport and metabolism in cells

  36. Protein Structure • Chain of amino acids folded into 3D structure • Levels of structure: • Primary • Secondary • Tertiary • Quaternary

  37. 19.6 Primary Structure • The primary structure of a protein is the sequence of amino acids in the peptide chain Protein backbone Ala-Leu-Cys-Met

  38. Insulin Insulin: • The first protein to have its primary structure determined • 51 residues • 2 chains • 2 disulfide bridges

  39. 19.7 Secondary Structure • The secondary structure of a protein indicates the arrangement of the polypeptide chains in orderly patterns. • Alpha helix • Beta-pleated sheet

  40. Alpha Helix • The a-helixis a three-dimensional arrangement of the polypeptide chain that gives a corkscrew shape like a coiled telephone cord • The coiled shape of the alpha helix is held in place by hydrogen bonds between the amide groups and the carbonyl groups of the amino acids along the chain

  41. Beta-Pleated Sheet The b-pleated sheet: • Holds proteins in a parallel arrangement with hydrogen bonds • Has R groups that extend above and below the sheet • Is typical of fibrous proteins such as silk

  42. Learning Check Indicate the type of structure as: 1) primary 2) a-helix 3) b-pleated sheet A. Polypeptide chains held side by side by H bonds. B. Sequence of amino acids in a polypeptide chain. C.Corkscrew shape with H bonds between amino acids.

  43. Levels of Structure • So far… • Primary • Secondary • Next… • Tertiary

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