Chapter 19 Amino Acids and Proteins

1. Chapter 19 Amino Acids and Proteins 19.4Protein Structure: Primaryand Secondary Levels

2. Primary Structure of Proteins The primary structure of a protein is • the particular sequence of amino acids • the backbone of a peptide chain or protein + Ala─Leu─Cys─Met

3. Primary Structures The nonapeptides : oxytocin, vasopressin synthesized in hypothalamus, secreted by pituitary oxytocin – stimulate uterus contraction during childbirth, & milking vasopressin – anti-diuretic hormone(ADH, stimulate water absorption in nephron) • have similar primary structures • differ only in the amino acids at positions 3 and 8

4. Primary Structure of Insulin Insulin • was the first protein to have its primary structure determined F. Sanger (NP, 1958) • has a primary structure of two polypeptide chains linked by disulfide bonds • has an A chain with 21 amino acids and a B chain with 30 amino acids • Tertiary structure elucidated in 1969, D. Hodgkin (NP, 1964, X-Ray Crystallography)

5. Secondary Structure:Alpha Helix The secondary structures of proteins indicate the three-dimensional spatial arrangements of the polypeptide chains. Analpha helix (α-helix) has • a coiled shape held in place by hydrogen bonds between the amide groups and the carbonyl groups of the amino acids along the chain • hydrogen bonds between the H of an —NH group and the O of C═O of the fourth amino acid down the chain • A. A. with smaller R groups

6. Secondary Structure: Alpha Helix (continued)

7. Secondary Structure: Beta-Pleated Sheet Abeta-pleated sheet (β-pleated sheet) is a secondary structure that • consists of polypeptide chains arranged side by side • has hydrogen bonds between chains • has R groups (bulky) above and below the sheet • is typical of fibrous proteins such as silk

8. Secondary Structure: Beta-Pleated Sheet (cont’d)

9. Secondary Structure: Triple Helix A triple helix • consists of three alpha helix chains woven together to form braid-like fibers (fibrils), • contains large amounts of glycine, proline, hydroxyproline, and hydroxylysine that contain –OH groups for H-bonding Vitamin C is required for enzymes for synthesis of these. • found in collagen, connective tissue, skin, tendons, and cartilage: these protein become brittle, loose elasticity with Vit. C deficient diet of with old age.

10. Learning Check Indicate the type of protein structure as: 1) primary 2) alpha helix 3) beta-pleated sheet 4) triple helix 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 D. three peptide chains woven like a rope

11. Solution Indicate the type of protein structure as: 1) primary 2) alpha helix 3) beta-pleated sheet 4) triple helix 3 A. polypeptide chains held side by side by H bonds 1 B. sequence of amino acids in a polypeptide chain 2 C. corkscrew shape with H bonds between amino acids 4 D.three peptide chains woven like a rope