General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.1 Chapter 19 Amino Acids and Proteins 19.4 Protein Structure: Primary and Secondary Levels

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.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 +

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.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

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.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)

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.5 Secondary Structure: Alpha Helix The secondary structures of proteins indicate the three-dimensional spatial arrangements of the polypeptide chains. An alpha 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

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.6 Secondary Structure: Alpha Helix (continued)

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.7 Secondary Structure: Beta-Pleated Sheet A beta-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

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.8 Secondary Structure: Beta-Pleated Sheet (cont’d)

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.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.

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.10 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 Learning Check

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.11 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 Solution