1. 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

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

3. 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

4. Alpha Helix
The a-helix is 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

5. 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

6. Learning Check Indicate the type of structure as:
1) primary ) 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.

7. Levels of Structure
So far…
Primary
Secondary
Next…
Tertiary

8. 19.8 Tertiary Structure The tertiary structure:
Gives a specific overall shape to a protein
Involves interactions and cross-links between R groups in different areas of the peptide chain
Is stabilized by:
Hydrophobic and hydrophilic interactions
Salt bridges (electrostatic interactions)
Hydrogen bonds
Disulfide bridges

9. Tertiary Structure
(electrostatic interactions)
Disulfide Bonds:

10. Tertiary Structure
The interactions of the R groups give a protein its specific three-dimensional tertiary structure

11. Learning Check Select the type of tertiary interaction as:
1) disulfide 2) salt bridge 3) H-bonds 4) hydrophobic
A. Leucine and valine
B. Cysteine and cysteine
C. Aspartate and lysine
D. Serine and threonine

12. 19.9 Quaternary Structure
The quaternary structure contains two or more tertiary subunits (protein chains)
Held together by same interactions as tertiary structure
Hemoglobin contains four chains
The heme group in each subunit picks up oxygen in the blood for transport to the tissues

13. Summary of Structural Levels

14. Learning Check Identify the level of protein structure:
A. Beta-pleated sheet
B. Order of amino acids in a protein
C. A protein with two or more peptide chains
D. The shape of a globular protein
E. Disulfide bonds between R groups

15. Learning Check
In myoglobin, about one-half of the 153 amino acids have nonpolar side chains.
A. Where would you expect those amino acids to be located in the tertiary structure?
B. Where would you expect the polar side chains to be?
C. Why is myoglobin more soluble in water than silk or wool?

16. Learning Check
State whether the following statements apply to primary, secondary, tertiary, or quaternary protein structure:
A. Side groups interact to form disulfide bonds or salt bridges.
B. Peptide bonds join amino acids in a polypeptide chain.
C. Hydrogen bonding between carbonyl oxygen atoms and nitrogen atoms of amide groups causes a polypeptide to coil.
D. Hydrophobic side chains seeking a nonpolar environment move toward the inside of the folded protein.
E. Protein chains of collagen form a triple helix.
F. A protein contains four tertiary subunits.

17. 19.10 Reactions of Proteins
Hydrolysis
Denaturation

18. Hydrolysis of Amides Amide hydrolysis (review section 16.8):
Protein hydrolysis:
Splits the peptide bonds to give smaller peptides and amino acids
Catalyzed by enzymes
Occurs in the digestion of proteins
Occurs in cells when amino acids are needed to synthesize new proteins and repair tissues

19. Denaturation
Disruption of bonds in the native secondary, tertiary and quaternary protein structures
Covalent amide bonds (primary structure) are not affected
Loss of biological activity with loss of structure

20. Applications of Denaturation
Cooking food containing protein
Wiping the skin with alcohol (denaturation of bacterial proteins)
Hair permanents

21. Learning Check
The products of the complete hydrolysis of the peptide Ala-Ser-Val are:

22. Learning Check
What structural level of a protein is affected by denaturation?

23. End of Chapter 19!