1. Proteins
Section 3.4

2. Learning Objectives
Give an overall description of the structure & functions of proteins
Describe the features shared by all amino acids
Distinguish among the four levels of organization of protein molecules
Explain how the sequence of amino acids in a protein determines each level of that protein’s structure
Explain how the conditions of the environment a protein is in affect its structure & function

3. Protein Protein = polymer made from amino acid monomers
Proteins perform different functions based on their structure
Monomer is amino acids
20 different amino acids

4. Types of Proteins Enzymes – catalyze reactions
Transport proteins – move things through the cell membrane
Regulatory proteins – control activities of the cell (ex. hormones & transcription factors)
Protective proteins – defend against foreign invaders (ex. antibodies)
And many more…

5. Amino Acids Monomers of proteins Only 20 kinds of amino acids
Arranged in different orders to make different proteins
Are important biological buffers
Resist changes in pH

6. Every amino acid has: an amino group a carboxyl group
an R group (side chain)
what makes each of the 20 different amino acids different

7. The R group gives each amino acid specific properties
Leucine (Leu)
Serine (Ser)
Cysteine (Cys)
HYDROPHOBIC
HYDROPHILIC
Figure 3.12B
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

9. Essential Amino Acids
Prokaryotes & plants can usually make all of their amino acids from simpler substances
Animals can only make some of the amino acids on their own
Essential Amino Acids = amino acids that an animal species cannot make
Must get them from their diet
Different for each animal species

11. Peptide Bonds Join Amino Acids
Cells link amino acids together by condensation reactions
Peptide bond = The bond between joined amino acid monomers

12. Dipeptide = two amino acids joined together
Polypeptide = many amino acids joined together in a chain
Protein = one or more polypeptide chains folded in a specific way
Can be hundreds of amino acids long

13. A Protein’s Shape Determines Its Function
A protein consists of polypeptide chains folded into a unique shape
The shape determines the protein’s function
A protein loses its function when its polypeptides unfold
Figure 3.14A
Figure 3.14B
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

14. Proteins Have 4 Levels of Structure
Primary Structure = the specific sequence of amino acids in a protein
1st level of organization

15. A slight change in the primary structure of a protein can affect its ability to function properly
The substitution of one amino acid for another in hemoglobin causes sickle-cell disease.

16. Secondary structure = polypeptide coiling (α-helix) or folding (β-pleated sheet) produced by hydrogen bonding between atoms in the backbone
2nd level of organization
Primary structure
Amino acid
Secondary structure
Hydrogen bond
Pleated sheet
Alpha helix
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Figure 3.15, 16

17. Tertiary structure = the overall 3-D shape of a polypeptide
A result of interactions between R groups
3rd level of organization
Quaternary structure = the interaction of multiple polypeptide chains in a protein
4th level of organization
Tertiary structure
Polypeptide (single subunit of transthyretin)
Quaternary structure
Transthyretin, with four identical polypeptide subunits
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Figure 3.17, 18

18. The Environment Affects Protein Structure & Function
A protein’s shape is sensitive to the surrounding environment & can affect protein structure
Chaperones = proteins in a cell that help other proteins fold properly
Denaturation = the process where a protein unfolds
Caused by high heat or changes in pH or salt concentration
Usually cannot be reversed or undone
Ex. When you cook an egg the clear part turns white as the proteins unfold

19. Denaturation