1. Chapter 5
Proteins

2. Proteins More than 50% of the dry weight of most cells
Humans have tens of thousands of different proteins, each with a specific structure and function
All proteins are polymers (polypeptides) built from the same 20 amino acids
Used for structural support, storage, transport of substances, signaling from one part of the organisms to another, movement, defense against foreign substances, and as enzymes (see Table 5.1 on Page 72)

3. Amino Acids There are only 20 amino acids
Organic molecules possessing both carboxyl and amino groups
Amino acids are grouped according to the properties of their side chains
Some are polar, some nonpolar, some acidic and some basic

4. Nonpolar amino acids - hydrophobic

5. Polar amino acids - hydrophilic

6. Amino Acids
Electrically charged amino acids – side chains are negatively or positively charged causing them to act as acids or bases and to dissociate in water. They are hydrophilic.

7. Peptide Bond
Peptide bond – covalent bond linking amino acids together to form polypeptides
Polypeptides vary in length from a few monomers to thousands or more

8. Proteins
Each specific polypeptide has a unique linear sequence of amino acids
Chains of polypeptides make a protein
A protein’s specific conformation determines how it works
A functional protein consists of one or more polypeptides precisely twisted, folded, and coiled into a molecule of unique shape
Four Levels of Structure
Primary, Secondary, Tertiary, Quaternary

9. The primary structure of a protein is the unique amino acid sequence

10. Primary Structure
A change in the primary structure can affect a protein’s conformation and ability to function

11. Secondary Structure
Segments of the polypeptide chain repeatedly coiled or folded in unique patters that contribute to the protein’s overall conformation
Results from the hydrogen bonds at regular intervals along the polypeptide backbone, not the amino acid side chains
Can be in the form of an α helix or β pleated sheet

12. Tertiary Structure
Irregular contortions from interactions between side chains of the various amino acids.

13. Quaternary Structure
The association between two or more polypeptides that make up a protein

14. What Determines Protein Conformation?
Dependent upon the interactions among the amino acids making up the polypeptide chain
Usually arise spontaneously as soon as the protein is synthesized in the cell
These interactions can be disrupted by changes in pH, salt concentration, temperature, etc. causing the protein to denature, losing its native conformation and thus its function

15. The Protein-Folding Problem
Chaperonins, chaperone proteins that assist the folding of other proteins.

16. X-Ray Crystallography