1. Protein Structure and Enzyme Function
How Protein Activity Depends on Proper Folding

2. Four Levels of Protein Structure
Primary (Sequence)
Tertiary (Folding by R-group interactions)
Depending on the sequence of amino acids, the polypeptide chain can either form a spiral like the tube on the left, or a pleated sheet like on the right.
Some proteins have a spiral section followed by a sheet section, etc.
Fig 2.26, p35.
Quaternary (Two or more chains associating)
Secondary (Coiling by Hydrogen Bonding)

3. The “R” Group Differs for Each Amino Acid
Proteins: Structure
Primary structure = chain of amino acids
Amino acids have common features
Carboxylic Acid Group
Amino Group    
See p 34 R
Central Carbon
The “R” Group Differs for Each Amino Acid

4. Proteins: Structure Forming the Protein Chain
Phenylalanine
Leucine
Here we position two amino acids close together so that the amine group of one is close to the carboxyl group of the other.
Note that a -H is hanging out from one, and a -OH from the other.
These will be broken off (by an enzyme) to form water.
The remaining bonds of the two amino acids will be connected to form a dipeptide (two amino acids joined by a peptide linkage).
Dehydration Synthesis between COOH & NH2

5. Proteins: Structure Forming the Protein Chain
The Peptide Bond
Water
Phenylalanine-Leucine Dipeptide

7. Four Levels of Protein Structure
Secondary Structure:
Folded structure due to hydrogen bonds between the amino and acid groups of amino acids N C N C H O H or O O H O H C N C N

9. Four Levels of Protein Structure
Tertiary Structure:
Three dimensional folded structure due to attractions and repulsions between R groups
Can involve
covalent bonding hydrogen bonding ionic interactions hydrophilic and hydrophobic interactions

11. Four Levels of Protein Structure
Quaternary Structure:
Association of two or more protein chains
eg. Hemoglobin is composed of 4 protein chains
2 are called alpha hemoglobin
2 are called beta hemoglobin

12. Applying Your Knowledge
Primary
Secondary
Tertiary
Quaternary
Which structure results from hydrogen bonding?
Which structure involves an association of two or more protein chains?
Which structure describes the linear sequence of amino acids?
Which structure depends upon interactions between the R groups of the amino acids?

13. Enzymes Assist in Biological Reactions
Enzymes are biological catalysts.
biological: composed of protein
or, rarely, RNA
catalyst: speeds up a reaction
without being changed
by the reaction

14. Properties of Enzymes Most enzymes are proteins.
All enzymes are SPECIFIC for the reactants (substrates) in the reactions that they catalyze.
Enzymes speed up biological reactions by lowering the activation energy for the reaction.

15. Enzyme-Substrate Interactions
1 Substrates enter active site
Active Site
2 Shape change promotes reaction
Enzyme
Product released; enzyme ready again

16. Activation Energy: Controls Rate of Reaction
Energy content of molecules
high
low
Activation energy without catalyst
Activation energy with catalyst
Progress of reaction

17. Properties of Enzymes
Three dimensional structure of an enzyme preserves its ACTIVE SITE
active site: region on enzyme where
reactants bind
Conditions that can affect three dimensional structure include: heat, pH (acid/base balance) and other chemicals (salt, charged ions)

18. Effects of Temperature and pH on Enzymatic Activity
fewer collisions between enzyme and substrate
enzyme unfolds
enzyme unfolds

19. Applying Your Knowledge
Active Site
Activation Energy
Enzyme
Catalyst
A biological catalyst is a(n) __________ .
The region on an enzyme where reactants bind is the _________.
Enzymes (raise or lower) the (1, 2, 3 or 4) of a reaction.