Biochemistry: A Short Course

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Chapter 6 Basic Concepts of Enzyme Action
Proteolytic Enzymes Catalyze Peptide Bond
Hydrolysis
Jellyfish luminescence
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Gibbs Free Energy Indicates Reaction Spontaneity
?G ?H - T?S
?G' ?H' - T?S'
?G ?G' RTlnKeq
What determines if a reaction is spontaneous in
the forward direction reverse direction or at
equilibrium?
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Entropy (?S) is a measure of Disorder
Lower Entropy
Higher Entropy
Enthalpy (?H) is a measure of Heat Content
Negative ?H ? Exothermic reaction Positive ?H ?
Endothermic reaction
What is ?G, ?H and ?S for Burning paper?
Dissolving (NH4)2SO4 in H2O?
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Enzymes Accelerate the Reaction Rate
What catalysis difference can an enzyme make?
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Enzymes Enhance Reaction Rates
How are enhanced reaction rates possible?
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Enzymes Decrease the Activation Energy
How can enzymes lower the transition state? Is
external energy required?
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Mechanisms of Enzyme Catalysis

• Acid-Base Catalysis
• Covalent Catalysis
• Metal Ion Catalysis
• Orientation/Proximity Effects
• Preferential Transition- State Binding

Can the mechanism for an enzyme catalyzed versus
uncatalyzed reaction be different?
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• Substrate Specificity Observed with each
  Proteolytic Enzyme
• Papain cleave peptides non-selectively
• Trypsin cleaves carboxyl side of bulky
  charged R- groups
• Chymotrypsin cleaves carboxyl side of bulky
  aromatic R-groups

Thrombin
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Enzyme-Substrate Binding Critical for Catalysis
Lock and Key Model

• Enzyme Active Site
• 3-D cleft or crevice
• Small part of enzyme
• Unique micro-environment
• Substrate binding by weak forces

Induced Fit Model
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Inhibition by Transition State Analogs
Pyrrolidine the natural substrate binds 160 less
tightly than pyrrole a transition state
analog. What is the favored enzyme binding
geometry?
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Components that Facilitate Enzyme Catalysis
What is the cofactor in ATP hydrolysis? What is
the co-substrate in an oxidation-reduction
reaction? What is a apoenzyme versus a
holoenzyme?
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Rate of Enzyme Catalysis
Explain why enzyme activity increases with
temperature and then precipitously drops off
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RNAas A Digestive Enzyme Cleaving Mechanism
Why does ribonuclease catalyzes the hydrolysis of
RNA but not DNA
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Conversion of Adenosine to Inosine
What does the much greater binding affinity of
1,6-dihydropurine ribonucleoside than the
substrate indicate about the enzyme mechanism?
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Chapter 5 Problems 1-19