BIO 330 Cell Biology
Lecture Outline
Spring 2011
Chapter 6: Enzymes
I. Introduction
A. Bioenergetics tells us whether reactions CAN go & how much energy they release or require
We need to know about mechanisms and rates of reactions, too
Enzymes influence both mechanism and rate
B. Definition of enzyme
Protein (or RNA) catalyst for biochemical reaction
II. Activation Energy and the Metastable State
A. Even highly exergonic reactions do not take place quickly
B. Activation energy (EA)
Transition state
Reaction rate is proportional to fraction of molecules with energy ≥ EA
C. Metastable state
Themodynamically unstable, but lack the energy to exceed EA
D. Overcoming the activation energy barrier
Either increase temperature OR…
Lower activation energy
Catalysts
An agent that enhances reaction rate by lowering EA
Not consumed by the reaction
III. Enzymes and Biological Catalysts
A. Properties of catalysts
1. Increases rxn rate by lowering EA
2. Acts by forming transient, reversible complexes with substrates
3. Changes rxn rate, not position of equilibrium
B. Enzymes – specific organic catalysts
Active site
Prosthetic groups – nonprotein components at active site
Coenzymes – small organic molecules attached to enzymes
Cofactors – inorganic ions/molecules attached to enzymes
Substrate specificity
High specificity – enzyme will recognize only one substrate
Group (low) specificity – enzyme will recognize similar substrates
Enzyme diversity and nomenclature
6 classes based on general function
Denaturation
High temperature
BIO 330 Cell Biology
Lecture Outline
Spring 2011
pH and general ionic strength of solution
inhibitors and activators
C. Enzyme activity
Substrate binding
Lock and key model vs induced-fit model
Substrate activation
Bond distortion, proton transfer, electron transfer
Catalytic event
Products are released following reaction
IV. Enzyme Kinetics
A. Reaction rates are dependent upon…
Substrate concentration
Enzyme concentration
Product concentration
Inhibitor concentration
B. Michaelis-Menten kinetics
Initial reaction velocity (v)
Substrate concentration ([S])
Vmax = maximum velocity
Saturation: a fundamental property of enzyme-catalyzed reactions
V = Vmax[S] / (Km + [S])
Lineweaver-Burk double reciprocal plot
C. Enzyme inhibitors
Reversible vs irreversible inhibitors
Irreversible inhibitors may be competitive or noncompetitive
V. Enzyme Regulation
A. Allosteric regulation
Feedback inhibition (end-product inhibition)
Allosteric enzymes exist in a high-affinity form or a low-affinity form
Allosteric effectors bind enzymes at allosteric sites
Shift equilibrium to high-affinity state (activator) or low-affinity state (inhibitor)
In multimeric proteins: catalytic subunits vs regulatory subunits
Cooperativity – increased affinity as each substrate binds
B. Covalent modification
Phosphorylation / dephosphorylation
Kinases vs phosphatases
Proteolytic cleavage