BIOLOGY 12
Enzymes
Enzymes
Video
Enzyme Basics
Like all catalysts, enzymes work by lowering the
activation energy for a reaction, thus
dramatically increasing the rate of the reaction.
As a result, products are formed faster and
reactions reach their equilibrium state more
rapidly. Most enzyme reaction rates are millions
of times faster than those of comparable uncatalyzed reactions. As with all catalysts,
enzymes are not consumed by the reactions
they catalyze, nor do they alter the equilibrium of
these reactions.
Enzymes
 proteins
 catalysts for biological reactions
 substrate specific
 they initiate chemical reactions without being
used in the reaction (reusable)
 usually named after their substrate, most end in
“ase” (ex. Maltase)
 “Lock and Key” Fit  Induced Fit Model
Enzymes
 they lower the
activation energy,
therefore increase
the rate of reaction
Enzymes
 activity is lost if they are denatured

the unfolding and inactivation – by heating or
chemical denaturants, which disrupt the threedimensional structure of the protein
Enzymes
 How Enzymes Work
Enzyme Action
Lock and Key Model
 substrate binds to active site of enzyme
 enzyme-substrate complex forms
 substrate reacts and product is formed
 product is released (enzyme is unchanged by
reaction)
Enzyme Action
Enzyme Action
Induced Fit Model
 scientists have modified the “lock and key”
model
 enzyme structure is flexible – not rigid
 the shape of the enzyme’s active site and the
shape of the substrate do not need to match
exactly
Enzyme Action
 when the enzyme and substrate join, the
enzyme changes its shape slightly
(adjusts to fit)
 this increases the range of substrate
specificity
Enzyme Action
Catalytic Cycle of an Enzyme
Activation Energy (EA)
 the minimum amount of energy required to
start a chemical reaction
 examples:


Oxygen gas + Hydrogen gas + Heat (A.E) =
Water
Wood + Oxygen + Burning Match (A.E) =
Burning wood
Activation Energy (EA)
 enzymes use a variety of mechanisms that
lower activation energy and speed up a
reaction
 *see graphs on page 69
 Activation Energy Graph
 example of why we have enzymes:
 enzymes allow reactions to take place at lower
temperature (i.e. body temperature)



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the reactions of living organisms cannot depend
on heat as a source of activation energy
high temperatures would destroy cells
heat would be far too random of a source of
activation energy (not dependable)
cells must carry out specific reactions at definite
times
Recap on Enzymes
 each is specific – guides only one type of
cell reaction
 they are not permanently changed or used
up during a reaction
 can be used over and over again
 the shape of the enzyme is crucial to the
reaction
Factors Affecting Enzyme Activity
 environmental factors
 cofactors and coenzymes
 enzyme inhibitors
Factors Affecting Enzyme Activity
temperature
 up to a point, the speed of the reaction increases
with the increase in temperature
 most enzymes function optimally around body
temperature (between 35o – 40oC)
 extreme temperatures can denature an enzyme
Factors Affecting Enzyme Activity
pH
 the optimal pH is between 6 and 8 (near
neutral)
Factors Affecting Enzyme Activity
cofactors
 inorganic non-protein helpers are sometimes
needed for optimal enzyme activity
 examples:



vitamins
zinc
iron must be present in the quaternary structure
of hemoglobin in order for it to pick up oxygen
Factors Affecting Enzyme Activity
enzyme inhibitors
 competitive:



has a structure similar to the substrate
competes with the substrate for the active site
occupies the active site
Factors Affecting Enzyme Activity
enzyme inhibitors
 non-competitive:





does not have a structure like the substrate
binds to the enzyme but not active site
changes the shape of enzyme and active site
substrate cannot fit into altered active site
no reaction occurs
Factors Affecting Enzyme Activity
Factors Affecting Enzyme Activity
 How Enzymes Work
Lets Label It
Questions
1. You turn on a battery powered tape deck
and the reels in the cassette begin to rotate.
How does this example explain energy
transformation?
A. Potential energy (stored energy) in the
batteries was converted to kinetic energy
(mechanical energy).
Questions
2. Suppose you started a fire by using a
magnifying glass to focus the sun’s image
on some paper.
What is the source of activation energy for
this reaction?
A. The concentrated sunlight was absorbed
and changed to heat. This heat provided the
activation energy.
Questions
3. Explain how the enzyme maltase can control a
reaction involving maltose, but not one
involving sucrose.
A. Maltose consists of two bonded glucose units,
whereas sucrose consists of a glucose bonded
to a fructose.
If the active site of the enzyme maltase fits with
maltose, it will not fit with sucrose.
Compare and Contrast
 Kinetic Energy / Potential Energy
 Endergonic / Exergonic
 Enzyme/ Substrate
 Lock and Key Model / Induced Fit Model
 Enzyme / Coenzyme
 Competitive Inhibitors / Non-Competitive
Inhibitors
Questions
 Page 77 #1, 4, 5, 6, 7, and 8.