ENZYMES
Section 1.4
Pg. 69-77
Outline
Review – What is an enzyme?
Models of enzyme activity
◦ Lock and key
◦ Induced fit model
Factors affecting enzyme activity
◦ Temperature
◦ pH
Regulation of enzyme activity
◦ Inhibition
◦ Allosterically
◦ Feedback inhibition
What is an enzyme?
“Biological catalysts”:
Speed up reactions in the body
Type of protein
◦ Polymer of amino acids
Often globular (spherical) in shape
Contain “active sites” – depressions or
grooves on the surface where molecules
can bind
Binding to an enzyme is very selective:
◦ Active sites interact with specific molecules to
catalyze specific reactions
The molecule that binds to an enzyme is
called its “substrate”
Enzymes lower the activation energy
required for a reaction to occur.
◦ Stabilizes the transition state by stretching the
bonds of the substrate
Some enzymes need to bind to additional
molecules to function:
◦ Cofactors (usu. metal ions), or
◦ Coenzymes (act like shuttles btw. enzymes)
Apoenzyme – The enzyme, without its
cofactors/coenzymes
Holoenzyme – The active form, bound to
cofactors/coenzymes
MODELS OF ENZYME ACTIVITY
Lock and Key model
Induced fit model
The “Lock and Key” model
The enzyme has an active site that is
unchanging
Substrates bind chemical process
occurs
Induced fit model
The enzyme can change its shape
1. One substrate molecule binds weakly
2. The enzyme’s active site changes shape so
that a second substrate molecule can bind
3. The second substrate binds
Chemical process (rx) occurs
FACTORS AFFECTING ENZYME
ACTIVITY
1. Temperature
2. pH
Temperature
Increase in activity with temperature
(molecules have more energy)
... but only to a certain point.
Too much heat will disrupt the 2° , 3° and
4° structure
◦ Denaturation & loss of function
Every enzyme has a temperature at which
it functions best – this is the optimal
temperature
For human enzymes it is usually 37°C
(body temp.)
pH
Enzymes also have optimal pHs
These will vary according to the location
in which they function
Location Optimal pH
Pepsin Stomach 2
Trypsin Small intestine 8
REGULATION OF ENZYME ACTIVITY
1. Inhibition
2. Allosterically
3. Feedback inhibition
Competitive inhibition
Are very similar in shape to an enzyme’s
substrate.
Bind to the active site, and physically
block the correct substrate from binding.
Competitive Inhibition
Enzyme Efficiency / Rate of Rx
Noncompetitive inhibition
May be allosteric
OR
The inhibitor directly affects the enzyme so
that it cannot carry out its function.
(chemicals that alter the enzyme’s shape and
therefore function)
Noncompetitive inhibition
Noncompetitive inhibition
Comparing the 2 inhibitions
Allosteric Regulation
Allosteric site – Site on an enzyme that
can bind with an effector.
NOT the same as active site.
Effector:
◦ Activator, or
◦ Inhibitor
Inhibition vs Activation
Inhibitors vs Activators
Some substances can inhibit enzyme
function – Inhibitors
Some substances can enhance enzyme
function – activators
Inhibition can be either competitive or
noncompetitive
Allosteric activator Allosteric inhibitor
Stabilize the active-form Stabilize the inactive form
See pg. 73,
Figure 7
Feedback inhibition
Often biological reactions occur in a series.
Feedback
inhibition:
The product of a
series of enzyme-
catalyzed reactions
acts to allosterically
inhibit an enzyme
that acts earlier in
the series.
WHY IS FEEDBACK INHIBITION
important?
5 minutes discussions:
WHY DO CELLS NEED FEEDBACK?
WHY DO YOU NEED FEEDBACK?
“TWO-WAY” WORLD
ANYONE?
SOME APPLICATIONS
Cheese-making
Cow enzyme rennet
Insulin for Type 1 diabetics
Lactase to digest lactose
Homework
Pg. 77 #1-8