ENZYMES
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
What is a catalyst????
a substance which increase in the rate of
a chemical reaction due to it’s
participation.
 The effect of a catalyst may vary due to
the presence of other substances known
as inhibitors or poisons (which reduce the
catalytic activity)
 Promoters (which increase the activity).
 The opposite of a catalyst, a substance
that reduces the rate of a reaction, is an
inhibitor.

The antonym…

The opposite of a catalyst, a substance
that reduces the rate of a reaction, is an
inhibitor.
Literacy T.Y.P.
5 minutes.
List three synonyms and three antonyms
of a catalyst.
 Give an example of each.


Share with your peers.

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.
2.
3.
One substrate molecule binds weakly
The enzyme’s active site changes shape so
that a second substrate molecule can bind
The second substrate binds
 Chemical process (rx) occurs
FACTORS AFFECTING ENZYME
ACTIVITY
Temperature
2. pH
1.
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.
2.
3.
Inhibition
Allosterically
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 enzymecatalyzed 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
There are three main enzymes
found in a human body
1.
2.
3.
digestive enzymes
metabolic enzymes
food enzymes
Who’s ‘ase is it anyways?
 Naming enzymes most end in “ase”