Catalysts
Reduces the amount of activation
energy needed for a reaction to take
place.
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A type of catalyst.
Globular proteins (tertiary structure) that speed up a metabolic
reactions by lowering the activation energy needed.
Name usually ends in “-ase” and also indicates the substrate it acts
on. (ex: Lipase breaks down…?)
 This is not always the case.
 Ex. Amylase breaks down starch and Catalase breaks down
hydrogen peroxide
Metabolic pathways using an enzyme can work in both the forward
and reverse direction


A+B
C
or C
A+B
Very small amounts needed
 Reusable (are not changed or destroyed)
 Highly specific
 (ex: catalase only works on Hydrogen peroxide)
Types of Enzymes
Catabolic
1.
Enzymes that break down a molecule through
hydrolysis
•
•
•
•
•
Amylase breaks complex starches into simple sugars
Catalase breaks down hydrogen peroxide to water and
oxygen
More examples: Lactase breaks down lactose, Papain
breaks down papaya
Any type of digestive enzyme
Anabolic
2.
Enzymes that synthesize (build) molecules
through dehydration
•
•
•
ATP synthase creates ATP from ADP and a phosphate
Anabolic steroids build muscle
Lactose intolerance
The inability to metabolize lactose,
a sugar found in milk and other dairy
products.
The required enzyme lactase is absent in the
intestinal system.
Symptoms of lactose intolerance include loose
stools, abdominal bloating and pain, flatulence,
and nausea.
Activation energy energy required to start a reaction.
(with and without an enzyme)
• Is reaction shown endergonic or exergonic?
• Do these reactions have to catabolic, anabolic, or can they be
either?
Substrate & Active site
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The substrate is the substance that the enzyme is
working on.
Active site –
Where the enzyme attaches
to the substrate.
Allosteric site – a separate site on an enzyme where the
binding of a regulatory molecule can either inhibit or
stimulate the enzymes activity
Allosteric Site (continued)
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
The binding of an activator to the allosteric
site stabilizes the shape that has functional
active sites (remember, enzymes are globular
so they may have an active site on each
polypeptide).
Binding of an inhibitor stabilizes the inactive
form of the enzyme
Lock & Key Model
Induced Fit Model

When the enzyme and substrate “lock” together, the
enzyme changes it shape slightly to fit more tightly
around the substrate.


Ex. Glove changing shape once hand is inserted
Ex. Lock clicking when key is full inserted
Ways in which substrate can bind to
enzyme
1.
2.
3.
Hydrogen bonds
Van der Waals interactions
Ionic bonds
Types of Activators

1.
Enzymes can be “primed” to react by 2 factors.
Allosteric activation
As discussed earlier, the binding of an activator molecule to the
allosteric site can alter the shape of the enzyme as to make it
more likely to bind to its substrate.
1.
Cooperativity
A substrate molecule binding to one active site may stimulate the
catalytic powers of a multisubunit enzyme by affecting the other
active sites
In other words, if an enzyme has 2 or more subunits, a substrate
molecule causing induced fit in one subunit can trigger the same
favorable shape change in all the other subunits.
Ex. Hemoglobin binding to one oxygen promotes the pickup of
additional oxygen molecules (hemoglobin is not an enzyme, but
the concept of cooperativity is shown here)
Cofactors and Coenzymes

Cofactors – nonprotein helpers that aids enzymes in
their catalytic activity
 May be bound tightly to the enzyme as permanent
residents or may bind loosely and reversibly along
with the substrates
Inorganic examples: zinc, iron, and copper
Organic examples: Organic cofactors are specifically called
coenzymes
Most vitamins are coenzymes
Determining the reaction rate of a
reaction using an enzyme


Studying the rate of disappearance of the
substrate or appearance of the product.
Calculating the slope on a time vs. product
formed graph
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Remember “Dry Mix”
What does it mean when line is increasing linearly?
What about when line is flat?
Enzymes are affected by 1), temperature, 2), pH, 3)
concentration (of enzyme or substrate), or 4) salinity
*Can be denatured (change shape & lose function)
saturation point.
The effect of concentration on reaction rate
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Substrate Concentration
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Reaction rate increases as substrate concentration increases (b/c enzyme
works faster)
Equilibrium eventually reached b/c enzyme cannot work any faster.
Enzyme Concentration
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Reaction rate increases as enzymes concentration increases (b/c more
enzymes present to aid in breaking down the substrate).
Equilibrium eventually reached b/c all the substrate is being broken down and
adding more enzymes will not affect the reaction rate (Because those enzymes
will have no substrate to break down).
Competitive Inhibitor
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Molecule competes for
the “active site”
How can this kind of
inhibitions be
overcome?
Noncompetitive inhibitor
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The presence of a noncompetitive inhibitor changes the shape of
the enzyme and prevents the reaction from occurring.
Both noncompetitive inhibitors and allosteric regulators bind at a
site other than the active site to control the activity of an enzyme.
 They are different in that allosteric regulators are reversible
while most noncompetitive inhibitors are not. Also, allosteric
regulators can be serve to excite or inhibit an enzyme while
noncompetitive inhibitors only inhibit.
Inhibitors are used to control reaction
rates

Beneficial examples of inhibitor usage:

Negative feedback mechanism - Many enzymes work to monitor
their production because products of their reactions serve to inhibit
the enzyme themselves.

Ex. A + B

X
C
If product C inhibits enzyme X then the accumulation of product C
would stop this reaction from occurring and stops the enzyme from
“overworking”
Other examples: many prescription drugs work, some
types of chemotherapy, some pesticides
Harmful examples of inhibitor usage:
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Non-prescription drugs
Toxins
Poisons work by inhibiting enzymes as well.