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SBI4U
Enzymes
Factors that Affect Enzyme
Activity
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Factors that Affect Enzyme
Activity (reaction rate)
The factors we will test for the
enzyme lab are:

pH

temperature

substrate concentration

enzyme concentration
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Enzyme is said
to be denatured
– no longer a
catalyst
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Enzyme is said
to be denatured
– no longer a
catalyst
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Other Regulators of
Enzyme Activity
Enzyme Cofactors
Non-protein, bound
May
to enzyme
be organic or inorganic ions
Enhance
change
enzyme activity - “helper”
enzyme active site shape
make active site more reactive
Examples of Inorganic Cofactors
Mg in Chlorophyll
Fe in heme group of hemoglobin
Organic Cofactors
Active site
Coenzymes
e.g., NAD
Prosthetic group
(perm. attached)
Active site
Enzyme
Coenzyme
(detaches)
Prosthetic Groups
e.g., FAD
Enzyme
Important Organic Cofactors
Nicotinamide Adenine Dinucleotide (NAD)
coenzyme derived from vitamin B3
Coenzyme
e.g., NAD
carries and transfers electrons and
functions as oxidizing agent in redox
reactions
Active site
Coenzyme
(detaches)
Enzyme
Important Organic Cofactors
Flavin Adenine Dinucleotide
(FAD)
Active
site
 prosthetic group
Prosthetic
group
(perm.
attached)
Enzyme
 like NAD, FAD functions as a
reducing agent in cellular
respiration and donates electrons
to the electron transport chain
Prosthetic Group
e.g., FAD
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Covalent Modulation
 Enzymes
can be activated or inactivated by
covalent modification.
A
common example is phosphorylation of an
enzyme (addition of a phosphate group to the
amino acids serine, threonine, or tyrosine)
mediated by another enzyme called a kinase .
 The
phosphorylation is reversible, and other
enzymes called phosphatases typically catalyze
the removal of the phosphate group from the
enzyme.
Covalent Modulation
P
HSL
Glycerol
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P
FA
Triglyceride (TG)
FA
HSL
HSL
+ H2O (hydrolysis)
HSL
Hormone-sensitive lipase (HSL)
is phosphorylated during fasting or exercise
to catalyze the hydrolysis of TGs to release
FAs
FA
Glycerol
Fasting/
exercise
FA
FA
FA
Diglyceride
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Enzyme Inhibitors

Enzymes may become deactivated
 Temporarily
or Permanently
Types of Inhibitors
 Reversible
Inhibitors
 Irreversible Inhibitors
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Reversible Inhibitors

Used to control enzyme activity

Involves the substrate or the end
product of the reaction

For example: a build up of the end
product – called feedback inhibition
http://highered.mcgrawhill.com/classware/ala.do?alaid=ala_1032273
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Competitive Inhibitors

Competitive Inhibitors have a similar
shape as the substrate

Compete with the substrate to bind to
the active site, but no reaction occurs

Block the active site so no substrate
can fit
Competitive Inhibition
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Non-Competitive Inhibitors

Binds to a different site on the enzyme

Does not compete with the substrate to
bind to the active site
Two ways to non-competitively inhibit
the enzyme:
1. slow down the reaction
or
2. changes the shape of the active site
(allosteric inhibition)
Which of the following diagrams represents allosteric
inhibition?
(a) Reaction
Substrate
active
site
enzyme
(b) Inhibition
Substrate
Substrate binds with
the active site of
enzyme
Reaction occurs and product
molecules are produced
active
site
enzyme
Inhibitor Inhibitor binds with the inhibitor site of
the enzyme and changes the structure
of the active site
Inhibitor prevents binding of
the substrate by changing
the active site shape
(a) Reaction
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Substrate
enzyme
active
site
Inhibitor
site
(b) Inhibition
enzyme
Substrate binds with
the active site of
enzyme
Reaction occurs and product
molecules are produced
active
site
Inhibitor
site
Inhibitor
Inhibitor binds with the inhibitor
site of the enzyme
Substrate may still bind with the
enzyme but the reaction rate is
reduced
Recap - Distinguish between
competitive and non-competitive
inhibition
Competitive

inhibitor competes with the substrate for binding to
the active site of the enzyme and prevents reaction
Non-competitive
inhibitor
does not compete for the active site,

binds to a different site,

either slows down or completely prevents reaction.
Recap cont. Explain how allosteric
inhibitors differ from other noncompetitive inhibitors:
 While non-competitive inhibitors
reduce enzyme activity and slow down
the reaction rate, allosteric inhibitors
block the active site altogether and
prevent its functioning completely
+ Irreversible Inhibitors
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Also called poisons

For example: certain heavy metals
 E.g., cadmium, lead, mercury
Retained
in the body and lost slowly
Cyanide is a poison that prevents the activity of
cytochrome C oxidase, an enzyme in the electron
transport chain in the cell. It therefore inhibits ATP
production and cellular respiration.
Cytochrome c oxidase
Why are enzymes so tightly
regulated by co-factors and
inhibitors?
Control of Metabolism
 Biochemical
reactions are controlled in part by the
specificity of substrate binding, but the human body could
not function if all enzymes were present together and all
operating maximally with no regulation.

There would be biochemical chaos with substances being
synthesized and degraded at the same time.
 Instead, the
body tightly regulates enzymes through
metabolic pathways and by controlling specific enzymes
within a pathway.
 This
approach allows an entire pathway to be turned on or
off by simply regulating one or a few enzymes.
 Metabolic
pathways can also be regulated by switching
specific genes on or off.
Since the tight control of enzyme activity is essential for
homeostasis, any malfunction (mutation, overproduction,
underproduction or deletion) of a single critical enzyme can lead
to a genetic disease.
Lethal illness can be caused by the malfunction of just one type of
enzyme out of the thousands of types present in our bodies.
E.g., the disease phenylketonuria (PKU) results from a mutation of
a single amino acid in the enzyme phenylalanine hydroxylase,
which catalyzes the first step in the degradation of phenylalanine
The result is a build-up of phenylalanine and related products and
can lead to a number of ill effects
Why is it important to know how
enzymes are regulated?
 Enzymes
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play a critical role in everyday life.
Many heritable genetic disorders occur because there is a
deficiency or total absence of one or more enzymes.
Routine medical tests monitor the activity of enzymes in the
blood,involve the use of enzymes to diagnose diseases, and many
of the prescription drugs (e.g., penicillin,) exert their effects
through interactions with enzymes.
 Enzymes
and their regulators are important tools in
medicine, agriculture, and food science.
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The application of enzymes in food processing and pharmaceuticals is a
multi-billion dollar industry! Biotechnology is influencing countless
aspects of our daily lives. The use and impact of enzymes in these sectors
will be the focus of your biochemistry unit task.