Enzymes
Chapter 5
Lesson Objectives
Questions to answer!
1. What are enzymes?
a.
State the definition of enzymes.
2. How do enzymes work?
a.
Explain the mode of action of enzymes in terms of
active site, enzyme-substrate complex, lowering of
activation energy, and enzyme specificity.
b.
Describe the characteristics of enzymes.
3. What affects enzyme activity?
a.
Investigate and explain the effects of temperature,
pH, on the rate of enzyme-catalyzed reactions.
3. Where are enzymes used?
a.
b.
State 1 enzyme-catalyzed process.
Classify enzymes.
What are enzymes?
Recall: What are
proteins??
Definition of Enzymes
What are catalysts?
How do they work?
Enzymes are biological catalysts, commonly
made of protein. They alter the rate of
chemical reactions without themselves being
chemically changed at the end of the reaction.
Properties of enzymes
What are enzymes?
Recall: Proteins
•
•
•
•
•
Basic unit of proteins: amino acids
How amino acids are linked up: condensation reaction
Link between amino acids: peptide bond
Long chains of basic units: polypeptide
Polypeptides are folded and its 3D shape is held together
by weak bonds .
• Weak bonds can be broken by heat and chemicals. The
protein is said to be denatured .
• A denatured protein loses its function .
Source: http://www.monroecc.
edu/deps/pstc/backup/heme.gif
How do enzymes
work?
Enzymes are
Biological Catalysts
• A catalyst is a substance which can alter or speed up a
chemical reaction.
• Catalysts are not chemically-changed at the end of the
reaction.
Enzymes are
Biological Catalysts
How do enzymes
work?
Eg: Decomposition of Hydrogen Peroxide…
2H2O2 → 2H2O + O2
This reaction can be sped up by a catalyst,
manganese (IV) oxide.
Videos:
Dilute H2O2: H2O2 decomposition
Fresh H2O2: Last High School Chemistry Class
Source: http://pics.drugstore.
com/prodimg/73864/200.jpg
Enzymes are
Biological Catalysts
How do enzymes
work?
Eg: Decomposition of Hydrogen Peroxide…
2H2O2 → 2H2O + O2
This reaction can be sped up by a catalyst,
manganese (IV) oxide.
Inorganic Catalyst
2H2O2
MnO2 (catalyst)
2H2O + O2 + Heat
The same amount of manganese (IV) oxide remains
after the reaction. It is not changed during the
reaction.
Enzymes are
Biological Catalysts
How do enzymes
work?
The enzyme CATALASE can speed up hydrogen peroxide
decomposition.
Biological Catalyst
(Organic)
Enzymes are BIOLOGICAL CATALYSTS.
Video: Nosebleed Chemistry
2H2O2
Catalase (enzyme)
Note: Catalase is also
found in potatoes!
2H2O + O2 + Heat
How do enzymes work?
How do enzymes
work?
Activation Energy
• The energy needed to start a chemical reaction is called
activation energy.
• Once the reaction has received enough energy, the rest
of the reaction occurs spontaneously.
How do enzymes
work?
Activation Energy
• Enzymes lower the activation energy required to start a
chemical reaction.
• Enzymes do not
change the energy
level of reactants and
products.
Products
Reactants
• Enzymes only lower
the ‘height’ of the
‘activation energy hill.’
How do enzymes
work?
Lock and Key Hypothesis
• The substances on which enzymes act are called substrates.
• Enzymes bind to substrates by the lock and key hypothesis.
active sites
enzyme molecule is free
to take part in another
reaction
enzyme molecule
(the ‘lock’)
AB
B
A
substrate molecules
(A and B) can fit into
the active sites
enzyme-substrate
complex
a new substance (product) AB
leaves the active sites
How do enzymes
work?
Lock and Key Hypothesis
• Enzyme reaction depends on the presence of active sites.
• Active sites have a specific 3D shape.
• Specific substrate molecules fit into the active site like a
lock and key.
• Substrate binds to the enzyme, forming an enzyme
substrate complex.
• Reactions take place at the active sites to convert
substrate molecule(s) into product molecule(s).
• Product molecule(s) separate, leaving the enzyme
molecule unchanged and free to combine again with
more substrate molecules.
How do enzymes
work?
Induced Fit Model
• Current hypothesis for enzyme action
• When a substrate molecule fits into an enzyme molecule,
the enzyme molecule alters its shape slightly so that it fits
more tightly around the substrate molecule.
• This makes the chemical reaction easier.
Lock and key hypothesis
Source: http://neurobio.drexel.edu/GalloWeb/loudon_enzymes.htm
Induced Fit Model
How do enzymes
work?
Enzyme Characteristics
Enzymes are required in minute amounts
• Enzymes are very efficient molecules.
• They remain unchanged at the end of the reaction, and
thus can be reused over and over again.
• A small amount of enzyme can catalyze a large amount of
chemical reactions.
How do enzymes
work?
Enzyme Characteristics
Enzyme Specificity
• Enzyme action is highly specific: each chemical reaction is
catalyzed by a unique enzyme.
• The enzyme’s specificity is due to its 3-dimensional
surface configuration.
• The shape of the substrate must be complementary to the
shape of the enzyme’s active site.
• The shape of an enzyme can be affected by high
temperature and acids and alkalis.
How do enzymes
work?
Enzyme Characteristics
Enzymes catalyze reversible reactions
• Some reactions in living cells are reversible. They can
proceed in the forward or backward direction.
• Some enzymes catalyze both forward and backward
reactions until an equilibrium is reached. This occurs
when the amount of reactants equals the amount of
products formed.
How do enzymes
work?
Enzyme Characteristics
Some enzymes require coenzymes
• Some enzymes require a coenzyme (another compound)
to be bound to them before they can catalyze reactions.
• Coenzymes are non-protein organic compounds.
• Eg. of coenzymes: Vitamin B complex
What affects
enzyme activity?
4 Factors
Enzymes are affected by the following:
• Temperature
• pH
• Enzyme concentration
• Substrate concentration
What affects
enzyme activity?
Temperature
• Enzymes have an optimum temperature.
• This is the temperature at which the enzyme catalyzes the
largest number of reactions per second.
• The optimum temperature of enzymes varies in different
organisms, and is often but not always close to the
temperature at which the enzyme usually functions.
• Some enzymes may have very high or very low optimum
temperatures.
What affects
enzyme activity?
Rate of reaction
(enzyme activity)
Temperature
3
The optimum temperature is reached.
Enzyme is most active.
4
Beyond the optimum
temperature, enzyme
activity decreases.
2
1
An enzyme
is less active
at very low
temperatures.
0
As the temperature rises,
enzyme activity increases as
indicated by the increase in
the rate of reaction it
catalyses. Usually the
enzyme is twice as active
for every 10°C rise in
temperature until the
optimum temperature is
reached.
5
K (optimum
temperature)
D
At point D, the enzyme
has lost its ability to
catalyse the reaction.
Temperature
What affects
enzyme activity?
Temperature
As temperature rises…
• Particles of matter are in constant random motion.
• In the reaction, the enzyme and substrate molecules
move and collide with one another at random.
• Raising the temperature increases the kinetic energy of
the molecules.
• The substrate and enzyme molecules collide with each
other more often, increasing the chance of substrates
fitting into active sites.
• The rate of formation of enzyme-substrate complex
increases, increasing the rate of formation of products.
What affects
enzyme activity?
Temperature
As temperature rises…
• Enzyme activity increases as the temperature increases up
to the optimum temperature.
• At high temperatures, enzyme activity decreases.
• Enzymes are made of proteins , and its 3D shape are held
together by
.
weak bonds
• At high temperatures, the vibrations of the atoms in the
enzyme vibrate so violently that they break the weak
bonds in the enzyme.
• The enzyme loses its shape and becomes denatured.
• The higher the temperature, the faster the rate of
denaturation.
What affects
enzyme activity?
pH
• Enzymes are affected by the acidity or alkalinity of the
solutions they are in.
• Acid or alkali can interact with the enzyme molecule,
causing the protein to change its shape.
• Extreme changes in pH of the solutions denature the
enzymes.
• Enzymes also have an optimum pH – the pH at which the
enzyme can best function in.
What affects
enzyme activity?
pH
What affects
enzyme activity?
Enzyme & Substrate
Concentration
What affects
enzyme activity?
Enzyme & Substrate
Concentration
• In a solution of fixed enzyme concentration, as substrate
concentration increases, the rate of reaction increases to
a maximum.
• Beyond the maximum point, increasing the substrate
concentration will not increase the rate of reaction.
• This is because at any point of time, all the enzyme
molecules are in use. The enzyme molecules are
saturated.
• The enzyme concentration becomes the limiting factor.
• The reaction rate will increase if the enzyme
concentration is increased.
Where are enzymes
used?
Digestion
• Some food molecules are large and insoluble in water.
• These food molecules cannot diffuse through the cell
surface membrane.
• Some examples of large food molecules are proteins ,
and fats .
starch
• The large molecules must first be converted into simpler
smaller substances which are soluble in water and
diffusible.
• This is known as digestion, and is achieved by digestive
enzymes.
Where are enzymes
used?
Digestion
Enzymes are involved in two types of reactions in the body:
1. Synthesis of complex substances from simpler ones
2. Break down of complex substances to simpler ones
HYDROLYTIC REACTIONS
CONDENSATION
REACTION
Examples of condensation
reactions in the body:
• Forming
starch
from glucose
• Forming proteins from
amino acids
Examples of hydrolysis in the body:
• Large molecules of food are converted to
smaller molecules by digestive enzymes.
• In cell respiration, glucose is broken down to
carbon dioxide and water and release energy.
• Hydrogen peroxide can be produced in chemical
reactions in cells. It is toxic to tissues. Cells
produce enzyme catalase which catalyses the
breakdown of hydrogen peroxide.
Enzymes catalyze practically ALL the chemical reactions
that occur in an organism!
Where are enzymes
used?
Classification of Enzymes
• Enzymes are classified according to the chemical
reactions they catalyze.
• Enzymes that catalyze hydrolytic reactions are known as
hydrolases.
• Oxidation-reduction enzymes are involved in breaking
down glucose during respiration.
• Enzymes were previously named by the persons who
discovered them.
• Nowadays, enzymes are named according to a scientific
system.
Where are enzymes
used?
Type of hydrolase
Classification of Enzymes
Example(s)
Carbohydrases
• Salivary amylase (in the mouth) and
digest carbohydrates
pancreatic amylase. Both digest starch.
• Cellulases digest cellulose. It is produced by
some bacteria. It is not found in mammals.
Proteases
• Pepsin in the stomach
digest proteins
Lipases
digest lipids (fats)
• Lipase in pancreatic juice
Where are enzymes
used?
Industrial Uses
• Enzymes are used in industry as they can bring about
chemical changes at low temperature.
• Chemical reactions at low temperatures are easier to
control and cheaper to run.
• Enzymes can be extracted from living cells and used in
their pure form.
Where are enzymes
used?
Industrial Uses
• Microorganisms producing useful enzymes can be used
in industry to make cheese, yoghurt and beer.
• Catalase can be added to latex containing hydrogen
peroxide to produce oxygen to form foam rubber.
• Enzyme specificity makes them ideal for use in analysing
chemicals.
– Enzymes can be used to detect glucose in urine.
– This can help to detect diabetes.
Source: http://www.southernbiological.com/Products/
Kits&Equipment/SpecialLabFieldEquipt/G10_36.htm
Source: http://news.thomasnet.com/
images/large/011/11187.jpg
Pure Biology Chapter 5
Definition of enzymes
Enzymes are biological catalysts,
commonly made of protein. They
alter the rate of chemical reactions
without themselves being
chemically changed at the end of
the reaction.
Enzymes
have specific
are
Functions
Proteins
as
Remain chemically
unchanged at the
end of the reaction
Biological catalysts
form
speed up
Enzyme-substrate
complex
with the
help of
are
Inactivated
Enzyme
Substrate
concentration concentration
by
by lowering
Coenzymes
affected by
Rate of
reaction
Lock-and-key
hypothesis
Induced-fit
model
Activation
energy
of
pH
Temperature
are
at extreme
Chemical reactions
involve
Intracellular
enzymes
Extracellular
enzymes
found in
Digestive
system
examples
is the
Specific binding
of enzyme
to
Substrate
to form
Specific
product(s)
Lipases
digest
Lipids
to form
Fatty acids
and glycerol
Proteases
digest
Proteins
to form
Amino
acids
Carbohydrases
digest
Carbohydrates
to form
Simple
sugars
at low
Denatured
at high
Pure Biology Chapter 5
Definition of enzymes
Enzymes
have specific
are
Functions
as
Remain chemically
unchanged at the
end of the reaction
Biological catalysts
form
Enzyme-substrate
complex
with the
help of
are
speed up
Inactivated
Enzyme
Substrate
concentration concentration
by
by lowering
Coenzymes
affected by
Rate of
reaction
Induced-fit
model
of
at low
are
at extreme changes of
Chemical reactions
involve
Intracellular
enzymes
Extracellular
enzymes
found in
examples
is the
Specific binding
of enzyme
to
Lipases
digest
Specific
product(s)
digest
Proteins
Substrate
to form
Denatured
to form
to form
digest
Carbohydrates
to form
Simple
sugars
at high