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Enzymes
Topics 3.6, 7.6
Assessment Statements
3.6.1 Define enzyme and active site.
3.6.2 Explain enzyme-substrate
specificity.
3.6.3 Explain the effects of temperature,
pH and substrate concentration on
enzyme activity.
3.6.4 Define denaturation.
3.6.5 Explain the use of lactase in the
production of lactose-free milk.
Enzymes are organic molecules
which act as catalysts
o Enzymes are proteins which have taken
on a very specific 3-D shape
o Somewhere within the enzyme is an
area (active site) designed to match a
specific molecule (substrate)
o Active site of enzyme matches the
substrate the way a key fits into a lock,
only one key will fit
o As catalysts, enzymes influence the rate
of reactions
o A set of reactants in the presence of an
enzyme will form product(s) at a faster
rate than without the enzyme
o Role of an enzyme in a reaction is to
lower the energy level (activation
energy) needed to start the reaction
o Enzymes are not reactants and not used
up in the reaction
Factors affecting enzymecatalyzed reactions
o Temperature
o Reactions with or without enzymes will increase
their reaction rate as temperature (and thus
molecular motion) increases
o Reactions which use enzymes do have an upper
limit
o Upper limit is based on the temperature at which
the enzyme (as a protein) begins to lose its threedimensional shape due to intramolecular bonds
being stressed and broken
o This causes a structural change in a protein that
results in the loss of its biological properties and is
said to be denatured
o Effect of pH
o Active site of an enzyme includes many amino
acids some of which have areas that are either
positively or negatively charged
o + and – of a substrate must match the opposite
charge when the substrate is in the active site of
an enzyme in order for the enzyme to have
catalytic action
o When a solution has become too acidic, the large
# of H+ can bond with the negative charges of the
enzyme or substrate and not allow proper charge
matching the two
o Same as basic, but with large # of OHo Will result in an enzyme becoming:
o less efficient
o completely inactive in extreme situations
o denatured
o No one pH that is best
for all enzymes
o Many within human
body are most active
when in a neutral
environment
oException: pepsin is
active in stomach
which has a highly
acidic environment
o Effect of substrate concentration
o If there is a constant amt. of enzyme, as the
conc. of a substrate increases, the rate of
reaction will increase as well
o Has a limit due to the fact that enzymes have
a maximum rate at which they can work
Use of lactase to help solve the
problem of lactose intolerance
o Almost all humans on Earth are born with the
ability to digest lactose because we have the
ability to produce enzyme known as lactase
o Lactase digests disaccharide lactose into two
monosaccharides which are more readily
absorbed into the bloodstream
o By adulthood we no longer produce any
significant amount of lactase
o So how are we able to drink milk without the
symptoms as cramping, excessive gas, and
diarrhea?
o Milk and milk
produces can be
treated with lactase
before consumption
o Lactose intolerance
has been shown to
have an extremely
high incidence in
some ethnic groups
and be relatively low
in others (natural
variation in a
population)
Enzyme Review
o video
Assessment Statements
o 7.6.1 State that metabolic pathways consist of
chains and cycles of enzyme-catalyzed
reactions
o 7.6.2 Describe the induced-fit model
o 7.6.3 Explain that enzymes lower the activation
energy of the chemical reactions that they
catalyze
o 7.6.4 Explain the difference between
competitive and non-competitive inhibition,
with reference to one example of each
o 7.6.5 Explain the control of metabolic pathways
by end-product inhibition, including the role of
allosteric sites
Metabolism
o Def.- the sum of all the chemical
reactions that occur in you as a living
organism
Anabolic reactions
Catabolic reactions
Build complex molecules Break down complex
molecules
Are endergonic
Are exergonic
Are biosynthetic
Are degradative
Example: photosynthesis Example: cellular
respiration
Metabolic pathways
o Almost all metabolic reactions in organisms are
catalyzed by enzymes
o Many of these reactions occur in specific
sequences and are called metabolic pathways
o Substrate A → substrate B → final product
o Each arrow represents a specific enzyme that
causes one substrate to be changed to another
until the final product of the pathway is formed
o Some pathways consist of cycles of reactions
instead of chains of reactions
o Others involve both cycles and chains
(photosynthesis and cellular respiration)
Induced-fit model of enzyme
action
o As substrate “fits” into the active site, there
is a conformational change of the active
site, thus providing an induced fit
o Same way hand fitting into glove changes
shape of the glove
o Conformational changes and induced fit
are due to changes in the R-groups of the
amino acids at the active site of the
enzyme as they interact with the substrate
or substrates
Mechanism of enzyme action
1. The surface of the substrate contacts
the active site of the enzyme
2. The enzyme changes shape to
accommodate the substrate
3. A temporary complex called the
enzyme-substrate complex forms
4. Activation energy is lowered and the
substrate is altered by the
rearrangement of existing atoms
5. The transformed substrate – the product
– is released from the active site
6. The unchanged enzyme is then free to
combine with other substrate molecules
Activation energy (AE)
o Def. - Energy necessary
to destabilize the existing
chemical bonds in the
substrate of an enzymesubstrate catalyzed
reaction
o Enzymes cause
chemical reactions to
occur faster because
they reduce the amount
of energy needed to
bring about a chemical
reaction
Competitive inhibition
o A molecule called a
competitive inhibitor,
competes directly for the
active site of an enzyme
o Result is that the substrate then
has fewer encounters with the
active site and the chemical
reaction rate is decreased
o Competitive inhibitor must
have a structure similar to the
substrate to be able to
function in this way
o Example
o Use of sulfanilamide (a sulfa drug) to kill the bacteria
during an infection
o Folic acid is essential as a coenzyme to bacteria in
production of nucleic acids
o We take in folic acid through our diet
o It is also produced in bacterial cells by enzyme
action on para-aminobenzoic acid (PABA)
o The sulfanilamide competes with the PABA and
blocks the enzyme
o May be reversible (inhibition may be
overcome by increasing the substrate
concentration)
o May be irreversible
Non-competitive inhibition
(allosteric inhibition)
o Involves an inhibitor that does not compete for the
enzyme’s active site
o The inhibitor interacts with another site (allosteric site)
on the enzyme
o The allosteric site causes a change in the shape of the
enzyme’s active site, making it non-functional
o Example
o Strychnine, a convulsant poison, acting as
an allosteric inhibitor of glycine
o Glycine is a major post-synaptic inhibitory
neurotransmitter in mammalian spinal cord
and brain stem
o Strychnine’s binding lowers the affinity of the
glycine receptor for glycine.
o Strychnine thus inhibits the action of an
inhibitory transmitter, causing convulsions.
End-product inhibition
o Prevents the cell from wasting chemical
resources and energy by making more of a
substance than it needs
o When the end-product of the metabolic
pathway is present in a sufficient quantity, the
assembly line shuts down by inhibiting the
action of the enzyme in the first step
o As the existing end-product is used up by the
cell, the first enzyme is reactivated
o The enzyme that is inhibited and reactivated is
an allosteric enzyme
Commercial Uses
o Pectinase
o Acts on pectin
o Produces smaller, more soluble carbohydrates
o Used in fruit juice clarification to increase yield
o Endonuclease DNA
o Acts on DNA
o Produces DNA fragments
o Used in genetic engineering
o Protease
o Acts on proteins
o Produces amino acids
o Found in washing powders for stain removal