So – just what is an enzyme?
Questions to think about:
• What type of macromolecule is it?
• What do you think it’s role is?
• Why are they important?
If you are not sure think about the
following scenarios...
1. You need to light a fire, and the only tools
you have are two sticks that you can rub
together.
2. You need to light a fire, and the only tool you
have is a match.
3. You need to light a fire, and the only tool you
have is a flint (which when
struck, creates sparks).
Things to think about...
•
•
•
Would all three scenarios work?
Other than type of tool used, what is the main
difference between them?
If you needed to light another fire, could you do
so (assuming other one completely went out)?
Now lets find out what an
enzyme actually is – as we are
doing so, decide which
scenario most closely
represents
the role of enzymes.
What are they?
• Enzymes are proteins
• Function is to catalyze reactions
• A catalyst is a substance that speeds up the
rate of a chemical reaction by lowering the
activation energy required for the reaction to
begin
• Catalysts are reusable, that is they are not
consumed in processes, so they can be
recycled for other reactions
Why do we need enzymes?
• All reactions, even spontaneous ones, require
a certain amount of energy to begin a reaction
• This energy is called the Activation energy(EA)
• Most chemical reactions in cells reach the
state of activation too slowly on their own
• Increasing temperature, results in increased
rates of reactions (i.e. speeds up the process),
but this has draw backs in living organisms
....what might that be?
Why do we need enzymes?
• Another way – to increase rate of chemical
reactions without increasing temperature is to
use a catalyst
• catalysts function by lowering the activation
energy of the reaction
• Almost all chemical reactions in organisms are
facilitated by enzymes
• So – which scenario most closely matches the
function of an enzyme?
Exergonic Reaction – a chemical reaction that releases energy
Energy
Supplied
EA = Activation Energy
Energy
Released
0
Reactions with and without a Catalyst
Energy
Released
Energy
Supplied
Endergonic Reaction – chemical reaction that requires energy
0
Reactions with and without a Catalyst
What ways can you think of
that would allow an enzyme
to decrease the activation
energy of a reaction?
How does it all work?
• Like other proteins, enzymes have 3-D shapes
with tertiary and often quaternary structures
• Each enzyme contains an active site
• An active site is a 3-D pocket or indentation on its
surface
• It matches the shape of the substrate and is the
site on an enzyme where the substrate binds
• Originally thought of as a key-and-lock model
How does it all work?
• A substrate is a reactant that interacts with the enzyme
in an enzyme-catalyzed reaction
• Together the enzyme and substrate bonded to it form an
enzyme-substrate complex
• Each chemical reaction has its own enzyme – i.e. They
are highly specific
E.g. Enzymatic Hydrolysis of Sucrose
Q: Do you think
maltose could
also be a
substrate for this
enzyme? Why or
why not?
How does it all work?
Now the thinking is:
• As substrates bond to active sites on enzymes,
the intermolecular bonds formed between the
macromolecules allows enzymes to adjust their
shapes slightly to accommodate the substrates
• This adjustment allows the substrate to fit in the
active site, and the change in the active site of
the enzyme is called induced fit
Substrate Binding
induced-fit model - enzyme changes shape
upon substrate binding
enzyme-substrate complex
Model Comparison
• Two proposed models:
– Lock-and-key model: this assumes that the active site is a
perfect fit for a specific substrate (once the substrate binds
there is no further modification)
Induced fit model: developed from the lock and key model
Change in shape to:
 bring R-groups closer to substrate
 bend bonds to make them easier to break / react
 reduce EA (makes transition state easier)
 bring two reactants close together
 provide a microenvironment for reactions
How enzymes Work
• Regardless of model
• Enzymes prepare substrates for reaction by:
– Changing the substrate
– Its environment
– Or both in some way
With the end result being a lowering of the activation
energy of the reaction.
How enzymes Work
• E.g., bring two substrates together in the correct
position for a reaction to occur
• Add or remove hydrogen ions to or from the
substrate (i.e., act as an acid or base),
destabilizing it and making it more likely to react
How enzymes Work
• Transfer electrons to or from the substrate (i.e.,
reduces or oxidizes it), which destabilizes it and
makes it morel likely to react
• Contain amino acid R groups that end up close to
certain chemical bonds in the substrate, causing
these bonds to stretch or bend, which makes the
bonds weaker and easier to break
Steps to Enzyme Reactions
1.
Substrate binds to available active site pocket.
2.
Enzyme changes shape to envelope substrate(s)
3.
Reaction occurs
4.
Products lose affinity for the active site
5.
Enzyme is set for another substrate
Whole cycle called catalytic cycle.
Enzyme Factors
• Some enzymes require non-protein molecules
to operate
Coenzymes = organic molecules that assist an
enzyme
Cofactors = inorganic molecules that assist an
enzyme (e.g., metal ions, iron, zinc)
Enzyme Classification
• Enzymes are classified according to the type of
reaction they catalyze
• E.g., hydrolases = enzymes that catalyze
hydrolysis reactions
• Each enzyme has a unique name, ending in
“-ase”, beginning with first part of the substrate
name
• E.g., lactase (breaks down lactose)
Any ideas? There are four of
them. What might be the
consequences?
Factors Affecting Enzyme Activity
•
•
•
•
Temperature
pH value
Concentration of substrate
Concentration of the enzyme itself
Temperature
• As temperature rises  increase in rates of
reactions.....to a degree!
• Which causes the rate of enzymatic reaction to
increase as well
• Every enzyme has a specific or optimal temp.
where its activity is the greatest. E.g. body
enzymes has optimal temp. 37.5oC
• At higher temp. intra and intermolecular bonds
are broken as enzymes gain more kinetic energy,
they become denatured.
pH
• Each enzyme works within quite a small pH
range.
• Activities are greatest at optimal pH
• Changes in pH break intra- and intermolecular
bonds, changing the shape of the enzyme, and
effectiveness.
Effect of Temperature and pH on Enzyme Activity
Limitations of Enzymes
• only a set number of
each type of enzyme
in body
– reactions have a
maximum rate
Concentration of enzyme and substrate
• Rate of reaction increases with increasing
substrate concentration up to a point,
• Above which any further increase in substrate
concentration produces no significant change in
reaction rate.
• Why? – because the active sites of the enzymes
molecules at any given moment are virtually
saturated with substrate.
• Enzyme/substrate complex has to dissociate
before the active sites are free to accommodate
more substrate
Provided that the substrate concentration is high and that
temperature and pH are kept constant, the rate of reaction is
proportional to the enzyme concentration.
Can you think of anyway
enzymes can be controlled or
kept in check?
Regulation of Enzyme Activity
• Regulate enzyme activity through use of inhibitors
• Inhibitor = a molecule that binds to the allosteric or
active site of an enzyme and causes a decrease in
the activity of that enzyme
• Allosteric site = a site on an enzyme that is not the
active site, where other molecules
can interact with and regulate the
activity of the enzyme
Regulation of Enzyme Activity
Competitive inhibitor – binds to the same
active site as the substrate
noncompetitive inhibitor – binds to an
alternate site (allosteric site) on the
enzyme to keep it in an inactive form (no
longer has affinity for substrate)
Q: What happens
if a competitive
inhibitor is
present, but in a
lower
concentration
than the
substrate? In
higher
concentration?
Competitive Inhibition
Non-competitive Inhibition
Note: If binding and
reduction of
enzyme is
permanent, then
the substance
binding to the
enzyme is
considered a toxin
Regulation of Enzyme Activity
• Also regulate enzyme activity through use of
activator molecules.
– Molecules that promote the action of enzymes and
which bind to the allosteric site of an enzyme.
• The regulation of enzyme activity by activators
and inhibitors binding to allosteric sites is called
allosteric regulation
Allosteric Sites
Feedback Inhibition
 a method for cells to regulate metabolic
pathways (i.e., maintain homeostasis)
 often, products at the end of a series of a
reaction will act as an allosteric inhibitor to shut
the reactions down
A, B, C and D are molecules along
a metabolic pathway.
E1, E2 and E3 are enzymes
required for this metabolic
pathway.
D is an allosteric inhibitor of E1.
Feedback Inhibition
Use of Enzymes
• Includes industrial and commercial purposes
– E.g. proteases are used in the dairy industry to
produce cheese
– proteases, along with amylase, are also added to
detergents to help remove protein and
carbohydrate produced stains
What you should have learned
• Examine enzymatic pathways,
• Focus on how they inhibit or activate reactions
• Use appropriate terminology related to
biochemistry e.g. allosteric, substrate, substrateenzyme complex, inhibition
• Describe the chemical structures and
mechanisms of various enzymes
• Analyse technology applications related to
enzyme activity in the food and pharmaceutical
industries
Homework
• Pg. 77, Q 1, 2, 4-9
Unit 1- Quest 2 (Monday)
• Focus – macromolecules and biochemical
reactions, including enzymes (not
thermodynamics)
• To be given
– Multiple choice
– Short answers
– Application questions