Enzymes
Essential knowledge 4.B.1: Interactions between molecules affect their
structure and function.
Exergonic vs Endergonic
(Spontaneous vs. Nonspontaneous)
• Exergonic/Spontaneous:
•
•
•
•
releases energy
no energy needed
𝛥G = +
More stable
• Endergonic/Nonspontaneous:
•
•
•
•
absorbs (stores) energy
energy needed
𝛥G = Less stable
Chemical Reaction
•Reactants – go into the reaction (aka
substrates)
•Productions – comes out of a reaction
•Chemical reactions require bonds of
reactants to break and form new bonds to
create a product
Fig. 8-14
Activation Energy
• Energy to get a reaction started
• Typically if a bond between two
molecules exist it is because the
molecule is stable  however,
a molecule can form bond with
new molecules to become even
more stable
• Although this is the case we
must first break the bonds in
the molecule that is currently
stable…we call the energy
needed to break these bonds
activation energy
A
B
C
D
Transition state
A
B
C
D
EA
Reactants
A
B
∆G < O
C
D
Products
Progress of the reaction
Partner Share Time
Explain ways you can speed
up chemical reactions. (At
least TWO). Or ways to get
the reactants into a transition
state.
Ways to Speed Up Reactions
• Can be in the form of heat (thermal energy) causes
the movement of molecules to increase  results in
molecules colliding more often increases rxn rate.
• Why can’t we use this method in living things?
• Denatures proteins and kills cells
• Speed up ALL reactions  not just desired reactions
• What do we use instead?
• ENZYMES = Biological catalyst
Class Review –
What is an
enzyme??????
What is an enzyme?
• Macromolecule that acts as a catalyst (recall a catalyst speeds up
chemical reactions…if you put chocolate syrup in a cup of milk its
going to take a really long time before you have chocolate milk;
however, you can use a spoon to speed up the reaction-in this
example the spoon acts as the catalyst)
• Enzyme is not consumed…it can be used over and over again…it is not
part of the reaction…it just helps the reaction take place
• Usually a protein (which is going to be our main focus…but
recall…there is also RNA that can speed up reactions called
ribozymes)
How Enzyme Lower Activation Barrier
• lowers activation energy by enabling the reactant molecules to absorb
enough energy to reach the transition state even at moderate
temperatures
• Cannot change 𝛥G  does not turn an endergonic reaction
(nonspontaneous) into an exergonic reaction (spontaneous); can only
speed up a chemical reaction that was already destined to happen
• When the substrate enters the active site of the protein, interactions
between the chemical groups of the substrate and the R groups (side
chains) of the amino acids interact causing the enzyme to change shape
• Lowers the activation energy by causing the bonds of the reactant to break…new
bonds form to create more stable molecules (products)
Substrate Specificity of Enzymes
• Enzymes are very specific to particular reactions because they are
so specific they can control which reactions take place and don’t take
place and any particular time
Pair Share
Explain how enzymes are
so specific to the substrates
they recognize and
metabolize.
How are enzymes specific to Enzymes?
• Most enzymes are proteins  macromolecule with unique 3D
shape  DNA determines mRNA which determines codon which
determines order of amino acid  R groups on amino acid will
determine how the protein folds  resulting in protein shape
protein shape will have an activation site that allows the
substrate to fit in a way that lowers the activation energy
• The active site can lower an EA barrier by
•
•
•
•
Orienting substrates correctly
Straining substrate bonds
Providing a favorable microenvironment
Covalently bonding to the substrate
1 Substrates enter active site; enzyme
changes shape such that its active site
enfolds the substrates (induced fit).
Substrates
6 Active
site is
available
for two new
substrate
molecules.
2 Substrates held in
active site by weak
interactions, such as
hydrogen bonds and
ionic bonds.
Enzyme-substrate
complex
Enzyme
5 Products are
released.
4 Substrates are
converted to
products.
Products
3 Active site can lower EA
and speed up a reaction.
Pair share – What factors
would influence the rate at
which a particular amount of
enzyme coverts substrate into
products.
Pair Share - Answer
• Concentration of substrate – more substrate = more product
• UNLESS Enzyme is saturated
• Enzyme is said to be saturated if all enzyme molecules have their
active sites engaged  you will not get products faster by adding
more substrate because ALL enzymes are fully activated
• If we needed to produce products faster when enzymes are saturated
the body would need to create more enzymes
Local Conditions (Enzymes Environments)
Impact on Enzyme Activity
• As we have seen the shape of the protein determines its function
• Changing the shape of the protein would cause the enzyme to be less
effective or completely ineffective
• pH and temperature both have an impact of the enzyme and can
change its shape
• Each enzyme has an optimal temperature and pH value
• The optimal temperature of most human enzymes is 35-40 degrees
Celsius OR 98.6 degrees F.
• Increase temps cause reactions to happen faster  but at some point the
thermal agitation of the enzyme disrupts the bonds between the R group in
the amino acid changing the enzyme shape
Cofactors
•Nonprotein helpers for catalytic activity
•May be permanent or reversible
•Can be inorganic (metals such as zinc, iron,
copper) OR organic (vitamins)
• Organic cofactors are more specifically called
coenzymes
Pair Share: What does inhibition mean? What
does activation mean? Describe ways you can
inhibit or activate an enzyme. Why would you
want to inhibit an enzyme?
Enzyme Inhibitors
• Inhibitors – essentially turn off or inactivate the enzyme; some inhibitors are
irreversible (such as toxins or poisons), others are reversible
• Two main types of inhibition:
• Competitive:
• Resembles normal substrate; binds to activation site; reduces productivity of enzyme because blocks
substrate from entering active site; can be overcome by increasing the concentration of substrate
• Noncompetitive:
• Hinders enzyme by binding to another part of the enzyme other than the active site; results in a change
in the enzyme shape  active site changes preventing substrate from entering
Substrate
Active site
Competitive
inhibitor
Enzyme
(a) Normal binding
(b) Competitive inhibition
Noncompetitive inhibitor
(c) Noncompetitive inhibition
Regulation of enzyme activity helps control
metabolism
• Chemical chaos would result if all the cell’s metabolic pathways were
operating simultaneously
• Cell’s regulate enzymes by switching them on or off (gene regulation –
essentially making the enzyme or not – another way to say it is
expressing the gene or not expressing the gene)
• Also control metabolism by regulating the activity of the enzyme once
it is made
Allosteric enzyme
with four subunits
Active site
(one of four)
Allosteric Regulation
• Can be used to
activate (turn on) or
inhibit (turn off) an
enzyme
• Allosteric regulation is
the controlling of an
enzyme by the binding
of a regulatory
(activator or inhibitor)
molecule
Regulatory
site (one
of four)
Activator
Active form
Stabilized active form
Oscillation
Nonfunctional
active
site
Inactive form
Inhibitor
(a) Allosteric activators and inhibitors
Stabilized inactive
form
Feedback Inhibition
• End product shuts down the pathway
• Prevents cell from wasting chemical resource by synthesizing more
products than is needed
Fig. 8-22
Active site
available
Isoleucine
used up by
cell
Feedback
inhibition
Isoleucine
binds to
allosteric
site
Active site of
enzyme 1 no
longer binds
threonine;
pathway is
switched off.
Initial substrate
(threonine)
Threonine
in active site
Enzyme 1
(threonine
deaminase)
Intermediate A
Enzyme 2
Intermediate B
Enzyme 3
Intermediate C
Enzyme 4
Intermediate D
Enzyme 5
End product
(isoleucine)