Enzymes
Biological catalysts
Increase rate of reactions
by lowering activation energy (EA)
Spontaneous reactions can take a long time!
Need enzymes to speed reactions for cell survival
Activation Energy (EA)
• Needed to destabilize bonds of reactants
LE 8-14
A
B
C
D
Free energy
Transition state
A
B
C
D
Could raise temp.
to break bonds
EA
Reactants
A
B
DG < O
C
D
Products
Progress of the reaction
Why don’t cells rely on increases in temperature
to break bonds?
Because proteins could be denatured causing cell damage.
LE 8-15
Free energy
Course of
reaction
without
enzyme
EA
without
enzyme
EA with
enzyme
is lower
Reactants
Course of
reaction
with enzyme
DG is unaffected
by enzyme
Products
Progress of the reaction
LE 8-13
Example:
Sucrose
C12H22O11
Glucose
C6H12O6
Fructose
C6H12O6
Structure & Function of Enzyme DRAW
• Enzymes bind substrate molecules (the reactant)
• Substrates bind to active site on enzyme
• Binding induces conformational change in enzyme-better ”fit” for substrate
• Active sites are highly specific and discriminatory
i.e. sucrase does not accept lactose
LE 8-16
Substrate
Active site
Enzyme
Enzyme-substrate
complex
How does enzyme lower activation energy of reaction?
– Orients substrates for optimal interaction
–Strains substrate bonds
–Provides a favorable microenvironment
-May covalently bond to the substrate
LE 8-17
Substrates enter active site; enzyme
changes shape so its active site
embraces the substrates (induced fit).
Substrates held in
active site by weak
interactions, such as
hydrogen bonds and
ionic bonds.
Substrates
Enzyme-substrate
complex
Active
site is
available
for two new
substrate
molecules.
Enzyme
Products are
released.
Substrates are
converted into
products.
Products
Active site (and R groups of
its amino acids) can lower EA
and speed up a reaction by
• acting as a template for
substrate orientation,
• stressing the substrates
and stabilizing the
transition state,
• providing a favorable
microenvironment,
• participating directly in the
catalytic reaction.
Environmental Conditions Affect
Enzyme Function
?
Temperature: cold-->decreased chance of bumping into substrate
hot--> good chance of substrate interaction but
chance of denaturation at some point
pH->change in charge (H+ or OH-) can denature proteins
and substrate
Examples of pH sensitive enzymes?
LE 8-18
Optimal temperature for
typical human enzyme
What is
your
normal
body
temp.?
0
Optimal temperature for
enzyme of thermophilic
(heat-tolerant
bacteria)
40
60
Temperature (°C)
20
80
100
Optimal temperature for two enzymes
Optimal pH for pepsin
(stomach enzyme)
0
1
2
3
4
Optimal pH
for trypsin
(intestinal
enzyme)
5
pH
Optimal pH for two enzymes
6
7
8
9
10
Cofactors
• Non-protein enzyme helpers (like metal, Fe)
•Coenzymes
•organic cofactors (con-enzyme A)
•Vitamins
•e.g. Vitamin K: required for blood clotting &
Required in certain carboxylation reactions
Regulation of Enzymes
Enzyme Inhibitors
• Competitive inhibitor
– binds to active site of enzyme
– blocks substrate binding by competition
•Noncompetitive inhibitor
– binds to another part of enzyme
– causes enzyme to change shape
– prevents active site from binding substrate
–Allosteric effect
DRAW
LE 8-19
A substrate can
bind normally to the
active site of an
enzyme.
Substrate
Active site
Enzyme
Normal binding
A competitive
inhibitor mimics the
substrate, competing
for the active site.
Competitive
inhibitor
Competitive inhibition
A noncompetitive
inhibitor binds to the
enzyme away from the
active site, altering the
conformation of the
enzyme so that its
active site no longer
functions.
Example of allosteric effect
Noncompetitive inhibitor
Noncompetitive inhibition
Allosteric Regulation of Enzymes
• Where protein function at one site is affected
by binding of a regulatory molecule at another
site
• May inhibit or stimulate enzyme activity
Allosteric Activation and Inhibition
• Most allosterically regulated enzymes are made from
polypeptide subunits
• active and inactive forms
• binding of activator stabilizes active form of enzyme
• binding of inhibitor stabilizes inactive form of enzyme
LE 8-20a
Allosteric enzyme
with four subunits
Regulatory
site (one
of four)
Active site
(one of four)
Activator
Active form
Oscillation
Nonfunctional
active site
Allosteric activator
stabilizes active form.
Inactive form
Stabilized active form
Allosteric inhibitor
stabilizes inactive form.
Inhibitor
Allosteric activators and inhibitors
Stabilized inactive
form
LE 8-20b
Binding of one substrate molecule to
active site of one subunit locks all
subunits in active conformation.
Substrate
Inactive form
Stabilized active form
Cooperativity another type of allosteric activation
Shift from regulation of one enzyme to
regulation of an enzymatic pathway
Feedback Inhibition
• End product of a metabolic pathway shuts
down the pathway
• Prevents over-production of unneeded
molecules
LE 8-21
Initial substrate
(threonine)
Active site
available
Isoleucine
used up by
cell
Threonine
in active site
Enzyme 1
(threonine
deaminase)
Intermediate A
Feedback
inhibition
Enzyme 2
Active site of
enzyme 1 can’t
bind
Intermediate B
theonine
pathway off
Enzyme 3
Isoleucine
binds to
allosteric
site
Intermediate C
Enzyme 4
Intermediate D
Enzyme 5
End product
(isoleucine)
Metabolic pathways are often localized in cell
• Cellular structures organize and concentrate
components of enzymatic pathways
– e.g. organelles (mitochondria, chloroplast, lysosomes)
– Pathways: respiration, photosynthesis, hydrolysis
LE 8-22
Mitochondria,
sites of cellular respiration
1 µm
LE 8-22
It’s nice to get so much attention!