1. 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

2. Activation Energy (EA)
Needed to destabilize bonds of reactants

3. Could raise temp. to break bonds A B C D Transition state A B EA
LE 8-14 A B C D
Transition state
Could raise temp.
to break bonds A B EA
Free energy C D
Reactants A B
DG < O C D
Products
Progress of the reaction

4. Why don’t cells rely on increases in temperature to break bonds?
Because proteins could be denatured causing cell damage.

5. Progress of the reaction
LE 8-15
Course of
reaction
without
enzyme EA
without
enzyme
EA with
enzyme
is lower
Reactants
Free energy
Course of
reaction
with enzyme
DG is unaffected
by enzyme
Products
Progress of the reaction

6. LE 8-13
Example:
Sucrose
C12H22O11
Glucose
C6H12O6
Fructose
C6H12O6

7. 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

8. LE 8-16
Substrate
Active site
Enzyme
Enzyme-substrate
complex

9. 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

10. 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.
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.
Substrates
Enzyme-substrate
complex
Active
site is
available
for two new
substrate
molecules.
Enzyme
Products are
released.
Substrates are
converted into
products.
Products

11. 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?

12. What is your normal body temp.? LE 8-18 Optimal temperature for
typical human enzyme
Optimal temperature for
enzyme of thermophilic
(heat-tolerant
bacteria)
What is
your
normal
body
temp.?
Rate of reaction 20 40 60 80
100
Temperature (°C)
Optimal temperature for two enzymes
Optimal pH for pepsin
(stomach enzyme)
Optimal pH
for trypsin
(intestinal
enzyme)
Rate of reaction 1 2 3 4 5 6 7 8 9 10 pH
Optimal pH for two enzymes

13. 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

14. 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

15. Example of allosteric effect
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

16. 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

17. 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

18. Active site (one of four)
LE 8-20a
Allosteric activator
stabilizes active form.
Allosteric enzyme
with four subunits
Active site
(one of four)
Regulatory
site (one
of four)
Activator
Active form
Stabilized active form
Oscillation
Allosteric inhibitor
stabilizes inactive form.
Non-
functional
active site
Inhibitor
Inactive form
Stabilized inactive
form
Allosteric activators and inhibitors

19. Binding of one substrate molecule to
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

20. Shift from regulation of one enzyme to
regulation of an enzymatic pathway

21. Feedback Inhibition
End product of a metabolic pathway shuts down the pathway
Prevents over-production of unneeded molecules

22. LE 8-21 Initial substrate (threonine) Active site available Threonine
in active site
Enzyme 1
(threonine
deaminase)
Isoleucine
used up by
cell
Intermediate A
Feedback
inhibition
Enzyme 2
Active site of
enzyme 1 can’t
bind
theonine
pathway off
Intermediate B
Enzyme 3
Intermediate C
Isoleucine
binds to
allosteric
site
Enzyme 4
Intermediate D
Enzyme 5
End product
(isoleucine)

23. 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

24. sites of cellular respiration
LE 8-22
Mitochondria,
sites of cellular respiration
1 µm

25. It’s nice to get so much attention!
LE 8-22
It’s nice to get so much attention!