Enzyme Inhibition
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Transcript Enzyme Inhibition
Part 2
INHIBITION
ALLOSTERIC REGULATION
FEEDBACK INHIBITION
Remember…
Enzymes catalyze (speed up) biological reactions
The substrate (the reactants) must bind to the enzyme at
the active site.
Enzyme Animations:
http://highered.mcgraw
hill.com/sites/0072495855/student_view0/chapter2/animation__how_enzymes_wo
rk.html
http://www.youtube.com/watch?v=TLr7_2wnIXU
http://www.youtube.com/watch?v=z8lG8X9ZvxQ&feature=related
http://www.youtube.com/watch?v=CZD5xsOKres&feature=related
http://www.youtube.com/watch?v=XTUm-75-PL4
Remember….
Enzymes are NOT reactants or products
Enzymes are NOT used up in a reaction
Enzymes may be used again over and over again (so
long has they have not been denatured)
Enzymes are specific to a particular substrate (or group
of substrates)
1)
Cells must control enzyme activity to coordinate
cellular activities
This can be done by:
Restricting the production of a particular enzyme
(enzymes are proteins, so your body can control how
much you make of them)
1)
Inhibiting the action of an enzyme
This may involve
a) competitive inhibition
b) noncompetitive inhibition
- including allosteric regulation
c) feedback inhibition
ENZYME INHBITION
Sometimes enzyme function can be inhibited by other
molecules (reducing the rate of enzyme-controlled
reactions)
These molecules are called INHIBITORS
There are COMPETITIVE INHIBITORS and NON-
COMPETITIVE INHIBITORS
a) Competitive Inhibitors
These are molecules that are similar in shape to the
substrate
They bind to the enzyme’s active site preventing the
real substrate from binding
The molecule “competes” with the substrate for the
active site
This can be overcome by increasing the substrate
concentration
Competitive
Inhibitor
Now the substrate cannot
get to the active site
because it is blocked by
the inhibitor.
enzyme
substrate
Ex: PRONTOSIL (competitive inhibitor)
PRONTOSIL – is an antibacterial drug
Bacteria require folic acid for replication of genetic material
Prontosil binds to the enzyme that makes folic acid preventing
other substrates from binding
As a result, folic acid is not longer made and the bacterial cell dies
Since animal cells don’t make folic acid themselves, they do not
have this enzyme and so Prontosil has no effect on them
b) Noncompetitive Inhibition
A molecule binds to the enzyme at a location other
than the active site
This binding alters the shape of the enzyme, changing
the shape of the active site
The enzyme is now dysfunctional because the
substrate now cannot bind to the active site.
Adding more substrate will not affect the reaction
because the active site is unavailable.
Non competitive
inhibitor
substrate
The shape of the
active site has
changed
Therefore, the
substrate no
longer fits in
enzyme
Ex: Cytochrome C Oxidase
(noncompetitive inhibition)
Regular Function
Speeds up the reduction of oxygen to water in cellular
respiration
Without it, the reaction will not occur fast enough and
the organism will die because not enough energy is
released.
Ex: Cytochrome C Oxidase
(noncompetitive inhibition)
Inhibition
CN- attach to the –SH groups in the enzyme
This destroys the disulfide bridges and thus
changing the tertiary structure of the enzyme
Change in the shape results in the change in the
active site thus the substrate cannot bind and
cytochrome c oxidase is nonfuctional.
Animations
http://www.youtube.com/watch?v=PILzvT3spCQ
Allosteric Regulation
Some enzymes have receptor sites away from the active
site called ALLOSTERIC SITES
(These enzymes are usually proteins made of several
subunits each with an active site)
Substance that bind to the allosteric sites may inhibit
or stimulate (increase) the enzyme activity.
Allosteric Regulation - Activators
Binding an ACTIVATOR to an allosteric site
stabilizes the protein conformation
This keeps all the active sites available for the
substrates to bind to them.
Allosteric Inhibition
Binding of an allosteric inhibitor (a type of
noncompetitive inhibitor) stabilizes the
inactive form of the enzyme
Allosteric Regulation
The binding of an activator or an inhibitor affects the
activity of all the active sites on the enzyme
Animation: “ An example of allosteric activation of an
enzyme” (Text video)
c) Feedback Inhibition
Method of controlling metabolic pathways
Negative feedback inhibition is like a thermostat.
When it is cold, the thermostat turns on the heater to
produce heat.
When it is too warm, the heat will cause the
thermostat to turn off the heater
Feedback Inhibition
Cold
Thermostat
Heater
Heat
INHIBITS
Heat has a negative effect on the thermostat;
A build up of product inhibits the enzymes and the
reaction
Feedback Inhibition
Many enzymatic pathways are regulated by feedback
inhibition
As the enzyme’s end product accumulates, it inhibits
the enzyme by binding to the first enzyme in the
pathway.
This shuts down the entire sequence.
Feedback Inhibition
As the product is used up over time, the concentration
of product decreases
The inhibition product will detach from the enzyme
allowing it to become active once again and produce
product.
When the product concentration gets too high again,
the product will once again allosterically inhibit the
enzyme
Animations
Feedback inhibition animation (text)
http://highered.mcgraw-
hill.com/sites/0072507470/student_view0/chapter2/a
nimation__feedback_inhibition_of_biochemical_path
ways.html