Review



Enzymes are specific to a substrate
 Based on shape (lock & key)
Enzymes are NOT consumed or changed
by catalyzed reaction
 Great for speeding up reactions!
 Can be denatured…
Enzymes do not add energy
 They lower activation energy
 Results in higher reaction rate (faster)
Enzyme Regulation

How cell affects enzyme activity (rate)
“on purpose”  regulate activity Faster/slower?
Enzyme on/off ?
 Can send “signal molecule” 
binds enzyme in 2 ways:
1) Can bind directly to active site:
prevents substrate bind  only repress
2) Can bind “elsewhere” on enzyme to
change enzyme shape (allow OR prevent
Allosteric substrate binding): activate or repress
site
Allosteric enzymes: can change
to other shape by signal molecule
Enzyme Regulation Names


If repressor signal molecule binds
to active site…
 Called competitive inhibition
If repressor signal molecule binds
to allosteric site…
 Enzyme changes shape
 Active site no longer fits
 Called noncompetitive
inhibition
Not all enzymes have an allosteric site
 Not all enzymes are allosteric…
http://www.northland.cc.mn.us/biology/biology1111/animations/enzyme.swf Enzyme Inhibitors

http://bcs.whfreeman.com/thelifewire/content/chp06/0602002.html
http://course1.winona.edu/sberg/animtns/allostan.gif
Allosteric Enzyme Regulation


Active Site
Repressor signal molecule
 Will change shape so CANNOT bind
Activator signal molecule
 Will change shape so CAN bind
Allosteric
Site
Feedback Inhibition

System of enzyme regulation where product
of reaction can be signal molecule
(inhibition  repressor)
 Competitive feedback inhibition
Product
Product

Product
Product
Product binds directly
to active
site
Product
Product
Product
Product
Product
 Noncompetitive


feedback inhibition
Product
Product binds to allosteric site
Product
Product
Product
Allows for self-regulation


If too much product, stops catalysis (repress)
If too little product, continues catalysis
Binding not permanent (can diffuse away)
Product
http://www.northland.cc.mn.us/biology/biology1111/animations/enzyme.swf - Feedback Inhibition
(Biochemical) Metabolic Pathway

A “chain” reaction
 Involves several
enzymes (closeby)
 Product of one
catalyzed reaction
becomes
substrate for next
 Desired product
only made at end
Similar to Assembly Line
of pathway
Allows for more precise control
http://highered.mcgrawhill.com/classware/ala.do?isbn=0072986670&alaid=ala_1048052&showSelfStudyTree=true
http://highered.mcgraw-hill.com/classware/ala.do?isbn=0072986670&alaid=ala_1048055&showSelfStudyTree=true
Prepare for a mix/match of
different regulations

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Noncompetitive Inhibition = allosteric
enzyme with repressor (simple)
Noncompetitive Feedback Inhibition =
allosteric enzyme with product being
repressor
Noncompetitive Feedback Inhibition of
a Biochemical Pathway = several
enzymes with at least 1st enzyme being
allosteric and product of last enzyme
being a repressor to 1st enzyme
Environmental Affects on Enzymes



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Increase amount of
substrate
Increase amount of
enzyme
Increase temperature
Increase pH
Increase an repressor
molecule
Having a Feedback
Inhibition
Protein
Protein
works best
works best
Rate

Protein
denatures
Protein
works
slow
Protein
denatures
Substrate
Enzyme
Temperature
pHRepressor
Time
Substrate
An Example


H2O2 = Hydrogen peroxide (reactant)
 Can spontaneously break down to
 H2O water and O2 gas (products)…
 so this is an…
 Exergonic reaction
2H2O2  2H2O + O2
Enzymes speed up this reaction by…
 lowering activation energy…how exactly?
 In terms of motion of molecules, how is
it lowered?
Energy Profile of Exergonic Rxn

Note
release
of
energy

Note EA
Activation Energy
Energy required to start reaction =
The energy needed to get molecules
into transition state
Can accomplish by moving faster
(i.e. add heat: thermal energy)
Enzymes lower Activation Energy

Release of energy same as without enzyme
How
enzymes
lower
activ.
nrg?
(molecular
level)
Substrate-Specificity (by shape)




Substrate = reactant(s) catalyzed by enzyme
Substrate binds to enzyme on active site
Forms enzyme-substrate complex (H-bonds)
Induced fit: enzyme changes shape
SLIGHTLY to “cuddle” substrate
Active Site is shapespecific – catalyze on
specific substrate
Induced fit lowers
activation energy
because orients
substrate(s) correctly
Enzyme Catalysis
Enzyme does NOT change in catalysis (retains shape)
Enzyme is ready to catalyze another substrate after
reaction is complete (after products released)
Note on Molecular Motion:
It’s still all about “chance”
Substrates can hit enzyme in
“wrong” way and not be
catalyzed…
But since enzyme has active
site (fitted shape), more likely to
react than if substrate on own
Rate of Catalysis
Reaction

How quickly turn reactants into products
 NOT how much products are made

Rate of Catalysis = rate of enzymes
 How quickly do enzymes catalyze

Rate is affected by several environmental
factors
Rate of Catalysis
Rate = 2
1 rxn in 1 sec
Rate of Catalysis
Rate = 10 rxn in 1 sec
Rate of Catalysis
Rate = 20 rxn in 2 sec = 10 rxn in 1 sec
Rate of Catalysis



As you increase substrate concentration,
rate will increase until…
Enzymes are saturated and then rate will
level off (enzyme still working…)
As you increase enzyme concentration,
rate will increase (indefinitely?)
Only when Enzymes >>>>> Substrate,
rate will level off eventually
Saturation more likely if increase substrate
(not enzyme)
Rate of Catalysis
Rate = 20 rxn in 2
1 sec
Rate of Catalysis
With more enzymes, there is a higher chance
of substrate hitting correctly and reacting
-- this increases rate
But at some point, rate will level off
Compared to increasing substrate, this level
off will be (@ a much higher concentration of
enzyme and @ a much higher rate)
Rate = 5 rxn in 0.9
1 sec
sec
Graphing Enzyme Rates

Increase amount of
substrate
Increase amount of
enzyme
Rate

Enzyme Substrate
2 more factors affecting enzyme rate

Same 2 factors that cause denaturation…
 Temperature
 Increase temp. will
move molecules faster
but…break
 pH
H-bonds
in enzyme:
not functional
 Change
in H+
concentration will
disrupt H-bonds
affect cross-linking
+
Environmental Affects on Enzymes





Increase amount of
substrate
Increase amount of
enzyme
Increase temperature
Increase pH
Increase an repressor
molecule
Having a Feedback
Inhibition
Protein
Protein
works best
works best
Rate

Protein
denatures
Protein
works
slow
Protein
denatures
Substrate
Enzyme
Temperature
pHRepressor
Time
Substrate