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Enzymes
They do everything. All of the things. What did that
enzyme do? It did a thing!
Enzymes
All end in –ase
 Sugars end in –ose
 They are like a lock and key
 Enzymes are specific!

◦ They only attach to one thing (or a
mimic/blocker)
◦ They only do one thing
◦ Example – the enzyme that gives me red hair
doesn’t also break down my breakfast in my
stomach
More boring enzyme facts!
They are reusable!
 When they break we can break them
down and build new enzymes
 Can build hard bonds, strengthen
existing ones, speed up reactions, make
things more powerful
 There is a Daft Punk reference in that I’m
not going to subject you too.

The Laws of Thermodynamics

OH COME ON MAN THAT WAS SO
LAST YEAR
Law 1 – energy can’t be created nor
destroyed only transformed (we gotta
break down food)
 Law 2 – The amount of available energy
will always decrease after each reaction
(we gotta eat food)

Concept 8.4: Enzymes speed up metabolic reactions
by lowering energy barriers
• A catalyst is a chemical agent that speeds up a
reaction without being consumed by the reaction
• An enzyme is a catalytic protein
• Hydrolysis of sucrose by the enzyme sucrase is an
example of an enzyme-catalyzed reaction
LE 8-13
Sucrose
C12H22O11
Glucose
C6H12O6
Fructose
C6H12O6
The Activation Energy Barrier
• Every chemical reaction between molecules
involves bond breaking and bond forming
• The initial energy needed to start a chemical
reaction is called the free energy of activation, or
activation energy (EA)
• Activation energy is often supplied in the form of
heat from the surroundings
LE 8-14
A
B
C
D
Transition state
A
B
Free energyC
D
EA
Reactants
A
B
G < O
C
D
Products
Progress of the reaction
How Enzymes Lower the EA Barrier
• Enzymes catalyze reactions by lowering the EA
barrier
• Enzymes do not affect the change in free-energy
(∆G); instead, they hasten reactions that would
occur eventually
LE 8-15
Course of
reaction
without
enzyme
EA
without
enzyme
EA with
enzyme
is lower
Reactants
Free energy
Course of
reaction
with enzyme
G is unaffected
by enzyme
Products
Progress of the reaction
Substrate Specificity of Enzymes
• The reactant that an enzyme acts on is called the
enzyme’s substrate
• The active site is the region on the enzyme where
the substrate binds
LE 8-16
Substrate
Active site
Enzyme-substrate
complex
Enzyme
Enzyme Database
Catalysis in the Enzyme’s Active Site
• The active site can lower an EA barrier by
o Orienting substrates correctly
o Straining substrate bonds
o Providing a favorable microenvironment
o Covalently bonding 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.
Effects of Local Conditions on Enzyme Activity
• An enzyme’s activity can be affected by:
o General environmental factors, such as
temperature and pH
o Chemicals that specifically influence the enzyme
Factors Affecting Enzyme Function
Enzyme concentration
 Substrate concentration
 Temperature
 pH
 Salinity
 Activators
 Inhibitors

catalase
Factors affecting enzyme
function

Enzyme concentration
◦ as  enzyme =  reaction rate
 more enzymes = more frequently collide with
substrate
◦ reaction rate levels off
reaction rate
 substrate becomes limiting factor
 not all enzyme molecules can find substrate
enzyme concentration
Factors affecting enzyme
function

Substrate concentration
◦ as  substrate =  reaction rate
 more substrate = more frequently collide with
enzyme
◦ reaction rate levels off
reaction rate
 all enzymes have active site engaged
 enzyme is saturated
 maximum rate of reaction
substrate concentration
#jokes
Factors affecting enzyme
function

Temperature
◦ Optimum T°
 greatest number of molecular collisions
 human enzymes = 35°- 40°C
 body temp = 37°C
◦ Heat: increase beyond optimum T°
 increased energy level of molecules disrupts bonds in
enzyme & between enzyme & substrate
 H, ionic = weak bonds
 denaturation = lose 3D shape (3° structure)
◦ Cold: decrease T°
 molecules move slower
 decrease collisions between enzyme & substrate
LE 8-18
Optimal temperature for
typical human enzyme
Optimal temperature for
enzyme of thermophilic
(heat-tolerant
bacteria)
Rate of reaction
0
40
20
60
Temperature (°C)
80
100
Optimal temperature for two enzymes
Optimal pH for pepsin
(stomach enzyme)
Optimal pH
for trypsin
(intestinal
enzyme)
Rate of reaction
0
1
2
3
4
5
pH
Optimal pH for two enzymes
6
7
8
9
10
Enzymes and temperature

Different enzymes function in different
organisms in different environments
reaction rate
human enzyme
hot spring
bacteria enzyme
37°C
temperature
70°C
(158°F)
Enzyme Inhibitors
• Competitive inhibitors bind to the active site of an
enzyme, competing with the substrate
• Noncompetitive inhibitors bind to another part of
an enzyme, causing the enzyme to change shape
and making the active site less effective
pH
What’s
happening here?!
pepsin
trypsin
Oh what the heck that was in
German, dude
We
don’t want pepsin in your intestines
or trypsin in your stomach
Because they are pH activated any pepsin
that gets out turns off and any trypsin that
gets in is turned off
Factors affecting enzyme function

pH
◦ changes in pH
 adds or remove H+
 disrupts bonds, disrupts 3D shape
 disrupts attractions between charged amino acids
 affect 2° & 3° structure
 denatures protein
◦ optimal pH?
 most human enzymes = pH 6-8
 depends on localized conditions
 pepsin (stomach) = pH 2-3
 trypsin (small intestines) = pH 8
0 1 2 3 4 5 6 7 8 9 10 11
Factors affecting enzyme function

Salt concentration
◦ changes in salinity
 adds or removes cations (+) & anions (–)
 disrupts bonds, disrupts 3D shape
 disrupts attractions between charged amino acids
 affect 2° & 3° structure
 denatures protein
◦ enzymes intolerant of extreme salinity
 Dead Sea is called dead for a reason!
Compounds which help enzymes

Activators
Fe in
hemoglobin
◦ cofactors
 non-protein, small inorganic
compounds & ions
 Mg, K, Ca, Zn, Fe, Cu
 bound within enzyme molecule
◦ coenzymes
 non-protein, organic molecules
 bind temporarily or permanently to
enzyme near active site
 many vitamins
 NAD (niacin; B3)
 FAD (riboflavin; B2)
 Coenzyme A
Mg in
chlorophyll
Compounds which regulate enzymes

Inhibitors
◦
◦
◦
◦
◦
molecules that reduce enzyme activity
competitive inhibition
noncompetitive inhibition
irreversible inhibition
feedback inhibition
Competitive Inhibitor

Inhibitor & substrate “compete” for active site
◦ penicillin
blocks enzyme bacteria use to build cell walls
◦ disulfiram (Antabuse)
treats chronic alcoholism
 blocks enzyme that
breaks down alcohol
 severe hangover & vomiting
5-10 minutes after drinking

Overcome by increasing substrate
concentration
◦ saturate solution with substrate
so it out-competes inhibitor
for active site on enzyme
Non-Competitive Inhibitor

Inhibitor binds to site other than active site
◦ allosteric site
◦ allosteric inhibitor
 regulation of enzyme function
 keeps enzyme inactive
 some anti-cancer drugs
inhibit enzymes involved in DNA synthesis
 stop DNA production
 stop division of more cancer cells
 cyanide poisoning
irreversible inhibitor of Cytochrome C,
an enzyme in cellular respiration
 stops production of ATP
◦ causes enzyme to change shape
 conformational change
 active site is no longer
a functional binding site
Irreversible inhibition

Inhibitor permanently binds to enzyme
◦ competitor
 permanently binds to active site
◦ allosteric
 permanently binds to allosteric site
 permanently changes shape of enzyme
 nerve gas, sarin, many insecticides (malathion,
parathion…)
 cholinesterase inhibitors
 doesn’t breakdown the neurotransmitter, acetylcholine
Allosteric regulation

Conformational changes by regulatory
molecules
◦ inhibitors
 keeps enzyme in inactive form
◦ activators
 keeps enzyme in active form
Conformational changes
Allosteric regulation
Metabolic pathways






A
EF
FG G
A BB CCDDE
enzyme enzyme enzyme enzyme enzyme enzyme
1
2
3
4
5
 Chemical reactions of life
are organized in pathways

divide chemical reaction into
many small steps
 efficiency
 intermediate branching points
 control = regulation
6
Efficiency

Organized groups of enzymes
◦ if enzymes are embedded in membrane
they are arranged sequentially

Link endergonic & exergonic reactions
Whoa!
All that going on
in those little
mitochondria!
Feedback Inhibition

Regulation & coordination of production
◦ product is used by next step in pathway
◦ final product is inhibitor of earlier step
 allosteric inhibitor of earlier enzyme
 feedback inhibition
◦ no unnecessary accumulation of product
1
2
3
4
5






ABCDEFG
enzyme
X enzyme enzyme enzyme enzyme enzyme
6
allosteric inhibitor of enzyme 1
Feedback inhibition

Example
◦ synthesis of amino acid,
isoleucine from amino
acid, threonine
◦ isoleucine becomes the
allosteric inhibitor of the
first step in the pathway
 as product accumulates it
collides with enzyme
more often than substrate
does
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.
Noncompetitive inhibitor
Noncompetitive inhibition
Cooperativity

Substrate acts as an activator
◦ substrate causes conformational
change in enzyme
 induced fit
◦ favors binding of substrate at 2nd site
◦ makes enzyme more active & effective
 hemoglobin
Hemoglobin
 4 polypeptide chains
 can bind 4 O2;
 1st O2 binds
 now easier for other 3
O2 to bind