BIOLOGY 12 - ENZYMES & CELLULAR METABOLISM: CHAPTER NOTES
n order for cells to maintain homeostasis, they must constantly
convert chemicals from one form to another, in order to produce
necessary molecules, obtain usable molecules from food, and
produce energy rich molecules.
 These constantly occurring chemical reactions are collectively
known as metabolism. Enzymes control metabolism.
I
METABOLISM AND METABOLIC PATHWAYS
METABOLISM
 a term to collectively describe all the
reactions occurring constantly in the cell that maintain
.
METABOLIC PATHWAYS
 the orderly step-wise series of chemical reactions from
the initial
to the final
. One
reaction leads to the next. Highly structured.
Controlled by
.
 each step (i.e. each chemical reaction) within the
metabolic pathway requires a
enzyme.
Step 1
A

reactant
Enz. 1
Step 2
B

Enz. 2
Step 3
C

Enz. 3
Step 4
D

Enz. 4
Step 5
E

Enz. 5
Step 6
F

G
Enz. 6
Product
 one pathway can lead to several others (intermediate
products of one pathway can be
for
another pathway.
 having more than one step means that there are more
places where the overall reaction can be
.
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
ENZYMES:
ENZYME (abbr. = “E”): a
that can
up a
chemical reaction without being
.
Enzymes are the
of chemical reactions
Enzymes are Highly
. Each enzyme
speeds up only
reaction. Enzyme names usually
end with the suffix “
” (or sometimes “sin” e.g.
trypsin, pepsin)
SUBSTRATE (“S”)= the
(s) in an
enzyme’s reaction.
The equation for an enzyme-catalyzed reaction is
always:
E + S  ES E + P
where “ES” =
(the
chemical reaction occurs when the ES complex exists).
The place where the substrates actually bind on the
enzyme is known as the
.
Please Label the parts
of this diagram, and
identify the ES complex.
How do Enzymes Work? The LOCK AND KEY THEORY vs. the Induced Fit Theory
-The original model, called the “
Theory”
has been superseded by a slightly more sophisticated
model called the “
Theory.”
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Lock and Key Theory
S1
e
E
e
S2
e
Product
ES Complex
Enzyme hasn’t changed!
 E and S meet during the reaction, and fit together
perfectly from the very beginning, like a
.
 While this model is basically correct, we now believe
that the enzyme actually
slightly
when it binds the substrates, in order to get a tighter
“grip” on the reactants. This is called the Induced Fit
Theory.
Induced Fit Theory
E
e
S1
e
S2
Enzyme’s shape
e
doesn’t fit S perfectly
Product
ES Complex
Enzyme changes back to original shape!
E changes shape to
bind S more tightly!
 Upon binding , the enzyme undergoes a slight
change to more perfectly bind the
substrates.
 Then the reaction takes place, the ES complex
separates, and the enzyme
its original
shape. It is now free to catalyze another reaction.
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How does an Enzyme work?
without enzyme
Ea
Energy
Level
Energy
Level
Ea
Substrate
Reactants
Product
Progress of Reaction

with enzyme
Product
Progress of Reaction
is the energy required for a
reaction to proceed.
 Enzymes
the activation energy by bringing
the
molecules together and holding
them long enough for the
to take place.
 You must be able to interpret the above graph!!
FACTORS AFFECTING ENZYME ACTIVITY
 Change the enzyme
– change the
enzyme
. Some factors are:
1. pH: most human enzymes prefer pH’s of
(some exceptions:
in the stomach - pH
,
in the small intestine - pH
)
 if the pH is too low or too high, the enzyme
(-it loses its normal
, and therefore its
ability to form an
complex).
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2. TEMPERATURE
Rate
Normal Operating Temp.
of
 decreasing
Reaction
temperature will
rate of
Temperature
reaction. The lower
the temperature, the
the rate of reaction. Very
low temperatures
normally denature the
enzyme, however.
 increasing the temperature slightly will, at first,
the rate of reaction (as it speeds up the
at which
substrates
into enzymes). i.e. within E’s
, an increase in Temp. will
rate of reaction.
 However, temperature too high (above about 45 °C) will
the enzyme.
3. Concentrations of SUBSTRATES
 if the concentration (abbr. = “[ ]”) of substrate increases,
the rate of product formation will usually
too.
However, after a certain [ ], the rate
increase
anymore, as all the enzymes are “
” with
substrates and can’t work any
.
 if the concentration of substrate decreases, the rate of
product formation will generally
as well.
37C
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Rate of product
formation levels off
as enzymes
become saturated.
Rate
of
Reaction
Substrate Concentration
Why does this line not
start exactly at (0,0)?
Rate
of
Reaction
0
Enzyme Concentration
4. Concentration of ENZYME
 This is what
the overall rate of reaction.
Providing there is adequate substrate (and their is
typically millions
substrate molecules than
molecules), the more enzyme you add, the
more
you get, and the less enzyme you
have, the less
you get. The rate will only
level off if you run out of
, which is usually
not the case.
5. Presence of INHIBITORS
S1
e
E
e
Inhibitor
S2
e
e
EI Complex
e
The inhibitor binds to the
enzyme, blocking active
site from real substrate.
No product can be
formed. If the Inhibitor
doesn’t leave, the enzyme
is “dead”!
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 inhibitors are molecules that
to the enzyme
in some way to
or
the rate of
substrate binding to enzyme. There are several ways
in which inhibition can work.
a)
Inhibition
 a molecule that
the substrate can
for space at the
. This
will
the reaction rate. The inhibitor
binding to E can be
or
.
 Obviously, the more inhibitors are added, the
the rate of reaction.
b)
INHIBITION
S1
e
S1
e
E
e
Inhibitor
e
S2
e
E
e
S2
e
When the inhibitor binds
allosterically, it causes the
active site to change shape,
so the substrates can no
longer bind.
 in this case, the inhibitor binds to
on
enzyme (not the active site). The inhibitor may look
completely
from the substrate.
 When the inhibitor binds, it cause the enzyme to
at the active site so S
bind.
 binding may be
or
.
 This type of inhibition is also known as “
”
inhibition.
Examples of Inhibition:
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 Inhibitors can act as
or
.
e.g. penicillin works by binding
to the
enzyme that makes bacterial cell walls.
 HCN (hydrogen cyanide) is a lethal
inhibitor of enzyme action in human.
 Lead (Pb++) and other HEAVY METALS (like mercury
(Hg++) and cadmium) are
inhibitors
that cause poisoning when they bind irreversibly to
enzymes and make them
.
Substrates
What are Enzymes
Made of?
1) a protein part
called an
Coenzyme
that
Apoenzyme
gives it its
y
(i.e. exactly what
reaction it will
catalyze)
ES Complex
2) a
group called a
which may help out the reaction by accepting or
donating atoms (e.g. H+).
are often
coenzymes.
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