Enzymes
•
important functions of proteins > catalysts
(substance that enhances the rate of chemical
reaction but is not permanently altered by the
reaction).
•
enzyme contains a unique, intricately shaped binding
surface called an active site.
> a small cleft on a large protein where substrates
bind to
•
Active site also takes part in catalysis –side chains
of aa line with active site
•Enzymes work by the Key Lock Model (1890)
•Each enzyme binds to a single type of substrate
> both have complementary structure
•substrate overall shape and charge distribution allow it to
enter and interact with the enzymes active site.
E + S > ES > E+ P
Another model by which enzymes work is the INDUCED
KEY MODEL.
- the substrate does not fit the into the active site, but
binding of the substrate induces a conformational:
change in the enzyme that causes the active site to fit
around the substrate in a lock and key manner
• Some enzymes require non-protein components
for their activities.
• Cofactors- may be ions, such as Mg2+ or Zn2+
or complex organic molecules, referred to as
co-enzymes
• A protein component of an enzyme lacking
essential co-factor is called an apoenzyme.
•
Intact enzymes with their bound co-factor
are refereed to as holoenzymes
The six major enzyme categories are
1. Oxidoreductases -catalyses oxidation-reduction
reactions
A– + B → A + B–
2. Transferases – catalyses
reactions that involve the transfer
of groups from one molecule to
another. E.g. of such groups include
amino, carboxyl methyl etc..
A–X + B → A + B–X
3. Hydrolases- catalyze reactions in which the cleavage of bonds
is accomplished by adding water. E.g. esterases,
phosphatases and peptidases
A–B + H2O → A–OH + B–H
4. Lyases- catalyse reactions
in which groups (e.g. H2O,
CO2 and NH3) are
removed to form a double
bond or are added to a
double bond. E.g.
Decarboxylases
5. Isomerases- heterogeneous
group of enzymes that
catalyze intramolecular
rearrangements
6. Ligases- catalyzes bond
formation b/w two substrate
molecules. Energy for these
reactions is always supplied by ATP
hydrolysis.
Enzyme Activity
The properties of enzymes related to their
tertiary structure.The effects of change in
temperature,pH,substrate concentration,and
competitive and non-competitive inhibition on the
rate of enzyme action
HOW ENZYMES WORK
• Enzymes are ORGANIC
CATALYSTS. A CATALYST is
anything that speeds up a
chemical reaction that is
occurring slowly.
How does a catalyst work?
• The explanation of what
happens lies in the fact that
most chemical reactions that
RELEASE ENERGY
(exothermic reactions) require
an INPUT of some energy to
get them going. The initial
input of energy is called the
ACTIVATION ENERGY
Enzyme catalyzed reactions occur in two phases:
Where k+1, k-1 and
k+2 are the respective
rate constants which
quantifies the speed
of a chemical reaction
Substrate binds to active site of Enzyme; an intermediate ES forms.
During the transition state (intermediate between S and P) S forms P.
After brief time product leaves enzyme .
The ES complex is stabilized in the transition state by non-covalent
interactions between substrate the the aa inthe active site.glove around
a hand.
Properties of Enzymes relating to their
tertiary structure.
• The activity of enzymes is strongly
affected by changes in pH and
temperature. Each enzyme works best
at a certain pH and temperature,its
activity decreasing at values above and
below that point. This is because of the
importance of tertiary structure (i.e.
shape) in enzyme function and forces,
e.g., ionic interactions and hydrogen
bonds, in determining that shape.
The effects of change in temperature.
Temperature: enzymes work best at an optimum temperature.
Below this, an increase in temperature provides more kinetic
energy to the molecules involved. The numbers of collisions
between enzyme and substrate will increase so the rate will
too.
Above the optimum temperature, and the enzymes are
denatured. Bonds holding the structure together will be
broken and the active site loses its shape and will no longer
work
The effect of change in pH.
• pH: as with temperature, enzymes have an optimum
pH. If the pH changes much from the optimum, the
chemical nature of the amino acids can change.
This may result in a change in the bonds and so the
tertiary structure may break down. The active site
will be disrupted and the enzyme will be denatured.
The effect of change in concentration
Enzyme concentration:
•
at low enzyme concentration there is great competition for the active sites and
the rate of reaction is low.
•
As the enzyme concentration increases, there are more active sites and the
reaction can proceed at a faster rate.
•
Eventually, increasing the enzyme concentration beyond a certain point has no
effect because the substrate concentration becomes the limiting factor.
Substrate concentration:
• at a low substrate concentration there are many active sites that are not
occupied. This means that the reaction rate is low.
•
When more substrate molecules are added, more enzyme-substrate complexes
can be formed. As there are more active sites, and the rate of reaction
increases.
•
Eventually, increasing the substrate concentration yet further will have no
effect. The active sites will be saturated so no more enzyme-substrate
complexes can be formed.
Competitive and non-competitive inhibition
•
Inhibitors slow down the rate of a reaction. Sometimes this is a
necessary way of making sure that the reaction does not
proceed too fast, at other times, it is undesirable
•
•
Reversible inhibitors:
Inhibitors
•
Competitive reversible inhibitors: these molecules have a similar
structure to the actual substrate and so will bind temporarily
with the active site. The rate of reaction will be closer to the
maximum when there is more ‘real’ substrate, (e.g. arabinose
competes with glucose for the active sites on glucose oxidase
enzyme).
Reversible inhibitors of 2 kinds affects Vmax and Km
differently
• Competitive Inhibitors
•Inhibitor resembles S
structurally so binds to
the same active site as S
would.
•Form a EI complex
rather than ES
• since substrate and
inhibitor compete can
over come inhibition by
- diluting the inhibitor or
- increasing the [S]
• Non-competitive reversible inhibitors:
• these molecules are not necessarily anything like the
substrate in shape. They bind with the enzyme, but
not at the active site. This binding does change the
shape of the enzyme though, so the reaction rate
decreases.
• Non-competitive Inhibitors
Inhibitor binds elsewhere from
active side
Binding alters the enzyme 3-D
configuration and block the
reaction
Inhibition cannot be reversed
by increasing [S]
Irreversible inhibitors:
These molecules bind permanently with
the enzyme molecule and so effectively
reduce the enzyme concentration, thus
limiting the rate of reaction, for
example, cyanide irreversibly inhibits
the enzyme cytochrome oxidase found
in the electron transport chain used in
respiration. If this cannot be used,
death will occur
Study Guide
1.
Why do you think enzymes are important for chemical reactions in a living
cell?
2.
Describe the SIX classes of enzymes?
3.
What is the general role of enzymes in the cells?
4.
What is activation energy?
5.
Explain in terms of energy changes how enzymes are able to speed up
reactions?
6.
What is meant by the term “enzyme substrate complex”?
7.
Compare and contrast the “Lock and Key Model” with the “Induced Fit Model”
8.
An enzyme that hydrolyses protein will not act on starch. Why do you think
this happens?
9.
What property of enzyme does this statement depict?
10.
Why is this property important for enzymes?
11.
What part of an enzyme enables it to be substrate specific? What aspects of
this “parts” structure enable it to be so specific?
13.
Why do you suppose most enzymes are so large relative to their
substrate size?
14.
What are co-enzymes?
15.
Why are co-enzymes important in enzyme functioning?
16.
Describe FOUR methods enzymes use to lower the energy of
activation.
17.
Distinguish competitive, uncompetitive and non competitive
inhibitors.
19.
Why would a change in pH affect the rate of an enzyme reaction?
20.
How would the following temperature changes affect the activity of
an enzyme extracted from human cells: to 80 C; to 42 C; to 4 C
21. Explain why i) Refrigeration preserves food ii) Boiling can be used
to sterilize instruments