2. 1. Define the following:
a. Enzymes
b. Apoenzyme
c. Coenzyme
d. Holoenzyme
e. Metalloenzyme
f. Regulatory enzyme
g. Active site of the enzyme
h. Allosteric site of the enzyme
i. Substrate
2. Discuss the helpers (cofactors) of enzymes.
4. Enzymes – are biologic catalysts.
Catalysts are substances that increase the
speed of a chemical reaction, it is not
permanently changed, nor does it cause the
reaction to occur, that is, a catalyst can increase
the speed of a reaction but cannot cause that
reaction if it would not occur in the absence of
catalyst. Since catalysts are not used up, they can
be used over and over again.
5. Rate of Reaction = Amount of substrate
changed (or amount product formed) in a
given period of time.
Chemical reactions need an initial input of
energy = THE ACTIVATION ENERGY
During this part of the reaction the molecules
are said to be in a transition state.
6.
7. Enzymes catalyzed reaction
Activation Energy
with enzyme
Activation
Energy without
enzyme
Substrate
Products:
Energy
Time
• By lowering the activation energy
8.
9. Enzymes – are organic catalyst produced by an
organisms. The reactant in an enzyme-catalyzed
reaction is called “substrate”.
The small portion of the molecule that is
responsible for the catalytic action of the enzyme
is the “active site”.
Activators – inorganic substances that tend to
increase the activity of enzyme.
Inhibitors – any substance that will make the
enzyme less active or render it inactive.
10. Other enzymes are conjugated proteins – they
contain a protein and non-protein part. Both
parts must be present before the enzyme can
function.
The protein part is called the “apoenzyme”
and the non-protein (organic part) is called
“coenzyme”.
APOENZYMES and COENZYMES
11. CoFactor
are not proteins and so are not inactivated by
heat. Examples of coenzymes are the vitamins or
compounds derived from vitamins. The reaction
involving a coenzyme can be written as follows:
coenzyme + apoenzyme = enzyme
Tightly bound cofactors are called prosthetic groups
Cofactors that are bound and released easily are called
coenzymes
Many vitamins are coenzymes
Cofactors & Coenzymes
14. The enzyme that catalyzes the
rate-limiting or committed
step of a metabolic
pathway.
15. Different structural forms of an
enzyme which catalyze the same
chemical reactions → act on the
same substrate(s) and produce the
same product(s) but exhibit differing
degrees of efficiency.
Different isoenzymes are expressed in
specific tissues of the body.
16. Lactate dehydrogenase
(LDH) – catalyzes the
reversible conversion of
pyruvate to lactate.
Tetramer consisting of 2
subunits: M (found in
skeletal muscles and
liver) & H (heart).
5 distinct isoenzyme forms
(from combination of M &
H isozymes).
An increase of H4 in the
blood
indicates tissue damage
as in heart attack.
Enzymes can therefore serve
as markers for disease.
17. Lock-and key Model
Wherein the substrate must “fit” into the
active site of the enzyme – hence the specifity of
the enzyme.
Induced-Fit Model
Suggests that the active site is not rigid as the
Lock-and-Key Model, but flexible. That is, the site
changes in conformation upon binding to a
substrate in order to yield an enzyme-substrate fit.
18.
19.
20. Enzymes
1)speed up chemical reactions
2)are required in minute amounts
3)are highly specific in their action
4)are affected by temperature
5)are affected by pH
6)Some catalyse reversible reactions
7)Some require co-enzymes
8)Are inhibited by inhibitors
21. (1) Enzymes Speed up chemical reactions
Activation Energy
with enzyme
Activation
Energy without
enzyme
Substrate
Products:
Energy
Time
• By lowering the activation energy needed to
start the reaction.
22. (2) Enzymes are required in minute amounts
Sucrose Glucose + Fructose
• They remain chemically unchanged after
catalysing the reactions.
• The same enzyme molecules can be reused
over again.
• Therefore, only a small amount of enzyme is
required to catalyse a large number of reactions
Sucrase
Chemically
unchanged
23. Maltose Glucose + Glucose
(3) Enzymes are highly specific
• Each chemical reaction is catalysed by its own
specific, unique enzyme.
• This is due to every enzyme’s specific 3-d
configuration.
• How the shape of an enzyme affects its function can
be explained by the “LOCK & KEY HYPOTHESIS”.
Maltase
Starch Maltose
Amylase
24.
25. (4) Enzymes are affected by temperature
• Enzymes can function
over a range of
temperatures.
• But all enzymes have
their own optimum
temperature.
26. Optimum temperature
• The optimum temperature is the temperature at
which the enzyme is most active, catalyzing the
largest number of reactions per second.
• Different enzymes have different optimum temp.
• Example:
most enzymes in the human body
functions best at about 37-40oC, near
body temperature.
Enzymes of thermophilic bacteria that
live in hotsprings will have very high
optimum temperatures.
27. How enzyme activity is affected by temperature
• At low temperature, enzymes are INACTIVE.
• As temperature rises, the rate of enzyme activity
increases (usually 2x as active for every 10oC
rise).
• Enzyme reaching maximum rate of activity at OT
NOTE:
Describe the
enzyme activity with
respect to the Rate-
Temperature Graph
• Raising the temperature increases the kinetic
energy supplied to the substrate and enzyme
molecules.
• This increases the no. of collisions between
enzyme and substrate molecules.
• Increasing the rate of formation of enzyme-
substrate complex.
• Rate of formation of the products increases up till
the optimum temperature
Increasing temp up to optimum temp.
28. How enzyme activity is affected by temperature
• Beyond the optimum temperature, rate of
enzyme activity deceases.
• Until it is completely denatured by the extreme
heat
enzyme activity with
respect to the Rate-
Temperature Graph
• Increase in temperature increases the
vibrations of the atoms in the enzymes.
• Beyond the OT, the vibrations are so
violent that they break the hydrogen bonds
that hold the 3-D structrure in place.
• The enzyme loses its shape and active site
• The enzyme is DENATURED.
Increasing temp beyond optimum temp.
29. (4) Enzymes are affected by pH
• Different enzymes have
different optimum pH.
• Enzymes are affected
by the acidity or
alkalinity of the solutions
• M indicates the optimum
pH when the rate of
reaction is the highest
30. Different enzymes have different optimum pH
• Enzymes that work best at ACIDIC conditions
Renin and Pepsin: Found in the stomach
• Enzymes that work best at ALKALINE conditions
Intestinal Enzymes
• Enzymes that work best at NEUTRAL conditions
Amylase
Examples
31. How enzyme activity is affected by pH
At pH slightly above or below the
optimum
• Enzyme activity reduces
• Slight changes in the pH brings about reversible changes
• Can be restored by bringing enzyme back to the
optimum pH
32. How enzyme activity is affected by pH
At extreme pH conditions
• Enzymes are DENATURED
• Extreme changes in the pH causes:
A change in the charges at the active sites which
repels the substrate molecules, preventing them from
binding.
The irreversible alteration to the bonds that holds the
shape of the enzyme. Enzyme thus loses its original
3-D structure. The active site loses its shape.
33. (5) Some enzymes catalyze reversible reactions
A B C D
+ +
Reactants Products
Products Reactants
• Some enzymes catalyse both reactions until equilibrium
is reached.
• Reactions will proceed in the direction where the
products are constantly being removed
• E.g formation of glucose during photosynthesis
34. (6) Some enzymes require co-enzymes
•Some enzymes require co-enzymes to be bound
to them before they can catalyse reactions
Co-enzymes
•Small, non-protein, organic “helper” molecules
35. (7) Enzymes are inhibited by inhibitors
Type 1 (Competitive)
Substrate is prevented
from binding to active
site by inhibitor
Type 2 (Non-competitive)
Binding of inhibitor does not
prevent the binding of substrate
but slows down reaction speed
36. Any substrate that affects the configuration of an
enzyme affects its activity
Various factors that affect enzyme activity are;-
1-Substrate concentration
2-Enzyme concentration
3- pH (H ions concentration)
4-temperature
5-product concentration
6-inhibitors
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