Biochemistry- Enzymes

1. ENZYMES
By- Sanjesh Kumar

2. Mechanism of enzyme action
STEP-1
The substrate (S) binds
to the enzyme (E) at its
active catalytic site to
form activated
intermediate called as
enzyme substrate
complex (ES).
STEP-2
The activated complex
(ES) cleaved to the
products (P) and the
original enzyme (E)

3. Steps involved in enzymatic reaction
Date Your Footer Here 3
1.Substrate approaches active site
2. Enzyme-substrate complex forms
3. Substrate transformed into products
4. Products released
5. Enzyme recycled

4. Theories of enzyme substrate complex formation
Lock & Key Model of Enzyme Action
•The active site is fixed, with a rigid shape (LOCK)
•The substrate (KEY) must fit exactly into the rigid enzyme
(LOCK)
•Complementary shape & geometrybetween enzyme and
substrate
–Key (substrate) fits into the lock (enzyme)
•Upon completion of the chemical reaction, the products
are released from the active site, so the next substrate
molecule can bind.

5. Theories of enzyme substrate complex formation
Induced Fit Model of Enzyme Action
•It is a more flexible model, where the catalytic
site is not fully formed.
• The catalytic site of the enzyme is not
complementary to the substrate.
• Binding of the substrate to the enzyme
induces changes in the shape of the catalytic
site making it more fit for substrate.

6. ENZYMEACTION
•Enzymes increase the rate of reaction by
decreasing the activation energy of reaction.
•The activation energy is the energy barrier between
reactants and products.
Enzymes increase the rate of reaction by:
• It decreases the energy needed for activation
(activation energy).
• It decreases the energy barrier between
reactants and end products.

7. Factors Affecting Rate of
Enzyme Action
•Enzyme concentration
•Substrate concentration
•Temperature
• pH
• Concentration of co-enzymes
•Concentration of ion activators
•Time
• Inhibitors

8. Effect of enzyme
concentration
The rate of enzyme action is
directly proportional to the
concentration of enzyme
provided that there are
sufficient supply of substrate
& constant conditions.

9. Effect of substrate
concentration
The rate of reaction increases
as the substrate concentration
increases up to certain point
at which the reaction rate is
maximal (Vmax.) At Vmax, the
enzyme is completely
saturated with the substrate
any increase in substrate
concentration doesn't affect
the reaction rate.

10. Effect of substrate
concentration

11. Michaelis constant (Km)
It is the substrate concentration that
produces half maximum velocity of
enzyme.
•Enzymes with low Km: have high
affinity
to the substrate i.e. they act at maximal
velocity at low substrate concentration
• E.g. Hexokinase acts on glucose at low
concentration (fasting state)

12. Michaelis constant (Km)
•Enzymes with high Km: they have
low affinity to the substrate i.e.
they act at maximal velocity at high
substrate concentration.
• E.g. Glucokinase enzyme acts on
glucose at high concentration (fed
state)

13. Michaelis constant (Km)

14. Effects of temperature
Rate of reaction increases gradually
with the rise in temperature until
reach a maximum at a certain
temperature, called optimum
temperature.
- The optimum temperature is 37-
40 ℃ in humans.

15. Effects of temperature

16. Cont…
The effect of temperature on reaction rate is
due to:
1- Increase of temperature increase the initial
energy of substrate and thus decrease the
activation energy.
2- Increase of collision of molecules: more
molecules become in the bond forming or
bond breaking distance.
3-After the optimum temperature, the rate of
reaction decrease due to denaturation of the
enzyme (60-65 ℃).

17. Effect of PH
Each enzyme has an optimum PH at which its activity
is maximal
• E.g. Optimum PH of pepsin = 1.5 - 2
• Optimum PH of pancreatic lipase = 7.5 - 8
• Optimum PH of salivary amylase = 6.8
Change of PH above or below optimum PH decrease
rate of enzyme action due to:
1- The enzyme activity depends on the ionization
state of both enzyme and substrate which is affected
by PH.
2- Marked change in PH will cause denaturation of
enzyme.

18. Effect of PH

19. Effect of time
In an enzymatic reaction, the rate of
reaction is decreased by time.
• This is due to:
1- The decrease in substrate
concentration.
2- The accumulation of the end
products.
3- The change in PH than optimum PH.

20. Presence of enzymes inhibitor
Presence of enzyme inhibitor
decreases or stops the enzyme
activity.Enzyme inhibitors may be:
1- Competitive inhibitors.
2- Non competitive inhibitors.

21. Presence of enzymes inhibitor

22. Enzyme inhibition
ENZYME INHIBITOR
A substance that slows down or stops the
normal catalytic function of an enzyme
by binding to the enzyme
Three types of inhibition:
•Reversible competitive inhibition
•Reversible non-competitive inhibition
•Irreversible inhibition

23. Reversible Competitive
Inhibition
A competitive inhibitor resembles the substrate
–Inhibitor competes with the substrate for binding to the active
site of the enzyme
–If an inhibitor is bound to the active site:
•Prevents the substrate molecules to access the active site
–Decreasing / stopping enzyme activity
•The binding of the competitive inhibitor to the active site is a
reversible process
–Add much more substrate to out compete the competitive
inhibitor
•Many drugs are competitive inhibitors:
–Anti-histamines inhibit histidine decarboxylase, which
converts histidine to histamine

24. Reversible Noncompetitive
Inhibition
A non-competitive inhibitor decreases enzyme activity by
binding to a site on the enzyme other than the active site
–The non-competitive inhibitor alters the tertiary structure of
the enzyme & the active site
•Decreasing enzyme activity
•Substrate cannot fit into active site
–Process can be reversed only by lowering the [non-competitive
inhibitor]
•Example:
–Heavy metals Pb2+& Hg2+bind to –SH of Cysteine, away from
active site
•Disrupt the secondary & tertiary structure

25. Irreversible Inhibition
An irreversible inhibitorinactivates an enzyme
by binding to its active site by a strong covalent
bond
–Permanently deactivates the enzyme
–Irreversible inhibitors do not resemble substrates
•Addition of excess substrate doesn’t reverse this
process
–Cannot be reversed
•Chemical warfare (nerve gases)
•Organophosphate insecticides

26. REPRESENTATION
Dig.

27. Allosteric Enzymes
Allosteric enzymes have a quaternary structure
–Are composed of 2 or more protein chains
–Possess 2 or more binding sites
•2 types of binding sites:
–One binding site for the substrate
•Active site
–Second binding site for a regulator molecule
•Regulatory site
•Active & regulatory binding sites are distinct from each other in shape &
location. Binding of a regulator molecule to its regulatory site causes changes in
3-D structure of the enzyme & the active site
–Binding of a Positive regulator up-regulates enzyme activity
•Enhances active site, more able to accept substrate
–Binding of a Negative regulator (non-competitive inhibitor)
down-regulates enzyme activity
•Compromises active site, less able to accept substrate

28. Allosteric Enzymes
Dig.

29. Feedback Control
A process in which activation or inhibition of one of
the earlier reaction steps in a reaction sequence is
controlled by a product of this reaction sequence.
–One of the mechanisms in which allosteric
enzymes are regulated
–Most biochemical processes proceed in several
steps & each step is catalyzed by a different enzyme
•The product of each step is the substrate for the
next step / enzyme.

30. Proteolytic Enzymes&
Zymogens
2ndmechanism of allosteric enzyme regulation
–Production of an enzyme in an inactive form
–Activated when required (right time & place)
•Activated aka “turned on”
•Proteolytic enzymes catalyze breaking of peptide
bond in proteins
–To prevent these enzymes from destroying the
tissues, that produced them, they are released in
an inactive form = ZYMOGENS

31. Isoenzymes
Isoenzyme catalyze the same reaction in different tissues in the
body. e.g. lactate dehydrogenase (LDH) consists of 5 isoenzymes
•Each isoenzyme of LDH has the same function
–Converts lactate to pyruvate
•LDH1isoenzyme is more prevalent in heart muscle
•LDH5form is found in skeletal muscle & liver
•Isoenzymes can be used to identify the damaged or diseased
organ or tissue
•It is a marker for a particular location
•If LDH1isoenzyme was found in the blood >>> indicates heat
muscle damage.

32. PHARMACEUTICAL IMPORTANCE

33. Any questions
Drop question in comment box
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34. Thank You!
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