enzyme functioning

1. ENZYMES
Enzyme from Greek- in ferment
Special protein molecules whose function is to
facilitate or accelerate most chemical reactions
in cells.
A protein with catalytic properties due to its
power of specific activation

2. Chemical reactions
 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.

3.  Molecules need minimum energy for
collision - Potential or kinetic energy
 Fast collision –large energy and vice versa
 Kinetic energy greater than minimum energy
results in chemical reaction-transition state
energy
 Minimum energy required for bond breaking

4. Reaction pathway

5. Making reactions go faster
 Increasing the temperature make molecules move
faster
 Biological systems are very sensitive to temperature
changes.
 Enzymes can increase the rate of reactions without
increasing the temperature.
 They do this by lowering the activation energy.
 They create a new reaction pathway “a short cut”
 Enzymes binds to substrate

6. An enzyme controlled pathway
 Enzyme controlled reactions proceed 108 to 1011 times faster
than corresponding non-enzymic reactions.

7. Enzyme structure
 Enzymes are
proteins
 They have a
globular shape
 A complex 3-D
structure
Human pancreatic amylase

8. Enzymes Specificity
 Work in unique manner
 Important in diagnostics and research tools- unique
Four types of enzyme specificity reactions
 Absolute specificity - enzyme will catalyze only one reaction
 Group specificity- act on specific functional groups
 Linkage specificity- particular chemical bond
 Stereochemical specificity- particular steric or optical isomer

9. The active site
 One part of an enzyme,
the active site, is
particularly important
 The shape and the
chemical environment
inside the active site
permits a chemical
reaction to proceed
more easily

10. Cofactors
 An additional non-
protein molecule that is
needed by some
enzymes to help the
reaction
 Tightly bound cofactors
are called prosthetic
groups
 Cofactors that are bound
and released easily are
called coenzymes
 Many vitamins are
coenzymes Nitrogenase enzyme with Fe, Mo and ADP cofactors

11.  Metal as cofactor
 e.g. Alcohol dehydrogenase - Zn ++
 Kinases (phosphotransformer) - Mg++
 Cytochromes - Fe++ or Fe+++
 Cytochrome oxidase - Ge++

12. The substrate
 The substrate of an enzyme are the reactants
that are activated by the enzyme
 Enzymes are specific to their substrates
 The specificity is determined by the active
site

13. The Lock and Key Hypothesis
 Fit between the substrate and the active site of the enzyme is
exact
 Like a key fits into a lock very precisely
 The key is analogous to the enzyme and the substrate
analogous to the lock.
 Temporary structure called the enzyme-substrate complex
formed
 Products have a different shape from the substrate
 Once formed, they are released from the active site
 Leaving it free to become attached to another substrate

14. The Lock and Key Hypothesis
Enzyme may
be used again
Enzyme-
substrate
complex
E
S
P
E
E
P
Reaction coordinate

15. The Lock and Key Hypothesis
 This explains enzyme specificity….
 enzymes and substartes have natural geometric
shapes
 the loss of activity when enzymes denature
 Doesn’t Explain…..
 Stabilization of enzymes
 Denature as enzyme slightly change its shape

16.  Daniel Koshland (1958) suggested
-MODIFICATION to Lock and Key
hypothesis
 Enzymes are Flexible enough to wrap around
rigid substartes
 Complementary shape after binding not
before
 Amino acids are the part of active site –
molded in specific position

17. The Induced Fit Hypothesis
 Some proteins can change their shape
(conformation)
 When a substrate combines with an enzyme, it
induces a change in the enzyme’s conformation
 The active site is then moulded into a precise
conformation
 Making the chemical environment suitable for the
reaction
 The bonds of the substrate are stretched to make the
reaction easier (lowers activation energy)

18. The Induced Fit Hypothesis
 This explains the enzymes that can react with a
range of substrates of similar types
Hexokinase (a) without (b) with glucose substrate

19.  D-hexose + ATP D-hexose-6-phospahate + ADP
 Week binding without xylose
 Xylose not take part in phosphorylation –
 ATP binds at faster rate

20. – enzyme binds to the reactants, called the substrate(s), of a chemical
reaction
– the substrate joins with the enzyme at the enzymes active site
forming an enzyme-substrate complex
– after the enzyme-substrate complex forms the enzyme-catalyzed
reaction occurs
– enzyme releases the product(s) and the enzyme is ready to bind to
more substrate
Enzyme Activity

21. Factors affecting Enzymes
 substrate concentration
 pH
 temperature
 inhibitors

22. Substrate concentration: Non-enzymic reactions
 The increase in velocity is proportional to the
substrate concentration
Reaction
velocity
Substrate concentration

23. Substrate concentration: Enzymic reactions
 Faster reaction but it reaches a saturation point when all the
enzyme molecules are occupied.
 If you alter the concentration of the enzyme then Vmax will
change too.
Reaction
velocity
Substrate concentration
Vmax

24. The effect of pH
Optimum pH values
Enzyme
activity Trypsin
Pepsin
pH
1 3 5 7 9 11

25. The effect of pH
 Extreme pH levels will produce denaturation
 The structure of the enzyme is changed
 The active site is distorted and the substrate
molecules will no longer fit in it
 At pH values slightly different from the enzyme’s
optimum value, small changes in the charges of the
enzyme and it’s substrate molecules will occur
 This change in ionisation will affect the binding of
the substrate with the active site.

26. The effect of temperature
 Q10 (the temperature coefficient) = the increase in
reaction rate with a 10°C rise in temperature.
 For chemical reactions the Q10 = 2 to 3
(the rate of the reaction doubles or triples with every
10°C rise in temperature)
 Enzyme-controlled reactions follow this rule as they
are chemical reactions
 BUT at high temperatures proteins denature
 The optimum temperature for an enzyme controlled
reaction will be a balance between the Q10 and
denaturation.

27. The effect of temperature
Temperature / °C
Enzyme
activity
0 10 20 30 40 50
Q10 Denaturation

28. The effect of temperature
 For most enzymes the optimum temperature is about
30°C
 Many are a lot lower,
cold water fish will die at 30°C because their
enzymes denature
 A few bacteria have enzymes that can withstand very
high temperatures up to 100°C
 Most enzymes however are fully denatured at 70°C

29. Inhibitors
 Inhibitors are chemicals that reduce the rate of
enzymic reactions.
 The are usually specific and they work at low
concentrations.
 They block the enzyme but they do not
usually destroy it.
 Many drugs and poisons are inhibitors of
enzymes in the nervous system.

30. The effect of enzyme inhibition
 Irreversible inhibitors: POISIONS
 Bind covalently at active site
 Combine with the functional groups of the
amino acids in the active site, irreversibly.
Examples: nerve gases and pesticides,
containing organophosphorus, combine with
serine residues in the enzyme acetylcholine
esterase.

31. The effect of enzyme inhibition
 Reversible inhibitors: Non-covalent bonds
 These can be washed out of the solution of
enzyme by dialysis.
There are two categories.
- Competitive inhibitors
- Non-competitive inhibitors

32. The effect of enzyme inhibition
1. Competitive: These
compete with the
substrate molecules for
the active site.
The inhibitor’s action is
proportional to its
concentration.
Resembles the substrate’s
structure closely.
Enzyme inhibitor
complex
Reversible
reaction
E + I EI

33. The effect of enzyme inhibition
Succinate Fumarate + 2H+
+ 2e-
Succinate dehydrogenase
CH2COOH
CH2COOH CHCOOH
CHCOOH
COOH
COOH
CH2
Malonate

34. Competitive Inhibition

35. The effect of enzyme inhibition
2. Non-competitive: These are not influenced by the
concentration of the substrate. It inhibits by binding
irreversibly to the enzyme but not at the active site.
Examples
 Cyanide combines with the Iron in the enzymes
cytochrome oxidase.
 Heavy metals, Ag or Hg, combine with –SH groups.
These can be removed by using a chelating agent such
as EDTA.

36. Applications of inhibitors
 Negative feedback: end point or end product
inhibition
 Poisons snake bite, plant alkaloids and nerve
gases.
 Medicine antibiotics, sulphonamides,
sedatives and stimulants