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
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
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
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