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CHAPTER 3
ENZYMES

Learning objectives
1
...
Explain the time course of an enzyme-catalysed
reaction by measuring the rate of formation of
product(s) or rate of disappearance of substrate(s)
as the rate of reaction;
3
...
Explain the significance of Km and Vmax
5
...

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6
...
Explain the effects of competitive and noncompetitive inhibitions on the rate of enzyme
activity of reversible inhibition
8
...

9
...
Explain the application of enzyme in industries

Introduction
Enzyme= biological catalyst
Speed up chemical reaction in body
Globular proteins
Enzymes can be used over and over again
Usually end with suffix ‘ase’
Intracellular or extracellular
Enzyme molecules are larger
than substrate
Highly specific- active site

Mechanism of enzyme action
Specific site- section of enzyme comes into
contact with substrate

Mode of action
Lock and key mechanism
Substrate fits accurately into the active site
of enzyme
form enzyme substrate complex

Mechanism of enzyme action
Lock and key mechanism
Substrate fits accurately into the active site
enzyme- enzyme substrate complex
Product have different shape from the substrate
Leave the active site

The Induced Fit Model
• According to the induced fit model, the enzyme’s
active site is not a completely rigid fit for the substrate
• Instead, the active site will undergo a conformational
change when exposed to a substrate to improve
binding
• This theory of enzyme-substrate interactions has two
advantages compared to the lock and key model:
• It explains how enzymes may exhibit broad specificity
(e
...
lipase can bind to a variety of lipids)
• It explains how catalysis may occur (the
conformational change stresses bonds in the
substrate, increasing reactivity)

The Induced Fit Model

Enzyme reduce activation energy
Activation energy – minimum energy
required to make the substance react
 Enzyme= lower activation energy

Properties of enzyme
1
...
Reversibility
 Chemical reactants are reversible
A+B


C+D

Enzyme catalyze the forward and reverse
reaction equally

3
...
Substrate concentration
 Rate of reaction increase with increasing substrate
concentration up to one point
 Active site will be saturated
 Further increment of substrate- not increase the rate of
reaction
 Enzyme concentration is the limiting factor

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5
...


6
...
concentration of substrate
b
...
pH
d
...
The KM for the substrate
of the enzyme is 1
...

i
...
5 mM?
ii
...
0 mM?
iii
...
0 mM?

The effect of substrate concentration [S]
on the reaction velocity (V)

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Lineweaver- Burk Plot

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Question
• The following is a Lineweaver-Burk plot of kinetic data for
a particular enzyme
...

A
...

C
...


0
...
02 µM /sec
50 µM/sec

Enzyme inhibitors
Substance that inhibit/ slows down/ stop the
enzyme reaction
Inhibitors
Competitive
❖ similar shape to the natural substrate
❖ attach to active site
Non-competitive
❖no structural similarities to the natural substrate
❖ bind to allosteric site

Reversible inhibitor
➢ cause no damage to enzyme
➢Bind temporarily
Irreversible inhibitor
➢Cause permanent damage to enzyme
➢Bind tightly

 Competitive reversible inhibitors
Compete with substrate for active site
Bind temporarily- can overcome by adding
substrate
Eg
...
Enzymes are biological
...
cells
...
required for reactions to
occur
...
protein with a specific

...
Increasing temperature increases
the
...
This increases the

...
complex
...
violently
...
and

...
the active site will be
...

• What is the main role of enzymes?

• Describe how Linewaever-Burk plot used to
determine Vmax and KM

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❑ End product inhibition
The end product of the pathway may inhibit the
enzyme at the start
When the end product of metabolic pathway
accumulate, it may act as inhibitor of the
enzyme controlling the first step of the pathway
Non competitive reversible inhibitors

Example of end product inhibition

End Product Inhibition
• Isoleucine is an essential amino acid, meaning it is not
synthesized by the body in humans (and hence must be
ingested)
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