It’s time for new material!
Section 2.4
Chemical reactions and
Enzymes
We need to understand
some details about
enzymes that are NOT
in you book…so pay
ATTENTION!
Enzymes
• Enzymes are proteins that
act as biological catalysts
• They lower the activation
energy of a specific
chemical reaction
• Lowering the activation
energy has a profound effect
on how rapidly the reaction
is completed
In order to understand
enzyme activity, we
need to also understand
the energy
transformations that
occur during a chemical
reaction
What do you know about…
• Energy?
• The first law of thermodynamics?
• The second law of thermodynamics?
• Endergonic & Exergonic Reactions?
• Enzymes
Let’s Review….
What is energy?
• Etymology: Gk, energia
• the capacity to do work or to perform vigorous
activity. Energy may occur in the form of heat, light,
movement, sound, or radiation.
• Human energy is usually expressed as muscle
contractions and heat production, made possible by
the metabolism of food that originally acquired the
energy from sunlight. Chemical energy is that
released as a result of a chemical reaction, as in the
metabolism of food.
Energy is..
• The capacity to perform work
• Kinetic Energy: actually doing work
• Thermal (Heat) Energy: energy associated with
movement of molecules
• Potential Energy: capacity to perform work
• Chemical Energy: a form of potential energy related to
the structural arrangement of atoms or molecules.
Chemical energy can be transformed into other types
of energy during a chemical reaction
Thermodynamics
Thermodynamics is the field of physics that deals with
energy transformation – from heat to other forms
• 1st law of Thermodynamics: The principle of
conservation of energy
• 2nd law of Thermodynamics: Energy conversions
reduce the order of the universe (aka: increase disorder
[entropy]).
The First Law of
Thermodynamics
Energy is neither created nor destroyed
(but it can be transferred from one part
of the universe to another…)
The Second Law of
Thermodynamics
‘Energy spontaneously disperses from being localised to
being dispersed, provided it is not hindered from doing so’
Some real life examples of the
Second Law
• A rock falls if you pick it up then let it go
• A frying pan will cool down if you take it off the
stove
• Ice cubes melt in a warm room
• High pressure air escapes from a puncture until
pressure is equalised
So how does this apply to
chemical reactions?
• During a chemical reaction, one set of chemicals is
transformed into another
• Both mass and energy are conserved during a
chemical reaction (1st Law of Thermodynamics)
• Chemical reactions always involve energy transfer
• Chemical reactions always involve changes in
chemical bonds
Chemical reactions are classified
as exergonic or endergonic (I)
Chemical reactions can be endergonic or exergonic
Chemical reactions are classified
as exergonic or endergonic (I)
Enzymes activity
Chemical Reactions
Endergonic
Requires a net input of
energy (from
elsewhere).
Energy is absorbed by
the chemical products
Will not occur
spontaneously
Chemical Reactions
Exergonic
A reaction that
releases energy.
Occurs spontaneously
Enzymes
Lower the activation energy of a reaction
Enzymes Lower Activation Energy
What is the cellular energy source?
ATP – adenosine triphosphate
• Powers nearly all forms of cellular work
• It is a nucleotide
How does ATP work?
• ATP works by energy coupling
• Energy Coupling is the use of an exergonic process
to drive an endergonic process
• Bonds between phosphate groups are broken and
energy is released (exergonic)
• Called dephosphorylation
What are enzymes?
• Enzymes are typically
proteins
• Enzymes are specific
• Enzymes act as catalysts to
speed up the rate of
reaction of a biological
process
• Enzymes are not used up by
the reaction they catalyse
Enzymes are essential to all
forms of life…
Enzymes: Vocabulary Check
• Catalyst: A substance that speeds up a
chemical reaction without itself being changed
• Enzyme: A biological catalyst that is usually a
protein
• Substrate: The reactant(s) upon which an
enzyme has its action
• Product: A substance that results from a
chemical reaction
Enzyme-Substrate
Interaction
Factors affecting Enzymes
• Substrate concentration
• Enzyme availability
• pH
• Temperature
• Inhibitors
© 2007 Paul Billiet ODWS
Factors which affect enzyme
activity 1: Temperature
From: GCSE Bitesize:26.08.12
http://www.bbc.co.uk/schools/gcsebitesize/science/add_ocr_pre_2011/homeostasis/importancerev4.shtml
The effect of temperature
Q10
Enzyme
activity
0
© 2007 Paul Billiet ODWS
10
20
30
40
Temperature / °C
Denaturation
50
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 are fully denatured at 70°C
© 2007 Paul Billiet ODWS
Factors which affect enzyme
activity 2: pH
From: GCSE Bitesize:26.08.12
http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa_pre_2011/enzymes/enzymes1.shtml
The effect of pH
Optimum pH values
Enzyme
activity
Trypsin
Pepsin
1
© 2007 Paul Billiet ODWS
3
5
7
pH
9
11
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 its substrate molecules will occur
• This change in ionisation will affect the binding of the
substrate with the active site due to shape change.
© 2007 Paul Billiet ODWS
Factors which affect enzyme activity 3:
Substrate and enzyme concentration
From: http://www.skinnersbiology.co.uk/enzyme.htm
August 26th 2012
Complete this sentence…
The _____________
(more/less) enzyme,
the faster the
product is made.
Substrate concentration: Enzymic reactions
Vmax
Reaction
velocity
Substrate concentration
• 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.
© 2007 Paul Billiet ODWS
Enzyme action can be blocked
by:
• 1. Inhibition (many drugs work through enzyme
inhibition)
• 2. Denaturation -
Enzyme Inhibition
What does ‘enzyme
denaturation’ mean?
Denaturation is a change in the shape of an enzyme
which prevents it from fulfilling its function.
Enzymes (and other proteins) can be denatured by
heat, pH changes, or certain chemicals
NB: Denaturation is not the same as ‘killing’ – proteins and
enzymes are not living things, so can’t be killed!
What does ‘enzyme
denaturation’ mean?