It’s time for a new Topic!
Section 3.6 and
7.6:Enzymes
7.1: Metabolic
reactions consist
of chains and
cycles of
enzymecatalysed
reactions
 The SIGMA-Aldritch Imaginogram of metabolic
pathways
 An animation outlining buiochemical pathways
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: 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
Enzymes are essential to all
forms of life…
3.6.1 Explain enzyme substrate
specificity
7.6.2: Describe the ‘induced fit’
model
Enzyme substrate specificity
 Substrate specificity
 Induced fit versus Lock and key mechanism
 Induced fit in a moment....
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
7.6.3: Explain that enzymes lower
the activation energy of the
chemical reactions they catalyse
In order to understand enzyme activity, we
need to also understand the energy
transformations that occur during a
chemical reaction
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
Energy and enzymes animation
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
The energy stored in the
products is less than the
energy stored in the
reactants
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)
 This process is called dephosphorylation
3.6.3: Explain the effects of
temperature, pH and substrate
concentration on enzyme activity
The best way to understand temperature, pH and
substrate concentration effects is through paying with this
game....
and here's another....
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
 For many but not all enzymes the optimum
temperature is about 30°C. In warm blooded
animals, most enzymes are fully denatured at 70°C
 Optimal temperature is organism-deoendent.
Many enzymes function optimally at a lower
temperature. For example, cold water fish can die
at 30°C since many of their enzymes denature.
Many plant enzymes also function optimally at
lower temperatures.
 A few bacteria have enzymes that can withstand
very high temperatures up to 100°C
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
 pH change must change the structure of the enzyme,
associated with changes in bond
angles between amino acid R-groups
 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.
Factors which affect enzyme activity 3:
Substrate and enzyme concentration
From: http://www.skinnersbiology.co.uk/enzyme.htm
August 26th 2012
Substrate concentration: Enzymic reactions
Vmax
Reaction
velocity
Substrate concentration
3.6.4: Define denaturation
Denaturation
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
watch denaturation happen....
NB: Do NOT describe denaturation as ‘killing’ – proteins
and enzymes are clearly not living things, so can’t be
killed!
3.6.5: Explain the use of …to produce lactose-free milk
Lactase
 Lactase (beta-galactosidase) catalyses the hydrolysis of
lactose to glucose and galactose:
 Lactose -> D-glucose + beta-D-galactose
 Both of these sugars taste sweeter and are more readily
digestible than lactose
 Most people produce less lactase as they age
 An estimated 75 % of the world’s human population (and
most cats) are lactose intolerant in adulthood – it is lactose
tolerance that is unusual.
Methods for managing lactose
intolerance (1)
1. Take a lactase supplement
Produced industrially using yeasts and fungi (e.g. Aspergillus Niger)
Methods for managing lactose
intolerance (2)
Treat milk with lactase (produced by fungus/yeast)
Most commonly by running milk through beads coated with
immobilised enzyme (uses alginate beads, so that there is no
enzyme in the final product)
Immobilising enzymes or microscopic organisms involves trapping
them in a matrix of an inert material or binding them to its surface.
This makes it easier to remove the active catalysts from the
reaction mixture, and so makes it easier to purify the products.
It also allows us to set up systems for continuous processing,
packing the immobilised catalysts in a vessel through which a
steady stream of reactants can flow – collecting useful products at
the outlet.
Methods to manage lactose
intolerance (3)
genetically modify cows to
produce low-lactose milk
7.6.4: Explain the difference
between competitive and noncompetitive inhibition, with
reference to at least one
example of each
Some visuals to introduce you
to enzyme inhibition
 A walk-through the basics of enzymes inhibition
 A deeper look at enzyme inhibition
Types of enzyme inhibition
1. Competitive
reversible inhibitors
2. Non-competitive
reversible inhibitors
3. Competitive/ noncompetitive
irreversible inhibitors
1. Competitive (reversible) inhibitors
 Compete with substrate for the active site
 Similar in shape to the substrate so that they can fit into the
active site
 DIFFERENT from the substrate in that they are un-reactive,
thus reducing the number of enzyme molecules available for
reaction
 ‘Compete’ with the substrate; often they leave the enzyme if the
concentration of substrate is sufficiently high
Competitive inhibitors in
action
watch competitive inhibitors at
work
MANY DRUGS ARE COMPETITIVE
INHIBITORS…
 Antibiotics: sulphonamides,
penicillin, trimethoprim
 Anticholinesterase drugs:
Neostigmine
 Sildenafil (Viagra): competitive
phosphodiesterase inhibitor
 Sildenafil (Viagra): competitive
phosphodiesterase inhibitor
 Alcoholism treatment: Disulfiram
(antabuse) competes with aldehyde
oxidase, promoting nausea and
hangover…
2. Non-competitive inhibition
1. Don’t bind at the active site, rather to OTHER locations on the
enzyme (allosteric sites)
2. Change the active site by means of bonding resulting in a
conformation change
3. Will eventually leave their binding site
4. Their action is not affected by substrate concentration
5. Increasing substrate concentration will not displace the allosteric
non-competitive enzyme inhibitor
Examples: METALS: Lead (Pb), Mercury (Hg),
Chromium (Cr); also ACE inhibitors (vasodilators)
Allosteric inhibition
A quick animation of
allosteric inhibition
3. Irreversible enzyme inhibitors
How do irreversible inhibitors
work?
1. They bond to the enzyme covalently (the substrate
typically bonds with weaker bonds).
2. They cannot be displaced by substrate
3. By bonding (at the active site or elsewhere), the inhibitor
changes enzyme shape (conformation) so that it can’t work.
4. The inhibitor doesn’t readily leave the enzyme
 Examples: aspirin, acetylcholinesterase inhibitors
(Nerve gases, organophosphate insecticides)
Examples of irreversible
inhibitors
 scroll down for animated aspirin effects
7.6.5: Explain the control of
metabolic pathways by end
product inhibition, including the
role of allosteric sites
End-product inhibition
Enzyme pathways can be controlled
by concentration of products from the
end of the pathway.
 A wee introductory animation
 The metabolic pathways of life
Important metabolic pathways 1:
The clotting cascade
Important metabolic pathways 2:
The Krebs Cycle (Respiration)
Important metabolic pathways 3: The
Calvin Cycle (Photosynthesis)
Allosteric feedback inhibition is
common in metabolic pathways
How does allosteroic
feedback inhibition work?
 Many metabolites (end products of metabolic pathways) act
as allosteric inhibitors of enzymes earlier in a metabolic
pathway. This is called feedback inhibition.
 In feedback inhibition, an increase in the level of a
metabolite results in a decrease in the production of that
metabolite.
Examples of end-product
inhibition
1. Phosphofructokinase (key enzyme in
glycolysis – allosterically activated by
ADP and allosterically inhibited by ATP)
2. Dopamine and norepinephrine, once
present in high concentration, will inhibit
tyrosine hydroxylase