notes at end of 2.)
2.1 Introduction to enzymes - what
are they? and what do they do?
Aspects of the vitamin, food and drugs GCSE chemistry are on the "Extra
Organic Chemistry" page.
Living cells use chemical
reactions to produce new materials.
Living things produce catalysts called enzymes which allow chemical
reactions to occur quite quickly at ordinary temperatures and pressures.
Enzyme proteins are powerful 'biochemical catalysts' and are widely used in the
food industry and are being used more and more to manufacture many other
chemicals. These biological catalysts promote most of the reactions in
living tissue. The names of enzymes end
in ...ase e.g. amylase, protease, invertase, isomerase etc.
Enzymes are in all cells and keep
all living things working and are always extracted from a living
Enzymes are always complex 3D
molecules and are sensitive to pH (mustn't be too acid or too
alkaline) and temperature (mustn't be to low - too slow, or too high
which causes denaturing of the enzyme).
Each of these biological catalysts has a
unique complex 3D protein structure, particularly the
shape of the active site, into which the specific substrate molecule's
shape 'fits in' to give the enzyme substrate complex, and be chemically
(key and lock mechanism).
In forming this
enzyme-substrate complex, the enzymes provide a pathway of lower
activation energy, so, for any given temperature, a greater
proportion of reactant molecules have sufficient kinetic energy to
change when they collide with the active site on the enzyme.
Enzymes are true catalysts
because they are not consumed in the process and keep working as long as
the substrate reactant is present e.g. as long sugar is left in an
aqueous solution mixed with yeast.
Enzymes have been used by humans
since the beginning of recorded history e.g. use in fermentation to make
Enzymes are becoming increasingly
important in the “biotechnology” industry.
Enzyme reactions happen inside living
cells. However, dead cells that have been broken open to release their
enzymes are used to let the process happen in test tubes or for large
scale industrial production situations.
Enzymes have ability to reduce the
activation energy needed (See
rates of reaction notes)
The reaction profile diagrams
illustrate the point.
By lowering the activation energy,
more molecules have enough kinetic energy to react - bonds are more
easily broken - so greater probability of fruitful collision forming the
products of the reaction.
Cells contain protein
molecules that act as biological or biochemical catalysts, they are
known as ENZYMES.
Specific enzymes catalyse
specific chemical reactions, they only function with a specific
substrate molecule or molecules - the reactants for a specific
At low concentrations of either
enzyme or substrate the reaction mathematics is usually quite
The chemical reactions
brought about by living cells are quite fast in conditions that are
warm rather than hot.
This is because the cells use these enzyme catalysts.
and lock' mechanism is explained later on.
Although they are fantastic
catalysts, they can perform efficiently under quite narrow
conditions in terms of temperature and pH of the reaction medium.
Enzymes are protein molecules which are usually
damaged by temperatures above about 45º C. Although not damaged by
lower temperatures, the reactions may be too slow to be of any use.
(see rates notes at the end of
Different enzymes work
best at different pH values.
TOP OF PAGE
2.2 Some uses of enzymes
The enzymes in yeast cells
(living organism's) convert
sugar into ethanol ('alcohol') and carbon dioxide in the brewing and
This process uses the anaerobic respiration of yeast to manufacture alcohol (ethanol)
Wine is made from fermented grapes
and beer and whisky from fermented grain and hops.
similar process is used to convert sugar cane into ethanol and
distilled to use as biofuel.
The process does NOT need oxygen and
occurs best under anaerobic conditions in a fermenter.
e.g. in water and in the absence of air,
so absence of oxygen you get a form of
This process occurs
efficiently between 25 to 55oC and is
called fermentation and is used to produce the
alcohol in beer and wine. The carbon dioxide dissolved in the
final alcoholic drink produces the fizz!
on raising agents in cooking: It is this reaction
producing bubbles of carbon dioxide which make dough mixtures rise in the
kitchen or food industry when yeast is used in baking bread or
cake making etc.
So bread making uses the anaerobic respiration of yeast
to produce carbon dioxide gas to give the rising action of the
An alternative to
yeast is to use sodium
hydrogencarbonate ('sodium bicarbonate' or 'baking soda') in
baking. The rising action is also due to carbon dioxide gas
formed from its reaction with an acid (e.g. tartaric acid), and nothing to do with enzymes:
baking powder = carbonate base + a solid organic acid,
hydrogencarbonate + acid ==> sodium salt of acid +
water + carbon dioxide
A simple laboratory
test for carbon dioxide is that it forms a milky precipitate with
However other enzymes
in living material can also catalyse oxidation with the oxygen in
air. When alcoholic drinks turn sour it is due to the alcohol
being oxidised to the weak organic acid ethanoic acid, commonly
know as 'vinegar'!
Enzymes are involved
in the following processes in the home
Bread dough raising using yeast
(see above), the rising action is due to the formation of carbon
may contain protein-digesting protease enzymes and fat-digesting enzymes
In industry and agriculture,
enzymes are used to bring about reactions at normal temperatures and
pressures that would otherwise require more expensive and more energy demanding
In the dairy industry
yoghurt and cheese are formed by the action of bacteria (and
their enzymes) on milk.
break down proteins and are used to
'pre-digest' the protein in some baby foods.
used to convert starch syrup into sugar syrup.
is used to make the sugar
for the soft centres of chocolates, but is quite expensive.
Isomerase* is used to
convert glucose syrup into fructose syrup, which is much sweeter
and therefore can be used in smaller quantities e.g. in slimming foods.
breaks down insoluble pectin polysaccharides and so is used in
clarify fruit juices.
Amylases break down carbohydrates and
break down fats.
are used in genetic
engineering and penicillin production.
The dairy industry
uses enzymes made by microorganisms (bacteria) to
produce yoghurt and cheese from milk.
in biological detergents - the enzyme lipase (together
with protease) is used in biological detergents to break
down (digest) the substances in stains into smaller water
penicillin is made by the action of enzymes in the
penicillium mould which is mixed with a particular sugar
solution and other ingredients in a fermentation tank and as
the mould grows the penicillin is produced. The penicillin
is then extracted from the penicillium mould and ends up as
a solution in a little bottle ready for injection into a
processes often depend on the immobilization of the enzyme:
Traditional use of enzymes have
all been “batch” processes, which is not very efficient on a
commercial scale. Modern biotechnology industries have developed
techniques to isolate and then immobilise enzymes, thereby
allowing continuous processes to be developed. The enzyme is
isolated, usually from a culture of bacteria, then 'immobilised'
or trapped in an unreactive material, so that they remain
The enzymes can be
trapped in a silica gel lattice or a collagen matrix (a mesh that
traps the enzymes) or cellulose
fibres (again enzyme trapped in a molecular mesh). In principle all
that is needed is a stable inert surface (the larger the better for
a more efficient rate of reaction) on which the enzyme is attached
beads of alginate or polymer microspheres can also be used immobilize
enzymes but these beads need to be selectively permeable allowing
substrate molecules in and reactant products out.
enzyme keeps it
functioning for a longer period because it can be easily recovered
for further use.
To immobilise the enzyme
allows a continuous
process, this means a continuous input of raw materials and output of product,
so can run 24 hours a day for many weeks or months efficiently.
process means loading the reactor vessel with reactants, extract
clean out the reactor/fermenter, re-load with reactants etc. etc. i.e. less efficient,
costs time and so is less economic!
There is less
contamination of the product with the enzyme because of ease of
Rates of Reaction - Kinetics of Enzymes -
Optimum Conditions (full
rates of reaction notes)
extra notes on the kinetics of
enzyme reactions, for advanced level students only
TOP OF PAGE
2.3 Effect of
substrate or enzyme concentration on reaction rate
If either the substrate reactant
e.g. sugar, or
the yeast cell (enzyme) concentration is increased, the rate of reaction
increases in a simple proportional way.
The graph on the left shows what
happens as you gradually increase the substrate molecule concentration
for a fixed constant concentration of enzyme. Initially the rate of
reaction steadily increases, in fact it rises proportionately with
increase in substrate concentration.
Kinetic particle theory: The greater
the concentration of substrate or enzyme, the greater the probability of
a fruitful collision leading to the formation of products.
Assume the grey background is the solvent water
See Rates of Chemical Reactions
Notes (GCSE chemistry notes)
What happens if the enzyme gets
overloaded with substrate molecules?
However, if the concentration of
enzyme is low but the substrate concentration becomes very high, the rate of
reaction rises to a maximum and then stays constant.
is because the maximum number of catalyst sites for the 'key and lock' mechanism
are all in use and the rate of reaction depends on the rate of diffusion of
substrate in and diffusion product out of the active sites.
TOP OF PAGE
Effect of pH - what is the optimum pH of an enzyme?
pH effect: The structure of the protein enzyme
can depends on how acid or alkaline the reaction medium is, that is, it is
pH dependent. If the medium is too acid (low pH) or too alkaline
(high pH), the structure of the protein
is changed and it is 'denatured' and becomes less effective.
If the complex 3D shape of the enzyme protein is changed by the
action of an acid or an alkali, both the 3D shape of the enzyme and
the active site are changed so the substrate-enzyme complex is more
difficult to form, hence the reduction in reaction rate.
optimum pH range, the enzyme catalysis is at its most efficient. In the
denaturing process the 'active site' may be damaged or changed so the
enzyme cannot perform its catalytic function on the substrate molecules.
If the enzyme does not have the correct 'lock' structure, it
cannot function efficiently by accepting the 'key' substrate molecule. Most enzymes have an optimum pH of between 4
and 9, and quite frequently near the neutral point of pH 7.
enzyme pepsin has a peak at pH 2 and can operate in the very acid
(hydrochloric) conditions of the stomach to help breakdown proteins for
digestion in the small intestine.
|Amylase (pancreas) enzyme
||pH 6.7 - 7.0
This would be the graph for an enzyme with
an optimum pH of 7.0
A pancreatic enzyme that
catalyzes the breakdown and hydrolysis of starch into soluble sugars
that can readily be digested and metabolised for energy
|Amylase (malt) enzyme
||pH 4.6 - 5.2
breakdown and hydrolysis of starch into soluble sugars in malt
Catalyses the breakdown of
potentially harmful hydrogen peroxide to water and oxygen.
Important in respiration metabolism chemistry.
==> 2H2O(l) + O2(g)
breakdown/hydrolysis of sucrose into fructose + glucose, the
resulting mixture is 'inverted sugar syrup'.
+ H2O ==> C6H12O6 +
|Lipase (pancreas) enzyme
||Lipases catalyse the breakdown
dietary fats, oils, triglycerides etc. into digestible molecules
in the human digestion system.
|Lipase (stomach) enzyme
||pH 4.0 - 5.0
||As above, but note the
significantly different optimum pH in the acid stomach juices,
to optimum pH in the alkaline fluids of the pancreas.
||pH 6.1 - 6.8
||Breaks down malt sugars.
||pH 1.5 - 2.0
breakdown/hydrolysis of proteins into smaller peptide fragments.
||pH 7.8 - 8.7
breakdown/hydrolysis of proteins into amino acids. Note again,
the significantly different optimum pH to similarly functioning
||Catalyzes the hydrolysis breakdown of urea
into ammonia and carbon dioxide.
(NH2)2CO(aq) + H2O(l) ==> 2NH3(aq) + CO2(aq)
||catalyses the fermentation of
sugars into ethanol
==> 2C2H5OH(aq) + 2CO2(g)
I've seen on the internet quoted values
from 4 to 8 for the
optimum pH for the maximum, most economic, speed of reaction for
fermentation. The pH in yeast cells is apparently 6 which is the
mid-value of those quoted. There are also different strains of
chemistry of ethanol
|Note in the
data/information table above, that there are several instances,
amongst many, where two different enzymes perform the same
function, BUT at very different optimum pHs. This results in
versatility ie where same chemistry is needed in different
organs of the body - which incidentally, functions very nicely
with its ~3000 different enzymes. Almost every chemical reaction
in living organisms requires a specific catalyst or enzyme.
TOP OF PAGE
What is the effect of temperature?
What is the optimum temperature of an enzyme?
Temperature: The structure of the protein enzyme can
depend the temperature. If the enzyme does not have the correct 'lock'
structure, it cannot function efficiently. The shape of the graph is
due to two factors.
(a) The initial rise in rate of reaction is what you
normally expect for any chemical reaction. The increase in temperature
increases the average kinetic energy (KE) of the molecules. This causes
more particles to have the necessary activation energy to break bonds and increase the chance of
the product forming from the higher KE substrate-enzyme 'fruitful' collisions.
See Rates of
Chemical Reactions Notes
(b) However as the
temperature rises further, the increasing thermal vibration of the enzyme
molecule causes its protein structure to break down (denature) and so the 'lock'
is damaged so the enzyme is less efficient in interacting with the
substrate molecule (see key-lock below).
be due to the failure of weak intermolecular forces or actual
ionic/covalent bonds, but the 3D molecular structure of the
enzyme is changed so that the substrate molecule cannot 'dock in' to the
active site and be
changed into the products.
1. The enzyme 3D shape is intact and the
active site will be available.
2. The enzyme is denatured losing the the
original 3D shape, including the active site, so the substrate molecule
(the key) cannot temporarily combine with
active site (the lock), hence the
biochemical reaction cannot take place.
(c) The graph that you see is effectively the result
of adding two graphs together,
(a) the increase in rate due to increase
(b) the decrease in rate as denaturing of the enzyme
increases with increase in temperature.
The resulting graph than has two
minimums and one maximum, the hump in the graph being the optimum
The optimum temperature for
the fastest rate of reaction is often around 30-40oC (note our
body temperature is about 37oC, no coincidence!). Eventually at high
temperatures the enzyme completely ceases to function.
TOP OF PAGE
- the 'key and lock' mechanism
E = free
= free substrate reactant molecule, E-S
= enzyme-reactant complex, E-P
= enzyme-products complex, E
= free enzyme, P
= free product
The enzyme is a complex
protein molecule, but there is a particular 'active site' where the reactant
molecule 'docks in' by random collision. The enzyme is sometimes referred to as the
and the initial reactant substrate molecule as the 'key', hence
this is called the 'key and lock mechanism'. This is also explains why enzymes are
very specific - you need the right molecular key for a particular
(1) Once the 'reactant-enzyme
complex' is formed the enzyme function changes the reactant molecule
into the new product molecule or molecules (2).
(3) The 'enzyme-new molecule
complex' breaks down to free the new product molecule and
the enzyme reactive site can now be re-used by another reactant molecule.
(a). Compared to the
un-catalysed reaction, the enzyme provides a 'chemical change
route' with a much lower activation energy, and
so this greatly increases the rate of reaction as more molecules
have enough kinetic energy to react at the same temperature.
The products are shown as two molecules, because there are quite
often two products for each step of the breakdown of a bigger
molecule into smaller molecules e.g. protein to 'smaller
protein' + amino acid, or
starch to 'smaller starch' plus a glucose molecule
etc. But there can be just one
product molecule e.g. when isomerase changes glucose into fructose.
There can also be two substrate reactant molecules being combined
to form a bigger molecule or a long natural polymer molecule like
starch being broken down to small sugar molecules. In other words there are lots of
(c). Many drugs
work by blocking the sites normally used by enzymes. The
molecular key (the drug) goes onto the reactive enzyme site, but
stays there, so inhibiting enzyme activity which promotes harmful
chemical-organism effects in the body. The harmful effect might be
the production of toxic chemicals from a bacteria or the
reproduction of a harmful organism etc.
(d). "Rates of
Reaction Notes" fully
explains all the factors affecting the rate of any chemical
reaction, including explaining experimental methods and reaction
profile diagrams and activation energy.
(e). Different reactions
need different enzymes, and also if enzymes, which bring about the same
chemical change, are quite likely to have different optimum rate pH's or
temperatures. this phenomena is known as the specificity of
enzymes is related to the unique structure of each enzyme and its
'reactivity' limited to interaction with particular substrate
ENZYMES - structure, function, optimum conditions,
advanced A level chemistry notes on Enzyme structure on the stereochemistry
extra notes on the kinetics of
enzyme reactions, for advanced A level students only
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