Download f212 molecules biodiversity food health 2.1.3 enzymes

Unit F212: Molecules,
biodiversity, food & health
2.1.3 Enzymes
By Mr. Wilson
Enzymes
• All GLOBULAR proteins of tertiary
structure, usually with HYDROPHOBIC
amino acid R-groups in centre of the
molecule and HYDROPHILIC ones
around the outside.
• Tertiary structure (shape) gives an
enzyme its properties by forming a
receptor site (ACTIVE SITE ), which is
usually an area of fewer than 10 amino
acids and composed of their R-groups.
Enzymes
• Hydrogen bonds, ionic bonds and disulphide bonds hold the tertiary
structure shape together.
• Act on SUBSTRATES to make
PRODUCTS.
• Are biological catalysts & have many
industrial uses as they are fast and
don’t produce any unwanted byproducts.
Properties of enzymes
• They are specific. One enzyme will only
catalyse one type of reaction.
• Not used up in reactions.
• Only small amounts are needed to
catalyse a lot of substrate.
• They have a high turnover number.
• Affected by pH and temperature
changes.
Question:
?Why do
temperature
changes affect
enzyme action?
Naming enzymes
• Enzymes are usually named after the
substrate they act upon:
• Amylase acts on amylose (starch).
• Lactase acts on lactose.
• Protease acts on proteins.
• ATPase acts on ATP.
• Cellulase acts on cellulose.
• READ PAGES 122 to 123.
Where enzymes work best
• Essential to metabolic reactions in ALL
types of organism.
• Endotherms regulate temperature & so
their enzymes can function at optimum
levels in a variety of environments.
• Can be EXTRACELLULAR (eg. digestion)
or INTRACELLULAR (eg. catalysing
reactions in the cytoplasm of cells).
Where enzymes work best
Where enzymes work best
• Phagocyte
white blood
cells use
enzymes to
digest
microbes.
• READ
PAGES 124
to 125.
Enzyme action: Lock & key theory
Enzyme action: Lock & key theory
• The small area of the enzyme that
comes into contact with the substrate
is called the active site.
• While substrate is attached to enzyme
in the active site an enzyme/substrate
complex is formed.
• Because products have a different
shape they are released as they no
longer fit.
Enzyme action: Induced-fit theory
Enzyme action: Induced-fit theory
• Suggests the active site may not
exactly fit the substrate, but is
slightly flexible and can mould
around the substrate with charges
on active site amino acids also
being important.
• The reaction only proceeds when
tight bonding is achieved.
Enzyme action: Activation energy
• To break & reassemble important
biological molecules (such as maltose +
water > glucose + glucose) takes an
additional input of energy.
• This is called the ACTIVATION
ENERGY.
• Enzymes are able to lower activation
energy & thus enzyme controlled
reactions can proceed at much lower
temperatures.
Enzyme action: Activation energy
Enzyme action: Activation energy
• Why can we not just increase the
temperature in cells to speed up
these reactions?
• Enzymes are able to carry out
this function due to the
specificity of the active site for
its substrate molecule.
• READ PAGES 126 to 127.
Enzymes & temperature
• Enzymes & substrates collide & react. The
number of collisions, and their force, is
increased with an increase in temperature
(kinetics).
• Heating enzyme controlled reactions
therefore increases the rate of reaction, but
only to the optimum temperature.
• For most enzymes this is around 40ºC, but it
depends on an organism’s internal and
external environment.
Enzymes & temperature
• Graphs show the rate of enzyme catalyzed
reactions steadily increasing until the
optimum temperature is reached.
• After this point the reaction slows
considerably and at high temperatures the
reaction stops altogether.
• At this point the enzyme’s tertiary structure
has collapsed. We call this DENATURATION
of the enzyme.
Enzymes & temperature
Enzymes & temperature
• DENATURATION occurs because the
increased heat gives increased kinetic
energy to the atoms in the enzyme
molecules.
• As they vibrate more & more vigorously
weaker bonds such as H-bonds and Ionic
bonds break, the tertiary structure is
compromised and the active site is no
longer the shape it should be.
• READ PAGES 128 to 129.
Enzymes & pH
• pH 1 to 6 is acidic and indicates the
presence of positively charged Hydrogen
ions (H+).
• H+ can interfere with the H-bonds and
Ionic bonds that stabilise an enzyme’s
tertiary structure and thus active site.
This can, for example, ‘replace’ essential
H-bonds.
• Increasing acidity will therefore alter the
rate of the enzyme-controlled reaction.
Enzymes & pH
• Furthermore, the H+ ions can alter the
essential & specific charges on amino
acid R-groups that make up the active
site. This will reduce bonding and slow
down the enzyme-controlled reaction.
• All enzymes have an optimum pH; the
concentration of H+ ions in the solution
that give the enzyme it’s best shape,
and thus active site shape, to carry out
its function.
Enzymes & pH
Enzymes & pH
• The optimum pH gives the maximum
rate of reaction.
• Most enzymes have a narrow pH range
for the reasons outlined previously.
• Extremes of pH can DENATURE
enzymes, but small changes can be
tolerated (when pH returns to optimum,
bonds can reform).
• READ PAGES 130 to 131.
Effect of substrate
concentration
• With a fixed amount of enzyme,
increases in substrate concentration will
increase the rate of an enzyme catalysed
reaction up to a point.
• When enzyme active sites are all fully
occupied at all times and working at
capacity the rate of reaction will level
off and the reaction rate reaches a
maximum value.
Effect of substrate
concentration
Effect of enzyme concentration
• The number of substrate molecules an enzyme
can turn into products in one minute is called
the turnover number.
• Chymotrypsin – 6000
• Catalase – 5600000
• Carbonic anhydrase – 36000000
• With unlimited substrate, optimum pH and
optimum temperature, rate of reaction will
be directly proportional to enzyme
concentration.
Effect of enzyme concentration
Effect of enzyme concentration
• If the concentration of substrate is
fixed however, increasing the enzyme
concentration will increase the rate of
reaction to a certain point before
reaching a maximum.
• In this situation active sites will begin
to remain empty and thus the reaction
rate cannot continue to increase.
• READ PAGES 132 to 133.
Focus on: Investigating enzymes
• It is very common to investigate the
action of Catalase (usually sourced from
liver or potato) on hydrogen peroxide.
• This is because oxygen gas is a product;
its volume can be measured.
• It’s also common to investigate the
effect that changing the enzyme or
substrate concentration has on the rate
of this reaction.
Focus on: Investigating enzymes
• You may remember doing this by some
method at GCSE.
• There are other ways to investigate the
rate of reaction using this enzyme &
substrate too.
• Watch the demonstrations.
• The measurements for concentration of
H2O2 are complicated so often arbitrary
(made up, but in scale) units are used.
Enzymes: review
Enzymes at work: Inhibitors
• Can reduce the rate of enzyme-controlled
reactions.
• Combine with enzymes and in some way stop
them attaching substrate.
• Can be specific or affect a range of enzymes.
• Can be reversible and non-reversible
(permanent).
• When a reversible inhibitor is removed from
an enzyme it will work effectively again.
• Reversible inhibitors can be competitive and
non-competitive.
Enzymes at work: Inhibitors
• Competitive reversible
inhibitors have a substratesimilar structure and so fill
active sites stopping substrate
from attaching. ROR slows
until inhibitor is removed.
• Increasing the substrate
concentration will increase the
rate of reaction. Explain why.
• Example; Malonate is a
competitive reversible
inhibitor of succinate in the
active site of succinate
dehydrogenase, an enzyme in
respiration.
Enzymes at work: Inhibitors
• Non-competitive reversible inhibitors
attach to a site on the enzyme other
than the active site and change the
tertiary structure (shape) of the
enzyme so that its active site changes
shape and substrate can no longer bind.
• Concentration of substrate has no
effect on the rate of reaction in this
case. Explain why.
Enzymes at work: Inhibitors
• Most competitive inhibitors are reversible (they are
temporarily in the active site). Many non-competitive
inhibitors are permanent (irreversible/nonreversible) inhibitors that alter enzyme shape
permanently so that they will no longer work at all.
You could say the enzyme is effectively DENATURED
by them.
• Examples: Heavy metal ions such as Silver & Mercury
are non-competitive, non-reversible inhibitors,
breaking disulphide bridges. Potassium cyanide is a
non-competitive, non-reversible inhibitor of
cytochrome oxidase in the essential metabolic
process; respiration, and as such can be a deadly
poison.
• READ PAGES 134 to 135.
Focus on: Investigating enzymes
• Copper sulphate is commonly used to
investigate the effect of inhibitors on
enzyme activity.
• This is because it yields copper ions which act
as non-competitive, non-reversible inhibitors
to a variety of enzymes.
• Increasing concentrations of copper sulphate
are often used to demonstrate increasing
levels of inhibition (slowed rate of enzymecontrolled reaction).
Enzymes at work: Cofactors
• Some enzymes require these additional
substances to work effectively.
• They are all non-protein molecules.
• They act to modify the chemical
structure of enzymes in a way that
allows them to function at an
appropriate rate.
• 3 types – prosthetic groups, coenzymes
& inorganic ion cofactors (activators).
Enzymes at work: Cofactors;
coenzymes
• Coenzymes – small organic
molecules that attach
temporarily to the active site
of an enzyme (just before or
with the substrate).
• They are changed in the
reaction, but are recycled to
be used again.
• The enzyme can only work
effectively when it is
attached.
• Many are derived from
vitamins. NAD is an example.
It comes from nicotinic acid
and is important for many
dehydrogenase enzymes.
Enzymes at work: Cofactors;
prosthetic groups
• Prosthetic groups are
coenzymes that are
permanently attached
to an enzyme.
• They contribute to the
shape, charge &
properties of the
enzyme.
• Other proteins have
these too. The
prosthetic haem group
in haemoglobin allows it
to carry oxygen. Hb
could do this without
it.
Enzymes at work: Cofactors;
inorganic ion cofactors
• These may bind to
substrate or enzyme and
allow more effective binding
to form the enzymesubstrate complex.
• They do this by changing
the overall charge (they are
ions) and sometimes the
overall shape of the ES
complex.
• Obtained from
food/minerals/supplements.
• Examples: Mg2+ is needed
for the enzymes of protein
synthesis to work. Amylase
requires Cl- ions to work.
Homework.
• READ PAGES 136 to
137.
• Complete a short essay
to explain the action of
some poisons and drugs
on enzyme activity.
• Start by addressing the
info on pages 138 to
139.
• READ PAGES 144 to
145. Try to answer the
Qs, in your head only,
after reading.
Focus on: Investigating enzymes
• You will be asked to bring this
presentation back in for a review lesson
in the week beginning 16th March ‘09.
• In preparation for the practical skills
assessment:
• Read through pages 140 to 143.
• Answer Qs 1, 2 & 3 on page 141 and Qs 1
& 2 on page 143.