Download Enzymes - A Level Notes

Enzymes
 Globular proteins that act as biological catalysts
 Increases the rate of reaction by lowering the activation energy without
being used in the process, holding the reactants in the correct orientation
for the reaction to take place, straining bonds so that they can be broken
more easily
 Precise 3D structure due to tertiary structure
 Soluble in water
 Not very stable and may become denatured
 Ionic bonds between ionised R groups (between hydrophilic and
hydrophobic side chains
 Disulphide bonds between R groups containing –SH groups
 Denaturation is the irreversible change in tertiary structure of the enzyme
due to high temperatures and extremes of pH
 The substrate binds to the active site, or cleft
 Enzymes are specific – they can only bind to one type of substrate and the
substrate is complementary in shape to the active site
 Only a few amino acids contribute to the active site
 DNA structure determines the specific shape of the enzyme because:
o DNA codes for amino acids/ polypeptides
o An enzyme is a globular protein
o DNA determines the primary structure
o Primary structure is folded/coiled to make the secondary structure
o DNA determines the projecting side groups
 Enzymes catalyse all metabolic reactions that take place inside the cell
(intracellular) and are also excreted out of the cell to catalyse
extracellular reactions too
 Lock and key model - The shape of active site is complementary to the
shape of the substrate before the substrate binds
 Induced fit model – The active site changes shape when the substrate
binds to become complementary. This strains the bonds in the substrate
(which may also change shape) so a reaction occurs more easily.
 Charged amino acids at the active site also help substrate binding and
facilitate the reaction
 The products that are formed have a different shape to the substrate (no
longer complementary), so are released
 Exergonic reactions – energy is released
 Endergonic reactions – energy is taken in
 For chemical reactions to take place, some energy is required initially –
this is the activation energy
 Factors that affect enzyme activity:
o Temperature
o pH
o Concentration of enzyme
o Concentration of substrate
o Presence of inhibitors
 Temperature – all molecules possess kinetic energy and are continually
moving randomly around and colliding with each other. For two
substances to react they must first collide with each other.
 At a lower temperature the substrate and enzyme have less kinetic energy
and are moving increasingly slowly, so fewer collisions, fewer enzymesubstrate complexes formed and a slower rate of reaction.
 At high the enzyme vibrates more. This breaks the weak bonds that
maintain the tertiary structure e.g. hydrogen bonds, ionic bonds. This
changes the shape of the enzyme and the shape of the active site. The
active site is no longer complementary in shape to its substrate so the
substrate cannot bind, so fewer enzyme-substrate complexes form so the
rate of reaction decreases - The enzyme may become denatured
 The optimum temperature for enzymes is slightly above normal body
temperature, so that if body temperature is raised, by fever or exercise,
the enzymes are not denatured
 During hibernation, metabolism slows down so less energy is used
 pH - H+ ions may disrupt the hydrogen and ionic bonds that maintain the
tertiary structure of the enzyme. The enzyme becomes denatured at
extremes of pH when many weak bonds are broken. H+ ions may also
associate with certain amino acid R groups at the active site altering the
charge distribution and interfering with substrate binding so fewer
enzyme-substrate complexes form
 If substrate is present in excess, increasing enzyme concentration will
mean that more active sites available to bind substrate, so more collisions
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between active site and substrate, more enzyme-substrate complexes
(ESC) form, and this increases the initial rate of reaction
Increasing substrate concentration, there are more collisions between
active site and substrate, so more enzyme-substrate complexes form,
increasing the rate of reaction up to a maximum rate (Vmax). Vmax is when
all the active sites are occupied. If the enzyme concentration (limiting
factor) was increased, it would increase the rate of reaction further.
Enzymes are usually kept at a low concentration in the cells because
enzymes can catalyse reactions very quickly (high turnover). They are
unchanged at end so can be reused; therefore they are only needed in low
concentrations. It also saves resources synthesising enzymes, its easier to
control enzyme activity
The rate of a reaction is a measure of how fast substrate is converted into
product
A variable is any factor that may change and affect the rate of reaction
Independent variable - Controlled by investigator
Dependent variable - Measured by investigator
All other variables must be controlled (kept constant), so any variables
that are not controlled adequately are referred to as limitations
Controlling variables
o Temperature – use a thermostatically controlled water bath
o pH – Use a buffer
o Enzyme concentration - Accurately measure volumes in solution,
use a graduated pipette; its more precise than a syringe, small
syringe more precise than a larger one, keep mass/surface area of
tissue constant
o Substrate concentration - accurately measure volumes in
solution/mass
Other points to consider
o Equilibration - Allow time for enzyme and substrate to reach set
temperature
o Use of controls - Show that any changes observed are due to the
activity of the enzyme, Replace enzyme with distilled water
o Repeat results and calculate a mean - To identify anomalous results
and increase reliability
Inhibitors are substances that bind to an enzyme and reduce its activity;
they may be reversible by only binding temporarily to the enzyme via
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weak H or ionic bonds; or irreversible by binding permanently to the
enzyme via strong covalent bonds
Competitive inhibitors - Binds to the active site and prevent the substrate
binding, they are often a similar shape to the substrate. Increasing the
concentration of the inhibitor decreases the rate of reaction, increasing the
substrate concentration increases the rate of reaction
Non-competitive inhibitors - If it is reversible, the inhibitor does not bing
to the active site - Bind elsewhere (to allosteric site) and change the shape
of the enzyme and its active site. The substrate is no longer
complementary in shape to the active site and can no longer bind to form
an enzyme-substrate complex. If irreversible, the inhibitor binds
permanently to the enzyme via covalent bonds. It may bind to the active
site or allosteric site. They are always non-competitive since changing
substrate concentration has no effect. It denatures the enzyme. It may
include many toxins or poisons
A cofactor is a non-protein substance that is required by an enzyme for its
activity. Without the cofactor, the enzyme is not active
Metabolism is all the chemical reactions that take place inside the cells of
an organism. Anabolic reactions build large molecules from smaller
ones. Catabolic reactions breakdown large molecules into smaller ones
Metabolic reactions are linked together to form metabolic pathways Each step is catalysed by a different, specific enzyme, the product of one
enzyme is the substrate for the next
End product inhibition is where the final product of the pathway can act
as a non-competitive, reversible inhibitor of an enzyme near the
beginning of the pathway; this is an example of feedback regulation.
Inhibiting an enzyme at the start of the sequence means that intermediates
do not build up, as well as the end product. Avoids wasting resources
making unnecessary products, gives control over the whole metabolic
pathway
The effect of a mutation in the gene that codes for an enzyme could cause
the enzyme not to be produced, or it may have a different tertiary
structure and not function. Such conditions can be inherited