CIE Biology A-level
Topic 3: Enzymes
Notes
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Enzymes
Enzymes are globular proteins that increase the rate of reaction by lowering the activation
energy of the reaction they catalyse. The active site is the area of the enzyme where the
reaction with the substrate takes place. Each enzyme has a specific shape that must be
complementary to the substrate, meaning that only one type of substrate fits into the
active site of each enzyme. When the enzyme and substrate form a complex, the structure
of the enzyme is altered so that the active site of the enzyme fits around the substrate. This
is called the induced fit model.
Enzymes can be intracellular (function inside cells), for example DNA polymerase. They can
also be extracellular, such as the enzymes used in digestion.
Lock and Key Theory:
Proposed by Fischer in 1894
Active site and substrate have complementary shapes prior to binding
Only one substrate can fit each active site
Induced Fit Theory:
Proposed by Koshland in 1958
Enzyme has active site
Enzyme is moulded around substrate as it enters to become complementary
Bonds form between oppositely charged groups on substrate and R groups to induce
a better fit. This puts a strain on the substrate molecule so reactions occur more
easily.
Factors affecting the rate of enzyme-controlled reactions:
Enzyme concentration the rate of reaction increases as enzyme concentration
increases as there are more active sites for substrates to bind to, however increasing
the enzyme concentration beyond a certain point has no effect on the rate of
reaction as there are more active sites than substrates so substrate concentration
becomes the limiting factor.
Substrate concentration as concentration of substrate increases, rate of reaction
increases as more enzyme-substrate complexes are formed. However, beyond a
certain point the rate of reaction no longer increases as enzyme concentration
becomes the limiting factor.
Temperature rate of reaction increases up to the optimum temperature as kinetic
energy increases. Rate of reaction decreases beyond the optimum temperature. At
very high temperatures, bonds in the enzymes tertiary structure break, changing the
shape of the active site so reactions cannot occur. This is called denaturation.
pH As the pH moves away from the enzymes optimum, rate of reaction decreases.
The pH is a measure of the concentration of hydrogen ions. Each enzyme has an
optimum pH: the wrong pH alters the charges on the amino acids which make up the
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active site, breaking the bonds in the enzyme's tertiary structure and leading to
denaturation. Thus, when the enzyme is not in its optimum pH, the substrate can no
longer become attached to the active site and the enzyme-substrate complex cannot
form.
Concentration of competitive reversible inhibitors as concentration of
competitive reversible inhibitors increases, rate of reaction decreases as the active
sites are temporarily blocked by inhibitors so substrates cannot bind to them.
Concentration of non-competitive reversible inhibitors as concentration on non-
competitive reversible inhibitors increases, rate of reaction decreases as the shape
of the enzyme (not the active site) is altered by the inhibitors.
Inhibitors
Inhibitors are substances which stop the enzyme from binding to its substrate. They can
therefore control the progress of a reaction.
Types of inhibition:
Competitive inhibition this is when an inhibitor molecule binds to the active site of
the enzyme and stops the substrate from binding to it; it can be reversed by
increasing the substrate concentration as the inhibitor is diluted.
Non-competitive inhibition- an inhibitor doesn’t bind to the active site but binds to
a different part of the enzyme which changes the shape of the enzyme; it decreases
the reaction rate as the substrate cannot bind to the enzyme.
Feedback inhibition this occurs when the end product binds to the enzyme at the
start of the reaction/pathway and this stops the pathway until the concentration of
the end product decreases.
Michaelis-Menten Equation
Michaelis-Menten equation can be used to
calculate the maximum rate of reaction (Vmax) by
relating the velocity of enzyme reactions (V) to
concentration of a substrate [S]. Vmax represents
the maximum rate of reaction achieved by the
system at maximum substrate concentration.
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Immobilising enzymes in alginate
When enzymes are in solution, they can only be used once as it is very difficult and time
consuming to separate them from the product. Therefore they are immobilised by
attaching them to an insoluble, inert material e.g. calcium alginate which forms a gel
capsule around them thus holding them in place during the reaction. This process enables
enzymes to be reused as they can be easily separated from the products. Immobilised
enzymes are used in industry because it enables the reaction to flow continuously.
Moreover, the use of immobilised enzyme is much cheaper than using enzymes in solution
as they can be reused.
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