Notes B4.1- Enzymes
Catalyst: a substance which alter or speed up a chemical reaction, without itself being chemically changed at end
of the reaction.
Catalyst alters the rate of reaction by allowing an alternative pathway for reaction to happen. Alternative pathway
with presence of a catalyst is usually with lower activation energy.
The Activation energy is the amount of energy needed to start/initiate a chemical reaction. Reaction pathway with
higher the activation energy is slower as more time is needed to gather enough energy to start/initiate a reaction.
1. What are enzymes?
1. Biological catalysts
2. A type of protein
3. Alter or speed up the rate of chemical reactions
4. Without being chemically changed at the end of the reactions
2. What types of reactions do enzymes catalyze?
Enzyme catalyzes almost every metabolic reaction in animals and plants.
1. Anabolic enzymes catalyze anabolic reactions – altering the speed of synthesis of complex substances from
2. Catabolic enzymes catalyze catabolic reactions – altering the speed of breaking down of complex substances
to simple substances
Note: Enzymes catalyse the chemical reactions but they do not cause the reactions to occur.
For instance, sometimes during chemical reactions in the cells, hydrogen peroxide is produced. This is poisonous
to body tissues and has to be broken down to harmless water and oxygen in the cells. Even without the help of
enzymes, the hydrogen peroxide will still be broken down, but this is an extremely slow process that may
eventually intoxicate the cells.
3. What are the characteristics of enzymes?
1. Alter the rates of chemical reactions that occur.
2. Enzymes are required in minute amounts. That is a small quantity of an enzyme is capable of catalyzing a
huge number of chemical reactions (do you know why?)
3. Enzymes are highly specific.
For instance, amylase (found in saliva) will act only on starch and not on proteins. Similarly, lipase (found in
the small intestine) will only act on fats.
The substances on which the enzymes act are called substrates.
The depression on the surface of an enzyme molecule into which the substrate fits is called the active site.
How enzymes work is explained by the two models: the lock- and-key model and *the induced fit model.
Of both model, the enzyme is the lock and the substrate is the key.
Emil Fischer's Lock and Key Model
Below illustrates the lock and key model. It describes a situation in which the enzyme and the molecule that it
acts on in a reaction, the substrate, fit together perfectly. For this system to work, the enzyme has an active site,
which is like a keyhole for the substrate. The substrate's shape, which is formed by the specific arrangement of
atoms and bonds between the atoms, is like a key that fits exactly into the enzyme active site. This specificity
means that like a house key, only the correct key will fit the lock.
*Daniel Koshland's Induced Fit Model
The induced fit model is an elaboration on the basic idea of the lock and key model. In this model, though, the
key and the enzyme active site do not fit perfectly together. Instead, the substrate interacts with the active site,
and both change their shape to fit together. This still means that only particular substrates can fit each enzyme
4. What factors affecting enzymes activities?
1. Enzyme activity is affected by temperature
Enzymes are less active at low temperatures.
Enzymes have an optimum working temperature. They are most active at this temperature. For your
information, enzymes found in human body cells work best around 37C.
Beyond the optimum working temperature, enzyme activity decreases. Enzymes are denatured at high
temperatures, that is, its
activity is irreversibly
destroyed. When enzyme
denatures, it loses the three
of its active site.
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2. Enzyme activity is affected by pH
Enzymes may be denatured at extreme pH.
Enzymes have an optimum working pH
The optimal working pH varies with the enzyme type.
3. Enzyme activity is affected by both the amount of enzyme used (enzyme concentration) and also by the
amount of substrate used (substrate concentration) depends on which one of the two is the limiting factor.
From either graph A or graph B, increasing the concentration of substrate can give a corresponding increase
in reaction rate, but only when the substrate concentration remains comparatively small.
When larger substrate concentrations are used, the reaction rate becomes less
dependent upon the concentration of the substrate but tends towards a fixed maximum determined by the
amount of enzyme present.
At low substrate concentrations many of the active sites of the enzymes are
unoccupied and the restricted supply of substrate molecules largely determines the reaction rate.
At high substrate concentrations almost all of the active sites of the enzymes are occupied, thus in order to
further increase the rate of the reaction, the amount of enzyme used has to be increased (i.e. increase the
When the enzyme concentration is increased, there will be a proportionate
increase in the maximum rate (this effect can be seen by comparing curves A and B).
5. *Some enzymes need coenzymes or cofactors for activity.
Coenzymes are organic non-protein compounds. Many of them are vitamin B complex.
Cofactors are non-organic compounds. Many of them are the essential metals ions human needs, such as
magnesium, iron, zinc etc.