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Enzymes
Enzymes are globular proteins, with a specific tertiary structure, which
catalyse metabolic reactions in living organisms. They may be intracellular
or extracellular
Describe, with the aid of diagrams, the mechanism of action of
enzyme molecules, with reference to specificity, active site, lock
and key hypothesis, induced-fit hypothesis, enzyme-substrate
complex, enzyme product complex and lowering of activation
energy:
In a reaction catalysed by an enzyme, the substrate binds to a region of
the enzyme molecule called the active site. This active site is made up of
a number of specific amino acids that give the active site its unique shape
(due to the protein having specific tertiary structure) and properties.
Enzymes are therefore highly specific in the types of reaction that they
catalyse.
To explain this, Fischer proposed the lock and key hypothesis. This theory
suggests that the active site of the enzyme (lock) has a particular shape,
due to its specific tertiary structure and it will therefore only accept a
particular substrate (key).
It now seems probably that the match between enzyme and substrate
isn’t quite so exact. Instead, the shape of the active site is altered slightly
to fit the substrate molecule as it attaches. The change in shape is small,
but it helps to bind the substrate tightly. Moulding the active site around
the substrate could put a strain on bonds making them more likely to
break. This is known as the induced fit theory.
When a substrate binds to the active site of an enzyme, an enzyme
substrate (E/S) complex will be formed. The enzyme will then break the
bonds in the substrate, to form an Enzyme product (E/P) complex.
Enzymes work by decreasing the activation energy of a reaction.
E+S  E/S Complex  E/P Complex  E+P
Describe and explain the effects of pH, temperature, enzyme
concentration and substrate concentration on enzyme activity:
1. pH
Enzymes are severely affected by the concentration of H+ ions in the
solution around them. Many of the bonds holding the enzyme in its 3D
shape depend upon the presence of ionic charges on amino acids at
particular point, e.g. R groups.