(a) What
is meant by cofactor?
(b) Give an
account of three different types of cofactors and their functions.
Answer
a. A cofactor is a non-protein substance which is necessary to
the function of an enzyme. Cofactor may be organic or non-organic.
b.
There are three types of cofactor, which are prosthetic
groups, coenzymes and enzyme activators.
A prosthetic group
is a non-protein organic molecule
that binds tightly on a permanent basis to the protein part of
the enzyme. The prosthetic group is involves in the catalytic function of the
enzyme. For example, haem is found in the prosthetic group of cytochromes which
are electron carriers and it takes part in oxidation – reduction reactions.
Haem is also present in the enzyme catalase which catalyses hydrogen peroxide
into oxygen and water.
On the other hand, coenzymes
are small, non-protein organic molecules
which bind loosely and temporary to the active site of the
enzyme. The coenzyme helps to transfer chemical groups, atoms or electrons from
one enzyme to another. NAD (nicotinamide adenine dinucleotide), for example,
which is derived from niacin (vitamin), is a coenzyme for a number of
dehydrogenase enzymes and act as a hydrogen acceptor.
Enzyme activators
are inorganic ions such as Ca2+,
Zn2+, Fe2+ and Cl-. They may attach temporarily to the enzyme and change
its active site to make the shape more suitable for a reaction to take place.
The ion may also bind the enzyme and substrate together. For example, calcium
ions are needed to activate thrombokinase which converts the prothrombin to
thrombin in blood clotting.
Assignment submitted by group 1 2011/2012 (IBM)
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