Enzyme Cofactors
Active
site
Enzyme is protein only
Example: lysozyme
Some enzymes require cofactors
to be active.
Cofactors are a nonprotein
component of an enzyme.
Cofactors can be:
organic molecules (coenzymes).
Enzyme
Active
site
inorganic ions (e.g. Ca2+, Zn2+).
Cofactors may be:
Permanently attached, in which
case they are called prosthetic
groups.
Temporarily attached coenzymes,
which detach after a reaction, and
may participate with another
enzyme in other reactions.
Enzyme
Prostheti
c group
Enzyme + prosthetic group
Example:
flavoprotein + FAD
Active
site
Coenzym
e
Enzyme
Enzyme + coenzyme
Example:
dehydrogenases + NAD
Enzyme Inhibitors
Enzymes can be deactivated by
enzyme inhibitors.
There are two types of enzyme
inhibitors
Native
arsenic
:
Irreversible inhibitors bind
tightly and permanently to the
enzymes destroying their
catalytic activity. Irreversible
inhibitors usually covalently
modify an enzyme.
Many drug molecules are enzyme
inhibitors
.
Mercury
Photo: US EPA
Reversible inhibitors are used
to control enzyme activity.
There is often an interaction
between the substrate or end
product and the enzymes
controlling the reaction.
Some heavy metals (above) are
examples of poisons which act as
irreversible enzyme inhibitors.
Irreversible Enzyme Inhibitors
Substrate
Some heavy metals, such as
cadmium (Cd), arsenic (As), and lead
(Pb) act as irreversible enzyme
inhibitors.
The lipothiamide
pyrophosphatase
enzyme with substrate
bound to its active site.
Enzyme
They bind strongly to the sulphydryl (SH) groups of the protein, destroying its
catalytic activity.
Most heavy metals, e.g. arsenic, act as
non-competitive inhibitors.
Mercury (Hg) is an exception.
It acts as a competitive inhibitor,
binding directly to a sulphydryl group
in the active site of the papain
enzyme.
Heavy metals are retained in the body,
and lost slowly.
As
Arsenic binds to the
enzyme and causes
its shape to change,
preventing the
substrate from binding
to the active site.
Poisons, such as arsenic (As), act as an
irreversible enzyme inhibitor. It binds to the
lipothiamide pyrophosphatase enzyme altering
its shape so the substrate cannot bind.
Reversible Inhibitors
Reversible inhibitors are used to control enzyme activity.
There is often an interaction between the substrate or end product
and the enzymes controlling the reaction.
Buildup of the end product or a lack of substrate may deactivate the enzyme.
Competitive inhibition involves competition for the active site.
Noncompetitive inhibitors work either to slow down the rate of reaction, or block
the active site altogether and prevent its functioning (allosteric inhibition).
Substrate
Competitive inhibitor
blocks the active
site. The substrate
cannot bind.
S
The substrate can still
bind to the active site
but the rate of reaction
is lowered.
S
Enzyme
S
S
Enzyme
Enzyme
Noncompetitive
inhibitor
No inhibition
The substrate
cannot bind to the
active site because
the active site is
distorted.
Competitive
inhibition
Noncompetitive
inhibition
Enzyme
Noncompetitive
inhibitor
Allosteric enzyme
inhibitor