Alterations in enzyme activity

:
Enzyme inhibition
Molecule that binds to enzyme and interferes with its activity to prevent either:
1) formation of ES complex E + I ---> EI
2) breakdown of ES --> E + P ES + I ---> ESI
Used to regulate metabolism.
Many drugs act by enzyme inhibition.
These molecules can be
1) irreversible - bind to enzymes by covalent means and modify enzyme
2) reversible - noncovalent binding to enzyme
There are three types of reversible inhibition:
1) competitive
Competes with substrate for active site of enzyme.
Both substrate and competitive inhibitor bind to active site.
These inhibitors are often substrate analogs (similar in structure
substrate), but still no product is formed.
Can be overcome by addition of more substrate (overwhelm inhibitor; a
numbers game).
e.g. malonate inhibition of succinate dehydrogenase
succinate ----------------------> fumarate
FAD FADH2
succinate malonate
e.g. AZT inhibition of HIV reverse transcriptase
actual substrate is dTTP (deoxythymidine triphosphate)
Can be represented by the following equation:
E + S ES E + P
+
I
EI
Graphical representation of competitive inhibitors:
affects Km (increases Km --> decreases affinity; need more substrate to
reach half-saturation of enzyme)
Vmax unaffected
5
2) uncompetitive inhibitor
Typically seen in multisubstrate reactions (here, there is a decrease in
product formation because the second substrate cannot bind).
Inhibitor binds to ES, but not enzyme.
E + S ES E + P
+
I
ESI
Graphical representation of uncompetitive inhibitors:
Lineweaver-Burke plot:
both Km and Vmax are lowered, usually the same amount
ratio Km/Vmax unchanged --> no change in slope
3) pure noncompetitive inhibitor
Can bind to enzyme and ES complex equally.
Does not bind to same site as substrate and is not a substrate analog.
Cannot be overcome by increases in [substrate].
e.g. lead, mercury, silver, heavy metals
Lineweaver-Burke plot:
No effect on Km, because those enzyme molecules unaffected have
normal affinity.
Vmax is lowered.