Control of enzyme activity

Question 1

What are inhibitors?

Answer 1

Enzyme inhibitors block key enzymes for the biology of the particular organism
Can be used to study enzyme mechanisms
Important as they can regulate metabolism, make drugs, poisons and toxins, study enzyme mechanism and metabolic pathways

Question 2

What are the two main classes of inhibitors?

Answer 2

The two main classes of inhibitors are irreversible inhibitors which bind covalently to the enzyme (reduce enzyme concentration) and reversible inhibitors which are not covalently bound to the enzyme (don’t covalently react, change kinetics of enzymes)

Question 3

What are reverse inhibitors

Answer 3

Reversible inhibitors can be either competitive or non-competitive, which in turn can be pure or mixed. Competing for binding to active site where substrate bind to into. Non competitive inhibitors bind elsewhere (not on active site), bind to the enzyme but can subsequently be released (leaving the enzyme in its original condition - not effectively changing the enzyme concentration)

Question 4

What are irreversible inhibitors?

Answer 4

Binds to enzyme and permanently inactivate a pool of enzyme. Inhibitor reacts with specific amino acid side chain, usually in the active site, and forms a covalent bond

Question 5

What is competitive inhibition?

Answer 5

Inhibitor competes directly with the substrate, for the active site
Mutually exclusive possibilities : active site can bind to substrate (Ks) or bind to inhibitor (Ki)
A competitor inhibitor will not change Vmax of the enzyme as if we have infinite [S] we can outcompete the inhibitor

Question 6

How are kinetic parameters affected with competitive inhibition?

Answer 6

Infinite substrate concentration will outcompete the inhibitor
More substrate is needed to get to V= Vmax/2 as substrate and inhibitor are still competing which increases Km

Question 7

What is non-competitive inhibition?

Answer 7

Inhibitor binds at a different place than the substrate
Means enzyme can bind to substrate or to the inhibitor or to both

Question 8

What is pure non-competitive inhibition?

Answer 8

In pure non-competitive inhibition the binding of inhibitor has no effect on the binding of substrate (not changing how well substrate can bind but changes how well enzyme can catalyse reaction)
Binding changes the structure of the active site such that substrate still binds, but transition state stabilisation is no longer optimal -> Vmax decrease (reduces enzymes ability to catalyse reaction); Km stays the same

Question 9

What is mixed inhibition ?

Answer 9

Doesn’t stop substrate binding completely but change conformation of active site reside so substrate doesn’t bind as well, Vmax and Km changes

Question 10

What are some modes of regulation?

Answer 10

Finer control = turning enzyme on and off
A sensible strategy is to avoid making unnecessary metabolic intermediates -> feedback and feed forward regulation
Changes in enzyme structure that affects enzymes ability to do its job