4.3 enzyme inhibitors & cofactors

What are inhibitors?

Inhibitors are molecules that bind to enzymes to reduce their activity.

What are reversible inhibitors?

Reversible inhibitors are those that form weak bonds with the enzyme.

They temporarily prevent that enzyme from catalysing another enzyme-controlled reaction.

What are irreversible inhibitors?

Irreversible inhibitors are those that that form strong bonds with the enzyme.

They permanently prevent that enzyme from catalysing another enzyme-controlled reaction.

What are competitive inhibitors?

Competitive inhibitors are molecules that bind to the active site of the enzyme.

What are non-competitive inhibitors?

Non-competitive inhibitors are molecules that bind to enzymes away from the active site. 

They bind to another site called the allosteric site.

How do competitive inhibitors affect the rate of enzyme-catalysed reactions?

Competitive inhibitors decrease the rate of reaction by preventing the formation of enzyme-substrate complexes. 

How do competitive inhibitors prevent the formation of enzyme-substrate complexes?

Competitive inhibitors have a similar shape to a substrateand bind to the active site of the enzyme.

This prevents the substrate(s) from binding to form enzyme-substrate complexes. 

How can the effect of competitive inhibitors be reduced?

The effect of competitive inhibitors can be reduced by by increasing the substrate concentration.  This means there will be more substrate molecules, making substrates more likely to bind to active sites than inhibitor molecules. 

How do non-competitive inhibitors affect the rate of enzyme-catalysed reactions?

Non-competitive inhibitors decrease the rate of reactionby preventing enzyme-substrate complexes from forming.

How do non-competitive inhibitors prevent the formation of enzyme-substrate complexes? 

Non-competitive inhibitors bind to enzymes away from the active site which changes the tertiary structure of the enzyme, causing the active site to change shape.

This means the active site is no longer complementary to the substrate(s), so the substrate(s) cannot bind to form an enzyme-substrate complex. 

How does increasing substrate concentration affect non-competitive inhibitors?

Increasing substrate concentration has no effect on the rate of reaction because non-competitive inhibitors do not compete with the substrate to bind to the active site. 

What are cofactors?

Cofactors are non-protein substances that bind to enzymes to increase their activity.

For example, Cl^- is a cofactor for the enzyme amylase.

What are coenzymes?

Coenzymes are organic cofactors derived from vitamins.

What are prosthetic groups?

Prosthetic groups are cofactors that are tightly bound to enzymes.

For example, Zn²⁺ is a prosthetic group for the enzyme carbonic anhydrase.