Posts Tagged ‘Inhibitors’

This quiz is purely on enzymes only.

1. What class of enzymes breaks down substrates by adding or losing, hydrogen and oxygen?

A) Hydrolases

B) Transferases

C) Oxidoreductases



2. Ligases is the only one of the six major classes of enzymes to

A) Undergo hydrolysis

B) Combine molecules

C) Oxidise a substrate

D)Rearrange the structure of a substrate.


3.Uncompetitive Inhibitors binds to

A) Active site only

B) Enzyme-substrate complex only

C) Both the free enzyme and enzyme-substrate complex

D) To the substrate molecule only


4. Mixed Inhibitors binds to either the free enzyme or to the enzyme-substrate complex. How does this affect Vmax and Km?

A) Nothing is affected

B) Vmax increases and Km can increase and decrease

C) Vmax is reduced and Km can increase and decrease

D) Vmax remains constant, while Km increases


5) Which type inhibitor binds only to the active site?

A) Non-competitive

B) Uncompetitive

C) Competitive



Best of luck.

Thank you for you time.


What is an Inhibitor?

An inhibitor is any substance that diminishes the velocity of an enzyme catalysed reaction.

There are two mainĀ categoriesĀ of inhibition- Reversible and Non-reversible. This entry deals with reversible inhibitors only. There are four types of reversible enzymes.

These are:

  • Competitive Inhibtors
  • Non-competitive Inhibitors
  • Uncompetitive Inhibitors
  • Mixed Inhibitors


Competitive inhibitors.

These inhibitors compete with the substrate molecules for the active site of the enzyme. Competitive inhibitors only bind to the active site of the free enzyme, note that it never binds to the enzyme-substrate complex. Competitive inhibitors do not change the Vmax, maximum velocity, of the reaction, however the substrate affinity binding to the active site of the enzyme, decreases. This means Km increases. (Remember high affinity = low Km and vice versa) .

In the case of allosteric enzymes. Both substrate and inhibitor cannot bind to the enzyme at the same time. When the Inhibitor binds to one active site, it temporarily changes the shape of the other active site, preventing the substrate from binding.

Taken from "Describe an Induced fit model" ,

Taken from “Describe an Induced fit model” ,


Non-Competitive Inhibitors.

Unlike competitive inhibitors,which only bind to the active site of the enzyme, non-competitive inhibitors can bind to the free enzyme as well as the enzyme substrate complex. Since the substrate can still bind to the enzyme, this tells us that Km is unchanged, however Vmax is reduced as the enzyme does not function with the inhibitor.


Uncompetitive Inhibitors.

These inhibitors only bind to the enzyme substrate complex. This means that the substrate can bind to enzyme although the inhibitor is already bound to it. This also tells us that both Vmax and Km are reduced.


Mixed Inhibitors.

Mixed inhibitors get the term “mixed” because they can act as competitive inhibitors, and only bind to the active site, or act and uncompetitive inhibitors and bind to the enzyme substrate complex. As a result of this, substrate affinity to the enzyme can either increase or decrease,(i.e. low or high Km values). From uncompetitive inhibitors, Vmax will always be reduced as the presence of the inhibitor decreases the velocity.