Enzyme Inhibition (HL) (HL IB Biology)

Enzyme inhibitors

• Inhibitors are chemical substances that can bind to an enzyme and reduce its activity
• Inhibitors can be formed from within the cell or can be introduced from the external environment
• An enzyme's activity can be reduced or stopped, temporarily, by an inhibitor
• There are two types of inhibitors: competitive and non-competitive

Allosteric sites and non-competitive inhibitors

• Non-competitive inhibitors bind to the enzyme at an alternative site which is not the active site
  
  • These sites are called allosteric sites and they are usually located quite far from the active site
  • Only specific substances (called effectors) can bind to an allosteric site
  • Binding to the allosteric site is reversible
• Binding to the allosteric site causes interactions within an enzyme which leads to conformational changes
  • These conformational changes will alter the shape of the active site
• This therefore prevents the substrate from binding to the active site
  • This will apply for as long as the effector is bound to the allosteric site

Non-competitive inhibition diagram

Non-competitive inhibitors bind to the allosteric site of an enzyme to alter the active site

Competitive inhibitors

• Competitive inhibitors have a similar shape to that of the substrate molecules
• They bind to the active site of the enzyme, interfering with it and competing with the substrate for the active site
• The substrate, therefore, cannot bind to the active site if a competitive inhibitor is already bound

Statins as an example of competitive inhibition

• Statins are drugs that are prescribed to lower the cholesterol levels of patients with high cholesterol levels in their blood
  • They bind to the active site of the enzyme needed to synthesise cholesterol
• Binding to the active site is possible because statins have a shape that is similar to the substrate of this enzyme
  • It therefore blocks access to the active site and the substrate is unable to bind
• The enzyme cannot catalyse the reaction that synthesises cholesterol, leading to cholesterol levels decreasing in the blood
  • Note that the cholesterol that is being referred to here is called low-density lipoproteins (LDLs), more commonly known as "bad cholesterol"
  • High LDL levels have been linked to atherosclerosis and may increase the risk of developing coronary heart disease

Competitive inhibition diagram

Competitive inhibition is a consequence of the reversible binding of an inhibitor to the active site

Identifying types of inhibition

• The effect of competitive and non-competitive inhibitors on enzyme controlled reactions can be represented graphically
• Both types of inhibitors slow down or stop enzyme activity, decreasing the rate of reaction
• Increasing the concentration of an inhibitor reduces the rate of reaction and eventually, if inhibitor concentration continues to be increased, the reaction will stop completely
  • For competitive inhibitors countering the increase in inhibitor concentration, by increasing the substrate concentration, can increase the rate of reaction but the substrate needs to reach a high enough concentration in order to displace the inhibitor (more substrate molecules mean they are more likely to collide with enzymes and form enzyme-substrate complexes)
  • For non-competitive inhibitors increasing the substrate concentration cannot increase the rate of reaction, as the shape of the active site of the enzyme remains changed and enzyme-substrate complexes are still unable to form
• A graph can be used to distinguish between the two different types of inhibitors and their effect on the rate of reaction
• The patterns shown are notably different for each type of inhibitor and also for an uninhibited enzyme

Rate of reaction of enzyme-catalysed reaction with inhibitors present diagram

Graph showing different types of inhibitors and their effect on rate of reaction

• A competitive inhibitor will lower the initial rate of reaction (by occupying some of the available active sites), whilst the maximal rate is not affected
  • Eventually, the same amount of product will be produced as would have been produced without the competitive inhibitor
• Non-competitive inhibitors lower the initial rate of reaction and the maximal rate of reaction
  • A lower amount of product is produced than would normally be produced

Comparing Competitive and Non-competitive Inhibitors Table

• End-product inhibition occurs when the end product from a reaction is present in excess and itself acts as a non-competitive inhibitor of the enzyme
• The end product binds to an allosteric site on the enzyme and causes inhibition of the pathway, so they are referred to as allosteric inhibitors
• Allosteric inhibitors are important to prevent the build-up of intermediate products in a metabolic pathway, as each small step of the pathway may produce a new product
• The product therefore does not accumulate and the pathway can continue
  • An outline of the process is as follows:
    • As the enzyme converts substrate to an end product, the process is itself slowed down as the end-product of the reaction chain binds to an allosteric site on the original enzyme, changing the shape of the active site and preventing the formation of further enzyme-substrate complexes
    • The inhibition of the enzyme means that product levels fall, at which point the enzyme begins catalysing the reaction once again; this is a continuous feedback loop
      • The end-product inhibitor eventually detaches from the enzyme to be used elsewhere; this is what allows the active site to reform and the enzyme to return to an active state

End-product inhibition diagram

End-product inhibition where the end-product of an enzyme controlled pathway inhibits the starting enzyme and limits the reactions

An example of end-product inhibition

• The amino acid isoleucine can be synthesised from threonine in bacteria
• Isoleucine can bind to the allosteric site of the enzyme threonine deaminase
  • Threonine deaminase catalyses the first stage of the metabolic pathway that produces isoleucine
  • If the enzyme is inhibited, then the production of isoleucine stops
• At the start of the process, isoleucine levels are low so the metabolic pathway can proceed without being inhibited too much
• However, as the concentration of isoleucine increases, it begins to regulate the metabolic pathway by acting as a non-competitive inhibitor
• Isoleucine is an essential amino acid, so as it is used by cells for protein synthesis, its concentration decreases which decreases the number of allosteric sites occupied
• More enzymes are free to bind to threonine, and the production of isoleucine can continue

Example of end-product inhibition diagram

Example of end-product inhibition between threonine and isoleucine

• Molecules that are able to form covalent bonds with the active site of an enzyme are known as a substrate analogue
• The substrate analogue can now be changed by the enzyme to produce a reactive group
  • This reactive group leads to the formation of a stable inhibitor-enzyme complex
• This form of inhibition is called mechanism-based inhibition and it is irreversible

Penicillin as an example of mechanism-based inhibition

• Penicillin is an antibiotic that is very effective at killing bacteria
• Bacterial cell walls are composed of peptidoglycans (long molecules of peptides and sugars)
• These peptidoglycan molecules are held together by cross-links that form between them
• When a new bacterial cell is growing, it secretes enzymes known as autolysins that create small holes in the bacterial cell wall
• These holes allow the bacterial cell wall to stretch, with new peptidoglycan molecules then joining up via the cross-links described above
• Penicillin stops these cross-links forming by inhibiting the enzymes (DD-transpeptidase) that catalyse their formation
  • This happens because penicillin has a similar structure to parts of the growing peptide chain of the cell wall
  • DD-transpeptidase will bind to penicillin and modify it to form a stable enzyme-penicillin complex
  • This will permanently block the enzyme from creating more cross-links
• However, the autolysins keep creating holes in the bacterial cell wall, making the walls weaker and weaker
• As bacteria live in watery environments and take up water by osmosis, their weakened cell walls eventually burst as they can no longer withstand the pressure exerted on them from within the cell
  • This is known as death by lysis
• This means penicillin is only effective against bacteria that are still growing (autolysins no longer create holes and no more cross-links between peptidoglycan molecules are formed once the growth of a bacterium is complete, as the bacterial cell wall no longer needs to expand)

The affect of penicillin on bacteria diagram

Penicillin works by irreversibly inhibiting transpeptidase and thereby prevents the formation of cross-links between peptidoglycan molecules

• Bacteria however, may develop resistance to penicillin
• DNA mutations in the bacterial genome may lead to changes in the shape of the active site of DD-transpeptidase, which will make it difficult for penicillin to bind and inhibit the enzyme
• Bacteria are able to share these mutations that promote antibiotic resistance quickly if it occurred in plasmid DNA 
  • This is because bacteria are able to transfer plasmids to other bacteria by the process of conjugation
  • These antibiotic resistant mutations could even be shared between different species of bacteria