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enzyme (lysozyme)

  Enzyme classes:
  EC 1: Oxidoreductases
  EC 2: Transferases
  EC 3: Hydrolases
  EC 4: Lyases
  EC 5: Isomerases
  EC 6: Ligases
  General information:
  Catalytic mechanism
  Enzyme kinetics
  Enzymes in industry

Enzyme inhibitors

Inhibitor overview

Enzyme inhibitors are molecules that reduce the catalytic activity of enzymes. Reducing of effective enzymatic activity or complete blocking of enzyme may cause either complete death of cell either modifications in the pathways. This fact is widely used for development of many types of drugs. Drugs with cause complete inactivation of enzymes from essential pathways will cause cell death and therefore such drugs can be used as an antibiotics. Drugs which are reduce the overall activity of non-essential enzymes are usually used for correction of metabolic processes in organism.

All inhibitors can be classified on the basis of reversibility of their action and on the basis of competitiveness of inhibitors and substrates.

Inhibitors classification based on the competitiveness

Competitive inhibitors

The competitive inhibitor and substrate cannot bind to the enzyme at the same time. This does not mean that substrate and inhibitor bind at the same place. These inhibitors can act by two major ways.
  • Substrate like inhibitors. These inhibitors are very similar to the substrate and therefore can easily bind instead of substrate. Usually it is very easy to construct such type of inhibitors by modifying of the substrate structure by modifying of some bonds, or atom configuration or atom itself.
  • Other inhibitors. These inhibitors can bind with enzyme almost anywhere, but only with apo-form of enzyme. Usually these inhibitors either change conformation of enzyme to non-active, either block somehow the access to the active site.
In general these inhibitors do not change the maximal velocity of enzymatic reaction, but the effective concentration of enzyme is decreased.

The overall reaction can be described by following equation:
No products <- EI <- I + E + S -> ES -> E + Products

Uncompetitive inhibitors

This is very rare type of inhibition, usually take place in multimeric enzymes. The inhibitor can only binds to the enzyme-substrate complex which leads to the increasing of formation of enzyme-substrate complexes (reduction of Km) and formation of non-productive enzyme-substrate-inhibitor complex with subsequent decreasing of enzymatic velocity.

Non-competitive inhibitors

In this case substrate and inhibitors bind together at different binding sites with enzyme. Usually the inhibitor binding cause structural changes in active site which inactivate enzyme completely or partially. This effectively will result in keeping the same Km and reducing of Vm.

Inhibitors classification based on reversibility

Reversible inhibitors

Reversible inhibitors bind to enzyme with only non-covalent interactions. Reversible inhibitors do not perform any chemical changes in enzyme or themselves and they are in dynamic equilibrium with solution and can be removed from enzyme completely by reducing inhibitor concentration in solution.

Reversible inhibition is often used as a self-regulated process in living cells, when the substrate or product of some enzymes acts as inhibitors for other enzymes.

Irreversible inhibitors

Irreversible inhibitors binds to enzyme via covalent bonds and prevent enzyme from further performing of catalytic acts. It is necessary to understand, that irreversible inhibitors act on some selected group of enzymes, with no effect on other enzymes and proteins. According to this classification it is impossible to consider chemical or physical agents which inactivate all enzymes and protein as an inhibitors. For example strong detergents or heat which cause partial or full denaturation of all proteins can not be classified as inhibitors.
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