Enzyme inhibition and types of enzyme inhibitors

What is enzyme inhibition?

• Enzyme inhibitors are the substance which when binds to the enzyme reversibly or irreversibly, decreases the activity of enzyme and the process is known as enzyme inhibition.
• Enzyme inhibitors are used to gain information about the shape of active site of enzyme and amino acids residues in active site.
• They are used to gain information about regulation or control of metabolic pathway.
• They can be used for drug designing.
• They are important for correcting metabolic imbalance.
• They are used for designing herbicides, pesticides and for killing pathogen.

Types of enzyme inhibitors:

I. On the basis of specificity:

II. On the basis of origin:

1. Natural enzyme inhibitor:
   • E.g. Alfatoxin, – amanitin
2. Artificial enzyme inhibitor (synthetic):
   • E.g. drugs

III. On the basis of whether the inhibition is reversible or irreversible

1. Reversible inhibition:

• The enzyme inhibition in which the enzymatic activity can be regained after removal of inhibitors.
• Types of reversible inhibition:
• i). Competitive inhibition
  • Competitive inhibitors are substrate analog that bind to substrate binding site of enzyme i.e. active site so competition occurs between inhibitor and substrate for binding to enzyme.
  • This type of inhibitor is overcome by increasing the concentration of substrate.
  • The kinetics of reaction is V_max remains same and K_m increases.
  • In this reaction, initially inhibitor binds to enzyme but with increase in concentration of substrate causes release of inhibitor.
  • Then, substrate bind enzymes so that the V_max remains same while K_m increases.
  • Example:
    • Succinate dehydrogenase convert succinate to fumarate.
      Succinate —succinate dehydrogenase————–> Fumarate + NADH +H⁺
    • Malate is competitive inhibitor of succinate due to structural analogy.
    • Malate + NAD⁺ —–succinate dehydrogenase———> Oxaloacetate
    • Sulphonamide is competitive inhibitor of PABA during tetrahydrofolate synthesis.
  • Example:
    • Treatment of methanol poisoning:
    • Methanol —–alcohol dehydrogenase————-> Formaldehyde (toxic)
    • Ethanol ——alcohol dehydrogenase———> Acetaldehyde
• ii). Non-competitive inhibition:
  • In this inhibition, there is no competition between substrate and inhibitor because the inhibitor binds to enzyme other than substrate binding site.
  • Since the binding site of substrate and inhibitor to enzyme is different, inhibitor don’t affect the affinity of enzyme to substrate.
  • In this case, the inhibition cannot be overcome by increasing substrate concentration.
  • The kinetic reaction is V_max decreases and K_m remains same. This means that substrate concentration has no effect on inhibition.
  • Binding of substrate and inhibitor are equal.
  • The inhibitor changes the conformation of enzyme after binding so that substrate cannot bind to enzyme.
  • This results in decrease of V_max.
  • Example:
    • Heavy metal poisoning. Hg, Pb etc. distort the -SH group containing enzyme at allosteric site.
    • Deoxycycline is non-competitive inhibitor of proteinase enzyme of bacteria.
    • The non-competitive inhibitor can be removed by pH treatment or by hydrolysis.
    • In case of metal poisoning, chelator is used.
• iii). Uncompetitive inhibitor:
  • This type of inhibition is seen in multi-substrate reaction.
  • It is rare type of inhibition.
  • The process of inhibition is same as non-competitive but it only binds to ES-complex.
  • At first substrate binds to enzyme to form ES-complex.
  • After binding of substrate to active site of enzyme, the binding site for inhibitor forms at allosteric site so that inhibitor bind.
  • The binding of inhibitor distorts the active as well as allosteric site of enzyme, inhibiting catalysis.
  • In this inhibition, V_max as well as K_m both decreases.
  • Examples:
    • Inhibition of lactate dehydrogenase by oxalate.
    • Inhibition of alkaline phosphatase by L-phenylalanine.
• iv. Mixed inhibition:
  • This type of inhibition is commonly seen in multi-substrate reaction.
  • It is the combination of competitive as well as non-competitive inhibition.
  • The mixed inhibitor can bind to both active site and allosteric site.
  • The kinetics of reaction is V_max decreases and K_m increases.
  • The V_max decreases because inhibitor non-competitively bind to allosteric site and distort enzyme.
  • Similarly, K_m increases because inhibitor can also bind to active site competiting with substrate.
  • This type of inhibition cannot be removed by increasing substrate concentration.
  • Examples:
    • Ketoconazole is mixed inhibitor bind to 5–α reductase enzyme.
    • Pallidium ion is mixed inhibitor of oxidoreductase enzyme.

2. Irreversible inhibition:

• In this type of inhibition, inhibitor bind to functional group of active sites by strong bond such as covalent bond and permanently destroy the catalytic property of enzyme.
• The functional group of active sites are -OH, -SH, -NH₂, etc.
• The irreversible inhibitor is non-specific and cause dead end of enzyme activity.
• The inhibitor can bind free enzyme and ES complex and destroy it permanently.
• Examples:
  • Iodoacetamide (CH₂ICOONH₂) bind with -SH group of enzymes permanently.
  • Enzyme-SH + CH₂ICOONH₂ —-> Enzyme-SCH₂COONH₂ + HI

Enzyme inhibition and types of enzyme inhibitors