3.8.1.4. Meaning of K_I

3.8.1.5. Allosteric Enzyme Kinetics

3.8.2. Irreversible Inhibition

• Irreversible inhibitors are bind via covalent linking to the enzyme causing modification of the enzyme and inactivating it.
• Many Enzymes contain -SH, -OH,- or -COOH groups as part of their active sites, any chemical which can react with them acts as an irreversible inhibitor. Heavy metals such as Ag⁺, Hg²⁺, Pb²⁺ have strong affinities for -SH groups.
• Irreversible inhibitors display time-dependent inhibition and their potency therefore cannot be characterised by an IC₅₀ value, because the amount of active enzyme at a given concentration of irreversible inhibitor will be different depending on the time of pre-incubation.
• By plotting the log of % activity vs. time, k_obs/[I] parameter can be derived, wherek_obs is the observed pseudo-first order rate of inactivation and [I] is the concentration of inhibitor.
• The k_obs/[I] parameter is valid as long as the inhibitor does not saturate binding with the enzyme wherein k_obs = k_inact.

• Types of Irreversible Inhibition:
  • Group-specific covalent modifying agents
  • Affinity labels
  • Transition state analogs
  • Suicide inhibitors (mechanism-based inhibitors.