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Irreversible Enzyme Inhibition

For medical students2 min readUpdated 2026-10-10

Irreversible inhibition is a process in which an inhibitor forms stable covalent bonds with an enzyme. As a result, a strong complex is formed, and the catalytic activity of the protein is permanently lost.

Type of bondStrong covalent bond between the inhibitor and the enzyme
EquationE + I → E-I (where E is the enzyme and I is the inhibitor)
SpecificityCan target exclusively the active site or any region of the molecule

General Characteristics of the Process

Unlike reversible processes, in irreversible inhibition, the inhibitor molecule binds permanently to the enzyme. A stable complex is formed, described by the simple equation:

`E + I → E-I`

The main feature of this interaction is that the catalytic activity of the enzyme is not restored. The protein is permanently inactivated. Depending on where the blocking substance attaches, irreversible inhibitors are divided into two types: specific and nonspecific.

Specific Irreversible Inhibitors

These substances act with maximum precision: they block strictly defined functional groups located exclusively within the active site of the enzyme.

A classic example of such a substance is diisopropylfluorophosphate (DFP). It belongs to the group of specific inhibitors of so-called "serine" enzymes, a prominent representative of which is chymotrypsin.

Mechanism of action of DFP:

  1. The inhibitor penetrates the active site of the enzyme.
  2. It locates the hydroxyl group (-OH) belonging to a serine amino acid (this specific residue takes direct part in catalysis).
  3. It forms a strong covalent bond with it, releasing hydrogen fluoride.

The reaction scheme is as follows: > Enzyme-CH₂-OH + DFP → Enzyme-DFP + HF

As a result, inactive diisopropylphosphoryl-chymotrypsin is formed. The catalytic activity of the enzyme drops to zero because the key participant in the reaction—serine—is reliably blocked.

Nonspecific Irreversible Inhibitors

Unlike specific agents, nonspecific inhibitors are not restricted to the active site. They form covalent bonds with specific chemical groups anywhere in the protein molecule.

A typical representative of this group is iodoacetate. Its targets are sulfhydryl groups (-SH) belonging to cysteine residues.

Mechanism of action of iodoacetate:

  1. The reagent attacks any accessible protein SH-groups, regardless of whether they are located in the active site or on the periphery of the molecule.
  2. Covalent modification (S-alkylation) occurs, accompanied by the release of hydrogen iodide.
  3. Massive alteration of the structure of side chains leads to disruption of the conformation of the entire enzyme molecule.

The reaction scheme: > Enzyme-CH₂-SH + Iodoacetate → Acetylated enzyme (S-linked) + HI

Changes in the spatial structure of the molecule inevitably lead to deformation of the active site. Consequently, substrate affinity may decrease, and the ultimate result is a marked reduction in the catalytic activity of the enzyme.

Mnemonic

Specific DFP strikes right on target (the serine OH-group in the active site), whereas nonspecific iodoacetate "fires" at all available cysteine SH-groups in the molecule.

Frequently asked questions

What well-known medications act via irreversible inhibition?

Well-known medications that act via irreversible inhibition include aspirin and nialamide.

  • Aspirin — irreversibly inhibits the enzyme cyclooxygenase by acetylating a serine residue in its active site, which blocks the synthesis of prostaglandins and thromboxanes.
  • Nialamide — causes irreversible inhibition of monoamine oxidase by forming stable covalent bonds with the functional groups of the enzyme's active site.
Which group of poisons does diisopropylfluorophosphate belong to, and what symptoms does its poisoning cause?

Diisopropylfluorophosphate belongs to the group of organophosphate inhibitors and is classified as a specific inhibitor of "serine" enzymes. In poisoning, this substance forms covalent bonds with acetylcholinesterase, irreversibly blocking its activity. This leads to an accumulation of acetylcholine in the synaptic cleft because it is no longer degraded. Physiologically, this excess neurotransmitter causes hypercontraction of smooth muscle, manifesting as spasm.

Which heavy metals can act as nonspecific irreversible inhibitors?

Heavy metal cations such as mercury, lead, and copper can act as nonspecific irreversible inhibitors. They inactivate virtually any enzyme, causing protein denaturation. Ions of these metals (specifically lead and mercury) interact with the sulfhydryl groups of enzymes, forming stable compounds that lead to chemical modification and loss of catalytic activity, ultimately blocking various enzyme systems in the body.

Why cannot irreversible inhibition be reversed?

Because a strong covalent bond forms between the inhibitor and the functional groups of the enzyme, which cannot be broken under normal physiological conditions.

How does a specific inhibitor differ from a nonspecific one?

A specific inhibitor binds strictly to defined groups within the enzyme's active site. A nonspecific inhibitor interacts with appropriate groups (e.g., SH-groups) across the entire surface of the protein molecule.

Which enzyme is inhibited by diisopropylfluorophosphate (DFP)?

DFP inhibits "serine" enzymes, such as chymotrypsin. It binds to the serine OH-group in the active site.

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