Inhibitor Calculators

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Enzyme inhibitors are molecules that reduce or eliminate enzyme activity by binding to the enzyme or enzyme-substrate complex. Inhibition is classified by mechanism: competitive inhibitors compete with substrate for the active site; noncompetitive inhibitors bind at a separate allosteric site; uncompetitive inhibitors bind only to the enzyme-substrate complex. Each mechanism has distinct effects on the Michaelis-Menten kinetic parameters Km and Vmax. Understanding inhibitor type guides drug development, toxicology, and metabolic regulation analysis.

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Types of Enzyme Inhibition

  • Competitive: Inhibitor binds reversibly to the active site. Apparent Km increases; Vmax unchanged. Overcome by excess substrate. Example: statins inhibiting HMG-CoA reductase.
  • Noncompetitive: Inhibitor binds equally to E and ES at an allosteric site. Vmax decreases; Km unchanged. Cannot be overcome by substrate. Example: cyanide inhibiting cytochrome c oxidase.
  • Uncompetitive: Inhibitor binds only to ES complex. Both Vmax and Km decrease proportionally. Rare in single-substrate enzymes; more common in multi-substrate reactions. Example: lithium inhibiting inositol monophosphatase.
  • Mixed inhibition: Inhibitor binds both E and ES but with different affinities. Vmax decreases; Km may increase or decrease depending on relative binding affinities.

Lineweaver-Burk Plots

The double-reciprocal (Lineweaver-Burk) plot of 1/v vs. 1/[S] linearizes the Michaelis-Menten equation. Inhibition type is identified by how lines shift: competitive inhibitors rotate the line around the y-intercept (same Vmax, different Km); noncompetitive inhibitors raise the y-intercept (lower Vmax, same x-intercept/Km); uncompetitive inhibitors shift the line parallel (same slope = Km/Vmax).

IC50 vs. Ki

IC50 is the inhibitor concentration causing 50% reduction in activity under specific assay conditions — it depends on substrate concentration. Ki is the inhibitor dissociation constant — an intrinsic property of the inhibitor-enzyme interaction, independent of assay conditions. The Cheng-Prusoff equation converts IC50 to Ki: Ki = IC50 / (1 + [S]/Km) for competitive inhibitors.

Glossary

Competitive Inhibitor
An enzyme inhibitor that competes with substrate for the active site; increases apparent Km without changing Vmax; inhibition is overcome by excess substrate.
Ki (Inhibitor Constant)
The dissociation constant of an inhibitor from its enzyme complex; a measure of inhibitor binding affinity independent of substrate concentration; lower Ki means tighter binding.
Lineweaver-Burk Plot
A double-reciprocal plot of 1/v vs. 1/[S] used to linearize Michaelis-Menten kinetics; the pattern of line shifts with inhibitor present identifies the type of inhibition.

Frequently Asked Questions

Competitive inhibition: the inhibitor competes with substrate for the active site. Adding more substrate can displace the inhibitor — Vmax is unchanged, but apparent Km increases. Noncompetitive inhibition: the inhibitor binds a separate allosteric site and reduces enzyme activity regardless of substrate concentration. Vmax decreases; Km is unchanged. Adding more substrate cannot overcome noncompetitive inhibition.

Competitive inhibition: increases apparent Km (lower affinity for substrate), Vmax unchanged. Noncompetitive inhibition: decreases Vmax, Km unchanged. Uncompetitive inhibition: decreases both Km and Vmax proportionally. Mixed inhibition: decreases Vmax; Km may increase or decrease. These distinctive patterns are diagnostic and can be identified from Lineweaver-Burk double-reciprocal plots.

IC50 is the inhibitor concentration causing 50% inhibition under specific experimental conditions (substrate concentration, pH, temperature). It depends on the assay conditions and is not a fundamental property. Ki is the inhibitor's dissociation constant from the enzyme — an intrinsic binding affinity independent of substrate concentration. For competitive inhibitors, Ki = IC50 / (1 + [S]/Km) by the Cheng-Prusoff equation. Ki is preferred for comparing inhibitor potencies.

Irreversible inhibitors form covalent bonds with the enzyme's active site, permanently inactivating it. Recovery requires new enzyme synthesis, not just removal of inhibitor. Examples: organophosphates (nerve agents, pesticides) covalently inhibiting acetylcholinesterase; aspirin acetylating cyclooxygenase (COX); penicillin acylating bacterial transpeptidase. Irreversible inhibitors are described by kinact (inactivation rate constant) and KI (concentration for half-maximal inactivation rate) rather than Ki.