AP Syllabus focus:
‘Competitive inhibitors bind active sites, while noncompetitive inhibitors bind allosteric sites, changing enzyme activity.’
Enzymes speed up cellular reactions, but their rates can be reduced by inhibitors. Competitive and noncompetitive inhibition alter catalysis in distinct ways, helping explain regulation, toxicity, and how many medicines work.
Core idea: inhibitors reduce enzyme activity
What an inhibitor does
Inhibitor: a molecule that decreases enzyme-catalysed reaction rate by interfering with enzyme–substrate binding and/or the catalytic process.
Inhibition changes how readily enzyme–substrate complexes form and how efficiently the enzyme converts substrate to product, without changing the reaction’s overall free-energy change.
Key enzyme regions involved
Practice Questions
FAQ
Often, but not always. Similarity can be:
shape-based (fits the active site)
chemical (mimics charge/polar groups)
Some inhibitors resemble the transition state more than the substrate.
Mixed inhibition occurs when an inhibitor binds allosterically but changes both substrate binding and catalysis. It can lower $V_{max}$ and also change apparent $K_m$, unlike the idealised pure noncompetitive case.
Irreversible inhibitors form covalent or extremely tight bonds to an enzyme site (active or allosteric), permanently reducing functional enzyme until new enzyme is synthesised.
Allosteric binding does not require competing with substrate. Even when substrate fluctuates, inhibitor occupancy can stabilise a less-active enzyme conformation.
They measure initial rates across multiple substrate concentrations and fit kinetic models. Patterns in how $V_{max}$ and apparent $K_m$ shift across conditions indicate likely binding behaviour.
