What determines substrate specificity in enzymes?

• A. Three-dimensional shape determines specificity
• B. Substrates bind randomly
• C. Enzymes are unaffected by pH
• D. Substrates always bind irreversibly

Answer: (A)

Substrate specificity comes from the active site's unique three‑dimensional shape that matches the substrate. The pocket’s geometry, along with the chemical environment of amino acids inside the site, creates precise complementary interactions—hydrogen bonds, ionic interactions, and van der Waals contacts—that orient the substrate correctly and stabilize the transition state. This shape-and-fit (often with an induced-fit adjustment) ensures the enzyme binds the right molecule and carries out the reaction efficiently.

That’s why substrates don’t bind randomly; only substrates with the proper geometry and chemistry fit well enough to be positioned for catalysis. Enzymes are also sensitive to conditions like pH, which can alter charge and binding, and most normal enzyme–substrate interactions are reversible rather than irreversible.