Lock-and-key versus induced fit models of specificity
Enzyme specificity starts at the active site, a pocket on the enzyme's surface shaped to accommodate the substrate. The lock-and-key model, proposed by Emil Fischer, imagines the active site as a rigid cavity that matches only the correct substrate, like a lock accepting only its key. This model explains why enzymes are selective: wrong substrates do not fit.
The induced-fit model, proposed by Daniel Koshland, refines this: the active site is somewhat flexible. When the correct substrate binds, the enzyme changes shape to optimize binding and catalysis. This conformational change can position catalytic residues, exclude water, or stabilize a transition state. The model explains why enzyme specificity is even higher than the lock-and-key model predicts and why some enzymes can catalyze related reactions with different substrates depending on conditions.
Specificity through binding and catalysis
Enzyme specificity works at two levels. Binding specificity: only the correct substrate binds tightly to the active site. Catalytic specificity: once bound, the enzyme accelerates the reaction, often by a factor of 10^12 or more. The enzyme does this by stabilizing the transition state (the highest-energy intermediate along the reaction pathway), lowering the energy barrier that molecules must overcome.
This dual specificity makes enzymes extraordinarily selective. Amylase cleaves alpha-1,4-glycosidic bonds in starch but not alpha-1,6 bonds (different enzyme). Proteases cleave peptide bonds next to certain amino acids but not others. The specificity is encoded in the shape, charge distribution, and hydrophobic properties of the active site and surrounding residues.