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Why do most enzymes act on only certain substrates?

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A multi-panel diagram illustrates how a substrate binds to an enzyme's active site through induced fit, leading to product formation, and shows how non-substrates and allosteric regulators can alter enzyme activity by binding to different sites on the enzyme. — Enzyme In the induced-fit theory of enzyme-substrate binding, a substrate approaches the surface of an enzyme (step 1 in box A, B, C) and causes a change in the enzyme shape that results in the correct alignment of the catalytic groups (triangles A and B; circles C and D represent substrate-binding groups on the enzyme that are essential for catalytic activity). The catalytic groups react with the substrate to form products (step 2). The products then separate from the enzyme, freeing it to repeat the sequence (step 3). Boxes D and E represent examples of molecules that are too large or too small for proper catalytic alignment. Boxes F and G demonstrate binding of an inhibitor molecule (I and I′) to an allosteric site, thereby preventing interaction of the enzyme with the substrate. Box H illustrates binding of an allosteric activator (X), a nonsubstrate molecule capable of reacting with the enzyme.

Enzymes act on specific substrates due to the unique structure and chemical properties of their active site, a specialized groove or pocket formed by the enzyme's amino acid folding pattern. This specificity is governed by two primary models:12

Key-Lock Hypothesis: Proposed by Emil Fischer, this model suggests that the substrate must physically fit into the preformed shape of the active site, much like a key fits into a lock.32
Induced-Fit Theory: This widely accepted model proposes that the substrate induces a conformational change in the enzyme upon binding. This change ensures the proper alignment of catalytic groups necessary for the reaction to proceed.3

Specificity is maintained because molecules that are too bulky, lack essential attractive forces, or possess incorrect charges cannot properly bind or align within the active site. The attraction between the enzyme and substrate is facilitated by physical and chemical forces, including electrostatic bonds and hydrophobic interactions. If these conditions are not met, the enzyme cannot catalyze the reaction, ensuring that enzymes regulate only specific chemical reactions within the cell.321