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Why can an enzyme be used repeatedly after it catalyzes a reaction?

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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.

An enzyme can be used repeatedly because it remains chemically unchanged by the reaction it catalyzes.12

The process functions as follows:
* Binding: The enzyme attracts specific substrate molecules to its active site, forming an enzyme-substrate complex.12
* Catalysis: The enzyme facilitates the chemical transformation of the substrate into products. While some enzymes may form a temporary covalent intermediate during this process, they are restored to their original state once the reaction is complete.12
* Dissociation: Once the products are formed, they dissociate from the enzyme’s surface.1

Because the enzyme is not consumed or permanently altered, it is immediately free to bind with new substrate molecules and repeat the cycle. This efficiency allows a very small quantity of an enzyme to catalyze a large number of reactions, with some enzymes capable of converting thousands of substrate molecules per second.12