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Why do enzymes not change the equilibrium point of a chemical 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.

Enzymes do not change the equilibrium point of a chemical reaction because they function strictly as catalysts that accelerate the rate at which a reaction reaches that equilibrium, rather than altering the thermodynamic properties of the reaction itself.1

In any reversible chemical reaction, a point of chemical equilibrium is eventually reached where the rate of the forward reaction equals the rate of the reverse reaction. Because enzymes are active for both forward and reverse reactions, they increase the speed at which this balance is achieved without shifting the final ratio of reactants to products.12

Enzymes work by lowering the activation energy required for a reaction to proceed, allowing molecules to form an unstable intermediate complex more easily. Once the reaction reaches equilibrium, the net rate of change becomes zero, and the enzyme remains unchanged, ready to catalyze further reactions. Therefore, while an enzyme significantly reduces the time required to reach the equilibrium state, it has no influence on the equilibrium constant defined by the thermodynamics of the system.31