How enzymes speed up reactions

The chemical an enzyme converts is its substrate (S), and the result is the product (P): S → P.

The substrate must diffuse to the active site and bind there, so the formation of an enzyme-substrate complex (ES) is obligatory. It lasts only a short time.

While bound, the substrate takes on a new, unstable transition state structure. Once the bonds are broken or made, the product is released from the active site.

On a graph of potential energy against the progress of the reaction, the substrate must climb to the higher energy of the transition state before falling to the product. If P lies lower than S the reaction is exothermic and needs no heating.

Whether the reaction releases energy or needs it, the energy gap between S and the transition state is the activation energy. Enzymes lower this barrier, which makes the change from S to P easier.

The overall scheme is E + S ⇌ ES → EP → E + P, where EP is the enzyme-product complex. Formation of ES is essential for catalysis.

The catalytic cycle: the substrate binds to the active site; binding makes the enzyme change shape and fit more tightly around it (induced fit); the active site breaks the substrate's bonds and an enzyme-product complex forms; the enzyme releases the products and is free to bind another substrate molecule.