Nature of Enzyme Action

An enzyme does its work by physically holding on to the molecule it acts upon and then rearranging its chemistry. Every enzyme (written as E) carries a specific pocket on its surface where its substrate (written as S) can settle. When the two come together, a very reactive, short-lived partnership called the enzyme-substrate complex (written as ES) is formed. This joining is not an optional extra; the formation of the ES complex is the essential first event without which catalysis cannot happen.

Once bound, the substrate does not simply sit unchanged. It is pushed through a high-energy, unstable arrangement of its atoms known as the transition state, and from there it is converted into the reaction's product or products. During this changeover the system briefly exists as an enzyme-product complex (written as EP) before the product finally comes away. The overall course can be pictured as the enzyme and substrate combining into ES, ES passing through to EP, and EP finally separating into the free enzyme and the product P:

E + S ⇌ ES → EP → E + P

The catalytic cycle, step by step

• Binding. The substrate first docks into the active site of the enzyme, fitting neatly into that specially shaped region.
• Induced fit and transition state. The arrival of the substrate makes the enzyme adjust its own shape so that it wraps more snugly around the substrate. This close, strained embrace corresponds to the transition state, in which the substrate is being distorted towards its product form.
• Bond breaking and product formation. Now held tightly and very close to the reactive groups of the enzyme, the substrate has its chemical bonds broken (or made), and the new enzyme-product complex is created. …