An enzyme has several cavities on its outer surface. These cavities, which are the enzyme’s active sites, contain functional groups such as -COOH and -SH. The substrate, which has a complementary charge to the enzyme, fits into these cavities just like a key fits into a lock. When the substrate binds to the enzyme at its active site, the interaction forms an enzyme-substrate complex. The active groups within the active site facilitate the decomposition of this complex, leading to the formation of the product.

This process occurs in two steps:

1. Combination of enzyme and reactant - The enzyme (E) binds with the reactant or substrate (R) to form an enzyme-substrate complex (ER).
2. Disintegration of the complex to yield the product - The enzyme-substrate complex (ER) breaks down to release the product (P) and regenerate the enzyme (E). In a catalysis reaction, the enzyme acts as a catalyst by forming a temporary complex with the substrate, which then decomposes to produce the final product, allowing the enzyme to be reused in subsequent reactions.