3.1.1—Enzymes as globular proteins

An enzyme is a globular protein that acts as a biological catalyst. It speeds up a chemical reaction by providing a pathway with lower activation energy, without being used up or permanently changed.

• Substrate binding: A substrate binds temporarily at the enzyme’s specific active site, forming an enzyme–substrate complex. The folded protein gives the active site its shape and chemical environment.
• Catalysis: Interactions at the active site help the reaction reach its activated state more readily, so products form faster.
• After the reaction: Products leave and the enzyme remains available to catalyse another reaction. Lowering activation energy changes the reaction rate; it does not change the overall energy difference or the position of equilibrium.
• Where the job occurs: Intracellular enzymes are made and act inside cells. Extracellular enzymes are secreted and act outside cells, for example during digestion of large food molecules.

Catalase is an intracellular example that breaks down hydrogen peroxide, a harmful by-product of cellular reactions. Amylase is an extracellular digestive example that breaks starch into smaller sugars. The examples illustrate location and role; they do not mean an enzyme is consumed by the reaction.

“Intracellular” and “extracellular” describe where an enzyme acts, not whether an organism is single-celled or multicellular. Enzymes are proteins, so conditions that disrupt protein structure can impair activity, but the catalytic reaction itself does not use the enzyme up.