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3.1 Enzymes

Syllabus
2025
Topic
3.1
Level
—

Learning objectives

3.1A—Explain how enzymes affect the rate of biological reactionsExplain how enzymes affect the rate of biological reactions.• The structure and function of enzymes contribute to the regulation of biological processes. Enzymes are proteins that are biological catalysts that facilitate chemical reactions in cells by lowering the activation energy.• For an enzyme-mediated chemical reaction to occur, the shape and charge of the substrate must be compatible with the active site of the enzyme. This is illustrated by the enzymesubstrate complex model.

How Enzymes Speed Biological Reactions

Enzymes are protein catalysts: they increase the rate of biological reactions by providing a reaction pathway with lower activation energy. With a smaller energy barrier, a greater proportion of reacting molecules can reach the transition state under the same cellular conditions.

Catalysis begins when a substrate enters the enzyme's active site. The substrate's shape and charge must be compatible with the active site, allowing an enzyme-substrate complex to form. Interactions in this complex position the substrate so the reaction can proceed more readily; products are then released and the enzyme can catalyze another reaction.

Compatible substrate and active site → enzyme-substrate complex → lower-activation-energy pathway → faster product formation. Because active sites differ in shape and charge, an enzyme catalyzes only a limited set of compatible reactions.

An enzyme does not supply energy to the reaction and is not used up by it. It changes the pathway and therefore the rate; it does not make an incompatible substrate fit its active site.