Topic 3.1 Enzyme Structure: describe the structure of enzymes, the role of the active site, and how the structure of an enzyme determines its specificity for a substrate.

What this topic is asking

The College Board (Topic 3.1) wants you to describe the structure of enzymes, identify the active site, and explain how an enzyme's three-dimensional structure determines its specificity for a particular substrate. This is the foundation for everything in Unit 3: the rest of the unit (catalysis, environmental effects, photosynthesis and respiration) depends on enzymes working.

Enzymes are protein catalysts

Because an enzyme is a protein, its function depends on the four levels of protein structure. The amino-acid sequence (primary structure) folds into a precise three-dimensional shape (tertiary, and sometimes quaternary, structure), and that shape is what makes the enzyme work. Lose the shape, lose the function, which is exactly why temperature and pH matter so much (Topic 3.3).

The active site

The active site is usually only a few amino acids, but those amino acids may come from distant parts of the chain that the folding brings together. When the substrate binds, the enzyme and substrate form a temporary enzyme-substrate complex.

Induced fit, not lock and key

An older model described the active site as a rigid lock and the substrate as a key. The accepted induced-fit model is more accurate: the active site is somewhat flexible, and when the correct substrate enters, the enzyme changes shape slightly to grip it more tightly.

This subtle change does real work. It positions the substrate's reacting groups correctly, puts strain on the bonds that must break, and creates a chemical environment that stabilizes the high-energy transition state. All of this lowers the activation energy of the reaction (the detail of how rate increases is Topic 3.2).

Why structure matters for the rest of Unit 3

Every metabolic pathway, including photosynthesis and cellular respiration, is a chain of enzyme-catalyzed steps. The fact that each enzyme is specific means a cell can run many different reactions at once without them interfering, and can switch pathways on or off by controlling individual enzymes. The same structure-function logic that explains specificity also explains how inhibitors and environmental conditions change enzyme activity, which the next two topics develop.

Try this

Q1. Identify the part of an enzyme that binds the substrate and state what determines its shape. [2 points]

• Cue. The active site; its shape is determined by the enzyme's amino-acid sequence and the folding that results.

Q2. Explain why an enzyme is described as a catalyst. [2 points]

• Cue. It speeds up a reaction by lowering activation energy and is not consumed or permanently changed, so it can be reused.