IB Biology Topic C1.1.7: The Structure of the Active Site, Enzyme–substrate Specificity and Denaturation Notes

Each Enzyme's Active Site Fits One Substrate, So It Catalyses One Reaction

1. An enzyme works because of the three-dimensional shape of its active site.
2. This shape lets the enzyme bind one substrate, or one class of similar substrates, and catalyse a single reaction.

The Active Site Is a Precise Pocket Built by Protein Folding

1. The active site is a small region on the enzyme where the substrate binds.
2. Its shape and chemistry are set by the enzyme's amino acid sequence and the way the protein folds.

What Makes the Active Site Specific

1. Shape complementarity
   1. The active site's shape matches the substrate, so only that substrate fits.
2. Chemical compatibility
   1. The amino acids lining the site have charges and other properties that match the substrate.

Specificity Means One Enzyme Catalyses One Reaction

1. Because the active site fits only one substrate, each enzyme catalyses one reaction or a group of closely related reactions.
2. Molecules that do not match the active site are not turned into product.
3. This keeps metabolic pathways controlled, since each step is handled by its own enzyme.

Denaturation Changes the Shape of the Active Site

1. Denaturation is the loss of an enzyme's three-dimensional conformation.
2. When the shape changes, the active site changes, so the substrate no longer fits.

High Temperature and Non-Optimal pH Cause Denaturation

1. High temperature
   1. Heat breaks the weak bonds, such as hydrogen bonds, that hold the enzyme's shape.
2. Non-optimal pH
   1. A pH far from the optimum changes the charge on the amino acids and disrupts ionic and hydrogen bonds.

A Denatured Enzyme Can No Longer Bind Its Substrate

1. Once the active site changes shape, the substrate can no longer bind, so no product forms.
2. This is why enzyme activity falls sharply at high temperature, even though warmth speeds reactions up to the optimum.

Structure Determines Function Across Biology

The active site is a clear case of a rule that runs through all of biology: the shape of a molecule sets what it can do.