Practice a College Board-style free response question on Chemistry of Life. Write your response, then reveal the model answer to see exactly what earns each point.
An enzyme called enzyme X catalyzes a reaction in a cell with a normal temperature of 37°C. A researcher exposes separate samples of enzyme X to different temperatures for 10 minutes, then measures enzyme activity once each sample is returned to 37°C. Her results are shown below.
| Treatment temperature (°C) | Relative enzyme activity (%) |
|---|---|
| 25 | 100 |
| 37 | 100 |
| 50 | 62 |
| 65 | 0 |
| 65, then cooled back to 37 | 0 |
Heat most directly disrupts the secondary, tertiary, and (if present) quaternary structures of enzyme X. These levels are held together by relatively weak interactions — hydrogen bonds, ionic bonds, and hydrophobic interactions — that are easily broken by added thermal energy. The primary structure remains intact because peptide bonds are strong covalent bonds that mild heating cannot break.
Once enzyme X is denatured at 65°C, its polypeptide chain unfolds and the active site loses its specific three-dimensional shape. Even after cooling, the protein typically cannot spontaneously re-fold to its original conformation because multiple stable but incorrect interactions can form between R groups during cooling, trapping the protein in a non-functional shape. Since enzyme function depends entirely on the active site fitting its substrate, the enzyme cannot catalyze the reaction even though its amino acid sequence is unchanged.
Enzyme activity will most likely decrease or be eliminated. The mutation changes the primary structure by substituting one amino acid for another with a different R group. Because the new R group is nonpolar instead of polar, it will form different interactions with both the surrounding amino acids and with the substrate. Two consequences are likely: the active site may fold incorrectly (because hydrophobic R groups typically cluster away from water, changing how the region folds), and the active site may no longer form the correct hydrogen bonds or charge interactions with the substrate. Either outcome disrupts the structure–function relationship: a changed shape produces a changed (or absent) function.