Chapter 7 · Macromolecules
Enzymes at Work
Tell enzyme from substrate in any wording, explain specificity, and read temperature and pH graphs.
What enzymes do
Enzymes are proteins that catalyze (speed up) chemical reactions by lowering the activation energy, the energy needed to start a reaction. This lets reactions happen at normal body temperature.
Key concept
What does not change
The enzyme lowers only the hill in the middle. The energy of the reactants and the energy of the products stay exactly where they were. An enzyme adds no energy.
Enzyme or substrate?
The substrate is the reactant, the thing being acted on. The enzyme holds it at the active site, which changes shape slightly. When the products leave, the enzyme is unchanged and free to bind another substrate.
| Enzyme (-ase) | Substrate it breaks down |
|---|---|
Lactase | Lactose |
Sucrase | Sucrose |
Amylase | Starch |
Lipase | Lipids |
Protease | Proteins |
Worked example
Assigning roles
Maltase turns maltose into two glucose molecules. Name the enzyme, the substrate and the product.
Ask which molecule does the work and comes out unchanged: maltase. It is the enzyme (-ase).
Ask which molecule gets changed: maltose. It is the substrate (-ose, a sugar).
Ask what is new at the end: glucose. Those are the products.
Answer
Enzyme: maltase. Substrate: maltose. Product: glucose. Not every substrate ends in -ose, so when the name does not help, ask what gets changed.
Specificity
Enzymes are substrate specific. The active site fits its substrate like a puzzle piece. Lactase will not break down sucrose, because sucrose does not fit.
Quick check
A student adds lipase to a tube of starch. What happens?
Temperature and pH
Enzymes are proteins, so anything that changes a protein's shape changes how well the enzyme works. If the active site loses its shape, the substrate no longer fits.
Key concept
Heat does not always mean faster
Below the best temperature, warming speeds the enzyme up. Above it, the rate falls. Human enzymes generally work best near 37 °C, so a high fever moves them out of their optimal range and they work less well. Much higher heat denatures them.
pH works the same way. Each enzyme has its own best pH, and it falls off on either side. Always read the graph you are given, because different enzymes have different peaks.
Worked example
Decoding a scenario
A patient's temperature rises to 40.5 °C. What happens to their enzymes?
Translate: a fever is a temperature change. Nothing in the question says enzyme, so you have to make the link yourself.
Compare with the optimum: human enzymes work best near 37 °C, and 40.5 °C is above that.
Apply the curve: past the peak, activity falls.
Answer
The enzymes work less well, not faster. Sauna, boiling and ice are temperature. Stomach acid, vinegar and lemon juice are pH.
Explore deeper: Cold, heat and enzyme amount
Optional. This goes beyond what most courses ask for, and mastery here does not depend on it.
Cold is different from heat. Cooling slows an enzyme without unfolding it, so it recovers when warmed. Very high heat denatures it, and cooling back down does not usually restore the shape.
More enzyme means more active sites working at once, so the rate rises with the amount of enzyme, until the substrate itself runs short.