Week 3, Session 5: Enzymes and the Chemistry of Cells

Estimated time: 75 minutes
NGSS connection: HS-LS1-6

Start with a mystery

A sealed bottle of hydrogen peroxide can remain stable for months. Place it on a cut, and bubbles appear quickly because cells and microbes contain catalase. The enzyme did not create a new law of chemistry. It provided a faster route for a reaction that could already occur.

By the end, you can

• explain activation energy and enzyme action;
• predict how temperature, pH, and concentration affect reaction rate; and
• interpret enzyme data without assuming that “more” is always “better.”

Learn the idea

Chemical reactions require particles to collide with enough energy and the correct orientation. The initial energy barrier is activation energy. An enzyme is a biological catalyst—usually a protein—that lowers this barrier. It speeds the approach to equilibrium without being permanently consumed and without changing the reaction's overall energy difference.

Reactants called substrates bind to an enzyme's active site. The active site is not a rigid lock. Binding can slightly adjust the enzyme's shape in an induced fit that positions chemical groups, strains bonds, or creates a useful microenvironment. Products no longer fit as well and leave; the enzyme can work again.

Enzyme activity depends on molecular shape and motion. At low temperatures, particles collide less often, so reactions slow. Warming usually increases rate until an optimum is reached. Beyond that range, stabilizing interactions in the protein can be disrupted, changing the active site. Extreme pH can alter charges and shape in a similar way. “The enzyme dies” is not accurate; enzymes are not alive. We say the protein denatures or loses activity.

Adding substrate increases reaction rate while many active sites are empty. Eventually nearly every enzyme is busy, and the rate levels off. More substrate then has little effect unless more enzyme becomes available. Cells regulate pathways using inhibitors, activators, compartmentalization, and feedback. In feedback inhibition, a pathway's end product slows an earlier enzyme, preventing wasteful overproduction.

Study the visual

Alt text: A substrate fits an enzyme, forms a temporary complex, and leaves as two products while the enzyme remains available.

Trace what changes and what stays the same. The substrate's bonds change. The enzyme may flex during binding, but it returns to a usable form. The enzyme does not supply energy that appears in the products; it changes the reaction pathway.

Safe investigation

If available, place equal pieces of fresh and thoroughly cooked potato in separate cups and add equal small amounts of 3% household hydrogen peroxide. Wear eye protection, avoid skin contact, and have an adult supervise. Fresh tissue typically bubbles more because heat has denatured much of the cooked potato's catalase. If you cannot run the test, predict the result and explain the molecular reason.

Check yourself

1. Does an enzyme make an unfavorable reaction release more energy?
2. Why does enzyme rate level off at high substrate concentration?
3. A human enzyme works best near 37°C. Must 37°C be best for every enzyme?
4. How can an end product regulate its own pathway?

Answer and feedback

1. No. It lowers activation energy but does not change the overall energy difference or equilibrium.
2. Active sites become occupied almost continuously; enzyme concentration becomes limiting.
3. No. Enzymes from cold-adapted, heat-adapted, or different cellular environments have different optima.
4. It may bind to and reduce the activity of an earlier enzyme through feedback inhibition.

Key vocabulary: catalyst, activation energy, substrate, active site, induced fit, denaturation, inhibitor, feedback inhibition

Unit checkpoint: Explain how water's polarity, macromolecular structure, and enzyme shape work together to make cellular chemistry possible.