AP Biology · Lesson 1

Enzymes and what changes their rate

Know Know it

An enzyme is a catalyst, almost always a protein, that speeds up a reaction without being used up. Every reaction has an activation energy, the energy the reactants need to start reacting. An enzyme lowers that barrier, so many more molecules react each second at body temperature.

The substrate binds the enzyme's active site, a pocket whose shape and charges fit that substrate. When it binds, the enzyme shifts slightly to grip it (induced fit). The enzyme does not change how much free energy the reaction releases; it only changes how fast the reaction gets there.

Temperature: warming speeds molecules up, so substrate reaches the active site more often and the rate rises. Past the optimum, the weak bonds that hold the enzyme's fold start to break, the active site changes shape, and the rate falls. pH: each enzyme has an optimum pH. A pH far from it changes the charges on R groups and can distort the active site.

Substrate level: more substrate raises the rate until nearly all active sites are busy; then the rate levels off. Enzyme level: more enzyme gives more active sites and a higher rate when substrate is plentiful.

Inhibitors: a competitive inhibitor binds the active site and blocks the substrate; a large excess of substrate can outcompete it. A noncompetitive inhibitor binds elsewhere and changes the enzyme's shape, so extra substrate does not undo it. Many enzymes also need a cofactor, such as a metal ion, to work.

Trap Cold slows, heat can ruin

A cold enzyme is slow because molecules move slowly, but its shape is intact, so it speeds up again when warmed. An enzyme heated far past its optimum can denature, and many such enzymes do not recover when cooled. Do not explain a low rate in the cold by denaturing.

Worked example Easy

How does an enzyme make a reaction in a cell go faster?

  1. It adds heat energy to the reactants
  2. It lowers the activation energyAnswer
  3. It lowers the energy stored in the products
  4. It is used up as the reaction runs

How to solve it

  1. Ask what limits how fast a reaction starts: the activation energy.
  2. An enzyme speeds a reaction without being changed or adding energy.
  3. So it must lower the activation energy.

Why each choice is right or wrong

  • A. Incorrect. Enzymes do not supply heat; they work at the cell's own temperature by lowering the energy barrier.
  • B. Correct. The active site holds the substrate so the reaction's starting barrier is smaller, and more molecules can clear it each second.
  • C. Incorrect. An enzyme does not change the energy of the reactants or products, so the free energy released stays the same.
  • D. Incorrect. An enzyme comes out of each reaction unchanged, so being used up is not how it works.

Explain Explain it

Task verbs: explain, predict

To explain an enzyme result, link three things: the condition (temperature, pH, substrate or inhibitor), what it does to the enzyme's shape or to how often substrate reaches the active site, and the effect on the rate. For a prediction, give the direction first, then that chain.

Worked example Medium

An inhibitor slows an enzyme, but its effect disappears when a large excess of substrate is added. Where does this inhibitor bind?

  1. At a site away from the active site, changing the enzyme's shape
  2. To the enzyme's bonds, unfolding the whole protein
  3. In the active site, where the substrate also bindsAnswer
  4. To a metal ion the enzyme needs, removing it

How to solve it

  1. Extra substrate can only help if substrate and inhibitor fight over one place.
  2. The only place the substrate binds is the active site.
  3. So this inhibitor binds the active site: it is competitive.

Why each choice is right or wrong

  • A. Incorrect. An inhibitor bound away from the active site changes the enzyme's shape at any substrate level, so more substrate would not undo it.
  • B. Incorrect. An unfolded, denatured enzyme stays inactive however much substrate is added, so the effect would not disappear.
  • C. Correct. The inhibitor and the substrate compete for the same site, so a large excess of substrate fills most active sites and the effect fades.
  • D. Incorrect. An enzyme missing a needed metal ion stays inactive, and adding substrate would not bring that ion back.

Predict Your turn

A protein-digesting enzyme from the stomach works fastest at pH 2. A student mixes it with its substrate in a solution at pH 7. Predict whether the reaction will be faster or slower than at pH 2. Then explain how the change in pH affects the enzyme's R groups, and how that affects the active site and the binding of the substrate.

Write it, then compare. Nothing you type here is saved or graded.

Compare with a model answer

A full answer does this

  • Predicts that the reaction will be slower at pH 7 than at pH 2
  • Explains that the pH change alters the charges on the enzyme's R groups, or the ionic and hydrogen bonds between them
  • Explains that this changes the shape or charge of the active site, so the substrate binds less well

Model answer

The reaction will be slower at pH 7 than at pH 2. At pH 7 there are far fewer H+ ions, so some R groups in the enzyme lose H+ and change their charge, and some of the ionic and hydrogen bonds between R groups break. Because those bonds hold the fold, the active site changes shape and charge. The substrate then fits and binds less well, so fewer reactions happen each minute.

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