recap

8.3 recap

A chemical reaction requires a “push” over the energy barrier to get started. An enzyme reduces the activation energy needed to start a reaction by binding the reactants (substrates), which speeds up the reaction. Biological catalysts are highly specific; only a narrow range of substrates can fit into the enzyme’s active site, so generally only one specific reaction can be catalyzed by a given enzyme. Enzymes affect only the rates at which reactions proceed—they do not affect the equilibrium or free energy associated with a given reaction.

learning outcomes

You should be able to:

  • Describe a catalyst.

  • Describe how an enzyme–substrate complex forms.

  • Describe the relationship between an enzyme and the equilibrium point of the reaction it catalyzes.

Question 1

Explain how the structure of an enzyme makes that enzyme specific.

Enzymes have a three-dimensional (tertiary) structure with an active site in which the substrate fits. Chemical groups at the active site also bind to the substrate non-covalently.

Question 2

What is the relationship between an enzyme and the equilibrium point of a reaction?

While an enzyme-catalyzed reaction proceeds more rapidly to its equilibrium than an uncatalyzed reaction, the actual equilibrium point is unaffected.

Question 3

When potatoes are peeled, the enzyme polyphenol oxidase causes discoloration by catalyzing the oxidation of certain molecules, using O2 as a substrate. Explain these observations:

  1. Browning is reduced in potatoes that are peeled under water and kept there.

    The presence of water may prevent O2 from reaching the enzyme.

  2. Potatoes that have been boiled at 100°C and then cooled do not brown when they are sliced.

    Boiling denatures proteins, so polyphenol oxidase is irreversibly altered by boiling, and its active site is destroyed.

Now that you have a general understanding of the structures, functions, and specificities of enzymes, let’s look more closely at how they work.