Enzyme action

MDCAT Biology · Biology · Enzymes. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Enzymes are globular proteins that speed up reactions in living cells. They are biological catalysts: a tiny amount converts a lot of substrate, because the enzyme is unchanged at the end and is used again. An enzyme lowers the activation energy, the energy needed to start the reaction. It does not change the products or the overall energy change.

The substrate binds at the active site, a small pocket made by a few amino acids. Its shape and charges fit only one substrate, so enzymes are specific. The site has a binding site and a catalytic site.

Many enzymes also need a non-protein cofactor before they can work.

Rules to remember

Activation energy = energy of the peak − energy of the reactants. An enzyme lowers the peak only.
Lock-and-key model (Fischer): the active site is rigid and the substrate (key) fits it exactly.
Induced-fit model (Koshland): the active site changes shape slightly as the substrate binds, to fit it closely.
Holoenzyme (active) = apoenzyme (protein part) + cofactor (non-protein part).
Activator: a detachable inorganic ion, e.g. Mg²⁺, Cl⁻.
Coenzyme: an organic cofactor that is loosely attached, e.g. NAD, FAD; many are made from vitamins.
Prosthetic group: a non-protein part bound tightly (covalently) to the protein, e.g. haem.

Common mistake

Do not say an enzyme gives energy to the reaction or changes the energy of the products. It only lowers the activation energy; reactants, products and the overall energy change stay the same.

Worked example

On an energy diagram the reactants are at 20 kJ mol⁻¹. The peak is at 95 kJ mol⁻¹ without the enzyme and at 55 kJ mol⁻¹ with it. By how much does the enzyme lower the activation energy?

Without enzyme: activation energy = 95 − 20 = 75 kJ mol⁻¹.
With enzyme: activation energy = 55 − 20 = 35 kJ mol⁻¹.
Difference = 75 − 35 = 40 kJ mol⁻¹. The levels of reactants and products do not move.

Answer: The enzyme lowers the activation energy by 40 kJ mol⁻¹, from 75 to 35 kJ mol⁻¹.

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