Enzymes

Lessons

Senior 4 · Biology

Enzymes

4 lessons

Lesson 2 of 4

Mode of action of enzymes

25 min

Why sugar does not burst into flames

Burning a spoonful of sugar releases a lot of energy, yet sugar sits in its bowl for years without catching fire. To react with oxygen, its molecules first need a push of energy, such as a flame, to break some bonds. Your cells release the same energy from glucose at 37 °C, without any flame, in a series of small steps, each catalysed by an enzyme. This lesson explains how an enzyme provides that easier route.

Definition

Activation energy ( $E_{a}$ ): the minimum energy that reacting molecules must gain before they can react; the "hill" they must climb.

Enzyme–substrate complex: the temporary structure formed when a substrate binds to the active site. It becomes an enzyme–product complex before the products are released.

Lock-and-key model: the idea that the active site is a rigid shape exactly complementary to the substrate. Induced-fit model: the idea that the active site is flexible and changes shape as the substrate binds, closing tightly around it.

Cofactor: a non-protein substance an enzyme needs in order to work. A coenzyme is an organic cofactor, often made from a vitamin; a prosthetic group is a cofactor bound permanently to the enzyme. An enzyme without its cofactor is an apoenzyme; with it, a holoenzyme.

Lowering the activation energy

An enzyme does not supply energy. It provides a different route for the reaction, one with a lower activation energy (Figure 1), so at body temperature many more substrate molecules have enough energy to react each second.

Figure 1. The enzyme lowers the activation energy, $E_{a}$ . It does not change the energy of the substrate or of the products, so the energy released is the same.

The reaction follows three steps:

\mathrm{E} + \mathrm{S} \;\rightleftharpoons\; \mathrm{ES} \;\longrightarrow\; \mathrm{EP} \;\longrightarrow\; \mathrm{E} + \mathrm{P}

While the substrate is held in the active site, the enzyme lowers the activation energy in several ways:

it holds two substrate molecules close together, in the right orientation to react;
binding strains the bonds of the substrate, so they break more easily;
charged or polar R groups of the active site give or take hydrogen ions or electrons, creating a chemical environment unlike the surrounding water.

Worked example 1: reading an energy profile

For the breakdown of hydrogen peroxide, suppose $E_{a}$ is $75\,\mathrm{kJ\,mol^{-1}}$ without a catalyst and $25\,\mathrm{kJ\,mol^{-1}}$ with an enzyme (illustrative values). (a) By how much, and by what percentage, does the enzyme lower $E_{a}$ ? (b) Does the enzyme change the energy released?

(a) Decrease $= 75 - 25 = 50\,\mathrm{kJ\,mol^{-1}}$ . Percentage $= \frac{50}{75} \times 100 = 66.7\,\%$ , about two-thirds.
(b) No. The energy released depends only on the energy of the substrate and of the products, which the enzyme does not change.
Because so many more molecules can now cross the lower "hill", the rate rises enormously, even though the temperature is the same.

Two models of how the substrate fits

Figure 2. In both models the substrate binds to the active site to form an enzyme–substrate complex, and the enzyme is unchanged at the end. They differ in whether the active site is rigid or flexible.

Emil Fischer proposed the lock-and-key model in 1894. It explains specificity well: only a substrate of the right shape fits. Daniel Koshland proposed the induced-fit model in 1958. Proteins are flexible, and X-ray studies show that many enzymes really do change shape when their substrate binds. Induced fit explains why some enzymes accept a small group of similar substrates (lipase acts on many different triglycerides), and why binding strains the substrate's bonds: the closing active site pulls on them. Today induced fit is the accepted model.

Worked example 2: which model fits the evidence?

Say which model each observation supports, and why. (a) Sucrase hydrolyses sucrose but not maltose or lactose. (b) When glucose binds, the two halves of hexokinase swing together and close around it. (c) One lipase hydrolyses triglycerides with many different fatty acids.

(a) Both models: the active site fits only sucrose. Specificity alone cannot tell the models apart.
(b) Induced fit only: the enzyme changes shape on binding, which a rigid lock cannot do.
(c) Induced fit fits better: a flexible active site can mould to a range of similar substrates; a rigid lock should take only one key.

Cofactors, coenzymes and prosthetic groups

Type	What it is	Examples
Inorganic ion (activator)	A metal or other ion that helps the substrate bind or the reaction happen	Chloride ions activate salivary amylase; magnesium ions are needed by enzymes that use ATP
Coenzyme	A small organic molecule that binds briefly, carrying atoms or electrons from one enzyme to another; often made from a vitamin	NAD (from vitamin B3, niacin) and FAD (from vitamin B2) carry hydrogen for dehydrogenases; coenzyme A (from vitamin B5)
Prosthetic group	A cofactor bound permanently, as part of the enzyme's structure	The haem group of catalase; zinc in carbonic anhydrase

This is why a lack of B vitamins slows respiration: the coenzymes made from them cannot be produced.

Worked example 3: does amylase need chloride?

Equal volumes of starch and amylase were mixed in three tubes, and samples were tested with iodine every 30 seconds until the blue-black colour no longer appeared (illustrative results):

Tube	Added	Time to digest the starch / min
A	Distilled water	6.0
B	Sodium chloride solution	2.0
C	Sodium sulfate solution, same sodium concentration as B	6.0

(a) Compare the rates in A and B. (b) What is tube C for? (c) Conclude.

(a) Rate is proportional to $\frac{1}{\text{time}}$ : A $= \frac{1}{6.0} = 0.17\,\mathrm{min^{-1}}$ , B $= \frac{1}{2.0} = 0.50\,\mathrm{min^{-1}}$ . B is 3 times faster.
(b) Tube C adds sodium ions without chloride. It tests whether the effect is due to $\mathrm{Na^+}$ or to $\mathrm{Cl^-}$ .
(c) C is no faster than A, so sodium has no effect: chloride ions activate amylase. They act as a cofactor.

Amoeba Sisters: "Enzymes (Updated)". Watch for the active site, the induced-fit model, how enzymes lower activation energy, and the roles of cofactors and coenzymes.

Warning

Common mistakes

Writing that enzymes "give energy" to the reaction. They lower the activation energy; they do not add energy.
Saying the active site and substrate are the "same shape". They are complementary in shape, like a hand and a glove.
Drawing the energy released as smaller with an enzyme. Only the peak changes; the start and end levels stay the same.
Confusing a coenzyme with an enzyme. A coenzyme is a small non-protein helper; it is not a catalyst by itself.

Tip

Exam technique. Describe enzyme action in four linked steps: "The substrate is complementary to the active site → it binds and forms an enzyme–substrate complex → the active site changes shape (induced fit), straining bonds and lowering the activation energy → the products are released and the enzyme is unchanged." Each step usually earns a mark.

✦

Practice: mode of action

Define activation energy, and explain how lowering it speeds up a reaction.
Sketch an energy profile for a reaction with and without an enzyme. Label the activation energies and the energy released.
An enzyme lowers $E_{a}$ from $86\,\mathrm{kJ\,mol^{-1}}$ to $43\,\mathrm{kJ\,mol^{-1}}$ . Calculate the percentage decrease.
State two differences between the lock-and-key and induced-fit models.
Explain the difference between a coenzyme and a prosthetic group, with an example of each.
In an amylase experiment, tube X took 8.0 min to digest starch and tube Y 2.5 min. Calculate the rate in each as $\frac{1}{\text{time}}$ , and how many times faster Y is.
Evaluate the statement: "The lock-and-key model is wrong and should no longer be taught."

Show answersHide answers

The minimum energy that molecules need in order to react. With a lower activation energy, a much larger fraction of molecules has enough energy at the same temperature, so more successful reactions happen per second.
Two curves starting at the same substrate level and ending at the same, lower product level; the enzyme curve has a lower peak. $E_{a}$ is the height from the substrate level to each peak; the energy released is the drop from substrate level to product level.
$\frac{86 - 43}{86} \times 100 = \frac{43}{86} \times 100 = 50\,\%$ .
Any two: lock-and-key has a rigid active site, induced fit a flexible one; in induced fit the active site changes shape when the substrate binds; induced fit explains strain on the substrate's bonds and enzymes that act on several similar substrates.
A coenzyme is an organic cofactor that binds briefly and is shared between enzymes, such as NAD; a prosthetic group is bound permanently as part of the enzyme, such as the haem of catalase.
X: $\frac{1}{8.0} = 0.125\,\mathrm{min^{-1}}$ ; Y: $\frac{1}{2.5} = 0.40\,\mathrm{min^{-1}}$ . Y is $\frac{0.40}{0.125} = 3.2$ times faster.
Partly true. The idea of a rigid active site is not accurate: many enzymes change shape on binding. But the model still correctly explains specificity through complementary shapes, and it is a useful first step; induced fit refines it rather than replacing every part of it.

Quick check

Put the steps of an enzyme-catalysed reaction in order.

Tap the cards in the right order.

Key points

Enzymes lower the activation energy; they do not change the energy released.
E + S → ES → EP → E + P. The active site holds substrates in position, strains their bonds and provides a special chemical environment.
Lock-and-key (rigid site) explains specificity; induced fit (flexible site that changes shape) is the accepted, more complete model.
Cofactors: inorganic ions, coenzymes (often from vitamins, such as NAD) and prosthetic groups (such as haem).

Sources and credits

2.3.6: Enzymes and 2.3.3: Potential, Kinetic, Free, and Activation Energy, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for activation energy, the active site, induced fit and cofactors. Adapted by Ganzaa.
"Enzyme", Wikipedia contributors, licensed under CC BY-SA 4.0. Used for the history of the lock-and-key (Fischer, 1894) and induced-fit (Koshland, 1958) models. Adapted by Ganzaa.
Figures 1 and 2, the table, worked examples (the activation energies and amylase results are illustrative), practice and quick check: written by Ganzaa.
Image: Thomas Shafee, CC BY 4.0, via Wikimedia Commons.
Video: "Enzymes (Updated)", Amoeba Sisters, embedded from YouTube.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Why sugar does not burst into flames

2Lowering the activation energy

3Two models of how the substrate fits

4Cofactors, coenzymes and prosthetic groups

Why sugar does not burst into flames

Lowering the activation energy

Two models of how the substrate fits

Cofactors, coenzymes and prosthetic groups