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3.3.2Exchange and Transport

Enzymes in Digestion — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 22 of 48 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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3.3.2 Enzymes in Digestion — AQA A Level Biology 7402 notes: Explain how the structure, specificity and optimum conditions of digestive enzymes link to where they act in the gut.Enzymes are globular proteins that lower the activation energy of a reaction.The substrate binds to th…

  • active site
  • induced-fit model
  • Temperature
  • Substrate concentration
  • Enzyme concentration
  • competitive inhibitor

Part 1 of 6Enzymes as biological catalysts

Enzymes are globular proteins that lower the activation energy of a reaction.

Active site
The substrate binds to the active site, forming an enzyme–substrate complex.
Induced-fit model
Binding is explained by the induced-fit model: the active site changes shape slightly as the substrate binds, which strains the bonds in the substrate and helps the reaction.
The specificity of an enzyme is determined by the tertiary structure of the protein, which gives the active site its specific shape and charge distribution.
A change in the primary structure, such as a different amino acid, can alter the shape and stop the enzyme from working.

Part 2 of 6Digestive enzymes and their sites of action

EnzymeSourceSubstrateProductsOptimum pH
Salivary amylaseSalivary glandsStarchMaltoseAbout 7
PepsinStomachProteinsShorter polypeptides1–2
Pancreatic amylasePancreasStarchMaltoseAbout 7–8
TrypsinPancreasProteinsPolypeptidesAbout 8
LipasePancreasTriglyceridesMonoglycerides, fatty acidsAbout 7–8
Maltase, sucrase, lactaseEpithelium of ileumDisaccharidesMonosaccharidesAbout 7–8

Notice that each enzyme works best in the pH of the region of the gut in which it is released. Pepsin's low optimum matches the acid in the stomach, whereas pancreatic enzymes work in the slightly alkaline conditions made by hydrogencarbonate in pancreatic juice and bile.

Part 3 of 6Inhibitors

A competitive inhibitor has a shape similar to the substrate and binds to the active site, blocking it.

1
Its effect is reduced by increasing the substrate concentration.
2Non-competitive inhibitor
A non-competitive inhibitor binds elsewhere on the enzyme, changing the shape of the active site; increasing substrate concentration does not overcome it.
3
Digestive enzymes are also controlled in the body by being made in an inactive form (a zymogen), such as trypsinogen, which is activated by enterokinase in the small intestine.
4
This prevents damage to the cells that produce the enzyme.

Part 4 of 6Calculating rates and linking to practical work

The rate of an enzyme-controlled reaction is calculated as change in product (or substrate) ÷ time, or from the gradient of a tangent to the curve at a given time.

Calculating rates and linking to practical work
The initial rate is taken from the steepest part of the curve since the substrate concentration is greatest at the start.
Rate may also be expressed as 1/time to clot or to clear in a simple practical.
Variables and controls for practical work are covered in 3.3.3.

Enzymes are also used in industry

  • lactase is used to make lactose-free milk, and proteases are used in biological washing powders.
  • Immobilising enzymes on beads or in alginate allows reuse and easier separation of the product.

Part 5 of 6Worked interpretation of a rate graph

Imagine a graph of rate of product formation (mg min−1) against pH for pepsin and trypsin. The pepsin curve peaks at pH 2 and falls to almost zero by pH 5; the trypsin curve peaks near pH 8 and is almost zero at pH 4. To explain such a curve, state the optimum, say that on either side of it the rate is lower because changes in H+ concentration alter the ionisation of R groups in the active site and break ionic and hydrogen bonds, and conclude that each enzyme is adapted to its site of action.

For a substrate-concentration graph, state that the initial part is a straight line, because substrate is the limiting factor and every substrate molecule finds an active site. The plateau shows that the enzyme concentration, not the substrate, is limiting. If the question asks how to raise the plateau, answer: add more enzyme.

The Michaelis constant, Km, is the substrate concentration at half the maximum rate and shows the affinity of an enzyme for its substrate; a low value means high affinity. This extra detail is not required, but it shows how enzyme kinetics is analysed.

Part 6 of 6Exam-style questions

  1. Explain why pepsin works well in the stomach but not in the small intestine. [3]3 marks

  2. A competitive inhibitor is added to a digestion reaction. Explain how increasing substrate concentration affects its action. [2]2 marks

  3. In an investigation, 6.0 g of product forms in the first 3 minutes. Calculate the mean rate. [2]2 marks

Key terms

TermMeaning
Active siteRegion of an enzyme where the substrate binds.
Induced-fit modelModel in which the active site changes shape to fit the substrate.
DenaturationPermanent change in tertiary structure of a protein.
Competitive inhibitorMolecule that binds to the active site and blocks the substrate.
Non-competitive inhibitorMolecule that binds elsewhere and changes the active site shape.
ZymogenInactive precursor of an enzyme, such as pepsinogen.
Activation energyEnergy needed to start a reaction.

Avoid these mistakes

1
Saying that enzymes are killed by high temperature; they are denatured because they are not alive.
2
Describing the active site as being destroyed by pH instead of changing shape because of broken bonds.
3
Writing that all enzymes have the same optimum pH.
4
Mixing competitive and non-competitive inhibition.
5
Saying the substrate fits the active site like a rigid lock and key, with no mention of induced fit.

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