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1.4.9Biological Molecules

Limiting Factors Affecting Enzymes- Enzyme Concentration — AQA A Level Biology 7402 Revision Notes & Topical Questions

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📚 AQA A Level Biology 7402 Hub / Topical Revision
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1.4.9 Limiting Factors Affecting Enzymes- Enzyme Concentration — AQA A Level Biology 7402 notes: Explain and interpret the effect of enzyme concentration on the rate of reaction when substrate is in excess and when substrate is limiting.If the substrate is present in excess, the …

  • substrate becomes the limiting factor
  • initial rate
  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3
  • Proportional

Section 1 of 9Enzyme concentration and rate

If the substrate is present in excess, the rate of reaction is directly proportional to the enzyme concentration, as long as temperature and pH are constant.

Enzyme concentration and rate
More enzyme molecules mean more active sites available,
  • so more collisions between substrate molecules and active sites occur per second, and more enzyme-substrate complexes are formed.
When the concentration of enzyme is doubled, the initial rate doubles,
  • so the graph of rate against enzyme concentration is a straight line passing through the origin.

Section 2 of 9When the line levels off

If enzyme concentration is raised high enough that there is not enough substrate to occupy all active sites, substrate becomes the limiting factor. Adding more enzyme then causes no further increase in rate, and the graph plateaus. In practice, a rate-enzyme graph may also curve for these reasons:

When the line levels off
Substrate is used up quickly, so the reaction finishes sooner but the initial rate may be hard to measure.
Another factor, such as temperature or the presence of a cofactor or coenzyme, may be limiting.
At high concentrations, enzyme molecules compete for substrate molecules.

Section 3 of 9Using initial rates

Because the substrate is used up over time, the proportional relation is seen in the initial rate.

Using initial rates
Measure the volume of product formed in the first moments, or draw a tangent to the progress curve at time zero.
Different enzyme concentrations produce progress curves that rise at different speeds but, if the same amount of substrate is present, level off at the same final volume of product.
That is an important point: enzyme concentration changes the rate but not the total amount of product when substrate is the same.
Enzyme concentrationInitial rateFinal product
LowSlowSame
HighFastSame

Section 4 of 9Investigating the effect

1
Prepare a range of enzyme concentrations by diluting a stock solution (for example 100, 80, 60, 40, 20 and 0 per cent).
2
Keep substrate concentration and volume, temperature (water bath) and pH (buffer) constant.
3
Mix and measure the product formed or substrate used after a fixed time, or the time for a colour change.
4
Repeat and calculate the mean rate; plot rate against enzyme concentration.

Catalase from different masses of liver or yeast, or amylase dilutions, are common systems. A zero-enzyme control shows that there is no uncatalysed reaction.

Section 5 of 9Cell and industrial context

Cells control how fast reactions happen partly by making more or less enzyme, by changing gene expression.

Cell and industrial context
In industry, the amount of enzyme used is a trade-off between the speed gained and the cost, so immobilised enzymes that can be reused are often used.
Link this to enzyme-controlled pathways and gene regulation topics.
Because the rate is proportional to the enzyme amount only when substrate is plentiful, in a cell where substrate concentration is often low, adding more enzyme may not speed a reaction.

Section 6 of 9Worked analysis and links

A typical data question gives rates at different enzyme concentrations and asks you to say what limits the reaction.

Worked analysis and links
Look at the shape: if the points fall on a straight line through the origin, enzyme concentration is the limiting factor and the substrate is in excess.
If the graph curves and flattens, a different factor, usually substrate, is limiting.
A good answer says what the limiting factor is and how you can tell,
  • then suggests an experiment to test it, such as adding more substrate and seeing whether the rate rises.

Consider also how a cell deals with changing demand.

Worked analysis and links
In the liver, the amount of enzymes that break down alcohol or drugs can increase after repeated exposure, a process called enzyme induction,
  • which is why tolerance develops.
Genes can be switched on by the presence of the substrate,
  • as in the lac operon of E. coli,
  • where lactose triggers the production of the enzyme that breaks it down.
Conversely, if an enzyme is not needed, its gene is switched off, which saves energy and resources.

In quantitative work, if the rate with 2 per cent enzyme is 0.15 cm3 s-1, then 4 per cent would give about 0.30 in the proportional region, and 6 per cent about 0.45. Say 'assuming substrate is in excess'.

Section 7 of 9Exam-style questions

  1. Describe and explain the effect of increasing enzyme concentration on the initial rate of a reaction when substrate is in excess. [3]3 marks

  2. The rate rose from 2 to 10 units as enzyme concentration rose from 1 to 5, then stayed at 10 when concentration rose to 8. Explain the plateau. [2]2 marks

  3. Two tubes contain the same amount of substrate. Tube B has twice the enzyme concentration of tube A. Sketch or describe the product against time for each and explain. [3]3 marks

Section 8 of 9Key terms

TermMeaning
Enzyme concentrationAmount of enzyme per unit volume.
Limiting factorA factor in shortest supply that restricts the rate.
ExcessMore than is needed to react with all the other reactant.
Initial rateRate at the start of the reaction.
PlateauA levelling off of a graph.
ProportionalChanging by the same factor as another quantity.
Immobilised enzymeAn enzyme fixed to an inert support.

Section 9 of 9Exam technique

1
Saying the final amount of product is greater with more enzyme when substrate is the same.
2
Saying that enzyme concentration changes the activation energy of each molecule.
3
Forgetting to state that substrate must be in excess for rate to be proportional to enzyme concentration.
4
Describing the plateau as denaturation.

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