Understanding Enzymes: A Key Topic in Unit 3

Jan 19

4 min read

The Importance of Enzymes in Science Education

Enzymes are a crucial topic in Unit 3. They allow students to showcase both their biological understanding and practical investigation skills. Understanding enzymes not only enhances your grasp of biology but also helps in practical applications.

At a basic level, enzymes are proteins. They consist of long chains of amino acids joined together by peptide linkages. These chains fold into a specific three-dimensional shape, and it is this shape that gives the enzyme its function. There are different levels of protein structure, each differing in complexity.

The Active Site: Where the Action Happens

Within the folded structure of an enzyme is an area called the active site. This is where the substrate binds. The shape of the active site is highly specific, meaning only one type of substrate will fit correctly.

Enzymes are described as being specific. This specificity is often explained using the lock-and-key model. In this model, the substrate fits exactly into the active site. Alternatively, the induced fit model suggests that the active site slightly changes shape to accommodate the substrate as it binds. In both cases, the enzyme lowers the activation energy of the reaction. This allows the reaction to occur more quickly and at lower temperatures than would otherwise be possible.

Factors Affecting Enzyme Activity

Because enzymes rely on their shape, changes in conditions can affect their efficiency. If an enzyme is exposed to high temperatures or extreme pH values, the bonds that maintain its structure can break. This causes the active site to change shape, a process known as denaturation. Once an enzyme is denatured, it can no longer bind to its substrate, and the reaction stops.

Enzymes act as biological catalysts. This means they speed up chemical reactions without being consumed in the process. They achieve this by increasing the number of successful collisions between enzyme and substrate particles. When a substrate binds to the active site, an enzyme–substrate complex is formed. This complex allows the reaction to occur more easily. After the reaction, the products are released, and the enzyme is free to be reused.

Measuring Enzyme Activity

When investigating enzyme activity, scientists often measure the initial rate of reaction. This is the rate at the very start of the reaction when substrate concentration is highest, and products have not yet built up. Measuring initial rates makes results more reliable. It reduces the effect of variables such as substrate depletion or product inhibition.

Several factors can affect enzyme activity, and these are commonly tested in Unit 3 investigations.

Temperature affects the kinetic energy of particles. Higher temperatures increase the rate of reaction up to an optimum point. Beyond this point, the enzyme denatures, and the rate rapidly decreases.
pH also affects enzyme structure. Changes in pH can disrupt the bonds within the active site. Each enzyme has an optimum pH at which it works best.
Substrate concentration affects how often enzyme–substrate complexes form. As substrate concentration increases, the rate of reaction increases until all active sites are occupied. At this point, the enzyme is said to be saturated, and the rate levels off.
Increasing enzyme concentration increases the number of available active sites. This can enhance the rate of reaction, provided there is enough substrate present.

Example Exam Question

A student investigated how pH affects the rate of an enzyme-controlled reaction. They used buffer solutions to create different pH conditions and measured how long it took for the substrate to be completely broken down at each pH.

Here is the student's method:

Label three beakers with pH values 3, 5, and 7.
Add a pH buffer solution at the pH specified on the label of each beaker.
Add the substrate into each beaker.
Add the enzyme into each beaker.
Check every minute until all of the substrate has been broken down.

The student tested pH values of 3, 5, and 7. Their results showed that the shortest time for the reaction occurred at pH 7, as shown here:

The graph shows enzyme activity increasing as pH increases over the range tested, suggesting the enzyme is more active under less acidic conditions.

The student concluded that the enzyme works best at pH 7.

Evaluate the Student’s Investigation

In your answer, you should consider:

the method and equipment used
the results obtained

Model answer

The method uses buffer solutions to control pH. Additionally, a range of pH values were tested rather than just one, demonstrating good experimental practice. However, no repeats were conducted, which may affect the reliability of the results. Testing only three pH values means the optimum pH may lie between or above these values.

The method also does not specify the name of the enzyme and substrate. It lacks details on the amounts used, making it less reproducible and reliable.

Data was collected on how long it took for the substrate to be completely broken down at each pH. However, the y-axis shows enzyme activity, which does not reflect the data collected in the experiment. The method states that the student measured the time taken for the substrate to be completely broken down.

Although the rate is highest at pH 7, no further pH values were tested. This leaves uncertainty about whether values higher than pH 7 would yield even higher rates.

The conclusion is supported by the data collected, but more data would be needed for certainty.

Suggested Improvements

State the volumes/amounts of reagents used.
State the names/concentrations of reagents used (which substrate, which enzyme).
Explain how the student "checked every minute until all of the substrate has been broken down."
State the variables that were kept constant (e.g., temperature? - how was this done?).
Repeat experiments and calculate a mean.
Test a wider range of pH values.
Explain the measurement of enzyme activity.