2.1.4(d)(i)-(ii) - Enzyme activity factors and practicals

Activity as rate

An enzyme is active when it converts substrate into product. In experiments, enzyme activity is measured as a rate: product formed per unit time, substrate used per unit time, or the reciprocal of the time taken for an endpoint to be reached.

The method determines the rate unit. A catalase investigation might measure oxygen volume in $\mathrm{cm^3\,s^{-1}}$. An amylase investigation might record the time taken until starch no longer gives a blue-black colour with iodine, then use $1/\text{time}$ as a relative rate. A colorimeter or pH probe could collect continuous readings, which can later be processed with software.

The first OCR skill is to state the rate clearly. The second is to remember that a low rate is not the same as "no enzyme". The active site may still exist, but fewer successful enzyme-substrate complexes are forming per second.

Now practise the same rate idea with a new set of values.

Temperature and pH

Temperature affects enzyme activity in two linked ways. As temperature rises at first, substrate and enzyme molecules have more kinetic energy. They collide more often and more collisions have enough energy to form enzyme-substrate complexes, so rate increases.

Above the optimum temperature, the enzyme's tertiary structure is disrupted. Bonds such as hydrogen bonds and ionic bonds that help maintain the active site's shape can break. The active site changes shape, so the substrate is no longer complementary and fewer enzyme-substrate complexes form. This is denaturation.

Low temperature does not usually denature the enzyme. It lowers kinetic energy and reduces collision frequency. If the enzyme is warmed again without being damaged, activity can increase again.

pH affects enzyme activity because pH changes the concentration of hydrogen ions. These can alter the charges on R groups in the enzyme and disrupt bonds involved in tertiary structure. Each enzyme has an optimum pH range. Away from that optimum, the active site may be less complementary to the substrate. At extreme pH values, the enzyme may denature.

[DIAGRAM: enzyme_activity_factor_curves: Lesson 38: Factors affecting enzyme activity and enzyme practicals - diagram 01; asset_slug: 038_m02_1_4_factors_affecting_enzyme_activity_and_enzyme_practicals__diagram_01; recommended_method: matplotlib; description: 16:9 four-panel line-graph visual showing rate against temperature, pH, enzyme concentration and substrate concentration, with qualitative curves and all axes in a pale scientific palette.]

Use the diagram as a map of graph shapes, not as a set of universal numbers. Different enzymes have different optimum temperatures and pH values. OCR rewards the explanation: temperature and pH change enzyme activity because they affect kinetic energy, tertiary structure, active-site shape, enzyme-substrate complex formation and product formation.

Do not write that low temperature denatures enzymes. For low temperature, explain fewer successful collisions. For high temperature or extreme pH, explain disruption of tertiary structure and active-site shape.

Concentration effects

Enzyme concentration and substrate concentration affect rate by changing how often enzyme and substrate can meet.

If substrate is in excess, increasing enzyme concentration increases rate. There are more enzyme molecules and therefore more active sites available, so more enzyme-substrate complexes can form per second. In this region, rate is often approximately proportional to enzyme concentration.

If substrate becomes limiting, adding more enzyme will not keep increasing rate at the same pace because there are not enough substrate molecules to occupy the extra active sites. The factor in shortest effective supply limits the rate.

Substrate concentration has a different curve. At low substrate concentration, increasing substrate concentration increases rate because substrate molecules collide with active sites more often. At high substrate concentration, most active sites are occupied most of the time. The active sites are saturated, so increasing substrate concentration has little further effect unless more active enzyme is added.

Do not use Michaelis-Menten constants or Lineweaver-Burk plots here. For this specification point, the important skill is to explain the qualitative graph shape and to identify which factor is limiting.

Q₁₀ from rate data

Q₁₀ is the temperature coefficient. It compares the rate of a reaction at two temperatures that are 10 °C apart.

In this formula, $R_1$ is the rate at the lower temperature and $R_2$ is the rate at the temperature 10 °C higher. If the rate doubles over a 10 °C increase, Q₁₀ is 2.0. Do not assume Q₁₀ is always 2.0; calculate it from the data.

For a fixed endpoint, rate is proportional to 1/time. You can first calculate both reciprocal rates, or calculate Q₁₀ directly as the lower-temperature time divided by the higher-temperature time. Dividing the times in the opposite order would invert Q₁₀. Q₁₀ is a ratio and has no unit.

[DIAGRAM: q10_rate_data_graph: Lesson 38: Factors affecting enzyme activity and enzyme practicals - diagram 02; asset_slug: 038_m02_1_4_factors_affecting_enzyme_activity_and_enzyme_practicals__diagram_02; recommended_method: matplotlib; description: 16:9 plotted enzyme-rate data graph with axes, highlighted rates at temperatures 10 degrees C apart, and a Q₁₀ = R2/R1 annotation in a pale scientific palette.]

Near the optimum temperature, Q₁₀ may stop following a simple rise because denaturation begins to reduce the number of active enzyme molecules. That is why a Q₁₀ calculation must be interpreted in context rather than treated as a rule that rate always doubles.

PAG4 method decisions

PAG4 is about measuring the rate of enzyme-controlled reactions. The same design logic works whether the independent variable is pH, temperature, enzyme concentration or substrate concentration.

1. Choose one independent variable to change.
2. Keep the other rate-affecting variables constant.
3. Measure a dependent variable that can be converted into rate.
4. Repeat and calculate a mean, checking for anomalies.
5. Present the data with clear headings, units and graph axes.

For a temperature investigation, use water baths and allow solutions to reach the chosen temperature before mixing enzyme and substrate. For a pH investigation, use buffer solutions. For an enzyme- or substrate-concentration investigation, use a dilution series so the concentrations are planned rather than guessed.

[DIAGRAM: pag4_serial_dilution_workflow: Lesson 38: Factors affecting enzyme activity and enzyme practicals - diagram 03; asset_slug: 038_m02_1_4_factors_affecting_enzyme_activity_and_enzyme_practicals__diagram_03; recommended_method: drawn_biology; description: 16:9 deterministic catalase gas-syringe apparatus showing an airtight reaction flask in a water bath connected to a gas syringe in a pale scientific palette.]

Suitable rate measurements include oxygen volume from catalase and hydrogen peroxide, time for starch to disappear in an amylase-iodine test, pH change when lipase produces fatty acids, or absorbance change with a colorimeter. Safety points depend on the method, but common PAG4 answers include wearing eye protection, handling hot water baths carefully, treating enzymes/iodine/hydrogen peroxide according to hazard information, and cleaning spills quickly.

For the amylase method, place separate drops of iodine in a spotting tile. Equilibrate starch, buffer and amylase to the chosen temperature, mix to start the reaction and start the timer immediately. At fixed intervals transfer a fresh drop to iodine using a clean pipette. Record the first sample that remains orange-brown. Iodine belongs on the tile, not in the reacting tube. The endpoint lies between the last blue-black and first orange-brown sample, so a 10 s sampling interval cannot justify a time quoted to 0.01 s. Use a no-enzyme control to check that starch loss depends on the enzyme.

For catalase, connect an airtight reaction vessel to a freely moving gas syringe. Arrange mixing after sealing, or use the same short delay each time and acknowledge lost initial gas. Measure oxygen volume at several early times and obtain the initial gradient. A leak underestimates gas production even if repeated measurements agree.

Record the actual temperature, concentrations, volumes, sampling times and raw readings while working, including units and unexpected events. Keep raw results when calculating means, so another person can follow the method and check the processing. Written planning supports practical work; reliable handling of the apparatus must also be demonstrated in the laboratory.

Data and conclusions

A strong conclusion uses the data first, then the biology. Start by describing the trend with figures. Then explain the trend using active sites, enzyme-substrate complexes, denaturation or limiting factors, depending on the independent variable.

For these data, rate increases steeply from 0.16 to 0.42 cm³ s⁻¹ as substrate concentration increases from 10 to 30 arbitrary units. Then the increase becomes much smaller, from 0.47 to 0.48 cm³ s⁻¹ between 40 and 50 arbitrary units. The biological explanation is that substrate concentration increases the frequency of collisions with active sites at first, but at high substrate concentration the active sites are saturated and enzyme concentration is limiting.

Graph decisions matter. Put the independent variable on the x-axis and rate on the y-axis, with units in the headings. If product formed is plotted against time, the initial rate is the gradient near the start of the curve. If the curve is not straight at the point being measured, a tangent may be needed.

Uncertainty and limitations belong in the interpretation. Stopwatch timing can be limited by human reaction time. Gas volumes can be affected if gas escapes or dissolves in water. Endpoint colour changes can be subjective. Repeats, means and anomaly checks make the conclusion more reliable, but they do not fix an invalid design where a second variable has changed.

A rate at one instant

A product–time curve often becomes less steep as substrate is depleted. The rate at 30 s is the gradient of a tangent at 30 s, not the total product at 30 s divided by 30. Draw a straight tangent matching the local slope and choose two widely separated points on that line. If they are (10 s, 40 µg) and (50 s, 160 µg), then:

The points used for the gradient belong to the tangent; they need not lie on the measured curve. Larger triangles reduce the proportional effect of reading error. Keep unrounded values while calculating and round only the final rate to a precision justified by the graph.

[DIAGRAM: 038_tangent_rate: Tangent at 30 seconds measures the instantaneous rate from a product–time curve.; asset_slug: 038_tangent_rate; recommended_method: matplotlib; description: Tangent at 30 seconds measures the instantaneous rate from a product–time curve.]

Quick recap

Explain each factor through kinetic energy, active-site structure or availability, then use rate data, controlled variables, repeats and uncertainty to judge the conclusion.