Enzyme Activity Assay Discussion Examples | Effects of Temperature, pH, and Inhibitors

What Is Enzyme Activity?

Enzyme activity is an indicator of how much substrate an enzyme can convert into product within a given period of time.
In an enzyme reaction, a substrate binds to the active site of the enzyme to form an enzyme-substrate complex, after which the product is formed.
By measuring the rate of this reaction, it is possible to investigate the conditions under which the enzyme functions effectively.

Enzyme activity is greatly influenced not only by the properties of the enzyme itself but also by the reaction environment.
When temperature and pH are appropriate, activity becomes high, but when the conditions deviate from the optimum, the structure or charge state of the active site changes and the reaction rate decreases.
When an inhibitor is present, substrate binding or the catalytic reaction is also disturbed, resulting in decreased activity.

Main Items to Include in the Results

In the results of an enzyme activity assay, organize the reaction conditions, changes in absorbance, initial rate, enzyme activity, relative activity, optimum temperature, optimum pH, inhibition rate, and other data.
Presenting the activity under each condition in tables or graphs makes the discussion easier to write.

Main Items to Include in the Results

• Enzyme and substrate used
• Reaction temperature
• Reaction pH
• Presence or absence and concentration of inhibitor
• Reaction time
• Measurement wavelength
• Changes in absorbance over time
• Initial rate
• Enzyme activity
• Relative activity
• Temperature-enzyme activity graph
• pH-enzyme activity graph
• Inhibitor concentration-enzyme activity graph
• Inhibition rate
• Sources of error and points for improvement

Reference Experimental Values and Analysis Examples for Enzyme Activity Assays

Here, changes in absorbance, reaction rates, temperature dependence, pH dependence, substrate-concentration dependence, and the effects of inhibitors obtained in enzyme activity assays are organized as reference experimental values that are easy to discuss in reports.

Enzymes bind to substrates and promote reactions while maintaining their three-dimensional structures as proteins.
Therefore, activity changes greatly depending on temperature, pH, substrate concentration, inhibitors, reaction time, and enzyme concentration.
When evaluating enzyme activity, it is important to determine the reaction rate as the amount of change per unit time rather than considering only color intensity or absorbance.

Reference Experimental Conditions

Example of Determining Reaction Rate From Changes in Absorbance

Consider a case in which the product formed by an enzyme reaction develops color and the absorbance at 420 nm increases over time.
The change in absorbance per unit time is determined from the linear portion at the beginning of the reaction.

Using the linear region from 0 to 3 minutes, the reaction rate can be determined as follows.

Reaction rate = (0.320 − 0.050) ÷ 3 min = 0.090 Abs/min

The initial rate under these reaction conditions can therefore be expressed as 0.090 Abs/min.

Example of Blank Correction

Even in a blank without enzyme, absorbance may change because of spontaneous substrate decomposition or the color of the reagents.
In such cases, the change in the blank is subtracted from the absorbance change of the sample.

The corrected reaction rate is 0.240 ÷ 3 min = 0.080 Abs/min.
Blank correction makes it possible to evaluate the change originating from the enzyme reaction more accurately.

Changes in Enzyme Activity With Temperature

Enzyme reactions are strongly affected by temperature.
In general, molecular motion is slow and reaction rates are low at low temperatures, whereas at high temperatures the three-dimensional structure of the enzyme may collapse and the enzyme may become inactivated.

In this example, the highest activity is observed at 37°C.
The reaction rate is low at lower temperatures, whereas enzyme activity decreases on the high-temperature side because of thermal denaturation.

Example Calculation of Relative Activity

Relative activity is a method of comparing activity under each condition by defining the highest activity as 100%.

Relative activity (%) = Initial rate under each condition ÷ Maximum initial rate × 100

If the initial rate at 25°C is 0.060 Abs/min and the maximum initial rate is 0.090 Abs/min,

Relative activity = 0.060 ÷ 0.090 × 100 = 66.7%

Therefore, the relative activity at 25°C is approximately 67%.

Changes in Enzyme Activity With pH

Because acidic and basic amino acid residues are involved in the active sites of enzymes, substrate binding and reaction rates change depending on pH.

This enzyme shows the highest activity near pH 7.
When the pH deviates from the optimum range, the charge state of the active site and the three-dimensional structure of the enzyme change, resulting in decreased activity.

Changes in Reaction Rate With Substrate Concentration

In enzyme reactions, the reaction rate increases as substrate concentration increases in a low substrate-concentration range.
However, once the enzyme active sites are almost saturated with substrate, the reaction rate reaches a plateau.

If increasing the substrate concentration no longer results in a substantial increase in rate, the enzyme is considered to be saturated with substrate and approaching its maximum rate.

Changes in Reaction Rate With Enzyme Concentration

When sufficient substrate is present, increasing the amount of enzyme causes the reaction rate to increase approximately in proportion to the amount of enzyme.

Within the range where the amount of enzyme and reaction rate are proportional, activity can be compared easily.
However, if the absorbance change is too large, it may fall outside the linear range, so an appropriate enzyme amount should be selected.

Decrease in Enzyme Activity Caused by an Inhibitor

When an inhibitor is present, it may bind to the active site or another site on the enzyme and interfere with the substrate reaction.

The initial rate decreases as the inhibitor concentration increases.
The inhibition rate is determined using the following equation.

Inhibition rate (%) = (Activity without inhibitor − Activity with inhibitor) ÷ Activity without inhibitor × 100

If the initial rate is 0.050 Abs/min at an inhibitor concentration of 0.5 mmol/L,

Inhibition rate = (0.090 − 0.050) ÷ 0.090 × 100 = 44.4%

Example of Distinguishing Competitive and Noncompetitive Inhibition

The effect of an inhibitor can be discussed by examining the reaction rate when the substrate concentration is changed.

In competitive inhibition, increasing the substrate concentration may reduce the effect of the inhibitor.
In noncompetitive inhibition, the maximum rate is less likely to recover even when the substrate concentration is increased.

Example of Enzyme Inactivation by Heat Treatment

The following is a reference example in which the enzyme was heat-treated before the reaction and the remaining activity was compared.

The decrease in activity after high-temperature pretreatment is considered to have occurred because the three-dimensional structure of the enzyme protein changed, making substrate binding more difficult.

Effect of Excessively Long Reaction Time

Enzyme activity is basically evaluated using the linear region at the beginning of the reaction.
As the reaction time increases, the rate may decrease because of substrate depletion, product inhibition, or enzyme inactivation.

If the average rate over 0–10 minutes is used, the initial rate may be underestimated because it includes the slowing of the reaction during the later stages.

Example Results of Control Experiments

In enzyme activity assays, controls without enzyme, without substrate, or using heat-inactivated enzyme can be used to confirm whether the observed change is caused by the enzyme reaction.

If almost no reaction occurs in the absence of enzyme or with heat-inactivated enzyme, the increase in absorbance under normal conditions can more readily be judged to result from enzyme activity.

Example of Repeated Measurements and Variation

Enzyme activity tends to vary because of temperature, mixing timing, reaction-start timing, and pipetting operations, so multiple measurements are performed and the mean is determined.

If only the fourth measurement is low, the timing of reaction initiation, insufficient mixing, temperature changes, or deterioration of the enzyme solution should be checked.

Example of How to Write the Results

A420 increased over time as the enzyme reaction proceeded.
Because the absorbance increased almost linearly from 0.050 to 0.320 during the 0–3 minute interval, the initial rate was calculated as (0.320 − 0.050) ÷ 3 = 0.090 Abs/min.
On the other hand, because an absorbance change of 0.030 was also observed in the blank without enzyme, the rate after blank correction was 0.080 Abs/min.

When measurements were performed at different temperatures, the highest activity was observed at 37°C, activity was low at 10°C, and it decreased greatly at 70°C or higher.
At low temperatures, molecular motion is low and the frequency of collisions between enzyme and substrate is reduced, resulting in a lower reaction rate.
In contrast, at high temperatures, the enzyme protein underwent thermal denaturation and the structure of the active site changed, making substrate binding more difficult and decreasing activity.

Regarding the effect of pH, maximum activity was observed at pH 7, whereas activity decreased under strongly acidic and strongly basic conditions.
This is considered to have occurred because pH changed the charge states of amino acid residues in the active site and affected substrate binding and the catalytic reaction.
At extreme pH values, the three-dimensional structure of the enzyme itself may also have become unstable and the enzyme may have been inactivated.

Points for Connecting the Results to the Discussion

In a discussion of enzyme activity assays, it is important to explain not only the magnitude of absorbance but also the relationship among reaction rate, initial rate, control experiments, and the three-dimensional structure of the enzyme.

• Has the initial rate been calculated from changes in absorbance?
• Has blank correction been performed to evaluate changes originating from the enzyme reaction?
• Can it be explained that reaction rates are low at low temperatures and activity decreases at excessively high temperatures because of thermal denaturation?
• Can the difference between optimum temperature and thermal inactivation be explained?
• Can the effects of pH on the charge state of the active site and enzyme structure be discussed?
• Can the reason the rate becomes saturated at high substrate concentrations be explained?
• Can the decrease in reaction rate as inhibitor concentration increases be quantified as an inhibition rate?
• Can competitive and noncompetitive inhibition be discussed from substrate-concentration dependence?
• Can it be explained that the initial rate cannot be evaluated correctly in the later stages of the reaction because of substrate depletion or product inhibition?
• Can the meaning of control experiments without enzyme, without substrate, or using heat-inactivated enzyme be explained?

Example Discussion

In this experiment, changes in absorbance accompanying the enzyme reaction were measured and enzyme activity was evaluated as the initial rate.
Because A420 increased almost linearly during the 0–3 minute period after the start of the reaction, the initial rate was determined using this range.
The initial rate under normal conditions was 0.090 Abs/min.
Because the increase in absorbance became slower when the reaction time was extended, the reaction rate is considered to have decreased because of a decrease in substrate concentration and accumulation of product.

Regarding temperature dependence, maximum activity was observed at 37°C.
At low temperatures, molecular motion is low and the frequency of collisions between enzyme and substrate is reduced, resulting in a lower reaction rate.
In contrast, activity decreased greatly at 70°C or higher.
This occurred because the high temperature disrupted the three-dimensional structure of the enzyme protein and prevented the active site from binding appropriately to the substrate.
Therefore, both the temperature effect that increases the reaction rate and thermal denaturation that inactivates the enzyme are involved in enzyme activity.

Regarding pH dependence, the highest activity was observed near pH 7.
Acidic or basic amino acid residues are involved in enzyme active sites, and their charge states change depending on pH.
When the pH deviates from the optimum range, the charge state required for substrate binding or catalytic reactions cannot be maintained, resulting in decreased activity.
At extreme pH values, the three-dimensional structure of the entire enzyme may also become unstable.

When an inhibitor was added, the initial rate decreased as the concentration increased.
At an inhibitor concentration of 0.5 mmol/L, the initial rate decreased from 0.090 Abs/min to 0.050 Abs/min, corresponding to an inhibition rate of 44.4%.
This is considered to have occurred because the inhibitor acted on the enzyme active site or another site and interfered with the substrate reaction.
If inhibition becomes weaker at high substrate concentrations, competitive inhibition is possible, whereas if the maximum rate remains low even at high substrate concentrations, noncompetitive inhibition is possible.

Possible sources of error include differences in reaction-start timing, insufficient mixing, inadequate temperature control, variation in pipetting, and differences in the timing of absorbance measurements.
Because enzyme reactions proceed over a short period of time, differences in the time between mixing the substrate and enzyme and beginning measurement can readily affect the results.
In addition, because activity changes even with differences of several degrees in temperature, it is important to keep the reaction temperature constant.

Summary

In enzyme activity assays, the initial rate is determined from changes in absorbance during the initial stage of the reaction, and the effects of temperature, pH, substrate concentration, and inhibitors are compared.
Enzymes have optimum temperatures and optimum pH values, and deviations from these conditions cause decreases in reaction rate or enzyme inactivation.

This reference example covered changes in absorbance, blank correction, temperature dependence, pH dependence, substrate-concentration dependence, enzyme concentration, inhibitors, thermal inactivation, control experiments, and measurement errors.
In a report, after calculating the initial rate numerically, it is useful to discuss the results in relation to the enzyme’s three-dimensional structure, active site, substrate binding, and inhibition mechanism.

How to Determine Enzyme Activity

Enzyme activity is determined from the amount of product formed or the amount of substrate consumed per unit time.
When absorbance measurements are used, the reaction rate is determined from the slope of the change in absorbance over time.
When necessary, a calibration curve or molar absorption coefficient is used to convert the change in absorbance into a change in concentration.

When comparing enzyme activity, it is important to use the rate at the beginning of the reaction.
As the reaction proceeds, substrate concentration decreases and product accumulates, so the reaction rate may change.
Therefore, using the initial rate makes it easier to compare enzyme activity among different conditions.

Enzyme activity = Amount of product ÷ Reaction time

Alternatively, enzyme activity = Amount of substrate consumed ÷ Reaction time

Concept of Relative Activity

In experiments comparing the effects of temperature or pH, the highest enzyme activity may be defined as 100%, and the activities under other conditions may be expressed as relative activity.
Using relative activity makes it easier to see changes in activity caused by differences in conditions.

Relative activity (%) = Enzyme activity under each condition ÷ Maximum enzyme activity × 100

Relative activity is suitable for comparing changes caused by different conditions rather than absolute enzyme amounts.
However, if there is an error in the maximum activity measurement, all relative activity values will also be affected.

Effect of Temperature on Enzyme Activity

Enzyme activity is greatly affected by temperature.
At low temperatures, molecular motion of the enzyme and substrate is low and the frequency of collisions is also low, so the reaction rate is small.
As the temperature rises, molecular motion becomes more active and the enzyme and substrate encounter each other more easily, so enzyme activity increases up to a certain range.

However, if the temperature becomes too high, the enzyme protein denatures and the three-dimensional structure of the active site collapses.
As a result, the substrate becomes less able to bind or the catalytic reaction becomes less likely to occur, causing enzyme activity to decrease.
Therefore, enzymes have an optimum temperature at which activity is highest.

Discussion of Optimum Temperature

The optimum temperature is the temperature at which enzyme activity is highest.
At temperatures below the optimum, molecular motion is insufficient and the reaction rate is low.
When the optimum temperature is exceeded, denaturation and inactivation of the enzyme progress and activity decreases.

A temperature-enzyme activity graph may show a peak near the optimum temperature.
However, if the measurement intervals are wide, the true optimum temperature may lie between the temperatures that were actually measured.
In a report, it is natural to write that “the maximum activity was observed near ○○°C under the conditions of this experiment.”

Why Enzyme Activity Decreases at High Temperatures

The main reason enzyme activity decreases at high temperatures is denaturation of the enzyme protein.
Enzymes form their active sites by maintaining specific three-dimensional structures.
At high temperatures, hydrogen bonds, hydrophobic interactions, ionic bonds, and other interactions are disrupted, changing the three-dimensional structure of the enzyme.

When the shape or charge state of the active site changes, the substrate can no longer bind correctly and the reaction becomes less likely to proceed.
Inactivation at high temperatures may also be irreversible, and once the enzyme has denatured, its activity may not return even if the temperature is lowered.

Why Enzyme Activity Is Low at Low Temperatures

At low temperatures, molecular motion of the enzyme and substrate decreases.
When molecular motion is low, the frequency of collisions between enzyme and substrate decreases and fewer molecules have the energy required for the reaction.
Therefore, even if the three-dimensional structure of the enzyme is not damaged, the reaction rate becomes low.

In many cases, the decrease in activity caused by low temperature is reversible, unlike high-temperature denaturation.
Activity may increase again when the temperature is raised.
In a report, it is useful to distinguish decreased reaction rate at low temperature from inactivation caused by denaturation at high temperature.

Effect of pH on Enzyme Activity

Enzyme activity changes greatly depending on pH.
Acidic and basic amino acid residues carrying charges may be involved in the active site of an enzyme.
Because the charge states of these residues change with pH, changes in pH affect substrate binding and catalytic reactions.

In addition, under extremely acidic or basic conditions, the three-dimensional structure of the entire enzyme protein may change and activity may decrease.
Therefore, enzymes have an optimum pH at which activity is highest.

Discussion of Optimum pH

The optimum pH is the pH at which enzyme activity is highest.
At the optimum pH, amino acid residues in the active site have charge states suitable for the reaction, making substrate binding and catalytic reactions more likely to proceed.
When the pH shifts to the acidic or basic side of the optimum, the required charge states change and activity decreases.

A pH-enzyme activity graph may have a bell-shaped curve with a peak near the optimum pH.
However, some enzymes maintain activity over a broad pH range.
In a report, the shape of the obtained graph is discussed in relation to the charge state of the active site and the enzyme structure.

Why Activity Decreases Under Acidic Conditions

Under acidic conditions, the hydrogen-ion concentration increases and amino acid residues in the enzyme become more readily protonated.
When the charge states of residues involved in the active site change, interactions required for substrate binding and catalytic reactions also change.
As a result, enzyme activity may decrease.

Under strongly acidic conditions, the three-dimensional structure of the entire enzyme may also become unstable and denaturation may progress.
In this case, not only changes in charge state but also structural changes may contribute to the decrease in activity.

Why Activity Decreases Under Basic Conditions

Under basic conditions, amino acid residues in the enzyme become more readily deprotonated.
If the charge state of the active site shifts away from the optimum condition, acid-base catalysis required for substrate binding and the reaction may no longer function properly.
As a result, enzyme activity decreases.

Under strongly basic conditions, the three-dimensional structure and stability of the enzyme protein may change and irreversible inactivation may occur.
When discussing decreased activity caused by pH, it is useful to consider both the charge state of the active site and structural changes in the entire protein.

Role of the Buffer

In enzyme activity assays, a buffer is used to keep the pH of the reaction solution constant.
Even when products or substrates change the pH during the enzyme reaction, the buffer suppresses rapid pH changes.
Because changes in pH alter enzyme activity, buffers are important for accurate measurements.

However, the type and concentration of the buffer may also affect enzyme activity.
For enzymes that require metal ions, buffer components may interact with the metal ions.
In a report, it can be explained that the buffer served to maintain a constant pH.

Effect of Inhibitors on Enzyme Activity

An inhibitor is a substance that interferes with an enzyme reaction.
When an inhibitor is present, enzyme-substrate binding may be disturbed or the structure of the enzyme may change, resulting in decreased enzyme activity.
Types of inhibition include competitive inhibition, noncompetitive inhibition, uncompetitive inhibition, irreversible inhibition, and others.

In enzyme activity assays, the effects of an inhibitor on the enzyme reaction are investigated by comparing conditions with and without the inhibitor.
If activity decreases as the inhibitor concentration increases, the inhibitor is considered to interfere with the enzyme reaction in a concentration-dependent manner.

How to Determine the Inhibition Rate

The effect of an inhibitor may be expressed as the inhibition rate.
Using enzyme activity without inhibitor as the reference, the extent to which enzyme activity decreases in the presence of inhibitor is calculated.

Inhibition rate (%) = {1 − Activity with inhibitor ÷ Activity without inhibitor} × 100

A higher inhibition rate indicates a larger decrease in enzyme activity caused by the inhibitor.
However, if there is measurement error in the activity without inhibitor, the inhibition rate will also be affected.
In addition, if the inhibitor itself affects the absorbance measurement, the apparent inhibition rate may become larger or smaller than the actual value.

Discussion of Competitive Inhibition

In competitive inhibition, the inhibitor binds to the same active site as the substrate and interferes with substrate binding.
In this case, increasing the substrate concentration may make it easier for the substrate to compete successfully with the inhibitor, reducing the effect of inhibition.
Kinetically, the apparent Km may increase while Vmax may remain largely unchanged.

It may be difficult to conclude that inhibition is competitive from an enzyme activity assay alone, but if the inhibitory effect becomes weaker in experiments where substrate concentration is increased, competitive inhibition can be considered as a possibility.

Discussion of Noncompetitive Inhibition

In noncompetitive inhibition, an inhibitor binds to a site other than the active site and changes the three-dimensional structure or catalytic function of the enzyme.
In this case, increasing the substrate concentration does not completely eliminate the effect of inhibition.
Kinetically, Vmax may decrease while Km may remain largely unchanged.

In noncompetitive inhibition, the effective activity of the enzyme as a whole decreases, so the maximum rate remains low even at high substrate concentrations.
In a report, this can be described as a possibility that the inhibitor affected the enzyme structure or catalytic function rather than the active site.

Discussion of Irreversible Inhibition

In irreversible inhibition, an inhibitor binds strongly to the enzyme or forms a covalent bond and inactivates the enzyme for a long time or permanently.
In this case, activity may not readily recover even after the inhibitor is removed.
Because the number of enzyme molecules that can effectively function decreases in irreversible inhibition, enzyme activity decreases greatly.

Additional experiments may be necessary to strictly identify irreversible inhibition in student experiments.
However, if enzyme activity decreases greatly after inhibitor treatment and does not recover even after the conditions are restored, irreversible inactivation can be considered as a possibility.

Relationship Between Inhibitor Concentration and Enzyme Activity

As inhibitor concentration increases, the probability that inhibitor molecules bind to the enzyme increases and enzyme activity becomes more likely to decrease.
In an inhibitor concentration-enzyme activity graph, activity may show a tendency to decrease as inhibitor concentration increases.

However, if the inhibitor concentration is too high, effects other than inhibition of the enzyme reaction may occur.
Examples include changes in pH, solvent effects, interference with absorbance measurements, and reactions with the substrate or product.
In a report, it is useful to distinguish whether the inhibitor actually acted on the enzyme or interfered with the measurement system.

When Enzyme Activity Is Evaluated by Absorbance Measurement

In enzyme activity assays, an increase in product or decrease in substrate may be monitored as a change in absorbance.
If the product absorbs at the measurement wavelength, the absorbance increases as the reaction proceeds.
Conversely, if the substrate absorbs and the product does not, the absorbance decreases as the reaction proceeds.

When changes in absorbance are used as enzyme activity, attention must be paid to blank correction, measurement wavelength, contamination of the cell, turbidity of the reaction solution, bubbles, and the linear range of the measuring instrument.
When the absorbance change is small, the influence of measurement noise becomes larger.

Importance of Blanks and Control Experiments

In enzyme activity assays, blanks and control experiments are established to correct for absorbance changes not caused by the enzyme reaction.
Examples include conditions without enzyme, without substrate, and conditions using inactivated enzyme.
These make it possible to distinguish spontaneous substrate degradation, reagent absorbance, and nonenzymatic reactions.

When inhibitors are used, it is also necessary to confirm whether the inhibitor itself absorbs at the measurement wavelength or affects the color reaction.
If control experiments are insufficient, it becomes difficult to determine whether a change in enzyme activity or a change in the measurement system caused the observed result.

When Enzyme Activity Is Higher Than Expected

If enzyme activity is measured as higher than expected, possible causes include a larger enzyme amount, temperature or pH close to the optimum conditions, overreading of the absorbance change, insufficient blank correction, or inclusion of nonenzymatic reactions.
In absorbance measurements, bubbles, turbidity, or contamination of the cell may also make the absorbance appear higher.

When Enzyme Activity Is Lower Than Expected

If enzyme activity is measured as lower than expected, possible causes include partial enzyme inactivation, temperature or pH outside the optimum range, insufficient substrate concentration, contamination with inhibitory substances, or differences in reaction time or measurement-start timing.
Because enzymes may denature during storage or handling, sample handling is important.

Errors in Temperature Conditions

Because enzyme activity is sensitive to temperature, differences between the set temperature and the actual temperature of the reaction solution cause measurement errors.
Even when a constant-temperature bath or incubator is used, the reaction solution may not have sufficiently reached thermal equilibrium.
In addition, if the temperature changes during measurement, activity may vary even under what is intended to be the same condition.

Errors in pH Conditions

Errors in pH conditions include mistakes in buffer preparation, insufficient calibration of the pH meter, changes in pH during the reaction, and temperature-dependent changes in pH.
If the actual pH differs from the intended value, the charge state of the active site changes and the measured enzyme activity shifts.

Particularly when preparing a pH-enzyme activity graph, if each pH condition is not prepared accurately, the optimum pH may be judged incorrectly.

Errors in Reaction Time

Enzyme reactions begin at the moment the enzyme is added.
Therefore, if the time between enzyme addition and the start of measurement, the timing of reaction termination, or the timing of absorbance readings differs among conditions, errors occur in enzyme activity.
Particularly when the reaction rate is high, even a difference of several seconds may affect the results.

Errors Caused by Substrate Concentration

In enzyme activity assays, it is important that the substrate concentration be sufficient.
If the substrate concentration is too low, the active sites of the enzyme are not sufficiently occupied by substrate and the true maximum activity of the enzyme cannot be evaluated.
On the other hand, if the substrate concentration is too high, substrate inhibition or interference with absorbance measurements may occur.

When comparing activity under different temperature or pH conditions, the substrate concentration must be kept constant.
If the substrate concentration differs among conditions, the differences in activity may result from substrate concentration rather than temperature or pH.

Errors Caused by Enzyme Concentration

When enzyme concentration changes, the number of active sites available to participate in the reaction also changes, so the measured enzyme activity changes.
When comparing conditions, it is important to keep the amount of enzyme constant.
If the dispensed volume of the enzyme solution differs or the enzyme precipitates or adsorbs to surfaces, the amount of enzyme actually participating in the reaction may change.

Errors Caused by Enzyme Storage Conditions

Enzyme activity changes depending on storage conditions.
Prolonged exposure to room temperature, repeated freezing and thawing, high temperatures, extreme pH, and organic solvents may cause enzymes to denature or become inactivated.
If activity decreases during storage, enzyme activity will be measured as low under all conditions.

In addition, if enzyme activity decreases over time according to the order of measurement, samples measured later may show lower activity.
In a report, enzyme storage and handling can be considered as sources of error.

When the Inhibitor Interferes With Absorbance Measurement

In experiments using inhibitors, the inhibitor itself may absorb at the measurement wavelength.
The inhibitor may also affect the color reaction or the absorbance of the product.
In this case, the apparent decrease in enzyme activity may result from interference with the measurement system rather than actual inhibition.

Therefore, it is important to examine a blank containing the inhibitor or a condition containing the inhibitor but no enzyme.
In a report, distinguishing between the effect of the inhibitor and interference with the measurement system makes the discussion more persuasive.

How to Interpret Graphs

In enzyme activity assays, graphs of enzyme activity against temperature, pH, or inhibitor concentration make the trends in the results easier to understand.
A temperature-activity graph may show a peak near the optimum temperature.
In a pH-activity graph, activity becomes high near the optimum pH.
In an inhibitor concentration-activity graph, activity may decrease as inhibitor concentration increases.

If some points in the graph deviate greatly from the overall trend, possible causes include an operating error only under that condition, measurement error, a deviation in temperature or pH, bubbles, or differences in reaction-start timing.
The graph is used to discuss the conditions under which the enzyme was most stable or active.

When the Results Can Be Considered Good

Results of an enzyme activity assay can be considered good when the change in absorbance during the initial stage of the reaction is linear, changes in activity with temperature and pH are theoretically reasonable, and the decrease in activity caused by inhibitor addition can be clearly confirmed.
It is also important that blanks and control experiments are appropriate and that repeated measurements under the same conditions show little variation.

Example Discussion When the Experiment Did Not Go Well

If an enzyme activity assay does not go well, possible causes are considered from results such as almost no activity, unnatural graph trends, large variation under the same conditions, no change in activity after inhibitor addition, or absorbance changes even in the blank.
Organizing enzyme inactivation, substrate and enzyme concentrations, temperature and pH conditions, reaction time, absorbance measurement, and inhibitor interference separately makes the discussion easier.

How to Write Points for Improvement

In a discussion of enzyme activity assays, including points for improvement in addition to sources of error makes the report easier to organize.
Improvements are easier to organize when divided into temperature control, pH control, enzyme handling, inhibitor conditions, absorbance measurement, and standardization of reaction time.

Improvements to Temperature and pH Conditions

• Allow the reaction solution to equilibrate sufficiently to the set temperature before measurement
• Keep the temperature constant using a constant-temperature bath or incubator
• Prepare the pH of the buffer accurately
• Calibrate the pH meter
• Use an appropriate buffer to suppress pH changes during the reaction

Improvements to Enzyme and Substrate Handling

• Store the enzyme at an appropriate temperature
• Avoid prolonged exposure of the enzyme to room temperature
• Reduce the number of freeze-thaw cycles
• Keep enzyme and substrate concentrations accurately consistent
• Mix the reaction solution thoroughly

Improvements to Inhibitor and Measurement Operations

• Prepare inhibitor concentrations accurately
• Prepare a blank containing the inhibitor
• Check whether the inhibitor absorbs at the measurement wavelength
• Standardize the timing of reaction initiation and measurement initiation
• Check cuvettes or plates for contamination and bubbles
• Measure the same condition multiple times and take the average

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of enzyme activity assays, simply writing that “activity changed with temperature,” “activity changed with pH,” or “the enzyme was inhibited” results in a superficial discussion.
Relating the enzyme’s three-dimensional structure, active site, charge state, substrate binding, type of inhibition, and measurement errors produces a more persuasive discussion.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of an enzyme activity assay.
Adjust the necessary parts according to your own experimental results.

• Enzyme activity represents the ability of an enzyme to convert substrate into product per unit time.
• The initial rate was determined from the change in absorbance during the initial stage of the reaction and used to evaluate enzyme activity.
• As the temperature increased, molecular motion became more active and enzyme activity increased within a certain range.
• At high temperatures, the enzyme protein denatured and the structure of the active site changed, resulting in decreased enzyme activity.
• Changes in pH altered the charge states of amino acid residues in the active site and affected substrate binding and catalytic reactions.
• At the optimum pH, the active site is considered to have had a charge state suitable for the reaction.
• Because enzyme activity decreased after inhibitor addition, the inhibitor may have interfered with substrate binding or the catalytic reaction.
• If activity decreased as inhibitor concentration increased, the inhibitory effect is considered concentration-dependent.
• If blank correction is insufficient, absorbance changes not caused by the enzyme reaction may be included as activity.
• The storage condition of the enzyme and differences in reaction-start timing affect measured enzyme activity.

Points to Check When Discussing Enzyme Activity Assays

Checking the following points before writing the report makes the discussion easier to write.

• Have you explained how enzyme activity was determined?
• Have you used data from the initial stage of the reaction?
• Have you explained the relationship between temperature and activity in terms of molecular motion and denaturation?
• Have you determined the optimum temperature from the graph?
• Have you explained the relationship between pH and activity in terms of the charge state of the active site?
• Have you determined the optimum pH from the graph?
• Have you compared activity with and without inhibitor?
• Have you calculated the inhibition rate?
• Have you considered interference of the inhibitor with the measurement?
• Have you checked blanks and control experiments?
• Have you considered enzyme inactivation and storage conditions as sources of error?
• Do the points for improvement correspond to the sources of error?

Summary

In enzyme activity assays, the conditions under which an enzyme efficiently converts substrate into product are investigated.
As temperature increases, molecular motion becomes more active and enzyme activity increases within a certain range, but at high temperatures the enzyme protein denatures and activity decreases.
Therefore, enzymes have an optimum temperature at which activity is highest.

pH affects the charge states of amino acid residues in the active site and the three-dimensional structure of the enzyme.
At the optimum pH, conditions suitable for substrate binding and catalytic reactions are maintained and enzyme activity becomes high.
When the pH shifts to the acidic or basic side, charge states and structure change and activity decreases.

Inhibitors bind to the active site or another site on the enzyme and interfere with substrate binding or catalytic reactions, thereby decreasing enzyme activity.
In a report, changes in activity caused by temperature, pH, and inhibitors should be discussed in relation to the three-dimensional structure of the enzyme, the active site, charge states, and the type of inhibition.
Including blank correction, temperature control, pH preparation, enzyme storage conditions, and interference of inhibitors with the measurement produces a more persuasive discussion of enzyme activity assays.