Unit 6: Enzyme Activity - Subjective Questions

Salivary amylase, also called ptyalin, is a hydrolytic enzyme secreted mainly by the salivary glands. It catalyzes the breakdown of starch into smaller carbohydrates.

• It acts mainly on the -1,4 glycosidic bonds of starch.
• Its products include maltose, maltotriose, and short-chain dextrins.
• It does not efficiently hydrolyze the -1,6 branch-point bonds of amylopectin.
• The enzyme begins carbohydrate digestion in the mouth, where the pH is approximately neutral.
• Its activity decreases in the acidic environment of the stomach.

The iodine test is based on the formation of a blue-black complex between iodine and intact starch.

• Starch gives a blue-black color when iodine solution is added.
• Salivary amylase hydrolyzes starch into maltose and dextrins.
• These hydrolysis products do not produce the same blue-black color with iodine.
• As enzyme activity increases, less intact starch remains and the intensity of the blue-black color decreases.
• Therefore, the decrease in color intensity is proportional to starch hydrolysis and can be used to estimate salivary amylase activity.

The reaction may be represented as:

A saliva sample should be collected carefully to avoid contamination and variation.

• Rinse the mouth with distilled water and wait for a few minutes.
• Collect fresh saliva in a clean, dry container without coughing or scraping the throat.
• Dilute the saliva with a known volume of distilled water or buffer, if required by the protocol.
• Mix the diluted sample thoroughly to obtain a uniform enzyme solution.
• Keep the sample cool if there is a delay before the experiment.
• Label the sample clearly and use it within the recommended time.
• Avoid food, beverages, smoking, and vigorous mouth washing before collection because these factors may alter enzyme activity.

The dilution factor must be recorded because it is required for calculating the activity in the original saliva sample.

A general procedure for the iodine-starch method is as follows:

1. Prepare a known concentration of soluble starch solution.
2. Add a measured volume of diluted saliva to the starch solution.
3. Incubate the reaction mixture at the selected temperature for a fixed time.
4. Stop the reaction by adding iodine reagent or by transferring an aliquot into iodine solution, according to the laboratory protocol.
5. Observe the color produced or measure its absorbance using a colorimeter or spectrophotometer.
6. Compare the test reading with a starch blank and suitable controls.
7. Calculate the amount of starch hydrolyzed during the incubation period.

The assay must keep the volumes, pH, temperature, incubation time, and substrate concentration constant so that differences in the result are due mainly to enzyme activity.

Blanks and controls are necessary to identify color changes that are not caused by enzymatic hydrolysis.

• Substrate blank: Contains starch and iodine reagent but no active saliva. It shows the color produced by the original starch concentration.
• Enzyme control: Contains saliva that has been inactivated, or a suitable substitute specified by the method. It indicates any color contribution from saliva or non-enzymatic effects.
• Test sample: Contains active saliva and starch. Its lower color intensity reflects starch digestion.

The blank reading is used to correct the test reading. Without controls, background color, reagent impurities, and spontaneous changes in starch could be incorrectly interpreted as amylase activity.

If the amount of starch hydrolyzed is determined from a calibration curve, enzyme activity can be expressed as the amount of substrate converted per unit time.

Let be the amount of starch hydrolyzed, be the incubation time, and be the volume of enzyme sample used. Then:

If the enzyme sample was diluted, the activity in the original saliva is calculated as:

A common unit is saliva. The exact unit depends on the calibration method and the laboratory protocol. All volumes must be expressed consistently, and the incubation time must be measured accurately.

A standard curve relates known starch concentrations to their iodine color intensities.

• Prepare several starch standards of known concentration.
• Add equal volumes of iodine reagent to each standard.
• Measure the absorbance of each standard at the appropriate wavelength.
• Plot absorbance on the -axis against starch concentration on the -axis.
• Measure the absorbance of the enzyme assay mixture.
• Use the graph or its regression equation to determine the concentration of starch remaining.
• Subtract the remaining starch from the initial starch to obtain the amount hydrolyzed.

For a linear calibration equation, the relationship may be written as:

where is absorbance, is starch concentration, is the slope, and is the intercept.

Temperature affects enzyme activity by changing molecular motion and protein structure.

• At low temperatures, enzyme and substrate molecules move slowly, so the reaction rate is low.
• As temperature increases, collisions become more frequent and the activity generally increases.
• Salivary amylase usually shows maximum activity near physiological temperature, approximately .
• Above the optimum temperature, heat disrupts the weak bonds maintaining the enzyme structure.
• The active site changes shape, causing enzyme activity to decrease because of denaturation.
• Very high temperatures may cause irreversible loss of activity.

A temperature experiment should use identical substrate and enzyme concentrations, pH, and incubation times for all tubes.

pH affects the ionization of amino acid residues at the enzyme active site and may alter the shape of the enzyme.

• Salivary amylase works most effectively in a nearly neutral medium, commonly around pH to .
• At the optimum pH, the active site has the appropriate charge and conformation for binding starch.
• Strongly acidic or strongly alkaline conditions reduce enzyme activity.
• Extreme pH may cause irreversible denaturation of the protein.
• The stomach's low pH eventually inhibits salivary amylase after swallowed food mixes with gastric acid.

Buffers should be used to maintain a constant pH during the assay, because uncontrolled pH changes can produce unreliable results.

Effect of enzyme concentration:

• When substrate is present in excess, increasing the amount of salivary amylase increases the reaction rate.
• The increase is approximately proportional if other conditions remain constant.
• Eventually, the amount of substrate or assay time becomes limiting.

Effect of substrate concentration:

• At low substrate concentration, increasing starch concentration increases the reaction rate because more enzyme active sites can bind substrate.
• At high substrate concentration, the enzyme becomes saturated.
• The reaction rate then approaches a maximum, , and further substrate addition has little effect.

This behavior is commonly described by:

where is reaction velocity and is substrate concentration.

Enzyme activity is the amount of starch hydrolyzed by a given volume or amount of saliva per unit time. It may be reported as:

Specific activity is enzyme activity expressed per unit mass of total protein:

The distinction is important because:

• Activity depends on the volume of sample used.
• Specific activity indicates the purity or catalytic efficiency of the enzyme preparation.
• A partially purified salivary amylase sample may have a higher specific activity than crude saliva even if its total activity is lower.
• Specific activity is especially useful when comparing samples with different protein concentrations.

Visual observation provides only a rough estimate of color intensity, whereas spectrophotometry provides a quantitative measurement.

• A spectrophotometer measures absorbance at a selected wavelength.
• Absorbance is related to the concentration of the colored starch-iodine complex by the Beer–Lambert law:
  
  where is absorbance, is molar absorptivity, is path length, and is concentration.
• Small differences in starch concentration can be detected more reliably.
• Replicate readings can be averaged to improve precision.
• A standard curve can convert absorbance into starch concentration.
• Instrumental measurement reduces observer bias, although proper blanking and calibration are still essential.

Incubation time determines how long salivary amylase is allowed to hydrolyze starch.

• A longer incubation generally permits more starch breakdown.
• If different tubes are incubated for different periods, their results cannot be compared fairly.
• During the early phase of the reaction, product formation may be approximately proportional to time.
• At later stages, substrate depletion, product accumulation, or enzyme inactivation may cause the rate to change.
• The reaction should be stopped or analyzed at a fixed time for every sample.
• A stopwatch or timer should be used, and samples should be processed in the same order or simultaneously when possible.

Thus, controlled incubation time is necessary for accurate calculation of reaction velocity.

Initial reaction velocity, represented by , is the rate of product formation or substrate disappearance at the beginning of the reaction.

It is useful because:

• Substrate concentration has not changed substantially.
• Product inhibition is minimal.
• The enzyme is less likely to have lost activity.
• The rate is less affected by reverse reactions or accumulation of hydrolysis products.
• It provides a more accurate measure of the enzyme's catalytic activity.

For a short, controlled assay, the initial rate may be calculated as:

where is the amount of starch hydrolyzed and is the measured reaction time.

Important sources of error include:

• Unequal saliva collection: Collect and dilute samples using accurately measured volumes.
• Pipetting errors: Use calibrated pipettes and consistent technique.
• Variable incubation time: Start and stop reactions at fixed times.
• Incorrect temperature: Use a calibrated water bath or incubator.
• Uncontrolled pH: Use an appropriate buffer.
• Contaminated glassware: Use clean, dry, labeled tubes.
• Improper iodine volume: Add the same reagent volume to every tube.
• Poor mixing: Mix each reaction thoroughly but gently.
• Delayed color measurement: Read samples promptly because color may change with time.
• Incorrect dilution correction: Record every dilution and multiply by the correct dilution factor.
• Instrumental error: Blank and calibrate the colorimeter or spectrophotometer before use.

Replicate measurements and inclusion of controls improve reliability.

An inhibitor is a substance that decreases enzyme activity without necessarily being consumed in the reaction.

• A competitive inhibitor resembles the substrate and competes for the active site. Its effect may be reduced by increasing substrate concentration.
• A noncompetitive inhibitor binds at a site other than the active site and reduces catalytic activity. Increasing substrate concentration may not fully overcome its effect.
• Saliva may also contain substances from food, medicines, or oral products that interfere with the assay.
• Inhibitors can reduce starch hydrolysis, leaving more starch to react with iodine and producing a darker color.
• If inhibitors are present in one sample but not in another, comparisons may be invalid.

Samples should be collected under standardized conditions, and suspected interfering substances should be identified or removed where possible.

Both methods measure starch digestion but detect different reaction components.

Iodine method:

• Measures the decrease in intact starch.
• Starch produces a blue-black color with iodine.
• It is simple, rapid, and suitable for routine teaching laboratories.
• It may be less sensitive when only small amounts of starch are hydrolyzed.

Reducing-sugar method:

• Measures products such as maltose and other reducing sugars formed during hydrolysis.
• Reagents such as DNS react with reducing sugars to produce a colored compound.
• The color intensity generally increases as enzyme activity increases.
• It can provide a quantitative estimate using a maltose or glucose standard curve.

The iodine method follows substrate disappearance, whereas the reducing-sugar method follows product formation.

A buffer resists changes in pH when small amounts of acid or base are added.

• It maintains the reaction mixture near the optimum pH of salivary amylase.
• It prevents changes caused by the saliva sample, substrate, or reaction products.
• Stable pH helps maintain the correct charge and three-dimensional structure of the enzyme's active site.
• It improves reproducibility between tubes and between experimental trials.
• The buffer concentration should not be so high that it interferes with the enzyme or color reaction.

A suitable buffer is selected according to the required experimental pH, commonly near neutral for salivary amylase.

Enzyme saturation occurs when nearly all active sites of the enzyme are occupied by substrate molecules.

• At low starch concentration, many active sites are unoccupied, so adding more starch increases the reaction rate.
• At high starch concentration, most salivary amylase molecules are continuously engaged with substrate.
• The rate approaches a maximum value called .
• Further increases in starch concentration produce little or no increase in rate.
• The reaction is then limited by the amount of enzyme and its catalytic turnover rather than by substrate availability.

This concept explains why substrate concentration should be selected carefully when comparing enzyme samples.

A suitable experimental design is:

• Prepare equal volumes and concentrations of starch solution and diluted saliva.
• Place reaction tubes at several temperatures, such as , room temperature, , and a higher temperature.
• Equilibrate both the enzyme and substrate separately at each temperature.
• Mix them and incubate for the same length of time.
• Stop the reactions at the same time interval.
• Add iodine reagent and measure the color intensity or absorbance.
• Include a starch blank for each temperature if required.
• Convert absorbance into starch remaining and calculate the amount hydrolyzed.
• Plot enzyme activity against temperature.

The expected result is low activity at low temperature, maximum activity near the physiological optimum, and reduced activity at high temperature because of denaturation.