3.2 Factors Affecting Enzyme Action

Compare enzyme factors through the bottleneck they change: successful enzyme–substrate collisions, the number of usable active sites, or the shape and function of those sites. Enzyme rate means the initial change in product or substrate signal per unit time, not simply the final amount.

• Temperature: Below the enzyme’s optimum, warmer conditions usually increase molecular movement and successful collisions, so the initial rate rises. Above the optimum, bonds maintaining the tertiary structure can be disrupted; the active site changes shape and the rate falls sharply through denaturation.
• pH: Each enzyme has an optimum pH. Moving away from it changes interactions and charges that maintain the active site, so productive substrate binding becomes less likely; extreme pH can denature the enzyme.
• Enzyme concentration: More enzyme supplies more active sites, so initial rate can increase while sufficient substrate remains. Once substrate becomes limiting, adding enzyme no longer gives the same increase.
• Substrate concentration: More substrate increases the chance of enzyme–substrate complex formation while active sites are available. The rate then approaches a plateau when the fixed active sites are saturated.
• Inhibitor concentration: Increasing inhibitor concentration reduces the fraction of enzyme able to catalyse productively, so the initial rate falls. Whether extra substrate can offset the effect depends on the inhibitor’s binding mechanism.

For a fair factor comparison, vary one factor while controlling the others, including temperature, pH, enzyme and substrate concentrations, volumes, mixing and measurement time. Plot the chosen progress signal against time and compare initial gradients; do not infer a rate from unequal endpoint amounts or from a plateau alone.

Avoid absolute statements such as “higher temperature” or “more enzyme” always increases rate. The direction and curve shape depend on the optimum, denaturation, substrate availability, active-site saturation and inhibitor type. Exact optimum values are enzyme-specific and are not assumed here.

On a graph of initial reaction rate, v, against substrate concentration, [S], Vmax is the capacity ceiling approached when the enzyme’s active sites are saturated. Km is the substrate concentration at which the initial rate is half of Vmax.

1. Find Vmax: Inspect the high-[S] plateau or limiting value approached by the curve; this is the estimated Vmax.
2. Find half Vmax: Mark the y-value halfway between zero and Vmax.
3. Read Km: Move horizontally from half Vmax to the curve, then vertically to the [S] axis. That x-value is Km.
4. Explain the shape: At low [S], adding substrate raises productive complex formation. At high [S], most active sites are occupied, so the curve flattens toward Vmax.

Worked read-off: if a curve approaches a clear plateau, label that plateau Vmax, mark its midpoint on the rate axis, and project from the midpoint through the curve to the substrate axis. The substrate-axis reading at that intercept is Km; do not read Km at the plateau itself.

Km is not the substrate concentration at Vmax. A lower Km can indicate higher apparent affinity only when the same model and comparable conditions apply; Vmax also depends on the amount of active enzyme. Read initial-rate data and do not invent precision beyond the graph.

Initial rate comes from the earliest linear part of a product–time or substrate–time record, before substrate depletion, product accumulation or drifting conditions begin to change the rate.

1. Prepare a sensible range of one chosen factor; equilibrate temperature or pH before mixing.
2. Start every reaction in the same way and record the signal at short, equal intervals.
3. Calculate the gradient of the earliest linear section.
4. Repeat each condition, investigate anomalies and plot mean initial rate against the independent variable.

Keep the comparison fair:

• hold enzyme and substrate source, concentrations and volumes constant unless one is the independent variable;
• use buffer to control pH and a water bath to control temperature;
• use the same timing and measurement method for every treatment;
• include a blank when the method has background colour or signal.

If product concentration rises by 12 µmol dm⁻³ during the first 30 s linear interval, the initial rate is 12 ÷ 30 = 0.40 µmol dm⁻³ s⁻¹. Apply the same interval rule to every treatment.

An endpoint amount is not an initial rate. A fixed endpoint can hide an early rate difference, while uncontrolled temperature or pH can imitate a concentration effect.

Reversible inhibitors reduce enzyme activity by limiting productive enzyme–substrate complexes. The binding site determines whether adding more substrate can recover the maximum rate.

Active-site competition lowers the chance that a substrate occupies the site at a given concentration, so increasing substrate can restore productive binding and the original maximum. Alternative-site binding changes the active site on affected enzyme molecules, so increasing substrate cannot recover the lost catalytic capacity. Read Vmax and Km from matched initial-rate saturation curves, not from one inhibitor point.

End-product inhibition is a reversible control example: as the final product accumulates, it binds at an alternative site on an earlier enzyme and slows the pathway. When product concentration falls, inhibition is relieved and the pathway can resume.

The table uses the standard simplified comparison: real data must be judged from the whole curve and matched conditions. Do not identify inhibitor type from one rate value, call alternative-site binding harmless, or introduce irreversible inhibition into this objective.

A free enzyme is added to the reaction solution. An immobilised enzyme is bound to an inert, stationary, insoluble support such as alginate, while substrate is brought to the support and product is collected separately.

• Compare equal reaction times and measure product formed per unit time, not just the final product amount.
• Keep substrate concentration, pH, temperature, total volume and measurement method the same.
• Match the starting catalytic amount as closely as possible and keep support amount/size consistent when comparing immobilised preparations.
• Repeat both treatments and compare representative means; a lower observed rate may reflect access or support effects rather than an intrinsically different enzyme mechanism.

In a supported lactase process, milk is passed over alginate beads containing lactase. Lactose is hydrolysed to glucose and galactose while the enzyme remains associated with the support, allowing the product stream to be collected and the preparation reused.

Immobilisation does not make an enzyme permanently stable, guarantee a faster rate, or justify claims that enzyme can never leave the support. Judge the measured system using matched controls and the same time basis; do not invent column dimensions or other apparatus specifications.