3.2.1—Effects on enzyme reaction rate

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.

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.