Factors Affecting Enzyme Activity Revision Notes | A Level AQA Biology

1. Why Enzyme Activity Matters

Every reaction that keeps you alive — from digesting food to copying your DNA — is catalysed by an enzyme. Without precise control of enzyme activity, these reactions would either grind to a halt or spiral out of control.

Enzymes catalyse a vast range of intracellular reactions (inside cells, such as glycolysis in the cytoplasm) and extracellular reactions (outside cells, such as the digestion of starch by salivary amylase in the mouth). This means that factors affecting enzyme activity have consequences that ripple from the molecular level right up to whole-organism function — a drop in body temperature slows metabolic enzymes, and a shift in blood pH can denature haemoglobin-processing enzymes in red blood cells.

This lesson unpacks every factor the AQA specification requires you to know, builds the conceptual reasoning behind each graph shape, and prepares you to design and evaluate a required practical investigation.

2. The Active Site: The Foundation You Need First

Picture the active site as a lock and the substrate as a key. The lock only accepts one specific key shape — this is what gives enzymes their specificity. The induced fit model refines this: the active site is not perfectly rigid but changes shape slightly as the substrate binds, like a hand closing around a ball. This induced fit is why any factor that alters the enzyme's three-dimensional shape will directly affect its ability to catalyse reactions.

Keep this image in mind throughout the lesson. Every factor we study — temperature, pH, inhibitors — ultimately works by either increasing the frequency of successful enzyme-substrate collisions or by disrupting the active site's complementary shape.

3. Temperature and Its Effect on Reaction Rate

Temperature affects enzyme activity in two competing ways, and understanding both is what separates a grade 6 answer from a grade 9 answer.

As temperature rises from 0 °C towards the optimum, molecules gain kinetic energy. Enzyme and substrate molecules move faster, collide more frequently, and form enzyme-substrate complexes at a higher rate — so the rate of reaction increases. For many human enzymes, the optimum temperature is around 37 °C, though this varies between organisms (thermophilic bacteria have enzymes with optima above 70 °C).

Above the optimum, the increased thermal energy begins to break the hydrogen bonds and other weak interactions that maintain the enzyme's tertiary structure. The active site changes shape and is no longer complementary to the substrate. This is denaturation — at temperatures significantly above the optimum, denaturation is effectively irreversible under physiological conditions. The rate of reaction falls sharply and eventually reaches zero.

Notice the asymmetry of the curve: the rise to the optimum is gradual (kinetic energy increasing), but the fall after the optimum is steep (denaturation is rapid and irreversible). Examiners frequently ask you to explain this asymmetry — make sure you distinguish between the two mechanisms on either side of the peak.

4. pH and Enzyme Activity

pH works through a similar structural mechanism to temperature, but the chemistry is slightly different. The active site contains charged R groups (side chains) of amino acids. These charges are responsible for holding the substrate in place and for the precise shape of the active site.

When pH moves away from the optimum, the concentration of hydrogen ions (H⁺) changes. These ions interact with the charged R groups, altering the pattern of ionic bonds and hydrogen bonds within the enzyme. The tertiary structure — and therefore the active site shape — changes. The substrate can no longer bind, and the rate of reaction falls. At extreme pH values, denaturation occurs.

Each enzyme has its own optimum pH, reflecting where it works in the body. Pepsin (a protease in the stomach) has an optimum around pH 2, while trypsin (a protease in the small intestine) works best around pH 8. This is not a coincidence — it reflects the pH of the environment in which each enzyme operates.

5. Substrate Concentration and Enzyme Concentration

Now that you understand how the active site can be disrupted, consider what happens when the active site is functioning normally but the supply of reactants changes.

When substrate concentration increases (with enzyme concentration held constant), more substrate molecules are available to collide with and bind to free active sites. The rate of reaction increases. However, there is a ceiling: once all active sites are occupied simultaneously — the enzyme is saturated — adding more substrate has no further effect. The rate plateaus at the maximum rate (V-max). The graph shows a curve that rises steeply then levels off to a plateau.

When enzyme concentration increases (with excess substrate present), there are more active sites available. More enzyme-substrate complexes can form per unit time, so the rate of reaction increases proportionally. The relationship is approximately linear — double the enzyme concentration, roughly double the rate — provided substrate is not limiting.

6. Competitive and Non-Competitive Inhibition

Inhibitors are molecules that reduce the rate of enzyme-controlled reactions. The AQA specification requires you to distinguish clearly between two types, and examiners regularly set questions that penalise students who confuse them.

Think of competitive inhibition as two people trying to sit in the same chair. If you add more people who want the chair (more substrate), the inhibitor is less likely to be sitting there at any given moment. This is why increasing substrate concentration reduces the effect of a competitive inhibitor — the inhibitor's impact is concentration-dependent.

Non-competitive inhibition is more like someone bending the chair so that nobody can sit in it comfortably, regardless of how many people are waiting. The active site is distorted while the inhibitor is bound, and no amount of extra substrate can fix that.

On a rate-versus-substrate-concentration graph, competitive inhibition shifts the curve to the right (you need more substrate to reach the same rate) but V-max is eventually achievable. Non-competitive inhibition lowers V-max — the plateau is reached at a lower rate — and the curve cannot be rescued by adding more substrate.

7. Enzymes Across Scales: Intracellular and Extracellular

The AQA specification asks you to appreciate that enzymes catalyse reactions that determine structures and functions from the cellular to the whole-organism level. This is not a throwaway line — it is asking you to connect molecular biology to physiology.

Intracellular enzymes work inside cells. DNA polymerase in the nucleus catalyses DNA replication; enzymes of the Krebs cycle work in the mitochondrial matrix; carbonic anhydrase in red blood cells catalyses the conversion of carbon dioxide and water into carbonic acid, enabling efficient CO₂ transport. Each of these reactions is essential to cell survival and, by extension, to the organism.

Extracellular enzymes are secreted and work outside cells. Digestive enzymes — amylase, lipase, proteases — are released into the gut lumen and break down large food molecules into absorbable monomers. Without these enzymes, digestion would be impossibly slow. At the whole-organism level, the consequences of enzyme malfunction are visible as disease: phenylketonuria results from a deficiency of phenylalanine hydroxylase, and lactose intolerance arises from insufficient lactase activity in the small intestine.

8. Required Practical: Investigating a Variable Affecting Enzyme Activity

Required Practical 1 asks you to investigate the effect of a named variable on the rate of an enzyme-controlled reaction. The most common choices are temperature, pH, or substrate concentration, using enzymes such as catalase (breaking down hydrogen peroxide), amylase (breaking down starch), or lipase. The principles of good experimental design apply regardless of which variable you choose.

When evaluating this practical, consider both reliability and validity. Reliability is improved by repeating each temperature at least three times and calculating a mean. Validity depends on controlling all variables other than temperature — if the potato discs vary in mass, the amount of catalase varies, confounding your results. Using a colorimetric method (e.g. iodine with amylase and starch) instead of gas collection can reduce measurement error for some enzyme investigations.

Worked example: Calculating Initial Rate of Reaction from Practical Data

In a catalase investigation at 30 °C, a student collects the following data: 0 s → 0 cm³ O₂; 30 s → 3.6 cm³ O₂; 60 s → 6.8 cm³ O₂; 90 s → 9.0 cm³ O₂. Calculate the initial rate of reaction in cm³ s⁻¹.

1. Use the first time interval (0 to 30 s) where the rate is most linear and substrate is not yet limiting. — The initial rate is taken from the steepest, earliest part of the volume-time curve, before substrate depletion slows the reaction. [M1]
2. ΔV = 3.6 cm³ − 0 cm³ = 3.6 cm³ — This is the volume of O₂ produced in the first 30 seconds.
3. Δt = 30 s − 0 s = 30 s — Use seconds as the unit throughout for consistency.
4. Initial rate = ΔV ÷ Δt = 3.6 ÷ 30 = 0.12 cm³ s⁻¹ — Rate = gradient of the volume-time graph. [A1] Always include units.
5. 0.12 cm³ s⁻¹ × 30 s = 3.6 cm³ ✓ — Multiplying rate by time recovers the measured volume, confirming the calculation is consistent.

Answer: Initial rate of reaction = 0.12 cm³ s⁻¹

9. Exam Technique: Writing for Full Marks

Enzyme questions are among the most predictable on the AQA paper — the same concepts appear year after year, but students consistently lose marks through imprecise language. The guidance below targets the most common mark-losing errors.