Enzymes are biological catalysts: protein molecules that speed up chemical reactions in living cells without being consumed. Every metabolic process depends on them — digesting food, copying DNA, releasing energy from glucose. They are exquisitely sensitive to temperature and pH; shift either too far from the optimum and activity falls to zero.
What is an enzyme and how does it work?
An enzyme is a protein that acts as a biological catalyst. It speeds up a specific chemical reaction by providing an alternative reaction pathway with a lower activation energy — exactly as an inorganic catalyst does, but with remarkable specificity for a single reaction or group of closely related reactions.
The specificity of enzymes arises from their three-dimensional shape. Each enzyme has an active site — a precisely shaped region on its surface into which only a particular molecule (the substrate) can fit. The substrate that an enzyme acts upon is specific to that enzyme: amylase breaks down starch, not protein; protease breaks down protein, not starch.
When the substrate binds to the active site, an enzyme-substrate complex forms. The enzyme lowers the activation energy of the reaction; the reaction proceeds; the products are released; and the enzyme is free to act again. Because the enzyme is unchanged at the end, it can catalyse the same reaction repeatedly.
What are the lock-and-key and induced-fit models?
Two models describe how the substrate fits into the active site:
The lock-and-key model
The lock-and-key model (proposed by Emil Fischer in 1894) describes the enzyme's active site as a rigid, fixed shape — like a lock — and the substrate as a perfectly complementary shape — the key. Only the correct substrate can fit.
The induced-fit model
The induced-fit model (Daniel Koshland, 1958) is the more accurate modern model. The active site is not completely rigid — it changes shape slightly when the substrate binds, moulding itself around the substrate for a tighter, more precise fit. This explains why enzymes can act on slightly different but similar substrates, and why they are even more specific than the lock-and-key model suggests.
At GCSE, you are expected to know both models and understand that the induced-fit model is the currently accepted explanation.
How does temperature affect enzyme activity?
Temperature has a two-stage effect on enzyme activity:
Below the optimum temperature: As temperature increases, particles move faster, the rate of collision between enzyme and substrate increases, and more enzyme-substrate complexes form per second. Reaction rate increases.
At and above the optimum temperature: The enzyme molecule vibrates more violently. The weak bonds (hydrogen bonds and other interactions) holding the protein in its precise three-dimensional shape begin to break. The active site changes shape — it can no longer accept the substrate. The enzyme is said to be denatured. Denaturation is permanent and irreversible.
Most human enzymes have an optimum temperature of approximately 37 °C (body temperature), where enzyme activity is highest. Above ~45 °C, most human enzymes are rapidly denatured.
| Temperature | Effect on enzyme activity |
|---|---|
| Increasing (below optimum) | Rate increases — faster molecular movement, more collisions |
| At optimum (~37 °C for most human enzymes) | Maximum activity |
| Above optimum | Rate decreases — denaturation begins, active site distorts |
| Well above optimum | Activity falls to zero — enzyme fully denatured |
How does pH affect enzyme activity?
Enzymes also have an optimum pH at which their activity is highest. Changes in pH alter the charge on amino acids in the active site, disrupting the ionic and hydrogen bonds that maintain the enzyme's shape. If the pH is too far from the optimum, the active site distorts and the enzyme denatures.
Different enzymes have very different pH optima:
| Enzyme | Location | Optimum pH | Function |
|---|---|---|---|
| Salivary amylase | Mouth | ~7 (neutral) | Breaks down starch → maltose |
| Pepsin | Stomach | ~2 (strongly acidic) | Breaks down protein |
| Trypsin | Small intestine | ~8 (slightly alkaline) | Breaks down protein |
| Catalase | Most body cells | ~7 | Breaks down hydrogen peroxide |
The stomach's extremely acidic environment (pH ~2) is perfect for pepsin but would denature salivary amylase — which is why salivary amylase stops working once food reaches the stomach.
What factors affect enzyme activity in summary?
| Factor | Effect of increasing factor | Effect of decreasing factor |
|---|---|---|
| Temperature | Rate increases up to optimum; then denaturation reduces/stops activity | Rate decreases (slower particle movement) |
| pH | Rate increases towards optimum; denaturation if too far from optimum | Rate decreases; denaturation if too far from optimum |
| Substrate concentration | Rate increases (more enzyme-substrate complexes) until all active sites saturated | Rate decreases |
| Enzyme concentration | Rate increases (more active sites available) if substrate is not limiting | Rate decreases |
What are the practical applications of enzymes?
Enzymes have many industrial and medical applications:
- Biological washing powders contain proteases and lipases that digest protein and fat stains at low temperatures.
- Cheese and yogurt manufacture use enzymes from microbes to digest milk proteins and fats.
- Medical diagnostics — blood glucose monitors use the enzyme glucose oxidase to detect glucose levels.
- Food industry — amylase converts starch to glucose syrup; glucose isomerase converts glucose to fructose (a sweeter sugar used in soft drinks).
- Genetic engineering — restriction enzymes cut DNA at specific sequences, enabling gene cloning and CRISPR editing.
Frequently asked questions
What is enzyme denaturation and is it reversible?
Denaturation is the permanent change in an enzyme's three-dimensional shape caused by excessive heat or extreme pH. The bonds holding the protein in its active conformation break; the active site changes shape and can no longer bind the substrate. Denaturation is irreversible — the enzyme cannot return to its original shape when conditions improve. This is different from a temporary loss of activity at low temperatures, which reverses when the temperature rises back to the optimum.
What is the difference between the lock-and-key and induced-fit models of enzyme action?
In the lock-and-key model, the enzyme's active site has a fixed, rigid shape that exactly matches the shape of the substrate — like a lock accepting only one key. In the induced-fit model (now considered more accurate), the active site is flexible and changes shape slightly as the substrate binds, moulding itself for a tighter, more specific fit. The induced-fit model better explains enzyme specificity and the fact that some enzymes can act on several structurally similar substrates.
Why does enzyme activity increase with temperature up to a point and then decrease?
Up to the optimum temperature, increasing temperature gives molecules more kinetic energy — enzyme and substrate collide more frequently, forming more enzyme-substrate complexes per second, so the reaction rate rises. Above the optimum, the extra energy causes the enzyme's protein structure to vibrate so violently that the hydrogen bonds and other weak interactions maintaining the active site's shape are broken. The active site distorts, substrates can no longer bind, and the reaction rate falls. At high enough temperatures the enzyme is fully denatured and activity reaches zero.
How do washing powders containing enzymes work?
Biological washing powders contain enzymes — usually proteases (to break down protein stains such as blood and egg), lipases (fat stains), and amylases (starch stains). These enzymes digest large, insoluble molecules that ordinary detergents cannot remove, breaking them into smaller, soluble products that wash away. They work best at lower temperatures (30–40 °C), which saves energy. Non-biological powders use only chemical detergents and are less effective on biological stains, but do not carry the slight risk of skin irritation from residual enzymes.
For Socratic GCSE biology with Professor Darwin — tracing enzyme action from molecule to cell to organism — visit aitutors.me.