Cambridge O Level Biology · Syllabus 5090 · Enzymes
Enzyme
What is Enzyme?
A protein that functions as a biological catalyst: it increases the rate of a metabolic reaction inside or outside a cell and is not changed by that reaction, so a single enzyme molecule can catalyse the same reaction repeatedly. Its function depends on the shape of a region called the active site.
This definition is part of the Enzymes chapter in Cambridge O Level Biology.
Enzyme in context
Enzymes are proteins that function as biological catalysts: each one increases the rate of a metabolic reaction and is not changed by that reaction, so a single enzyme molecule can catalyse the same reaction again and again. Cambridge O Level Biology (5090) Chapter 5 explains how an enzyme works through its active site, the enzyme-substrate complex and the lock-and-key model; why temperature and pH change the rate of an enzyme-catalysed reaction; what denaturation is and why it is not the same as being killed; and how to investigate amylase and catalase activity and interpret a product-time graph.
An enzyme is a protein that works as a biological catalyst: it increases the rate of a metabolic reaction and is not changed by that reaction. Each enzyme has an active site with a particular shape. A substrate whose shape is complementary to that active site fits into it and an enzyme–substrate complex forms; the reaction happens, the products no longer fit in the same way and leave, and the unchanged enzyme is free to work again. Raising the temperature towards the optimum gives molecules more kinetic energy, so collisions are more frequent and more of them are effective, and the rate rises. Above the optimum, increased vibration disrupts the bonds holding the enzyme's three‑dimensional shape, the active site changes shape, the substrate no longer fits, and the rate falls rapidly — the enzyme has been denatured. pH behaves the same way around an optimum pH that differs from enzyme to enzyme.
Common mistakes with Enzyme
- “Enzymes speed up reactions by giving them energy.” RepairAn enzyme increases the rate of the reaction. It does not supply energy to it. WhyThe definition of a catalyst says nothing about energy, and saying it does contradicts “not changed by the reaction”. A source of energy would be used up.
- “High temperature kills the enzyme.” RepairHigh temperature denatures the enzyme: the bonds holding its three-dimensional shape are disrupted and the active site changes shape. WhyAn enzyme is a protein molecule, not a living organism. “Killed” contradicts the definition you gave earlier in the same answer.
- “Low temperature denatures the enzyme.” RepairLow temperature slows the reaction: less kinetic energy, less frequent collisions, fewer enzyme–substrate complexes per second. The enzyme keeps its shape. WhyWarm the mixture back to the optimum and the rate returns — which could not happen if the enzyme had been denatured.
- “The substrate is denatured at high temperature.” RepairThe enzyme is denatured. Denaturation is a change in the shape of a protein. WhyStarch, hydrogen peroxide and most other substrates are not proteins, so the word does not apply to them.
- “The enzyme is used up in the reaction.” RepairThe enzyme is unchanged and is free to catalyse the same reaction again. WhyThis is half of the catalyst definition. It is also why a small amount of enzyme can convert a large amount of substrate.
- “Above the optimum the molecules move too fast to fit into the active site.” RepairThe fall is caused by the active site changing shape through denaturation, not by speed. WhyFaster movement means more collisions, which would raise the rate. The only thing that can make it fall is losing working enzyme.
- “The optimum is the only temperature the enzyme works at.” RepairThe enzyme works over a range of temperatures, with its greatest activity at the optimum. WhyLook at the curve: it is above zero well either side of the peak.
- “All enzymes have an optimum of 37 °C and pH 7.” RepairOptima differ between enzymes. A stomach protease has an optimum of about pH 2; a small-intestine protease about pH 8. WhyIf every enzyme shared an optimum, digestion in an acid stomach and an alkaline small intestine could not both work.
- “The graph plateaus, so the enzyme has been denatured.” RepairA plateau means no further net product is forming. The usual cause in a controlled experiment is that the substrate has been used up. WhyIf the temperature and pH were held constant, nothing has happened that could denature the enzyme.
- “More collisions, so the reaction is faster.” RepairMore effective collisions, so more enzyme–substrate complexes form per second, so the rate increases. WhyTwo separate marking points are hiding in that sentence: effective collisions, and complex formation.
- “The enzyme recognises its substrate and picks it out.” RepairThe substrate reaches the enzyme by random movement; either its shape is complementary to the active site or it is not. WhyA molecule cannot search, choose or recognise. Specificity is a consequence of shape alone.
Questions students ask about Enzyme
What is an enzyme?
An enzyme is a protein that functions as a biological catalyst: it increases the rate of a metabolic reaction and is not changed by that reaction. Each enzyme has an active site with a particular shape. A substrate whose shape is complementary to that active site fits into it and an enzyme-substrate complex forms; the reaction happens, the products leave because they no longer fit in the same way, and the unchanged enzyme is free to work again.
Why does each enzyme catalyse only one reaction?
Because of shape. Only a substrate whose shape is complementary to the enzyme's active site can fit into it and form an enzyme-substrate complex; a substrate of any other shape does not fit well enough for the reaction to be catalysed. This is the lock-and-key model. The substrate reaches the enzyme by random movement — an enzyme does not search for or recognise its substrate. Specificity is a consequence of shape alone.
Why do enzymes stop working at high temperature?
Above the optimum, increased vibration disrupts the bonds holding the enzyme's three-dimensional shape. The active site changes shape, the substrate is no longer complementary to it, fewer enzyme-substrate complexes form and the rate falls rapidly: the enzyme has been denatured. It is not that molecules move too fast to fit — faster movement means more collisions, which would raise the rate. Only the loss of working enzyme can make the rate fall.
Does low temperature denature an enzyme?
No. Low temperature slows the reaction because molecules have less kinetic energy, so they collide less often and fewer enzyme-substrate complexes form each second. The enzyme keeps its shape, and warming the mixture back to the optimum restores the rate — which could not happen if the enzyme had been denatured. Below the optimum the enzyme is fine and the molecules are slow; above it the molecules are fast and the enzyme is finished.
Is it correct to say that heat kills an enzyme?
No. An enzyme is a protein molecule, not a living organism, so it cannot be killed. High temperature denatures it: the bonds holding its three-dimensional shape are disrupted and the active site changes shape. The same care applies to "the enzyme is used up" and "the enzyme gives the reaction energy": both contradict the definition of a catalyst, which is not changed by the reaction and does not supply energy to it.
Do all enzymes have the same optimum temperature and pH?
No. Optima differ between enzymes. A stomach protease has an optimum of about pH 2, while a small-intestine protease has an optimum of about pH 8; if every enzyme shared one optimum, digestion in an acid stomach and an alkaline small intestine could not both work. An enzyme also works over a range of temperatures and pH values either side of its optimum, with its greatest activity at the optimum, not only at it.
When a product-time graph levels off, does that mean the enzyme has been denatured?
Not in a controlled experiment. A plateau means no further net product is forming, and if temperature and pH were held constant nothing has happened that could denature the enzyme; the usual cause is that the substrate has been used up. Read the gradient as the rate: a falling gradient means the rate is dropping as substrate runs out. Give substrate depletion as the explanation unless the question tells you the conditions changed.