Cambridge O Level Biology · Syllabus 5090 · Enzymes
Active site
What is Active site?
The region of an enzyme molecule with a particular three-dimensional shape into which a substrate of complementary shape fits. Only a substrate that fits the active site can form an enzyme-substrate complex, so the shape of the active site is what makes an enzyme specific, and any change to that shape reduces the enzyme's activity.
This definition is part of the Enzymes chapter in Cambridge O Level Biology.
Active site 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 Active site
- “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.
- “Denatured means the active site is blocked or full.” RepairDenatured means the active site has changed shape, so the substrate is no longer complementary to it. WhyNothing has been put into the active site. The site itself is the wrong shape now.
- “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 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 Active site
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.
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.