Cambridge O Level Biology · Syllabus 5090 · Enzymes
Enzyme-substrate complex
What is Enzyme-substrate complex?
The temporary structure formed when a substrate molecule of complementary shape binds to the active site of an enzyme. While it exists the enzyme catalyses the conversion of the substrate into product or products, which then leave because their shapes no longer fit the active site in the same way.
This definition is part of the Enzymes chapter in Cambridge O Level Biology.
Enzyme-substrate complex 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.
Questions students ask about Enzyme-substrate complex
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.