In this A-Level Biology Lesson "The Models of Enzyme Action - Lock and Key & Induced Fit" we once again build upon a common topic in biology - an easy topic that bridges the gap from GCSE to A-Level biology pretty seamlessly. So we being with a few key points. We know by now that enzymes are very specific, meaning enzymes only bind to their complementary substrates. You must be able to define this specificity of enzymes. We take the opportunity next to emphasise that over time scientific knowledge changes, and because of this hypothesis are continually tested and new hypothesis formed. Enzymes action showcases this notion - that as the scientific community gather more evidence and information, theories can be updated and change slightly over time. So the lock and key model of enzyme action was “updated” to a more current idea, The induced fit model of enzyme action. (This idea that scientific theories can be updated over time is a key principle that you may have to apply to unfamiliar situations - another good example is the central dogma of molecular biology and the enzyme reverse transcriptase). So, by the end of this lesson you’ll know about the lock and key model of enzyme action and how that idea has been updated to the induced fit model of enzyme action. You must be able to describe and illustrate both models of enzyme action (compare and contrast if needed) and of course understand that over time theories can change (or more appropriately be updated based upon the new evidence gathered by the scientific community).

2. Geometry (Lock and Key / Induced Fit model of enzyme action).

The substrate molecule fits into the active site of the enzyme molecule like a key fitting into a

lock (hence the lock and key model of enzyme action). However, the enzyme changes shape slightly, distorting the molecule in the active site, and making it more likely to change into the product. e.g. if a bond in the substrate is to be broken, that bond might be 'forced' by stretching or twisting by the enzyme, making it more likely to break. Alternatively the enzyme can make the local conditions inside the active site quite different from those outside (i.e. pH), so that the reaction is more likely to happen. It is a bit more complicated than that in truth. In fact the active site doesn't really fit the substrate at all, but instead they "kind of fit"  - this is called the transition state. When a substrate (or product) binds, the active site changes shape and fits itself around the molecule, distorting somewhat and forming the transition state, thus speeding up the reaction. This called the induced fit model of enzyme action.