Enzymes (and Proteins) Part 2

How Enzymes Work

Models of Enzyme Action

These are two simple models that help us understand the physical process of how an enzyme actually catalyses a reaction. Each model sets out how the enzyme interacts with the substrate molecule. There are two you need to know.

The Lock and Key Model

In this model, the substrate is said to fit into the enzyme like a lock into a key. This then transforms the substrate into the product molecule(s). This is actually now deemed outdated, and the more recent "induced fit" model (discussed later) is supposedly more accurate. However, the lock and key model is still useful in conveying the basic ideas of enzyme action. Here is a diagram to illustrate the process:

First the enzyme and substrate molecules must 'collide' (I'm sure you are all familiar with particle models from GCSE). They then bind and form the Enzyme substrate complex. It has been simple up until now. But how does the E-S complex split the substrate into two product molecules? Magic? Well, we will find out about that in the next model.

It is also worth noting a couple of things. First, notice how the enzyme remains 'unused' by the reaction and finishes the reaction unchanged. You can also apply your other knowledge of enzymes to each of the models. For instance, the shape of the substrate 'key' must be complementary to the Enzyme 'lock' as shown in the diagram. From the notes in part one you should be able to explain how the enzyme gets this specific shape.

The Induced Fit Model

The induced fit model follows pretty much the same idea as the lock and key model. An enzyme must first collide with a substrate molecule. However, here is the key difference. The enzyme and substrate are not complementary to begin with. When they try and bind, there are minute changes in the shapes of both which allow the E-S complex to form, allowing the substrate to react and form product molecules. So, the models are essentially the same, except in Induced Fit the shape of the enzyme and substrate must change to become complementary. 

The idea behind this (the magic part :P) is that the change in shape places certain stresses on the bonds in the substrate. This stress makes the bonds more inclined to form new ones and become product molecules. It is comparable to a situation where you have a child safe screw lid on a medicine pot. Normally to get the lid off (or to make the substrate react) you have to put in loads of effort (or energy). But if you have the technique and push down on the cap first, then you can unscrew it more easily by simply providing a different route. (Just like how the enzyme, by pushing in on the substrate molecule, provides an alternate route for the reaction making it require less energy and thus take less time). Okay, so not my best analogy, but you get the picture.