Enzymes (and Proteins) Part 3

What affects how enzymes work?

Welcome to the final part on enzymes and proteins.

There are five factors which affect enzyme activity you need to know, perhaps three of which you will already be aware of.

pH and Temperature: Each of these factors operate in a similar fashion. Enzymes will have an optimum pH and temperature that they work most efficiently at (converting substrates into products at the highest rate). When looking at graphs pay attention to the variables given, any maximum/minimum points, how the graph changes, and the scale given. Here are some graphs now to help understand:

Be careful though, there are many different enzymes, and they all have slightly different optimum temperatures and pH. The enzymes in your body might have an optimum temperature of 37 degrees Celsius, but the enzymes of bacteria living in a hydrothermal vent would likely have an optimum temperature that is far higher.

The key points to understand here are:

• Enzymes have an optimum pH and temperature that they operate best at.
• The further away the temp./pH is from the optimum, the lower the enzyme activity is.
• Extremely high temperatures or an extreme pH value will denature an enzyme. As you can see from the temperature graph, this means that enzyme activity grinds to a halt. Why? A high temperature will damage the bonds in the enzyme, and cause it to change shape. As you might have guessed, this is bad news for the enzyme as it can no longer form E-S complexes- it ceases to function. So in short, to denature is to damage an enzyme causing it's shape to change so it can no longer function.

Substrate Concentration/Enzyme Concentration: All this means is the amount of substrate present compared to the amount of enzyme. The concept is simple enough to grasp, but sometimes when asked to apply the idea certain questions can throw people. So let's say you begin with a large amount of substrate and just a few enzymes. Then repeat the experiment many times, keeping the amount of substrate the same but ever increasing the amount of enzymes.  Consider how the rate at which product molecules are produced changes from experiment to experiment. 

Try drawing a graph and see if it matches the following explanation... At first, as you increase the enzyme concentration the rate of reaction will also increase. This increase in rate of reaction will continue until there are so many enzymes present that adding more has no effect, as all the substrates can immediately form E-S complexes. So your graph should increase at a constant rate then curve off into a 'flat' line as the rate no longer changes.

Substrate concentration is a similar ball game. If you have a set number of enzymes, and then add more and 

more substrate you get a graph like this: 

  

www.rsc.org

  Adding more substrate to the enzymes present will  increase the rate of reaction up until the point where  all the enzymes are saturated with substrate  molecules. After this, the reaction occurs at it's  maximum rate as there physically aren't any more  enzymes present to bind to the excess substrate  molecules.

Tip: With graphs (or indeed with all questions, but especially graphs :P) reading the question properly, and not just going into "I-revised-this-50-times-let-me-pump-out-an-answer" machine mode, is more important than with other questions. Graphs may sometimes look like simple recall, but they will try and fool you. What I'm trying to get at here, is lets say they give you a specific context, or change a certain parameter. For instance, what would happen to our "Enzyme concentration" example, if rather than the amount of substrate being kept the same, it was in excess. The graph would not level off at all, you would simply draw a straight diagonal line between the x and y axis, because there would always be enough substrate molecules (if they were in excess) to satisfy the enzyme's never ending thirst for them- regardless of how many enzymes you throw in there.

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.