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Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

Enzymes are proteins that catalyze chemical reactions by facilitating the positioning of molecules. In biochemical pathways, sequences of enzymes form a chain of chemical reactions. Enzymes, like all catalysts, lower the activation energy of a reaction.

Enzymes are the chemical workhorse of the cell and a lot of times when you first read about enzymes you read that they are called protein catalysts. But a lot of times students just don't know what the heck does that mean. Well, I'm going to explain it to you. When you think about any chemical reaction, you know that if you have a piece of firewood in front of you, you know it has a lot of energy in it cause it burns and that's why we call it firewood. But you're not sitting there go "it is going to blow," no, because it takes something to get it started. If we diagram that on a graph you could talk about the log being here and you need to give it some energy in order to get all that energy back out. The amount of energy that you need to dump into a chemical reaction in order to start it or activate it is called the activation energy.

Now what a catalyst is, is it's a kind of chemical that doesn't give the energy but it just merely reduces the amount of activation energy required so that you have to dump much less energy in. The other thing about a catalyst is that it acts as, it is not used up in that process of doing it. So you can use the catalyst over and over, again and again and that's why cells use enzymes cause that way you don't have to keep remaking them. You make them once and they're good for a long time. Now, I can explain that that way but how about I use a little demo and in this case the chemical reaction we're going to be doing is ripping paper and my wonderful assistant Laura here shall demonstrate how hard it is to actually rip paper in half from the edges. Go ahead, use all of your strength. She can't do it, she can rip around her fingers but not the paper. This set of scissors is much like an enzyme. So I just give it a little slice and I due to all my muscles and the fact that I cheated by using the scissors much easier and you must use much less energy to get this started. Even Laura, once I help her out, go ahead. She can do it, alright, thank you. So if we go ahead and we take a look at it again, there's something about the enzyme that is called an active site. And an active site is specific to the substrate that that protein catalyst works on.

Now if we go back to my model that an enzyme is like a pair of scissors, hey, this right here is the active site. If I'm trying to cut paper, the handles don't work, so well this part doesn't work so well that part doesn't work so well. Only this part is the active site. In this contest, paper is the substrate and it's specific, these work great on paper. They do not work so well on mugs, my arm, no, just paper. Similarly, enzymes are specific on the substrate that they work on. Such as lactase is the enzyme that breaks down lactose sugar.

Now, the way they do this is something called the Induced Fit Theory. We take a look at this diagram. You can see here, the greatest blob here is the enzyme and this part here is the active site. And you can see it kind of fits that but it doesn't perfectly fit it. Used to be thought that it did and that was called the Lock and Key hypothesis. Scientists have realized now hey, atoms which make up molecules give off various charges and such. And as this molecule approaches, it interacts with the atoms that make up the enzyme to induce or cause the enzyme to change shape to fit the substrate. However, this causes stess on the substrate. Why? Because we've change, we bent the shape of the enzyme and so that stress is returned to the substrate which helps break it in half. So that's how the Induced Fit Theory works.

Now, there's a couple of little complexities to add into this. There is the various factors such as PH and temperature which can alter the shape and structure of the enzyme. So those are commonly used as factors in in an experiment to alter the rate which an enzyme does its job.