0610 · 5.1Enzymes

Catalysts and enzyme roles

0:00Starch is made of long chains of glucose. Without an enzyme, starch in pure water would take an extremely long time to break down into sugar on its own. But chew the bread for a minute, and it starts to taste slightly sweet. Something in your saliva breaks the starch down in seconds. What breaks it down is an enzyme, and without enzymes, almost none of the chemical reactions in your body would happen fast enough to keep you alive.

0:22By the end of this video you will be able to describe what a catalyst is, explain what enzymes are and why living things need them, describe how an enzyme works using the idea of an active site, and explain how temperature and pH affect enzyme activity. You need two ideas from earlier videos. First, proteins are large molecules made from amino acids joined in a chain, and the chain folds into a particular shape.

0:47Second, metabolism is all of the chemical reactions that happen in the cells of an organism. Let us start with catalysts. The syllabus describes a catalyst as a substance that increases the rate of a chemical reaction and is not changed by the reaction. So a catalyst makes a reaction go faster, but it is not used up. At the end of the reaction, the catalyst is still there, exactly as it was, ready to be used again.

1:10The syllabus describes enzymes as proteins that are involved in all metabolic reactions, where they function as biological catalysts. Biological just means they are made by living things. So an enzyme is a protein that speeds up a reaction in a living organism, and is not changed by it. Why are enzymes so important? Living things need thousands of chemical reactions to happen every second, for respiration, digestion, growth and much more.

1:36At body temperature, most of these reactions would happen far too slowly on their own. Enzymes make them happen fast enough to sustain life. Without enzymes, you could not release energy from your food quickly enough to stay alive.

Active sites and specificity

1:49So how does an enzyme work? Every enzyme has a particular shape, and on its surface is a region called the active site. The molecule that the enzyme acts on is called the substrate. The shape of the active site is complementary to the shape of the substrate. Complementary means the two shapes fit together, the way a key fits into a lock. Notice the word complementary. It does not mean the same shape.

2:12The active site and the substrate are not identical. They are opposite shapes that fit together, like two neighbouring jigsaw pieces, or a key in a lock. Here is what happens, step by step. First, the substrate and the enzyme move about randomly and collide. If the substrate fits into the active site, it binds there. For the extended syllabus, this is called an enzyme-substrate complex, the enzyme and its substrate fitted together.

2:37While the substrate is in the active site, the reaction happens. In a breakdown reaction, the substrate is split into smaller molecules. These new molecules are the products. The products then leave the active site, and the enzyme is free to bind with another substrate molecule. The enzyme itself is not changed, which is exactly what we expect of a catalyst. For the extended syllabus, you also need to explain the specificity of enzymes.

3:02Each enzyme only works on one substrate, or a few very similar ones. This is because the active site has a particular shape, and only a substrate with a complementary shape can fit into it. Amylase, the enzyme in your saliva, breaks down starch, but it cannot break down protein, because protein does not fit its active site. A different enzyme, called protease, is needed for that.

Temperature and enzyme activity

3:23Now let us look at what affects enzyme activity, starting with temperature. If you measure how fast an enzyme works at different temperatures, you get a graph with a peak. As the temperature rises, the activity increases, up to a peak. The temperature where the enzyme works fastest is called its optimum temperature. For most human enzymes, the optimum is about 37 degrees Celsius, which is body temperature. Above the optimum, the activity falls quickly, until it stops completely.

3:50For the extended syllabus, you need to explain this graph. As the temperature rises towards the optimum, the enzyme and substrate molecules gain kinetic energy. They move faster, so they collide more often. There are more effective collisions, where a substrate fits into an active site, so more enzyme-substrate complexes form each second, and the rate increases. Above the optimum, something different happens. The enzyme is a protein, and its shape is held in place by bonds.

4:17At high temperatures, the molecule vibrates so much that these bonds break, and the enzyme changes shape. The active site changes shape too, so it is no longer complementary to the substrate. The substrate can no longer fit, so there are fewer effective collisions and fewer enzyme-substrate complexes form. We say the enzyme has been denatured. Denaturation is permanent. Cooling the enzyme down again does not bring its shape back.

4:41A very common mistake is to say that the enzyme is killed. Enzymes are not alive, so they cannot be killed. They are denatured. Another mistake is to say that enzymes are denatured at low temperatures. They are not. At low temperatures, they just work slowly, because the molecules have less kinetic energy and collide less often. If you warm them up again, they work normally.

pH and enzyme investigations

5:02The second factor is pH. Each enzyme also has an optimum pH, where it works fastest. For many enzymes, the optimum pH is around 7, which is neutral. Amylase in your mouth works best at about pH 7. But pepsin, a protease in your stomach, works best at about pH 2, which is very acidic, because the stomach contains hydrochloric acid. For the extended syllabus, you need to explain this in terms of shape and fit.

5:28If the pH moves away from the optimum, in either direction, the bonds holding the enzyme in shape are affected, and the shape of the active site changes. The substrate no longer fits as well, so fewer enzyme-substrate complexes form and the activity falls. At extreme pH, the enzyme is denatured, and the active site loses its shape completely. How can you investigate this? A classic experiment uses amylase and starch.

5:53You mix amylase with starch solution, and every 30 seconds you take a drop of the mixture and add it to a drop of iodine solution on a spotting tile. At first, the iodine turns blue-black, because starch is still there. When all the starch has been broken down, the iodine stays orange-brown. You time how long that takes. To investigate temperature, you put tubes of starch and amylase in water baths at different temperatures, for example 20, 30, 40, 50 and 60 degrees Celsius, and leave them to reach that temperature before mixing.

6:24The temperature with the shortest time is closest to the optimum. To find it more precisely, you can repeat with smaller temperature steps around that value. To investigate pH, you keep the temperature the same and use buffer solutions, which hold the mixture at a fixed pH, such as pH 4, 5, 6, 7 and 8. As always, change only one factor. If you are testing temperature, keep the volume and concentration of starch and amylase, and the pH, the same in every tube.

6:52Otherwise you cannot tell which factor caused the difference.

Exam answers and recap

6:55Let us try an exam-style question. A graph shows the activity of an enzyme at different temperatures. The activity rises from 10 degrees to a peak at 40 degrees, then falls to zero at 60 degrees. The question asks you to explain the results. Start with the rising part. From 10 to 40 degrees, the molecules gain kinetic energy, so the enzyme and substrate collide more often, there are more effective collisions, and more enzyme-substrate complexes form.

7:21Then the peak. 40 degrees is the optimum temperature, where activity is highest. Then the falling part. Above 40 degrees, the enzyme starts to denature. More and more enzyme molecules change shape, so their active sites are no longer complementary to the substrate. The substrate cannot fit, so there are fewer effective collisions and fewer enzyme-substrate complexes, and the activity falls. By 60 degrees, all of the enzyme is denatured, so no enzyme-substrate complexes form.

7:49Notice how a good answer uses the key words: kinetic energy, collisions, optimum, active site, shape, and denatured. And notice that it explains each part of the graph separately. Let us collect the common mistakes. The first is saying that enzymes are killed. They are denatured. The second is saying that enzymes are denatured by low temperatures. Low temperatures only slow them down. The third is saying that the active site and the substrate are the same shape.

8:17They are complementary. The fourth is saying that the enzyme is used up in the reaction. It is not. It is a catalyst, so it can be used again and again. And the fifth is forgetting that enzymes are proteins. That is exactly why heat and extreme pH can change their shape. Let us recap. A catalyst increases the rate of a chemical reaction and is not changed by it.

8:39Enzymes are proteins that act as biological catalysts in all metabolic reactions, and they make reactions fast enough to sustain life. The active site of an enzyme has a shape that is complementary to its substrate. The substrate fits into the active site, forming an enzyme-substrate complex, and is turned into products, which then leave. Because only a substrate of the right shape can fit, each enzyme is specific. Increasing temperature increases the rate up to the optimum, because molecules have more kinetic energy and make more effective collisions.

9:11Above the optimum, and away from the optimum pH, the shape of the active site changes, the substrate no longer fits, and at extremes the enzyme is denatured. In the next video, we will see how plants use energy from light to make their own food, in photosynthesis.