Enzymes

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. You know that heating a reaction usually makes it go faster, right? Now warm up a reaction run by an enzyme. At first, yes, it speeds up. But above about three hundred and twenty kelvin, the rate falls sharply. More heat, and a slower reaction? We will solve this in a few minutes. Today's lesson is enzymes, the catalysts of living cells, and they do come up in JEE chemistry. So, let's begin.

0:33Enzymes: the catalysts of living cells

So, what is an enzyme? Enzymes are biological catalysts: they act as catalysts in living cells. In the lesson on protein structure, we met fibrous and globular proteins. Which class do you think enzymes belong to? Yes, exactly, they are globular proteins: compact, roughly spherical molecules. And notice the word almost: almost all of them, not every one. Okay? Next, enzymes are highly specific. Each enzyme catalyses one reaction, with one substrate. The substrate is simply the molecule that the enzyme acts on. Right? And they are needed only in small amounts. In a minute you will see why.

1:20Lock and key: the active site

Clear? Now look at the left part of the figure. The block marked capital E is the enzyme, and the orange piece marked capital S is the substrate. See the notch in the enzyme? That is the active site, the region of the enzyme surface where the substrate binds and reacts. Okay? The substrate fits into the active site like a key into a lock. In the middle picture they are locked together as the enzyme substrate complex. Only a substrate of the matching shape fits, which is why enzymes are so specific. With me? Now the same story as an equation. E plus S, through the double half arrows, which mean this step is reversible, gives E S, the complex. Then the single arrow: E S gives E plus P, with P for the products. Now, capital E is on the left and again on the right. What does that tell you about the enzyme? Yes, exactly, the enzyme comes out unchanged, and it is used again for the next substrate. That is why a small amount is enough. Got it?

2:24Enzymes lower the activation energy

Now the graph on the right: energy up the side, and the progress of the reaction along the bottom. The dark blue curve is without the enzyme, and the orange curve is with it. The height of the hump above the starting energy is the activation energy, E a, that is E with a small a written low. So the enzyme gives a much lower hump. Right? Now the numbers. For the hydrolysis of sucrose, that is splitting it with water, the activation energy with an acid catalyst is six point two two kilojoules per mole. Mole to the power minus one just means per mole. Okay? And with the enzyme sucrase, it is only two point one five kilojoules per mole, about a third. Remember these numbers for Example two. Clear?

3:09Best conditions: temperature and pH

Now, conditions. Each enzyme works best at an optimum temperature, about three hundred and ten kelvin, with K for kelvin. Up to the optimum the rate rises, and it is highest there. Okay? There is also an optimum pH, near seven for most enzymes. So every enzyme works best at pH seven, right? Oh no, careful, that is the trap: the screen says most, not all. Pepsin works in the stomach at about pH two, and trypsin in the intestine at about pH eight. So each enzyme has its own optimum pH. With me? Now, back to our puzzle. Above about three hundred and twenty kelvin, the rate falls sharply. Think about what an enzyme is made of. Why does this happen? Yes, exactly, the enzyme is a protein, and heat denatures it. Its shape is lost, so the active site is destroyed and the substrate no longer fits. So past the optimum the rate falls steeply, and a wrong pH does the same. That solves our puzzle. Got it?

4:15Names: most end in -ase

Next, names. Most enzymes are named after their substrate or their reaction, with the ending ase, as in sucrase, the enzyme for sucrose. The little dash just marks an ending. Okay? And the enzymes that catalyse the oxidation of one substrate together with the reduction of another are called oxidoreductases. Right? Now the ones to know. Invertase, also called sucrase, turns sucrose into glucose and fructose; the arrow means gives. So, what would you call the enzyme that splits maltose? Yes, it is maltase, and it turns maltose into two glucose units. In the same way, lactase splits lactose into glucose and galactose. Clear? Now two names to simply learn. Diastase, also called amylase, turns starch into maltose. And zymase turns glucose into ethanol and carbon dioxide, C O two. With me? Urease breaks urea into ammonia, N H three, and carbon dioxide. And pepsin and trypsin break proteins into peptides and amino acids. Okay?

5:20Example 1: name enzymes for starch, maltose, glucose, urea

Time for the first example. Name the enzyme for each change; the letters in brackets just number the parts. Starch to maltose, maltose to glucose, glucose to ethanol, and urea to ammonia and carbon dioxide. Pause the video and try it first. Done? Okay, let's check. The first change makes maltose, so its enzyme must be maltase, right? No, no, wait, that is the trap: maltase splits maltose, it does not make it. Starch to maltose is diastase, also called amylase. Then maltase turns maltose into glucose, so that is two enzyme steps. Okay? Glucose to ethanol is zymase, and urea to ammonia and carbon dioxide is urease. So the answers are diastase, maltase, zymase and urease, and that is the whole example.

6:13Example 2: rate factor when an enzyme lowers Ea

Now a harder one. The activation energy of a reaction is lowered from six point two two to two point one five kilojoules per mole by an enzyme. These are our sucrose numbers. By what factor does the rate constant increase at three hundred and ten kelvin, other things being equal? Capital R, the gas constant, is eight point three one four joules per kelvin per mole. Pause the video and try it first. Done? Okay, let's check. Many students simply divide: six point two two over two point one five is about two point nine; the wavy equals sign means about. So the rate constant goes up about two point nine times, right? Careful, that is the trap: the rate constant does not follow the plain ratio. From the Arrhenius equation, with all else equal, k two over k one equals e to the power delta E a over R T, okay? Here k two, k with a small two below, is the new rate constant, and k one the old one. The e is the base of natural logs, delta, the Greek triangle, means the change, and T is the temperature. With me? Delta E a is six point two two minus two point one five, which gives four point zero seven kilojoules per mole. Now the second trap: R is in joules, so this becomes four thousand and seventy joules per mole. Okay? Now R times T: eight point three one four times three hundred and ten gives about two thousand five hundred and seventy seven. Then four thousand and seventy over that gives one point five eight. Right? So k two over k one is e to the power one point five eight, which is about four point nine. Nearly five times, not two point nine. If you got four point nine, very well done. Clear?

8:02Quick revision: lock and key, activation energy, optimum pH

So, quick revision. Enzymes are specific catalysts, and almost all are globular proteins. Lock and key: E plus S gives E S, which gives E plus P, and they lower the activation energy. They work best near three hundred and ten kelvin and at their own optimum pH, and heat denatures them. Now go and try the practice problems on this page, and all the best for them!