Enzyme inhibition; chymotrypsin

Published on April 19, 2020 Reviewed on November 27, 2022   46 min

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Hello. Welcome to this Principles of Biochemistry lecture series. My name is Jerry Feigenson. I am a professor in the Department of Molecular Biology and Genetics at Cornell University in the USA. In the ninth lecture, you learned about using the initial velocity of enzyme catalyzed reactions, which leads to the useful Michaelis-Menten equation. Then you saw how to graph the double reciprocal versions of the Michaelis-Menten equation, to find useful enzyme parameters of maximal velocity and the Michaelis constant.
In this lesson, you are going to learn about the concept of an efficient enzyme. You will see how to obtain useful information by inhibiting enzymes. And you will see the mechanism of chymotrypsin step-by-step.
What is an efficient enzyme? This is an interesting concept and it will take me a couple of slides to show it to you. We'll start off with a bit of jargon. The jargon is turnover number. Turnover number is defined as the number of substrate molecules converted to product molecules per unit of time when the enzyme is saturated with substrate. Let me try to make that more clear. Well, first here on the upper right-hand side, here's a small table of some turnover numbers. Well, two of them we will look at, the carbonic anhydrase and chymotrypsin. So turnover number. Here is a little enzyme mechanism. Enzyme plus substrate forms ES complex. One product comes off, the second product is bound, and then the second product comes off. So when we say turnover number, we actually mean K catalytic. If you recall, K catalytic is what multiplies total enzyme concentration to give the maximum velocity. That means we have so much substrate present that every enzyme has a substrate bound, that's when we get the maximal velocity. We multiply that enzyme concentration by k catalytic to get the maximal velocity. Well, K catalytic is usually not a simple rate constant like k_2. It's usually a combination of rate constants. It tells us how fast the enzyme is working. How fast does the enzyme work? Let's consider that for the carbonic anhydrase case. Carbonic anhydrase catalyzes the chemical reaction of carbon dioxide with water to form bicarbonate and a proton. Actually, that reaction happens relatively fast without a catalyst. But in biology, this reaction has to happen much faster. So the enzyme carbonic anhydrase evolved. Its k catalytic is a large number, 600,000 per second. Another way to understand the meaning of this K_cat is if we consider a single carbonic anhydrase molecule that has a substrate bound. The reciprocal of K_cat, one over K_cat, tells us the amount of time for all the chemical steps to occur. In this case, one over K_cat is 1.7 microseconds. In 1.7 microseconds, all the chemical steps have occurred. So this is carbonic anhydrase, it has a large turnover number. We're going to be looking in detail at chymotrypsin. Carbonic anhydrase has a large turnover number. Chymotrypsin has a K_cat of 100 per second, a relatively small turnover number. So we might think carbonic anhydrase, wow, we're impressed. Chymotrypsin, oh, maybe we're not impressed. But we would be mistaken, because chymotrypsin is the more efficient enzyme.