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Enzyme Activity & Inhibition: Structure, Substrates, pH & Temperature

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  • 0:06 Enzymes and the Environment
  • 1:12 Effect of Substrate…
  • 2:50 Effect of Temperature
  • 4:07 Effects of pH
  • 5:29 Lesson Summary
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Lesson Transcript
Instructor: Kristin Klucevsek

Kristin has taught college Biology courses and has her doctorate in Biology.

Enzymes appreciate the right working conditions. Can you blame them? We all work best in the correct environment. In this lesson, we'll learn how substrate concentration, temperature, and pH affect enzyme activity and structure.

Enzymes and the Environment

The substrate binds to an enzyme at the active site
Enzyme Active Site

Sometimes we like things just right. If it's too hot, we swelter under the sun. If it's too cold, we shiver. If there's too much to eat, we hit a point where we can't swallow another bite. But when we're hungry, there never seems to be enough food in the house. It's not that we're nitpicky, right? It's just that we're human, and, after all, we know what we like and we know what conditions suit us best.

We are not the only ones who value the perfect conditions. Plenty of animals and other organisms do, too. And on a microscopic level, so do our cells. Even within our cells, enzymes, the proteins that catalyze chemical reactions, like things just right, too.

As a quick review, an enzyme works on a substrate, or substance or molecule on which an enzyme functions. An active site is where a substrate binds an enzyme in order to facilitate a reaction. Each enzyme works on a specific substrate in a specific condition. These conditions are optimal for the enzyme - just like how the perfect amount of sunshine and food keep us satisfied and happy.

Effect of Substrate Concentration

If you had two cookies in front of you, you could eat two cookies. If you had five cookies in front of you, you could eat five cookies. However, maybe there were 100 cookies in front of you. Could you eat 100 cookies in one sitting? Probably not. At some point, you'd become full, or saturated, and you wouldn't be able to eat one more cookie at this time, no matter how many more were put in front of you.

The reaction rate increases until the point of saturation is reached
Enzyme Point of Saturation

Enzymes - well, they don't work on cookies, but they work on substrates. Therefore, the substrate concentration in an enzyme's environment has a major effect on how much work can be done. There's only so much substrate an enzyme can work on, just like there's so many cookies that you could possibly eat. Let's say that one enzyme works on one substrate at a time. Let's also say in a cell, there are ten units of one type of enzyme but only four substrates for the enzyme to work on. In this case, all the substrates will bind to an enzyme and the reaction rate will be low.

If you add a few more substrates in - say, like twenty at a time - then the reaction rate will increase until the point where all the enzymes are currently busy catalyzing reactions. The reaction rate will increase until all the enzymes available are working on substrates. This is shown on this graph by the highest point of the curve. At this point, the reaction rate will plateau and reach its maximum. Here, the enzyme has reached a point of saturation. This point of saturation is when more substrate will not increase the rate of reaction. The rate of reaction will not increase with a higher substrate concentration unless the amount of enzyme also increases.

Effect of Temperature

Just like it affects us, temperature also affects enzyme function. Enzymes work optimally at a specific temperature. Again, different enzymes might have different optimum temperatures. Colder weather slows much of life down, and cold temperatures will also slow the reaction rate of enzymes. When things heat up, molecules move faster, similar to how we go outside or run around when the weather starts to warm up in the spring. An increase in temperature usually means more movement and a greater likelihood that molecules collide.

Higher temperatures can increase the rate of reaction between an enzyme and its substrate, but only to a point. This temperature point of maximum function is called an enzyme's optimum temperature. As you can probably guess, the optimum temperature for enzymes in our body is actually our body's temperature!

If the temperature gets too high, it can distort the enzyme, making it unable to function properly. In fact, if temperatures got too high, enzymes can become denatured, or undergo a structural change in shape that inhibits function. If the change in shape affects the active site, then an enzyme may no longer recognize a substrate. With more enzymes becoming more non-functional, the rate of the reaction decreases with an increase in temperature.

The reaction rate increases until the enzyme reaches its optimum temperature
Temperature Point of Maximum Function

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