Biology: Enzyme Characteristics

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10/23/2011

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Enzymes really do run your cells, but it wasn't until just before the turn of twentieth century that somebody figured out any kind of explanation at all as to the mechanism of the enzyme action. You see, until then, and still now, there are some things that enzymes did and reactions that--we could see that they were controlling things, but nobody was really sure why and nobody was really sure how. Well, here's what I want to do. I want to take just these next few minutes and I'm going to lay a puzzle on you. I'm going to tell you some of those bizarre things that enzymes seem to do. I'm going to make you scientists. I'm going to present you with a problem and I'm going to ask you to figure out some kind of mechanism to explain what enzymes may be doing to do that. If you can do that email me or something, because you deserve a Nobel Prize. This is going to be tough stuff.
Here's the thing. One of the things that has been always noticed about enzyme controlled reactions, in any enzymes or any reactions that had enzymes involved, was that they were sensitive to temperature. They noticed that in warm temperatures enzyme reactions went faster and in cold temperatures enzyme reactions went slower. So, enzymes were temperature sensitive. There's a degree of temperature sensitivity to enzymes. Let me show you a graph that illustrates this.
In this graph right here you can see that we have temperature in degrees Celsius on the x-axis and the rate of the reaction over here. Rate of the reaction, what does that mean? In other words, how quickly the reaction will turn. So, for example, if this happens to be a reaction where you're breaking down a substance, say in digestion. At 20° in a human, for this particular human enzyme, at 20° the rate is going along a little bit slower than it would at 30° or at 40°. Now look at, for example, this particular thermophilic bacteria which obviously is cooking at a pretty high temperature, this thing doesn't even start getting interested in reacting until about 40°. It's way up here, almost 80° before it reaches a maximum, but that's not all. Another bizarre thing about this effective of temperature and enzymes was this. Still is this. Enzymes stop functioning past a certain temperature. Now there's one that's tough to explain. They go faster, faster, faster, faster, faster, stop. Figure that one out, but that's only one. We've got more to go.
Now on to number two. The second thing we've noticed about enzymes, the second bizarre behavior, if you will, of enzymes is they are sensitive to pH. Remember what pH is? pH is how acidic or how basic a given solution is and this scale goes from one to fourteen. I want you to remember something about pH, and this is a hint. pH is the measurement of the hydrogen ions, or the hydronium ions, in a solution. So, another way to say this is, ions in solution effect enzymes. Well, what's interesting about that is certain enzymes have different pH's that they like. Now here are two digestive enzymes in the human digestive system. They're in different places in the human digestive system, but they're in your system.
This particular one, a stomach enzyme--look at this. Pepsin works best at an extremely acidic pH. Now we only have a pH scale going up to ten on this graph, but it's okay because after this you can see it plunges to zero down here. So in an extremely acid pH, the rate of reaction is high, gets higher and higher and higher and higher. You get to a certain concentration of hydrogen ions and its activity goes down. Well, meanwhile trypsin, an intestinal enzyme, you put that baby in a pH of three or four, you get no activity. The thing is useless to you. Of course, pepsin, you put in a pH of 5 or 6 or even neutral, it's useless to you, but trypsin--look at this. Once it starts getting toward neutral--there's neutral right there, and then into the alkaline or basic condition it gets more active, but it can get too basic, too alkaline and it doesn't work anymore. Bizarre behavior number two, enzymes are affected by pH.
Bizarre behavior number three. Certain things inhibit enzymes. There are certain factors, chemicals, certain chemicals act as inhibitors. In other words, added to chemistry or added to cells or added to test tubes where we were experimenting with enzymes, we found that some things temporarily inhibited enzymes and kept them from working and other chemicals permanently inhibited enzymes. Go figure that one. Some were temporary, some were permanent. The puzzle gets deeper. So, let's not get lost. Lets not lose track of the forest here. We have temperature sensitive chemicals that are also pH sensitive, that seem to react with other certain chemicals that we put in there and there are other chemicals that we put in there and make them better. There are certain chemicals that we put in there that actually promote enzyme activities. So, there are some things that promote enzyme activities.
Now I've saved the two most important clues for last. Clue number five. Enzymes are enormously specific. Specificity. One enzyme will only react with one, what we call, substrate. The substrate is whatever the enzyme reacts with. Certain enzymes will only react with certain substrates. So, I could take an enzyme that breaks down carbohydrates and put it in with proteins and it won't react with those proteins, and I can take an enzyme that breaks down proteins and it won't breakdown carbohydrates. They're specific. Now, the strangest thing of all--they're reversible. Enzyme reactions work both ways. Enzyme reactions can--I keep talking about breaking stuff down. Well, guess what? Apparently that very same enzyme can put stuff together. So they're reversible.
Here's your puzzle. Here's what I want you to do. I want you to try to think of some kind of explanation that will take these six aspects of enzymes, temperature, pH, this whole idea of inhibition and promotion, the fact that things help it and don't help it. I want you to think about the fact that some of these enzymes stop at certain--well, all the enzymes stop at certain temperatures depending on what temperature it is. I want you to think about the specificity of that enzyme. It will only react with a certain substrate and the fact that it can work in both directions. I want you to think about those because we have to explain this using a model of a molecule. Enzymes are just plain old molecules and I'll give you a hint. Shape, shape, shape. Good luck.
Inorganic and Organic Chemistry
Enzymes
Enzyme Characteristics Page [1 of 2]