Identify the following disaccharides as reducing sugars (RS) o...

Question

Identify the following disaccharides as reducing sugars (RS) or non-reducing sugars (NS)

Structures of maltose, lactose, and sucrose with labels for identifying reducing and non-reducing sugars.

Accepted Answer

Now I'm going to show you some of the most important disaccharides that you might need to know at some point, and I also have them drawn out down here in this practice problem. So as we talk about it, I'm going to be referencing the drawing as well just so that it makes more sense. Now guys disclaimer, this list is not meant to be comprehensive. Like I said, there are so many disaccharides out there and you may need to know more than these 3. And you may also not need to know any of them. So I'm going to leave that part up to you. I'm just going to give you the information of the most important disaccharides, some of the most popular, some of the most cited disaccharides so that you'll at least see a few examples for later. Okay. So let's go ahead and start off with, remember that we've already discussed Cellubios, which would be 2 d glucose with a Beta 14 linkage. Well, what happens if you, make 2 D glucose molecules come together and condensate into a disaccharide but instead using an alpha 14 linkage. Now what would that look like? Well, let's look down. This is actually the drawing. So you can see that once again I have d glucose. I'm just gonna put d glue. And here I also have D glucose and they look exactly the same, but notice that now the anomeric position is faced face trans to the stereo descriptor. This one is going down. This one is going up. That's the definition of an alpha linkage, but notice that everything else is still the same. I have the 1 and the 4 linked together. So notice how guys, first of all, the shape of my disaccharide has completely changed. Before the shape used to be a nice straight chain and now it's got like a little bit of a kink to it. That's gonna be important. Those kinks and those straight edges and and all those kind of structural changes actually significantly impact how these sugars react in in physiology, in the human body. It may determine if it can be digested or not. But anyway guys, what's interesting is that now if you change that one little thing, you've now made maltose. So maltose is the name of acellulbios. Don't don't quote me on that because it's not technically true, but it would be the same thing as cellulbios, but instead of beta, it's alpha. Cool? Awesome. So what else? Let's go to another one. Well, what if you wanna keep the 1, let's say you wanna keep the beta 14 linkage like we had for Sullibios and you want to keep sugar 2 as d glucose, but now we want to change sugar 1 into d galactose, which is just going to differ by the a few positions of Oh's facing up or down. Let's go down. Let's just see this for a second. So this is what it would look like. Notice that once again my linkage now looks just like Cellulubios, right? I have my one position, I have my four position, it looks like a beta once again. So this is a beta 14, and notice that this is also a D glucose over here. The only difference is that something looks a little bit weird on the one sugar one. Notice that this o h is facing up instead of down. That's the only difference. So instead of having it go down like a normal d glucose, it's now going up. That's the only difference. So basically this is a Cellulbios where the alcohol in position 4 is going up making it a galactose. Well guys, now we've just made lactose. Okay. So notice that just these small tiny little changes will completely change the sugar, will change the disaccharide. Okay. And then finally, one more that's just really important that you need to know is, D glucose plus a D fructose. This is is much different than the others. So remember fructose would actually be usually a 5 membered ring. With an alpha 1 beta 1 linkage. Now what does that mean? Well, notice that I'm using 2 completely different types of chains. So the reason that we call it an alpha 1 beta 1 is because let me show you. Because of the fact that over here, this is a beta one because for the for the what sugar is this again? For the d glucose, notice that it looks, oh, I'm sorry, this is an alpha one. This is my alpha one because notice that it's facing opposite to the stereo descriptor. The stereo descriptor is this way. Notice that this O is facing down, so the alpha one comes from the D glucose, right? For the fructose, we call this a beta 2 because the one position of fructose is actually this carbon here. Okay. So that's the one position, so this is my 2 position, so that means that it's actually attached to the 2, and it's beta 2 because the descriptor of fructose is this one right here. So do you have to absolutely memorize this right now? Probably not, but I want to expose you to the fact that sucrose, which is the one I'm hinting at here, which is a very, very important disaccharide, is made by 2 different types of linkages. It's like a linkage that you have to describe with 2 letters because you've got the alpha 1 on the d glucose and you've got the beta 2 on the fructose and you have to describe both of those. You wouldn't just call it alpha. You wouldn't just call it beta. You'd call it a combination of the 2. Okay. And this is called sucrose. Okay. Now guys, fun fact which, is related to these and it's actually going to be important later, so just keep this in mind. Humans cannot cannot digest carbohydrates or disaccharides with O beta glycosidic linkages. Okay. So remember that we were talking about the difference in structure between beta and alpha. Alpha has more kinks. Beta is more straight usually. Those straight ones, the human body can almost never digest because they're too rigid. It's too difficult to get the enzymes in there, so we can't digest it, except for one important exception. Can you guys think of what that exception might be? It's on this page. Lactose. Some people can digest lactose and some can't. So when you hear that someone is lactose intolerant, that's just because that person is normal. Most humans cannot digest beta glycosidic linkages. That's our default evolutionary state is that we would not be able to digest lactose. The reasons for that actually make a lot of sense that once you're past the age of 1 or 2, your body stops making that enzyme called lactase. So I'm just going to put here lactase is the enzyme. Lactase is the enzyme that digests, that breaks down lactose into its monosaccharides. Your body stops making it because it thinks that you're not gonna drink any more milk, right? So it it thinks, oh, we're done. Now we're gonna eat real food. But now the modern, American diet or even global diet, we're drinking, cow milk and and all these dairy products more than ever before. So our bodies aren't adapted to that because they stopped producing lactase a long time ago and a lot of people are struggling to digest lactose because of those beta glycosidic linkages. Now, another fun fact on top of that, whenever you get lactose free milk, it's not really lactose free. They didn't take out the lactose because that's actually the milk sugar. All they did is they added in lactase into the milk so that you would be able to digest it. Since you don't make your own lactase, they added in some extra lactase. So that's actually a big lie. Lactose free doesn't mean that it doesn't have lactose. They just added the enzyme so that you can digest the beta glycosidic linkage. Cool? So interesting, right? So if you had to predict which of these the body or human body would be able to use as energy, which one would they be? Maltose and sucrose because both of those contain to some extent alpha linkages. And alpha linkages we can digest. They're easier to digest for us. The beta linkages, we need a little bit of help with lactose. And for other types of disaccharides, we just won't be able to digest them at all. So for example, Cellubios, you absolutely cannot digest. That's too fibrous for us because it's so rigid. So even you would never eat cellulbios. That's more used for like structural elements and stuff like that. Cool? Awesome guys, so let's go ahead and move on to a practice problem.

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Identify the following disaccharides as reducing sugars (RS) or non-reducing sugars (NS)

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