Glycosides and Disaccharides – Complex Carbohydrates

by Kevin Ahern, PhD

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    00:01 Not all carbohydrates are simple sugars. Nutritionists tell us that eating complex carbohydrates is an essential part of our daily diet. In this talk, I will go through some of the simple modifications that happen to carbohydrates, and the higher order structures of the complex carbohydrates. I will discuss links of carbohydrates to lipids and finally conclude with proteins linked to complex carbohydrates.

    00:26 Now I start the talk here talking about glycosides, glycosides are modified sugars as can be seen here. So first of all I start with beta-D-glucose, a simple sugar. If I take the anomeric hydroxyide of beta glucose which is shown on the right part of the molecule and I combine it with a 10 carbon alcohol, I can create in this case, a glycoside known as beta-D-decylglucose.

    00:50 A glycoside is a sugar that has had its anomeric hydroxide modified.

    01:00 Sucrose is a good example of a glycoside. It's also an unusual example of a glycoside in the fact that it is a disaccharide that actually has two glycosidic bonds, I refer to it as a di-glycoside. We can see those two glycosidic bonds in the structure of sucrose as shown here. Now sucrose is comprised of one molecule of glucose linked to one molecule of fructose. The linkage is between carbons one and two which causes some funky things with respect to structure as we shall see. Glucose is numbered one through six in a clockwise fashion as you can see here. Fructose is normally numbered one through six in a clockwise fashion, but to make it fit on the slide, fructose was flipped causing the structure to be seen here. Nevertheless, the anomeric carbon number two was linked to the anomeric hydroxide on carbon number one of glucose to make a di-glycoside. Most disaccharide sugars that are glycosides do not have the two glycolytic bonds joined to each other. Now I've drawn this structure in another way so that you can see the glycosidic nature of this interaction a little bit better. In this case you can see the alpha bond of glucose bound to the beta bond of fructose to make the sugar known as sucrose.

    02:21 Now as I said most disaccharides don't have two glycosidic bonds, it's much more common that they will only have one. This is shown in the next slide where we examine two common sugars that are found in our diet. Lactose is a sugar comprised of glucose and galactose as can be seen here, lactose is also known as milk sugar. Maltose is a disaccharide comprised of two molecules of glucose. Now when we look at the glycosidic bonds that exist between these, we see in the case of lactose, that the glycosidic bond is between the hydroxyl on position one that was the anomeric carbon of lactose linked to the hydroxide of position four on the glucose. Position one of the glucose, where the anomeric hydroxide was located, was unaltered. This disaccharide only contain one glycosidic bond. Maltose, similarly, has a linkage between the alpha configuration of glucose on the left to the hydroxide of glucose position number four on the right. Again, the anomeric hydroxide on the far right of the molecule is unaltered in maltose.

    About the Lecture

    The lecture Glycosides and Disaccharides – Complex Carbohydrates by Kevin Ahern, PhD is from the course Biochemistry: Basics.

    Included Quiz Questions

    1. Chitobiose — fructose + galactose — β (1→2)
    2. Sucrose — glucose + fructose — α(1→2)β
    3. Maltose — glucose + glucose — α(1→4)
    4. Lactose — galactose + glucose — β (1→4)
    5. Cellobiose — glucose + glucose — β (1→4)
    1. They contain a sugar
    2. They contain nitrogen
    3. They are linear in structure
    4. They flip back and forth between alpha and beta forms
    1. Glucose and fructose
    2. Two glucoses
    3. Galactose and glucose
    4. Two fructoses
    1. Lactose
    2. Maltose
    3. Sucrose
    4. Lactase

    Author of lecture Glycosides and Disaccharides – Complex Carbohydrates

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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