Glycosaminoglycans – Complex Carbohydrates

by Kevin Ahern, PhD

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    00:01 An important modified form of carbohydrates are the glycosaminoglycans. These are polymers of modified sugars that have some very interesting properties associated with them. On the screen you can see a glycosaminoglycan known as hyaluronan also called hyaluronic acid.

    00:19 Now, in contrast to what we saw in the polysaccharides which only had a repeating unit of one sugar, the examples we saw were those of glucose, the glycosaminoglycans have repeating unit that's usually a disaccharide and it's usually a modified sugar in each case. Now for hyaluronan we can see the repeating unit shown in the brackets, the molecule on the left of the repeating unit is glucuronic acid. Glucuronic acid is an oxidized form of glucose, you can notice the carboxylate position six compared to the CH2OH that would normally be there.

    00:56 That carboxyl can ionize at physiological pH very readily and make a minus charge at that point. You can also see an interesting and unusual glycosidic bond between the glucuronic acid and the adjacent sugar to it, the adjacent sugar to it is a modified form of glucose known as N-acetylglucosamine. The linkage of the glycoside as you can see is a beta-1,3 where the anomeric hydroxyl of the glucuronic acid on position one is linked to hydroxide on position number three of the N-acetylglucosamine. The N-acetylglucosamine has on it an amine and an acetyl group as you can see. Now the negative charge on the carboxyl of glucoronic acid is repeated hundreds or thousands of times because this disaccharide is repeated hundreds or thousands of times. Now the more negative charges one has, the more polyanionic a substance is, and the more polyanionic a substance is, the more it interferes with or alters the chemical properties of the water in which it's dissolved. Most of the glycosaminoglycans have properties that relate to ability to withstand pressure and function as lubricants.

    02:14 If you were to feel a solution of hyaluronic acid for example you would find it to be oily or slimy. Hyaluronic acid is a very important component of the synovial fluid that's present in our joints because of its lubricating properties.

    02:31 A glycosaminoglycan that's related to hyaluronic acid is chondroitin sulfate. So we can see in this figure, it has a repeating unit that's also a disaccharide like hyaluronic acid and it's a modified set of sugars that's contained within that disaccharide. Like hyaluronic acid, the sugar on the left is related to glucuronic acid as we saw before, having the COOH carboxyl group present at position number six. At position number two however we see the presence of an R group, an R can of course stand for a variety of things meaning that different parts of this sugar can have different things attached to it here. If we look at the repeating unit on the right we see that it has two R groups. Now the R groups can be a variety of things and one of the things that R groups commonly are in chondroitin sulfate is sulfate. Sulfate of course is negatively charged. So in addition to having the negative charges associated with the carboxyl group glucoronic acid, other portions of this molecule can have a lot of negative charges as well. The complexing of these negative charges again has some important implications for the chemistry and the way that this interacts with water.

    03:40 Chondroitin sulfate is found in cartilage and has the property of helping to protect cartilage against impact, again, relevant to joints and use of cartilage in those joints.

    About the Lecture

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

    Included Quiz Questions

    1. Polyanionic disaccharide units
    2. Sphingolipid backbone joined to a single sugar
    3. Glycerophospholipid backbone joined to a single sugar
    4. Amino acid polymers
    5. Sphingolipid backbone joined to a complex carbohydrate moiety.
    1. Synovial fluid, cartilage
    2. Cartilage, synovial fluid
    3. Bones, muscles
    4. Muscles, bones
    5. Mucosal membranes, peritoneum

    Author of lecture Glycosaminoglycans – Complex Carbohydrates

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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