Monosaccharides, Disaccharides, and Polysaccharides: Overview and Classification

1. Chapter 12 Outline • 12.1Monosaccharides - Aldose and ketose • Glyceraldehyde, Stereoisomers (Mirror image = enantiomer), D and L isomers of Glyceraldehyde • - Linear forms of: Glucose, fructose, galactose, ribose • 12.3Monosaccharides in Cyclic Forms • Cyclic form of Glucose, Fructose and Ribose • a and b anomers of the Glucose, Fructose, and Ribose • 12.4Disaccharides and Oligosaccharides • -Maltose, Cellobiose, lactose, sucrose • 12.5 Polysaccharides: amylose, amylopectin, cellulose, glycogen, chitin, • chondroitin sulfate, hyaluronic acid

2. Classification of Monosaccharides • Aldoses -those that contain an aldehyde group. • Ketoses - those that contain a ketone group. • Trioses have 3 carbon atoms; tetroses have four carbon atoms; pentoses have 5 carbon atoms, and so on. • An aldohexose is an aldehyde sugar with 6 carbon atoms. • (The “ose” ending indicates that the molecule being named is a carbohydrate).

3. Chiral carbon gives rise to stereoisomers. (chiral carbon = asymetric carbon) Mirror image molecules = enantiomers -OH on the right side for D-isomers

4. Stereoisomers • Carbohydrates contain a chiral carbon atom and have stereoisomers. • Enantiomers are represented using a Fischer projection. • In Fischer projections, chiral carbon atoms sit at the intersection of a vertical line and a horizontal line. • The horizontal lines represent bonds pointing toward the viewer, and the vertical lines are for bonds pointing away from the viewer. • For monosaccharides, Fischer projections are drawn with the carbon atoms running vertically and the aldehyde or ketone group at or near the top.

5. D and L sugars • When drawn in a Fischer projection, the -OH attached to the chiral carbon atom next to the last carbon atom (just above the last carbon atom) points to the right in a D-monosaccharide and to the left in an L-monosaccharide. • In nature, most monsaccharides are D sugars, so D-glyceraldehyde is commonly found, while L-glyceraldehyde is not.

7. Glucose – blood sugar; the main fuel molecule for our bodies. Fructose – sugar from fruit Galactose – sugar found in lactose (in milk) Ribose – a composition of RNA 2-Deoxyribose – a composition of DNA Important Monosaccharides • D-Glucose is also known as blood sugar. One form of energy storage in the body involves combining glucose molecules to produce glycogen. • D-Galactose is combined with glucose to produce lactose, a disaccharide that gives milk its sweetness. • D-Fructose, or fruit sugar, is the ketose found most often in nature. Fructose plays a key role in energy production.

8. Important Monosaccharides • Pentoses and hexoses are the most abundant monosaccharides found in nature. • D-ribose and D-2-deoxyribose are aldopentoses that are often incorporated into larger biomolecules, including NADPH, NAD+, and the nucleic acids RNA and DNA. • The structural difference between these two monosaccharides is that 2-deoxyribose lacks an -OH group at carbon atom #2, hence the “2-deoxy” part of the name.

9. Examples of other kinds of monosaccharides or sugars. You don’t need to know any of these structures, just so that if you see the carboxylic acid, amide, amine, etc.. you recognize that these are different sugars.

10. 12.3 Cyclic Hemiacetal Formation • When the carbonyl group of an aldehyde or ketone reacts with the hydroxyl group (-OH group) of an alcohol, a hemiacetal forms. In D-glucose the #5 –OH group reacts with the carbonyl group of C #1.

11. Cyclic form of Glucose: - Mutarotation is the interconversion of the  and  anomer. These two forms of Glucose are said to be anomers (a and b anomers).

12. Anomers The one with the hemiacetal -OH on carbon 1 pointing down is called the alpha () anomer and that with the -OH group pointing up is the beta () anomer. 12

13. Cyclic forms of Fructose, Ribose and Deoxyribose

14. 12.4: Disaccharides are sugars which contain two monosaccharide residues. Examples: Maltose (2 glucoses, a(14) bond) Cellobiose (2 glucoses, b(14) bond) Lactose (galactose –glucose, b(14) bond) Sucrose (glucose-fructose) 14

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17. These are our Glycolipids! Can you spot the sphingosine and the fatty acyl group? 17

18. Roles of Disaccharides: * Maltose – form the constituents or building blocks of amylose, amylopectin and Glycogen; use for energy. * Cellobiose – form the constituents or building blocks of cellulose (cellulose fiber); leading to structural role, not energy role. * Galactose – milk sugar. * Sucrose – table sugar; energy role.

19. Carbohydrates that contain many monosaccharide residues linked together are called polysaccharides. In living things, some polysaccharides help provide structure (structural rigidity) and some act as energy storage molecules. We will study these polysaccharies only: Starch (consists of amylose and amylopectin), glycogen, cellulose, chitin, chondroitin sulfate, hiyaluronic acid Polysaccharieds: 19

20. In plants: Starch is produced by plants as a way to store energy. Starch consists of two different polysaccharides, amylose and amylopectin, each of which is composed entirely of maltose (or glucoses) building blocks. Amylose – single long chain of glucoses, no branching. Amylopectin – one long main chain of glucoses attached to branchings (every 25 – 30 glucose residues).. In animals or human: Glycogen – an energy storage polysaccharide in animals and human, not in plants. The structure of Glycogen looks like that of amylopectin, except it is more branching (every 8 – 12 glucose residues). Polysaccharides 20

21. Amylose structure Amylopectin or Glycogen structure. The branch is the one on top. 21

22. Cellulose is of major importance to the structure of plants (wood is about 50% cellulose) because it provides support to stems and stalks and provides a tough water-insoluble protective barrier. Most animals do not produce enzymes required to hydrolyze cellulose into cellubiose and individual glucose molecules, so cellulose is not a good animal food source. Bacteria found in the digestive tracts of horses, cows, and termites contain cellulases (enzymes) that break down the cellulose, which is digested. • Chitin makes up the exoskeleton of crustaceans and insects, and is present in the cell walls of some algae and fungi. 22

23. This is the structure of cellulose. There is no branching. 23

24. Structure of chitin Cartoon pictures of Glycogen and Amylopectin. Note glycogen is more branching. 24

25. Other polysaccharides Hyaluronic acid, found to be important components of the lubricating fluid that surrounds joints in animals (or human) body, and in the vitreous humor (clear gel) inside the eye; it is made up of alternating residues of N-acetyl-D-glucosamine and D-glucuronate; contains nitrogen. Heparin, is a natural anticoagulant substance; it inhibits blood clotting mechanisms. Chondroitin 4-sulfate consists of N-acetyl-D-galactosamine-4-sulfate residues alternating with D-glucuronate residues. These heteropolysaccharides are present in cartilage, tendons, and other connective tissues; contains nitrogen and sulfate. (Note: you don’t have to memorize anything about these two polysaccharides.) 25

26. Hyaluronic acid Chondroitin sulfate 26