1. Chapter 17: Carbohydrates

2. IMPORTANT FUNCTIONS OF CARBOHYDRATES
To provide energy through their oxidation
To supply carbon for the synthesis of cell components
To serve as a stored form of chemical energy
To form a part of the structural elements of some cells and tissues
Biomolecule – a general term referring to organic compounds essential to life
Biochemistry – a study of the compounds and processes associated with living organisms

3. CARBOHYDRATES
Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds upon hydrolysis.
Example:

4. CLASSIFICATION OF CARBOHYDRATES
Carbohydrates are classified according to size:
Monosaccharide – a single polyhydroxy aldehyde or ketone unit
Disaccharide – composed of two monosaccharide units
Polysaccharide – very long chains of linked monosaccharide units

5. STEREOCHEMISTRY
Many carbohydrates exist as enantiomers – stereoisomers that are mirror images.

6. A chiral object cannot be superimposed on its mirror image.
A chiral carbon is one that has four different groups attached to it.

7. The presence of a single chiral carbon gives rise to stereoisomerism.
If a carbon atom is attached to four different groups, it is chiral.
If any two groups are identical, it is not chiral.

8. Compounds can have more than one chiral carbon: • The maximum number of stereoisomers is 2n where n= number of chiral carbon atoms.
Therefore, this compound with two chiral carbon atoms has 22 or 4 stereoisomers.
The compound on the previous slide with four chiral carbon atoms has 24 or 16 stereoisomers.

9. FISCHER PROJECTIONS
Fischer projections depict three-dimensional shapes for chiral molecules, with the chiral carbon represented by the intersection of two lines.

10. Fischer projections of carbohydrates have the carbonyl (C=O) at the top. It is projecting away from the viewer behind the plane in which it is drawn.
The hydroxyl group on the chiral carbon farthest from the C=O group determines whether the carbohydrate is D (OH on right) or L (OH on left). The two horizontal bonds are coming toward the viewer out of the plane in which they are drawn.

11. D and L enantiomers rotate polarized light in opposite directions.

12. The enantiomer that rotates polarized light to the left is the levorotatory or (-) enantiomer.
The enantiomer that rotates it to the right is the dextrorotatory or (+) enantiomer.
The D and L designations do not represent dextrorotatory and levorotatory.
In some instances only the D or L enantiomers are found in nature. They are rarely found together in the same biological system.
For example, humans can only metabolize the D-isomers of monosaccharides.

13. MONOSACCHARIDE CLASSIFICATION
Is the monosaccharide an aldehyde (aldose) or ketone (ketose)?
How many carbon atoms are in the monosaccharide?

14. COMBINING MONOSACCHARIDE CLASSIFICATIONS

15. PHYSICAL PROPERTIES OF MONOSACCHARIDES
Most are called sugars because they taste sweet.
Because of the many –OH groups, they form hydrogen bonds with water molecules and are extremely water soluble.

16. MONOSACCHARIDE REACTIONS
All monosaccharides with at least five carbon atoms exist predominantly as cyclic hemiacetals and hemiketals.
A Haworth structure can be used to depict the  (-OH on the anomeric carbon pointing down) and  (-OH on the anomeric carbon pointing up) anomers of a monosaccharide.
Anomers are stereoisomers that differ in the 3-D arrangement of groups at the anomeric carbon of an acetal, ketal, hemiacetal, or hemiketal group.

17. SIX-MEMBERED PYRANOSE RING SYSTEM

18. FIVE-MEMBERED FURANOSE RING SYSTEM

19. MONOSACCHARIDE REACTIONS, cont.
The –OH groups of monosaccharides can behave as alcohols and react with acids (especially phosphoric acid) to form esters.

20. MONOSACCHARIDE REACTIONS, cont.
Cyclic monosaccharide hemiacetals and hemiketals react with alcohols to form acetals and ketals, referred to as glycosides.

21. IMPORTANT MONOSACCHARIDES
Ribose and Deoxyribose Used in the synthesis of DNA and RNA
Glucose Most nutritionally important monosaccharide Sometimes called dextrose or blood sugar

22. IMPORTANT MONOSACCHARIDES, cont.
Galactose A component of lactose (milk sugar)
Fructose The sweetest monosaccharide Sometimes called levulose or fruit sugar

23. DISACCHARIDES A glycosidic linkage is identified by:
Two monosaccharide units linked together by acetal or ketal glycosidic linkages
A glycosidic linkage is identified by:
the numbers associated with the carbon atoms joined
ﾊ together by the linkage
the configuration of the linkage for any anomeric carbon
atom joined by the linkage

24. IMPORTANT DISACCHARIDES
Maltose Two glucose units linked (14) Formed during the digestion of starch to glucose Found in germinating grain Hemiacetal means maltose is a reducing sugar

25. IMPORTANT DISACCHARIDES, cont.
Lactose Galactose and glucose units linked (14) Found in milk Hemiacetal means lactose is a reducing sugar
Sucrose Fructose and glucose units Found in many plants (especially sugar cane, sugar beets) Not a reducing sugar

26. POLYSACCHARIDES
Starch A polymer consisting of glucose units Has two forms
Unbranched amylose (10-20%)
Branched amylopectin (80-90%)
Amylose complexes with iodine to form a dark blue color

27. THE STRUCTURE OF AMYLOSE
The molecular conformation of starch and the starch-iodine complex: (a) the helical conformation of the amylose chain and
(b) the starch-iodine complex.

28. AMYLOPECTIN STRUCTURE

29. POLYSACCHARIDES, cont. Glycogen (animal starch)
A polymer of glucose units Used to store glucose, especially in the liver and muscles Structurally similar to amylopectin with (14) and
(16) linkages, but more highly branched

30. POLYSACCHARIDES, cont.
Cellulose A polymer of glucose units The most important structural polysaccharide Found in plant cell walls Linear polymer like amylose, but has  (14) glycosidic linkages Not easily digested, a constituent of dietary fiber

31. CELLULOSE STRUCTURE