Chapter 9. Carbohydrates and Glycoconjugates

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importance of carbohydrates?

role of carbohydrates

energy storage

structural polymers (e.g., cell wall, chitin)

lipid and protein conjugates (e.g., cell recognition)

components of other biomolecules (e.g., DNA, RNA, coenzymes)

A. Monosaccharides

characteristics:

3-6 carbons (larger do exist; typically displayed using Fisher projections)

multiple hydroxyl groups

carbonyl group in the form of an aldehyde or ketone

example:

1. nomenclature:

aldose (if aldehyde)

ketose (if ketone)

triose 3 carbons

tetrose 4 carbons

pentose 5 carbons

hexose 6 carbons

2. chirality: for a molecule with n chiral centers, there are 2n stereoisomers. The following examples show both possibilities for a carbohydrate with one chiral center (n=1) and one of the 16 possibilities when n=4.

enantiomers: mirror image related stereoisomers

diastereomers: non-mirror imagerelated stereoisomers

epimers: a special type of diastereomer that differs only at one chiral

center

3. chemistry of carbohydrates: acetal formation

intermolecular example

intramolecular example: aldenhyde and alcohol are in same molecule?

Do all hydroxyls in carbohydrates participate in hemiacetal formation?

no, typically only those leading to 5-membered rings (called furanoses) and 6-membered rings (called pyranoses)

consider case of glucose in detail

comments

cyclic forms (alpha- and beta-D-glucose shown above) have a new chiral center called the anomeric center

flat Haworth projections inaccurate – "chair" conformations would be more accurate

 

4. biologically important derivatives

a. sugar phosphates (key role in metabolism)

b. deoxy sugars (DNA building block)

c. amino sugars (structural building block)

d. sugar alcohols (lipids, coenzyme)

e. sugar acids

B. Disaccharides

Disaccharides arise from the combination of two monosaccharide units in which a new acetal bond is formed between the hemiacetal of one carbohydrate and a hydroxyl group of a second.

1. maltose: produced in hydrolysis of starch

2. cellobiose: repeating structure of cellulose

3. lactose

4. sucrose

C. Polysaccharides

terminology

homoglycans: same repeating unit

heteroglycans

reducing: has hemiacetal

non-reducing: has no hemiacetal

hemiacetal + Ag+ ® Ago + sugar acid

1. starch (plants, fungi; amylose + amylopectin)

2. glycogen (animals)

3. cellulose