Lecture 6

Question 1

General description of carbohydrate structure

Answer 1

general formula: (CH2O)n

contains other elements (N, S, P)

Question 2

Roles of carbohydrates

Answer 2

fuel source/storage (glucose, glycogen, starch)

structural compounds (cell walls, cellulose)

recognition/regulation of extracellular proteins

structural component in nucleic acids

Question 3

Describe the classes of carbohydrates

Answer 3

monosacch, disacch, oligosacch (generally soluble in water)

homopolysachh, heteropolysachh (polymers of one or more carboydrate unit, generally insol in water, large molecule)

glycoconjugates (carbohydrates joined to other molecular classes, proteins/peptides: proteoglycans, glycoproteins, peptidoglycans, lipid: glycolipids)

Question 4

Describe monosacch (i.e. structure and naming)

Answer 4

aldehydes or ketones with two or more hydroxyl groups

unbranched carbon chain connected by single bonds (multiple chiral centers)

one carbon attached to oxygen via carbonyl
aldehyde (C1): monosacch is an aldose
ketone (C2): monosacch is a ketose

named by number of carbon atoms (i.e. 3 carbons is a triose, 6 carbons is a hexose. hexose most common.)

numbering of carbons begins at end closest to the carbonyl

type and length are combined to refer to sugar length (glucose is an aldohexose)

Question 5

All monosaccharides except for what has at least one chiral center

For N chiral centers, there are how many possible steroisomers

Answer 5

dihydroxyacetone

2^N

Question 6

What are epimers

Answer 6

differ in config at only one chiral carbon (with two or more chiral centers)

Question 7

Saccharides are classed as D or L based on

Answer 7

config of the chiral carbon most distant from the carbonyl group

Question 8

In standard Fischer projections, where is C-1 drawn?

The chiral carbon furthest from the carbonyl will have the hydroxyl on the (left/right) for the D isomer, and on the (left/right) for the L isomer

Answer 8

at the top

right for D isomer

left for L isomer

Question 9

What are enantiomers

Answer 9

mirror reflection stereoisomers (invers all chiral centers)

Question 10

An aldohexose has how many chiral centers and how many possible stereoisomers?

Which carbon identifies an aldohexose as D or L?

Answer 10

4 chiral centers

16 possible stereoisomers

C5 identifies an aldohexose as D or L

Question 11

D-mannose is the C-2 epimer of D-glucose. What does this look like?

Answer 11

C2 hydroxyl group is on the other side

Question 12

How are hemiacetals and hemiketals formed?

Answer 12

alcohol and aldehyde/ketone

further addition of another alcohol can yield a full acetal or ketal

yields a new chiral center (previous carbonyl carbon)

Question 13

Monosaccharides will adopt cyclic structures. How?

Answer 13

in solution, many monosaccharides will adopt a cyclic conformation through an intramolecular reaction: hydroxyl group acts as nucleophile

pyranoses (6 membered ring) or furanoses (5 membered ring)

carbonyl is converted to either e hemiacetal (aldoses) or hemiketal (ketoses)

anomeric carbon is chiral in cyclic form

Question 14

How is the hemiacetal formed in glucose?

Answer 14

The c5 hydroxyl reacts with the c1 aldehyde group of glucose

this intramolecular reaction forms a 6 membered ring, called glucopyranose

after ring formation, c1 becomes a new chiral center. This carbon is called the anomeric carbon

two stereoisomers are formed depending on the orientation of the anomeric OH:

α-anomer, OH on opposite side of the ring relative to the C6 OH2OH group

β-anomer, OH on the same side of the ring as the C6 CH2OH group

α and βforms can interconvert. This occurs by opening of the ring, and reforming the hemiacetal. Process is called mutarotation

Question 15

Monosaccharides can form rings of two possible sizes:

Answer 15

furanose: 5 membered ring, named after furan

Pyranose: 6 membered ring, named after pyran

aldoses with ≥ 5 carbons can form: furanose rings and pyranose rings (pyranose ring usually more stable)

Question 16

What are Haworth projections

Answer 16

used to show cyclic sugar structures

display stereochem more clearly than Fischer projections

conversion: OH below ring in Haworth: right side in Fischer

OH above ring in Haworth: left side in Fischer

Question 17

Describe pyranose structure

Answer 17

sugar rings are not flat

pyranose rings adopt chair conformations, similar to cyclohexane

two chair conformations exists + they can interconvert without breaking bonds

substituents around the ring occupy axial (more crowded) or equatorial (more spacious) positions

reduce steric strain by putting bulky groups (like OH) in equatorial positions

Question 18

Fischer, Haworth, chair, and Mills projection

Answer 18

Fischer: linear form. Horizontal bonds = coming out of page, vertical bonds = going into page (note for D glucose, OH on C5 is on the right)

Haworth: cyclic form (note for α D glucose, OH on C1 is DOWN, and CH2OH (C6) is UP))

chair: ring behaves with cyclohexane, substituents are either axial or equatorial. (note in β D glucose, almost all OH groups are equatorial, making it more stable than α)

mills: modified Haworth, 3d stereochem. (solid bond is LEFT)

NOTE: D sugars:
α = anomeric OH DOWN
β = anomeric OH UP
relative to the CH₂OH group on C5.

Question 19

oxidized sugars, means?

Answer 19

oxidation means loss of electron, usually turning an aldehyde or alcohol into a carboxylic acid

there can be oxidation of aldonic acids, such as glucose to gluconic acid, or uronic acids, such as glucose to glucuronic acid

Question 20

What does oxidation of aldonic acids look like?

Answer 20

oxidation at C1, the aldehyde group (-CHO) at C1 is oxidized to a carboxylic acid (-COOH)

suffic -onic acid

example: glucose to gluconic acid

CHO to COOH

note: ONLY the anomeric carbon (C1) is oxidized

Question 21

What does oxidation of uronic acids look like?

Answer 21

oxidation of the primary alcohol (C6)

the primary alcohol group (CH2OH) becomes a carboxylic acid (COOH)

naming suffix -uronic acid

glucose to glucuronic acid

CH2OH to COOH

IMPORTANCE: glucuronic acid is important in detoxification in the liver

Question 22

Reduced sugars: what does formation of an alditol look like?

Answer 22

reduction of the carbonyl group to an alcohol

the aldehyde (or ketone) group is reduced to an alcohol

suffix -itol

examples of sugar to alditol
glucose to sorbitol
mannose to mannitol
ribose to ribitol

reaction idea: CHO to CH2OH

• sugar now has alcohol groups on both ends

Question 23

Which aldose can be reduced to generate sorbitol?

Answer 23

D glucose

Question 24

Monosaccharides can be chemically modified to create sugars with special biological roles. What are the modifications that could take place?

Answer 24

deoxy sugar: OH replaced by H

amino sugar: OH replaced by NH2

acetylated amino sugar: NH2 to NHCOCH3

sialic acid: 9 carbon derivative

Question 25

Describe the formation of deoxy sugars

Answer 25

formed when an OH group is removed from a monosachharide and replaced with H OH to H example: 2-deoxyribose: sugar in DNA, missing the OH at C2

Question 26

Describe the formation of amino sugars

Answer 26

an OH group (usually at C2) is replaced by an amino group OH to NH2or NH3+ suffix: amine i.e. glucosamine the amino group is often acetylated: NH2 to NHCOCH3 i.e. GlcNAc can occur in structural polysacch such as chitin and bacterial cell walls (peptidoglycan)

Question 27

Describe sialic acids

Answer 27

most common form: N-acetylneuaminic acid (Neu5Ac), a 9 carbon monosacch, formed from N-acetylmannosamine and pyruvate found at the ends of carbohydrate chains in glycoproteins, glycolipids, gangliosides (negative charge) biological importance: imporant in: cell recog, immune interactions, cell signaling often form the outermost sugar on cell surfaces

Question 28

How are glycosidic bonds formed?

Answer 28

forms when the anomeric carbon reacts with another alcohol group the hemiacetal/hemiketal becomes a full acetal or ketal this forms a glycosidic bond rxn: Sugar-OH + anomeric carbon = glycosidic bond once the glycosidic bond forms: the anomeric carbon cannot open the ring, therefore no mutarotation occurs reducing sugars are ones with a hemiacetal/hemiketal anomeric carbon nonreducing sugars have all anomeric carbons in glycosidic bonds

Question 29

What is the difference between a reducing and nonreducing sugar?

Answer 29

reducing sugar contain a free anomeric carbon (hemiacetal), can open to linear aldehyde form, can reduce oxidizing agents examples: glucose, maltose, lactose nonreducing sugars have all anomeric carbons involved in glycosidic bonds. They cannot open into aldehyde form, cannot act as reducing agents example: sucrose

Question 30

What are the types of glycosidic bonds?

Answer 30

O-glycosidic bonds - the anomeric carbon bonds to an oxygen - connects sugars to alcohol groups examples: disaccharides and polysaccharides N-glycosidic bonds - the anomeric carbon bonds to nitrogen - connects sugar to nitrogenous base

Question 31

What are polysaccharides?

Answer 31

long chains of monosacch also called glycans joined mainly by O-glycosidic bonds differ based on 1) type of monosaccharide 2) position of glycosidic bond (1→4, 1→6) 3) configuration of anomeric carbon (α or β) i.e. glycogen, starch, cellulose

Question 32

How are disaccharides formed?

Answer 32

2 monosacch combine via a condensation reaction (anabolic process, req energy, produces a O-glycosidic bond) bond forms between: anomeric carbon of one sugar, hydroxyl group of another sugar disacch may be reducing or non reducing

Question 33

How are disaccharides named?

Answer 33

1) config of anomeric carbon (α or β) 2) first monosaccharide - ends in -osyl 3) carbons involved in linkage (i.e. (1→4)) 4) second monosaccharide 5) suffix rule -ose = reducing sugar -oside = nonreducing sugar examples α-D-glucopyranosyl-(1→4)-D-glucopyranose *nonreudcing sugars are indicated with a double headed arrow in the abbreviated form)

Question 34

What is the disacch name for maltose?

Answer 34

α-D-glucopyranosyl-(1→4)-D-glucopyranose