Carbohydrate Theory

Question 1

Generally, what is a carbohydrate’s chemical structure?

Answer 1

• a polyhydroxy aldehyde (R-CH=O) or ketone (R-CR’=O)

Question 2

What is the simplest aldehyde-based carb?

And what are aldehyde-based carbs called generally?

Answer 2

glyceraldehyde

• glycerol w/ one end OH replaced by =O
• known as aldoses

Question 3

What is the simplest ketone-based carb?

And what are ketone-based carbs called generally?

Answer 3

dihydroxyacetone

• glycerol w/ middle OH replaced by =O
• known as ketoses

Question 4

How are carbs generally named based on the number of carbons they contain?

Answer 4

triose - 3 C

**tetrose **- 4 C

pentose - 5 C

hexose - 6 C

etc.

Question 5

How can L or D configurations be determined via a simple, 2D

backbone-style carb structure?

What configuration are most carbs in nature?

Answer 5

Look at the last chiral carbon in the chain.

The position of the functional group (OH) on this carbon determines L or D.

Most are D in nature.

Question 6

How would a 6 C aldehyde-based carb be named?

An 8 C ketone-based carb?

Answer 6

Aldohexose

Keto-octose

etc.

Question 7

How is D-Galactose different from D-Glucose?

What does this make it in relation to glucose?

How can its structure be rememberd?

Answer 7

• swapped OH position on C4 (left instead of right)
• this makes it the 4-epimer of glucose
• the OHs kind of make a G shape (according to Sipeki)

Question 8

What is an epimer?

Answer 8

• an isomer that differs in configuration from its pair epimer at only one stereocenter
• ALL other stereocenters, if there are any, are the same

Question 9

What is mannose in relation to glucose?

Answer 9

a 2-epimer

Question 10

What does reduction of a carb consist of?

What is the product?

Answer 10

• aldehyde or ketone is reduced into primary or secondary alcohol, respectively
• in other words =O becomes -OH and -H
• makes a sugar alcohol
  ex: glucose/fructose > sorbitol

Question 11

What does a sugar oxidation consist of?

What does it make (3 examples via 3 diff oxidations) ?

Answer 11

• generally a group that has either only -OH or =O gets BOTH and becomes carboxylic
• 1st group = -aldonic acid
• last group = -uronic acid
• both groups = -aldaric acid

Question 12

How is fructose different from glucose?

Using general nomenclature, what does this make it?

Answer 12

• the =O is on C2 via a spontaneous migration
• this makes it a ketohexose (rather than aldo-)

Question 13

What are some examples of important sugar alcohols and their functions in nature?

Answer 13

**ribitol **is a component of riboflavin (vit B2)

**sorbitol **is used as a sweetener for diabetics

Question 14

What are some important natural sugar acids and their functions?

Answer 14

**D-glucuronic acid **and uronic acid are mucopolysaccharide components

L-ascorbic acid is vitamin C

D-glyceric acid is important to glycolysis and O₂ binding in Hb

Question 15

Why don’t carbohydrates exist in the open chain form in which they are often represented?

Answer 15

• because aldehyde and ketone groups are quite reactive
• tend to form rings with other Os in the chain

Question 16

What are the two groups which bond to form the ring structure of an aldose?

And what is this kind of bond called?

Answer 16

the aldehyde and an alcohol

forms a hemiacetal

Question 17

What two groups bond to form the ring structure in ketoses?

What is this bond called?

Answer 17

ketone bonds with hydroxyl

forms hemiketale

Question 18

What is mutarotation?

Answer 18

a change in optical rotation due to shifts in equilibrium ratios of two anomers

Question 19

What is an anomer?

How are anomers denoted in chemical structure names?

Answer 19

carbohydrate epimers that differ specifically at the glycosidic OH

• ⍺ means OH is ‘up’ (or same position as the ‘D’ OH)
• β means OH is ‘down’ (or same position as ‘L’ OH)

Question 20

Why is beta-D-glucose the more stable than alpha?

Answer 20

all its OH groups are equatorial in chair conformation

Question 21

In what way does glucose isomerism change from open-chain to ring structure?

Answer 21

• addition of a 5th chiral center at the aldehyde C
• this means anomerism … ⍺ and β

Question 22

What are the general names for the two most stable carbohydrate ring structures?

Answer 22

Pyranose - 6 atom ring with one O, characteristic of aldohexoses

Furanose - 5 atoms, one O, from pentoses + ketohexoses

Question 23

What are the rules for drawing carbohydrate ring representations?

Answer 23

1. O in ring is at top right for pyranoses, top for furanoses
2. OH groups to right of open chain form are drawn below ring plane
3. CH₂OH group is written over ring plane for D, under for L

Question 24

What are reducing vs. non-reducing disaccharides?

Answer 24

reducing - have a free glycosidic OH group

non-reducing - no free glycosidic OH (only sucrose)

Question 25

What is glycogen?

What feature of a disaccharide unit of glycogen forms its backbone?

It contains two kinds of bonds between its monomers…what are they?

Why does the body store glycogen and not free monosaccharides?

Answer 25

• a highly branched storage carb of many glucose units
• axial groups of alpha-D-glucose form helical “trunk” of glycogen
• 1-4 glucose bonds make up “trunk”, 1-6 glucose bonds make up “branches”
• its single free glycosidic OH end makes it unreactive and it has lower osmotic effect than free monomers

Question 26

What are the two important pentoses in nucleic acids?

Answer 26

Beta-D-Ribose - RNA (shown below)

Beta-D-2-Deoxyribose - DNA (ribose minus the O on C2)