topic 7 - Carbohydrates

Question 1

what empirical formula do carbohydrates have?

Answer 1

Carbohydrates have the empirical formula: ~C(H2O) = carbo / hydrate.

• Most names end “..ose”
• Glycomics is the study of carbohydrates. Glyco… = to do with sugars
• Simplest sugars are called saccharides

Monosaccharides: (some examples)
- glucose
- mannose
- galactose
- fructose
- ribose

Question 2

what is equilibria - cyclisation?

Answer 2

C6 (hexose) sugars are in equilibrium through hemiacetal formation/cleavage

Open chain, pyranose (6-membered ring), and furanose (5-membered ring) forms exist

Pyranose forms typically dominate

Question 3

what is equilibria - anomers?

Answer 3

The product of pyranose hemiacetal formation can have the resultant OH axial or equatorial at the anomeric position

Axial OH = α, equatorial OH = β

The aldehyde carbon in the open chain form is known as the anomeric position.

Question 4

what is the anomeric effect?

Answer 4

• Substituents on 6-membered rings are usually more stable when equatorial (sterics)
• Inverted when an electronegative group is next to an oxygen in the ring

= the anomeric effect

• Reasons are not unambiguous:

Question 5

Aldo/keto and hexose/pentose

Answer 5

Sugars may have an aldehyde or ketone in open chain form

aldose or ketose respectively

Aldehydes are easy to oxidise = ‘reducing sugars’

Variation in no of C atoms

 triose (3), tetrose (4), pentose (5), hexose (6)

e.g. fructose = ketose and hexose = ketohexose
glucose = aldose and hexose = aldohexose

Question 6

what are oligosaccharides?

Answer 6

Saccharides are often found joined together via ether or acetal linkages

Vast range of possibilities by combining different sugars through different oxygen atoms

Question 7

what are polysaccharides?

Answer 7

Saccharides are often found joined together into polymers.

e.g. cellulose

• β(14) linked glucose

Plant cell walls

Paper, clothes

Question 8

what are glycosides?

Answer 8

Saccharides are often found attached to other molecules

Glycosides = small non-sugar molecule attached through anomeric position to a sugar

Small molecule part on its own is called the aglycon

This is the form in which many natural products usually exist, since it allows organisms to store compounds in an inactive form.

Glycosylation also happens during metabolism of compounds in the body.

Question 9

what are gylcans, glycoproteins and glycolipids?

Answer 9

Glycans are complex saccharides found on cell surfaces

Just 4 monosaccharides  15 million combinations

Key role is recognition of cells (e.g. cell to cell), proteins, viruses, etc.

Glycans can be attached to proteins (glycoproteins) or lipids (glycolipids)

Question 10

energy / metabolism

Answer 10

Sugars are a source of energy based upon the favourable oxidation of carbon

It provides an accessible source for the controllable equivalent of combustion.

Question 11

summary of carbohydrates

Answer 11

Sugars/carbohydrates have the formula ~[CH2O]n

They form monosaccharides with between 3-6 CH2O units

Monosaccharides exist in equilibrium between different 5- and 6-membered rings because of acetal chemistry

Pyranose forms usually dominate

The anomeric effect gives a preference for axial hydroxyl (α), but this varies

Oligo- and polysaccharides can be made by connecting monosaccharides through any of their hydroxyls.

Saccharides also get attached to small molecules (glycosides), proteins (glycoproteins), and lipids (glycolipids)

The main functions of carbohydrates are

Energy source – e.g. sucrose, starch

Structure – e.g. cellulose

Recognition – e.g. sialyl-Lewisx