Chapter 7: Carbohydrates

• “Sugars”– Nutrient source (energy source)– Structural component

• Eg. cellulose (pure polymeric sugar)• Or in combination with peptides

– Glycoproteins– Peptidoglycans

– Information content• Component of nucleotides/nucleic acids• “The sugar code”: cell/cell recognition

What is a carbohydrate?• Polyhydroxy aldehyde OR polyhydroxy ketone

• Often named with –ose– Generally: three carbons “triose,” six carbons “hexose,” etc– Specifically: glucose, ribose, fructose, sucrose, etc.

• Monosaccharides, oligosaccharides, polysaccharides

Carbohydrate structure

Carbohydrate structure

• Several chiral centers• Chirality at most

distant carbon determines L- or D-

• Chirality at other carbons determines identity

Five carbonsAll centers D- conformationD-ribose

1

2

3

4

5

Carbohydrate structure

Sugars tend to have cyclic (ring) structures in solution

Hydroxyl group on 5th carbon of glucose attacks the carbonyl carbon (aldehyde) to form a hemiacetal

Rings closed by hemiacetal or hemiketal formation

Ring formation introduces a new

stereocenter: can be - or -

That carbon is an anomeric center

Cyclic sugars

• Six-membered rings (glucose) = pyranoses

• Five-membered rings (fructose, ribose) = furans

• Not flat: ‘chair’ forms with axial and equatorial hydroxyl groups

Five-membered ring can come from five-carbon aldoses or six-carbon ketoses

Cyclic sugars

• Anomeric carbon is either:1. Free & therefore capable of mutarotation

2. Involved in a glycosidic bond

Hemiacetal/ketal can form a full acetal or ketal by a 2nd alcohol attack