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

Several chiral centers Chirality at most distant carbon determines L- or D- Chirality at other carbons determines identity Five carbons All centers D- conformation D-ribose

Carbohydrate structure

Sugars tend to have cyclic (ring) structures in solution Hydroxyl group on 5 th 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 2 nd alcohol attack