Carbohydrates

1. Carbohydrates • What are they? • Sugars, starches & much more • Most abundant molecules on Earth • End products of photosynthesis

2. Carbohydrates • Functional Roles • Carbohydrate polymers act as energy storage molecules • Cell walls and protective coatings • Cell recognition and interaction • Part of DNA, RNA, coenzymes

3. Classifications • Monosaccharides • Smallest unit of carbohydrate structure • Hydrates of carbon (CH2O)n n = 3-9 Most common n = 5-6 • Oligosaccharides • Polymers of 2-20 monosaccharides • Most common – disaccharides (2 monosaccharides

4. Polysaccharides • Polymers of many (usually >20) monosaccharides

5. Other Definitions • Homoglycan – a polymer of identical monosaccharides • Heteroglycans – a polymer of different monosaccharides • (note: glycan is a term for carbohydrate polymers)

6. Glycoconjugates – carbohydrate derivatives • One or more carbohydrate chains covalently linked to peptide, proteins or lipids

7. More Definitions • Stereoisomers – compounds that have the same molecular formula but different arrangements of their atoms in space • Enantiomers – 2 stereoisomers that are mirror images of one another • Diastereomers – 2 stereoisomers that are not mirror images of one another

8. Chiral Carbon- a carbon atom with 4 different groups bonded to it • Epimers – molecules that differ in configuration at only one of several chiral centers

9. Monosaccharides Water-soluble, white, crystalline solids with a sweet taste Are polyhydroxy alcohols with at least 3 carbons

10. Two Types • Aldoses • Contain an aldehyde on C 1 • Ketoses • Contain a ketone, usually on C2 • Smallest monosaccharides - trioses

12. Glyceraldehyde has 2 Forms • It contains one chiral carbon – is optically active • D-form rotates polarized light to the right • L-form rotates polarized light to the left

21. In solution, monosaccharides exist in cyclic conformations • Carbonyl carbons react with a hydroxyl group to produce: • Pyranose – a 6 membered ring • Furanose – a 5 membered ring • The carbon attached to 2 O’s is called an anomeric carbon – can be alpha or beta

26. Haworth Projections • A ring structure • The ring is projected as lying perpendicular to the page • Anomeric Carbon is on right or top • OH’s on right are drawn down • OH’s on left are drawn up

27. On anomeric carbon • Alpha configuration – OH is down • Beta configuration – OH is up

29. Other Furanose Conformations • Envelope – one of the five ring atoms is out-of-plane & others are coplanar • Twist – two of the five ring atoms are out-of-plane, one on either side of the plane formed by the other 3 atoms

31. Other Pyranose Conformations • Chair – • Most stable – minimize steric hindrance • Substituents may be: • Axial (above or below plane of ring) • Equatorial (in the plane of the ring) • Boat –

33. Derivatives of Monosaccharides • Sugar Phosphates • Phosphate ester of one of the –OH groups • Often on the terminal carbon • Important in glucose metabolism (e.g., glucose 6-phosphate, glucose 1-phosphate)

35. Derivatives of Monosaccharides • DeoxySugars • A hydrogen atom replaces a hydroxyl group • Very important in DNA synthesis (e.g., deoxyribose) • L-Fucose (6-deoxy-L-galactose) is common in polysaccharides

37. Derivatives of Monosaccharides • Amino Sugars • One of the hydroxyl groups in a monosaccharide is replaced by: • Amino group • Acetylated amino group • Important in glycoconjugates

38. N-Acetylneuraminicacid (NeuNAc) • Starts with N-acetylmannosamine and pyruvate • Cyclizes to a pyranose in a unusual way – carbonyl group at C-2 (from pyruvate) reacts with –OH at C-6 • NeuNAc & its derivatives = sialic acids • Important in glycoproteins

40. Derivatives of Monosaccharides Sugar Alcohols A carbonyl oxygen is reduced to an -OH

42. Derivatives of Monosaccharides Sugar Acids Derived from aldoses by either: oxidation of the aldehyde carbon oxidation of the highest numbered carbon