Chapter 8 (part 1) Carbohydrates

Carbohydrates Most abundant class of biological molecules on Earth Originally produced through CO2 fixation during photosynthesis

Roles of Carbohydrates Energy storage (glycogen,starch) Structural components (cellulose,chitin) Cellular recognition Carbohydrate derivatives include DNA, RNA, co-factors, glycoproteins, glycolipids

Carbohydrates Monosaccharides (simple sugars) cannot be broken down into simpler sugars under mild conditions Oligosaccharides = "a few" - usually 2 to 10 Polysaccharides are polymers of the simple sugars

Monosaccharides Polyhydroxy ketones (ketoses) and aldehydes (aldoses) Aldoses and ketoses contain aldehyde and ketone functions, respectively Ketose named for “equivalent aldose” + “ul” inserted Triose, tetrose, etc. denotes number of carbons Empirical formula = (CH2O)n

Monosaccharides are chiral Aldoses with 3C or more and ketoses with 4C or more are chiral The number of chiral carbons present in a ketose is always one less than the number found in the same length aldose Number of possible steroisomers = 2n (n = the number of chiral carbons)

Stereochemistry Enantiomers = mirror images Pairs of isomers that have opposite configurations at one or more chiral centers but are NOT mirror images are diastereomers Epimers = Two sugars that differ in configuration at only one chiral center

Cyclization of aldose and ketoses introduces additional chiral center Aldose sugars (glucose) can cyclize to form a cyclic hemiacetal Ketose sugars (fructose) can cyclize to form a cyclic hemiketal

Haworth Projections -OH up = beta -OH down = alpha 6 5 4 1 2 3 Anomeric carbon (most oxidized) For all non-anomeric carbons, -OH groups point down in Haworth projections if pointing right in Fischer projections

Monosaccharides can cyclize to form Pyranose / Furanose forms b = 36% a = 21.5% b = 58.5% a = 13.5% b = 6.5%

Conformation of Monosaccharides Pyranose sugars not planar molecules, prefer to be in either of the two chair conformations.

Reducing Sugars When in the uncyclized form, monosaccharides act as reducing agents. Free carbonyl group from aldoses or ketoses can reduce Cu2+ and Ag+ ions to insoluble products

Derivatives of Monosaccharides

Sugar Phosphates

Deoxy Acids

Amino Sugars

Sugar alcohols

Monosaccharide structures you need to know Glucose Fructose Ribulose Glyceraldehyde Dihydroxyacetone