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 O H C CH2OH H C* OH HO C* H H C* OH CH2OH D-ribose C O HO C* H H C* OH CH2OH D-ribulose 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) O H C CH2OH H C* OH HO C* H H C* H C* C O HO C* H OH H C* OH OH H C* OH CH2OH CH2OH D-glucose D-fructose Stereochemistry Enantiomers O H O C O H C HO C* H H C* OH HO C* HO C* Epimers Diastereomers O H C C C* OH HO C* H H C* OH HO C* H HO C* H HO C* H H C* OH H C* OH HO H H C* OH H C* OH H CH2OH L-glucose D-glucose O H C H CH2OH O H H C H C* OH HO C* H H HO C* H HO C* H C* H H C* OH H C* OH C* OH H C* OH H C* OH CH2OH CH2OH D-mannose D-galactose CH2OH D-glucose CH2OH D-mannose •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 H cyclic hemiacetal H NEW CHIRAL ALDEHYDE O O C H ALCOHOL H R1 C* R1 O R2 O H CARBON R2 HEMIACETAL • Ketose sugars (fructose) can cyclize to form a H H cyclic hemiketal NEW CHIRAL KETONE O O CARBON C R ALCOHOL R R1 R1 O R2 O H C* R2 HEMIKETAL Haworth Projections O H -OH up = beta -OH down = alpha C1 H C2 OH HO C3 H H C4 OH H C5 OH CH2OH 6 5 4 1 3 2 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 a = 64% 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