Sinh học - Chapter 8 (part 1): Carbohydrates

Chapter 8 (part 1)CarbohydratesCarbohydratesMost abundant class of biological molecules on EarthOriginally produced through CO2 fixation during photosynthesisRoles of CarbohydratesEnergy storage (glycogen,starch)Structural components (cellulose,chitin)Cellular recognition Carbohydrate derivatives include DNA, RNA, co-factors, glycoproteins, glycolipidsCarbohydratesMonosaccharides (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 sugarsMonosaccharidesPolyhydroxy ketones (ketoses) and aldehydes (aldoses)Aldoses and ketoses contain aldehyde and ketone functions, respectively Ketose named for “equivalent aldose” + “ul” insertedTriose, tetrose, etc. denotes number of carbons Empirical formula = (CH2O)nMonosaccharides are chiralAldoses with 3C or more and ketoses with 4C or more are chiralThe number of chiral carbons present in a ketose is always one less than the number found in the same length aldoseNumber of possible steroisomers = 2n (n = the number of chiral carbons)StereochemistryEnantiomers = mirror imagesPairs 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 centerAldose sugars (glucose) can cyclize to form a cyclic hemiacetal Ketose sugars (fructose) can cyclize to form a cyclic hemiketal Haworth ProjectionsAnomeric carbon(most oxidized)-OH up = beta-OH down = alpha123456For all non-anomeric carbons, -OH groups point down in Haworth projections if pointing right in Fischer projections Monosaccharides can cyclize to form Pyranose / Furanose formsa = 64%b = 36%a = 21.5%b = 58.5%a = 13.5%b = 6.5%Conformation of MonosaccharidesPyranose sugars not planar molecules, prefer to be in either of the two chair conformations. Reducing SugarsWhen 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 MonosaccharidesSugar PhosphatesDeoxy AcidsAmino SugarsSugar alcoholsMonosaccharide structures you need to knowGlucoseFructoseRibuloseGlyceraldehydeDihydroxyacetone