Carbohydrates

• Carbohydrates


  The most abundant biomolecules on earth
• Photosynthesis


  Converts over 100 billion tons of carbon dioxide and water into cellulose and other plant products each year
• Carbohydrates like sugar and starch are the entire staple in most parts of the world
• Carbohydrates make up about 75 percent by weight of dry plant materials
• Carbohydrates


  Polyhydroxylated aldehydes and ketones and substances that yield such compounds upon hydrolysis
• Carbohydrates


  Have a general formula of CH2On but some also contain nitrogen, phosphorus and sulfur especially for glycoconjugates
• Carbohydrates


  • Exist with at least one and often two or more asymmetric centers
  • These features give carbohydrates the capacity to perform a wide range of functions
• Functions of carbohydrates in the body

  • Carbohydrate oxidation provides energy source
  • Part of the structural framework for genetic material nucleic acids DNA and RNA
  • Serve as structural elements in cell walls
  • Part of nucleic acids, lipids and proteins
  • Serve as units on cell surfaces for cell to cell recognition processes
• Major classes of carbohydrates


  • Monosaccharides
  • Disaccharides
  • Oligosaccharides
  • Polysaccharides
• Monosaccharides


  Also called simple sugars, single polyhydroxylated aldehyde or ketone units
• Disaccharides


  Consist of two monosaccharides linked together by a glycosidic bond
• Oligosaccharides


  Made up of 3 to 10 monosaccharide units
• Polysaccharides


  Long chains of monosaccharide units
• Monosaccharide classifications based on carbonyl group


  • Aldos
  • Ketos
• Aldos


  Monosaccharides where the carbonyl group is at the end of the carbon chain
• Ketos


  Monosaccharides where the carbonyl group is at another position in the carbon chain
• Monosaccharide classifications based on number of carbon atoms

  • Trios
  • Tetros
  • Pentos
  • Hexos
  • Heptos
• Monosaccharides usually contain many chiral carbons and exist in a number of stereochemical forms
• D configuration


  Hydroxyl group is at the right side of the penultimate carbon
• L configuration


  Hydroxyl group is at the left of the penultimate carbon
• Number of stereoisomers


  2^n, where n is the number of chiral centers
• Number of stereoisomers


  • Aldo trios (2 stereoisomers)
  • Aldo tetros (4 stereoisomers)
  • Aldo pentos (8 stereoisomers)
  • Aldo hexos (16 stereoisomers)
• Epimers


  Sugars that differ in configuration in only one carbon other than the penultimate carbon
• Epimers of aldohexoses


  • D-glucose, D-mannose, D-galactose
• Aldohexoses

  Monosaccharides with an aldehyde group and 6 carbon atoms
• Ketohexoses


  Monosaccharides with a ketone group and 6 carbon atoms
• Anomers


  Isomeric forms of monosaccharides that differ only about the hemiacetal or hemiketal carbon atom in their configuration
• Alpha configuration


  Hydroxyl group at the anomeric center is on the same side as the hydroxyl group of the farthest chiral center
• Beta configuration


  Hydroxyl group at the anomeric center is on the opposite side of the hydroxyl group of the farthest chiral center
• Anomers


  • Alpha-D-glucopyranose
  • Beta-D-glucopyranose
  • Alpha-D-fructofuranose
  • Beta-D-fructofuranose
• In aqueous solution, simple carbohydrates exist in cyclic forms with the formation of internal hemiacetals or hemiketals
• Anomeric carbon is the hemiacetal or carbonyl carbon atom that reacts with the hydroxyl group
• Alpha and beta anomers interconvert through a process called mutarotation
• Reactions involving monosaccharides


  • Formation of sugar acids
  • Formation of sugar alcohols
  • Glycoside formation
  • Phosphate ester formation
  • Amino sugar formation
• Formation of sugar acids


  1. Oxidation of monosaccharides at carbon 1

  2. Oxidation of monosaccharides at carbon 6

  3. Oxidation of monosaccharides at both carbon 1 and 6
• Sugar acids


  Formed by the oxidation of monosaccharides
• Formation of sugar alcohols


  Reduction of the carbonyl group in monosaccharides to a hydroxyl group using hydrogen as a reducing agent
• Sugar alcohols


  Polyhydroxy alcohols formed by the reduction of monosaccharides
• Glycoside formation


  Reaction of cyclic monosaccharides (hemiacetals) with alcohols to form acetals
• Glycoside


  Acetal formed from a cyclic monosaccharide by replacement of the hemiacetal carbon hydroxyl group