Carbohydrates

• Carbohydrate


  Compounds of the general formula Cn(H2O)n
• Monosaccharides


  Simple sugars that fit the general formula Cn(H2O)n exactly
• Oligosaccharides


  Sugars formed when a few (Greek oligos) monosaccharides are linked
• Polysaccharides


  Sugars formed when many (Greek polys) monosaccharides are bonded together
• Reaction that adds monosaccharide units


  Involves the loss of one H2O for each new link formed
• Commonly encountered carbohydrates


  • Glycogen (in animals)
  • Starch (in plants)
  • Cellulose (in plants)
• Carbohydrate functions


  • Major energy sources
  • Oligosaccharides play key role in cell-cell interactions and immune recognition
  • Polysaccharides are essential structural components of several classes of organisms
• Monosaccharides


  Building blocks of all carbohydrates
• Aldose


  Monosaccharide that contains an aldehyde group
• Ketose


  Monosaccharide that contains a ketone group
• Trioses


  Monosaccharides with three carbon atoms
• Trioses


  • Glyceraldehyde (aldotriose)
  • Dihydroxyacetone (ketotriose)
• Stereoisomers (optical isomers)


  Molecules that differ from each other only in their configuration (three-dimensional shape)
• Enantiomers


  Mirror-image, nonsuperimposable stereoisomers
• D,L system


  System used by biochemists to denote stereochemistry
• Fischer projection


  Two-dimensional representation of the stereochemistry of three-dimensional molecules
• Diastereomers


  Nonsuperimposable, non-mirror-image stereoisomers
• Epimers


  Diastereomers that differ from each other in the configuration at only one chiral carbon
• Stereoisomers of aldoses


  • 3 carbons: 2 stereoisomers
  • 4 carbons: 4 stereoisomers
  • 5 carbons: 8 stereoisomers
  • 6 carbons: 16 stereoisomers
• D sugars predominate in nature, especially 5 and 6 carbon sugars like glucose and ribose
• Aldoses


  • Containing from three to six carbon atoms, with the numbering of the carbon atom shown. Note that the figure shows only half the possible isomers. For each isomer shown, there is an enantiomer that is not shown, the L series.
• Lewis Carroll (C. L. Dodgson), the author of Alice's Adventures in Wonderland: 'The relationship between mirror images is of interest to mathematicians as well as to chemists.'
• Sugars, especially those with five or six carbon atoms, normally exist as cyclic molecules rather than as the open-chain forms
• Hemiacetal


  A compound that is formed by reaction of an aldehyde with an alcohol and is found in the cyclic structure of sugars
• Hemiketal


  A compound that is formed by reaction of a ketone with an alcohol and is found in the cyclic structure of sugars
• Anomeric carbon


  The new chiral center created when a sugar cyclizes
• Anomers


  The possible stereoisomers formed when a sugar assumes the cyclic form
• Furanose


  A cyclic sugar with a five-membered ring, named for its resemblance to the ring system in furan
• Pyranose


  A cyclic form of a sugar containing a six-membered ring; it was named for its resemblance to pyran
• Haworth projection formulas


  A perspective representation of the cyclic forms of sugars
• In the Haworth representation, the -anomer is represented with the OH group downward, and the -anomer is represented with the OH group upward
• Figure 16.10 A comparison of the Fischer, complete Haworth, and abbreviated Haworth representations of a- and b-d-glucose (glucopyranose) and b-d-ribose (ribofuranose). In the Haworth representation, the -anomer is represented with the OH group (red) downward, and the -anomer is represented with the OH group (red) upward.
• Reducing sugars


  Sugars that have a free carbonyl group, one that can react with an oxidizing agent
• Oxidation of sugars


  1. Provides energy for organisms to carry out their life processes

  2. Highest yield of energy from carbohydrates occurs when sugars are completely oxidized to CO2 and H2O in aerobic processes

  3. Reverse of complete oxidation of sugars is the reduction of CO2 and H2O to form sugars, a process that takes place in photosynthesis
• Aldoses


  Reducing sugars because their aldehyde group can be oxidized to give a carboxyl group
• Ketoses


  Can also be reducing sugars because they isomerize to aldoses
• Lactone


  A cyclic ester linking the carboxyl group and one of the sugar alcohols
• Oxidation reactions of sugars

  • Oxidation of α-D-glucose hemiacetal to give a lactone
  • Deposition of free silver as a silver mirror indicates that the reaction has taken place
• Tollens reagent


  Uses the silver ammonia complex ion, Ag(NH3)2+, as the oxidizing agent
• Glucose oxidase


  A more modern method, specific for the detection of glucose but not other reducing sugars