Carbohydrates

• Carbohydrate

  A polyhydroxy aldehyde, a polyhydroxy ketone, or a compound that yields polyhydroxy aldehydes or polyhydroxy ketones upon hydrolysis
• Carbohydrates

  • Carbohydrate oxidation provides energy
  • Carbohydrate storage, in the form of glycogen, provides a short-term energy reserve
  • Carbohydrates supply carbon atoms for the synthesis of other biochemical substances (proteins, lipids, and nucleic acids)
  • Carbohydrates form part of the structural framework of DNA and RNA molecules
  • Carbohydrates linked to lipids are structural components of cell membranes
  • Carbohydrates linked to proteins function in a variety of cell–cell and cell–molecule recognition processes
• Classification of Carbohydrates

  • Monosaccharides
  • Disaccharides
  • Oligosaccharides
  • Polysaccharides
• Monosaccharides

  Classified as aldose or ketose on the basis of the type of carbonyl present
• Disaccharides

  Glycosides formed from the linkage of two monosaccharides
• Oligosaccharides

  Carbohydrates that contain three to ten monosaccharide units
• Polysaccharides

  Polymers in which monosaccharides are the monomers
• Glucose

  A polyhydroxy aldehyde
• Fructose

  A polyhydroxy ketone
• Chirality

  Handedness in molecules
• Chiral center

  An atom in a molecule that has four different groups bonded to it in a tetrahedral orientation
• Chiral molecule

  A molecule whose mirror images are not superimposable
• Stereoisomerism

  The atoms of stereoisomers are connected in the same way but are arranged differently in space
• Enantiomers


  Structures that are non-superimposable mirror images of each other
• Diastereomers

  Structures that are not mirror images of each other
• Fischer projection formula

  A two-dimensional structural notation for showing the spatial arrangement of groups about chiral centers in molecules

  
• D and L system

  Used to designate the handedness of glyceraldehyde enantiomers

  
• Dextrorotatory compound

  A chiral compound that rotates the plane of polarized light in a clockwise direction

  
• Levorotatory compound

  A chiral compound that rotates the plane of polarized light in a counterclockwise direction

  
• Classification of monosaccharides by number of carbon atoms

  • Triose
  • Tetroses
  • Pentoses
  • Hexoses
  • Heptoses

  
• 1. Glucose

  The most abundant in nature and the most important from a human nutritional standpoint
• 1. Fructose

  Biochemically the most important ketohexose, also known as levulose and fruit sugar

  
• 1. Galactose

  Milk sugar, synthesized in humans, used to differentiate between blood types, six membered cyclic form

  
• 1. Ribose

  Part of RNA, ATP, and DNA, five membered cyclic form
• Alpha-form of D-glucose

  • OH of C1 and CH2OH of C5 are on opposite sides
• Beta-form of D-glucose

  • OH of C1 and CH2OH of C5 are on same sides

  
• Pyranose

  A cyclic monosaccharide containing a six-atom ring
• Furanose

  A cyclic monosaccharide containing a five-atom ring
• Haworth projection formula

  A two-dimensional structural notation that specifies the three-dimensional structure of a cyclic form of a monosaccharide

  
• Alpha configuration

  The -OH group on C1 and the CH2OH group point in opposite directions
• Beta configuration

  The -OH group on C1 and the CH2OH group point in the same direction
• Reactions of monosaccharides

  • Oxidation to acidic sugars
  • Reduction to sugar alcohols
  • Glycoside formation
  • Phosphate ester formation
  • Amino sugar formation

  
• Alpha or Beta configuration

  Determined by the position of the —OH group on C1 relative to the CH2OH group that determines D or L series
• Beta configuration

  • Both the —OH group on C1 and the CH2OH group point in the same direction
• Alpha configuration

  • The —OH group on C1 and the CH2OH group point in opposite directions
• Monosaccharide identity

  Determined by the positioning of the other —OH groups in the Haworth projection formula

  
• Situations where α or β configuration does not matter

  • The —OH group on carbon 1 is placed in a horizontal position, and a wavy line is used as the bond that connects it to the ring
• Five important reactions of monosaccharides

  • Oxidation to acidic sugars
  • Reduction to sugar alcohols
  • Glycoside formation
  • Phosphate ester formation
  • Amino sugar formation

  
• Oxidation to Acidic sugars

  1. The redox chemistry of monosaccharides is closely linked to the alcohol and aldehyde functional groups present in them

  2. Oxidation can yield three different types of acidic sugars depending on the type of oxidizing agent used
• Reduction to sugar alcohols

  The carbonyl group in a monosaccharide (either an aldose or a ketose) is reduced to a hydroxyl group using hydrogen as the reducing agent