Carbohydrate

• Carbohydrates


  Polyhydroxy aldehydes or ketones or compounds which yield these on hydrolysis
• Functions of carbohydrates


  • Sources of energy
  • Intermediates in the biosynthesis of other basic biochemical entities (fats and proteins)
  • Associated with other entities such as glycosides, vitamins and antibiotics
  • Form structural tissues in plants and in microorganisms (cellulose, lignin, murein)
  • Participate in biological transport, cell-cell recognition, activation of growth factors, modulation of the immune system
• Glucose


  Provides energy for the brain and 1/2 of energy for muscles and tissues
• Glycogen


  Stored glucose
• Glucose


  Immediate energy
• Glycogen


  Reserve energy
• Classification of carbohydrates


  • Monosaccharides - carbohydrates that cannot be hydrolyzed to simpler carbohydrates
  • Disaccharides - carbohydrates that can be hydrolyzed into two monosaccharide units
  • Oligosaccharides - carbohydrates that can be hydrolyzed into a few monosaccharide units
  • Polysaccharides - carbohydrates that are polymeric sugars
• Simple carbohydrates


  • Monosaccharides - single sugars
  • Disaccharides - 2 monosaccharides
• Characteristics of carbohydrates


  • Consist of carbon, hydrogen, & oxygen
  • Energy containing molecules
  • Some provide structure
  • Basic building block is a monosaccharide (CH2O)n ; n = 3,5,6
  • Two monosaccharides form a disaccharide
• Monosaccharides


  Also known as simple sugars
• Classification of monosaccharides


  • By the number of carbons
  • Whether aldoses or ketoses
• Most (99%) monosaccharides are straight chain compounds
• 1. glyceraldehyde


  The simplest of the aldoses (aldotriose)
• All other sugars have the ending 'ose' (glucose, galactose, ribose, lactose, etc...)
• Glucose


  The chemical formula is C6H12O6. It is a six sided ring.
• Relative sweetness of different sugars

  • Sucrose - 100
  • Glucose - 74
  • Fructose - 174
  • Lactose - 16
  • Invert Sugar - 126
  • Maltose - 32
  • Galactose - 32
• Aldoses


  Have an aldehyde group at one end
• Ketoses


  Have a keto group, usually at C2
• Stereoisomers


  Compounds having same structural formula, but differ in spatial configuration
• D or L


  Refers to the asymmetric C farthest from the aldehyde or keto group
• Most naturally occurring sugars are D isomers
• D & L sugars are mirror images of one another. They have the same name, e.g., D-glucose & L-glucose
• The number of stereoisomers is 2^n, where n is the number of asymmetric centers
• The 6-C aldoses have 4 asymmetric centers. Thus there are 16 stereoisomers (8 D-sugars and 8 L-sugars)
• D vs L designation

  Based on the configuration about the single asymmetric C in glyceraldehyde
• Enantiomers


  Mirror images of each other
• Epimers


  Stereoisomers that differ only in configuration about one chiral center
• Hemiacetal


  An aldehyde can react with an alcohol to form a hemiacetal
• Hemiketal


  A ketone can react with an alcohol to form a hemiketal
• Anomers


  Stereoisomers formed when a ring is formed (α, β)
• Rules for drawing Haworth projections

  • For D-sugars the highest numbered carbon (furthest from the carbonyl) is drawn up. For L-sugars, it is drawn down
  • For D-sugars, the OH group at the anomeric position is drawn down for α and up for β. For L-sugars α is up and β is down
• Pentoses and hexoses can cyclize as the ketone or aldehyde reacts with a distal OH
• Glucose forms an intra-molecular hemiacetal, as the C1 aldehyde & C5 OH react, to form a 6-member pyranose ring, named after pyran
• Structural representation of sugars

  • Fisher projection: straight chain representation
  • Haworth projection: simple ring in perspective
  • Conformational representation: chair and boat configurations
• Oxygen of the hydroxyl group is removed to form deoxy sugars
• Simple carbohydrates


  • Monosaccharides - all are 6 carbon hexes, 6 carbons, 12 hydrogens, 6 oxygens, arrangement differs, accounts for varying sweetness (glucose, fructose, galactose)
• Glycosidic bonds


  The anomeric hydroxyl and a hydroxyl of another sugar or some other compound can join together, splitting out water to form a glycosidic bond
• Sugar derivatives


  • Amino sugar - an amino group substitutes for a hydroxyl, e.g. glucosamine
  • N-acetylglucosamine - the amino group may be acetylated
• Disaccharides


  • Cellobiose - a product of cellulose breakdown, with a (1->4) glycosidic linkage
  • Maltose - a cleavage product of starch, with an (1->4) glycosidic link between C1 - C4 OH of 2 glucoses
• Sucrose


  Cane sugar, also called "Invert Sugar" when hydrolyzed into glucose and fructose