Carbohydrates

• Carbohydrates


  Nutrients that provide energy for the body which enables it to perform work and maintain body processes
• Carbohydrates


  Saccharides, also called "sugar", one of the most common examples is sucrose or table sugar
• Carbohydrates (biochemical perspective)


  Polyhydroxy aldehydes or ketones
• Carbohydrates (oversimplification)


  Initially referred to as "hydrates of carbon" which follows stoichiometric formula (CH2O)n
• Glyceraldehyde


  Simplest sugar
• Dihydroxyacetone


  Simplest sugar
• Carbohydrates


  • Presence of aldehyde and ketone groups divide them into two major groups: aldoses and ketoses
• Glucose


  Aldehyde functional group, six carbons, ose suffix to indicate it is a carbohydrate
• Fructose


  Ketone functional group, six carbons, ose suffix to indicate it is a carbohydrate
• Tautomers


  Simple sugars like glyceraldehyde and dihydroxyacetone that have the same number of atoms per element but differ in functional groups
• Tautomerization in organisms


  1. Happens through an enediol intermediate

  2. Lobry-de Bruyn-von Ekenstein Transformation
• Carbohydrates


  • Have chiral or asymmetric carbons
  • Presence of chiral or asymmetric carbons leads to formation of enantiomers which are mirror images and not superimposable
• D and L designation

  D means dextrorotatory (clockwise), L means levorotatory (counterclockwise), pertaining to the direction of the plane polarized light as deflected by these enantiomeric forms
• In nature, the most abundant carbohydrates exist as D enantiomeric forms while L isomers are rare
• Cyclization of carbohydrates


  1. Carbohydrate structure often represented as open, long straight chain but in nature they form ring structures through reaction between hydroxyl and carbonyl groups

  2. Cyclization results in formation of a new chiral center in carbon 1, leading to formation of anomers designated as alpha (OH below the ring) or beta (OH above the ring)
• Cyclic carbohydrates


  May adopt a six-membered or a five-membered closed structures resembling pyran and furan rings
• Classification of carbohydrates


  • Monosaccharides
  • Disaccharides
  • Polysaccharides
• Monosaccharides


  Carbohydrates with only 1 sugar unit
• Monosaccharide derivatives


  Monomeric sugars with additional substituents
• Sugar acids


  Sugars with free anomeric carbon atoms that are good reducing agents and will reduce certain oxidizing agents
• Uronic acids


  Monosaccharides oxidized enzymatically at C-6, such as D-glucuronic and L-iduronic acids
• Aldaric acids


  Monosaccharides oxidized at both C-1 and C-6, such as D-glucaric acid
• Sugar alcohols


  Prepared by mild reduction of the carbonyl groups of aldoses and ketoses, cannot cyclize like aldoses
• Deoxy sugars


  Monosaccharides with one or more hydroxyl groups replaced by hydrogens, such as 2-deoxy-D-ribose, L-fucose, and L-rhamnose
• Sugar esters


  Phosphate esters of glucose, fructose, and other monosaccharides that are important metabolic intermediates
• Amino sugars


  Contain an amino group (instead of a hydroxyl group) at the C-2 position, such as D-glucosamine and D-galactosamine
• Disaccharides


  Carbohydrates composed of two sugar units joined by a glycosidic bond
• Disaccharides


  • Maltose
  • Trehalose
  • Sucrose
  • Cellobiose
  • Lactose
  • Gentiobiose
• Oligosaccharides


  Contain few monosaccharide units
• Polysaccharides


  Contain many monosaccharide units
• Glucosamines


  Linked to three-carbon acids at the C-1 or C-3 positions
• Disaccharides


  • Maltose
  • Trehalose
  • Sucrose
  • Cellobiose
  • Lactose
  • Gentiobiose
• Polysaccharides


  Carbohydrates composed of long chains of sugar molecules
• Polysaccharides


  • Serve as energy storage (glycogen and starch)
  • Provide structure (cellulose and chitin)
  • Provide protection to host (bacterial cell wall of Gram positive bacteria)
• Amylose


  A polymer made up of α-D-glucopyranose with α-1,4 glycosidic linkages
• Amylopectin


  A polymer made up of α-D-glucopyranose with α-1,4 and α-1,6 glycosidic linkages
• Starch has branch points every 24-30 residues


  Glycogen has branch points every 8-12 residues
• Chitin


  A polymer made up of N-acetyl-β-D-glucosamine
• Cellulose


  A polymer made up of long chains of β-D-glucose held together by hydrogen bonds
• The human body cannot digest cellulose due to the lack of the enzyme cellulase which cleaves β-1,6 glycosidic bonds