Carbohydrates

• Carbohydrate

  Molecules with the basic formula (CH2O)n, "hydrated carbon" e.g. glucose C6H12O6
• Saccharides

  Another name for carbohydrates
• Monosaccharides

  • The simplest carbohydrates, molecules with one aldehyde (aldose) or ketone (ketose) group and multiple hydroxyl groups, usually 3-7 carbon atoms, names end in -ose
• Aldoses


  Monosaccharides with an aldehyde group
• Ketoses

  Monosaccharides with a ketone group
• Triose

  Monosaccharide with 3 carbon atoms
• Tetrose

  Monosaccharide with 4 carbon atoms
• Pentose

  Monosaccharide with 5 carbon atoms
• Hexose

  Monosaccharide with 6 carbon atoms
• Heptose


  Monosaccharide with 7 carbon atoms
• Aldohexose


  Monosaccharide with an aldehyde group and 6 carbon atoms, e.g. glucose
• Most naturally occurring monosaccharides are in the D-form
• Optical isomers

  Molecules with n asymmetric centres (chiral atoms) and no plane of symmetry have 2^n isomers
• Glyceraldehyde

  Has one chiral carbon atom which means there are two optical isomers or enantiomers
• D isomers only, the L-isomers will be the mirror images of the D-isomers
• Cyclisation

  1. Pentoses and hexoses cyclise to form rings

  2. Forms a new asymmetric carbon at C-1 called the anomeric carbon

  3. Two different configurations can exist, α and β, called anomers

  4. Anomers can interconvert via the open-chain form in a process called mutarotation
• Anomeric carbon


  The new asymmetric carbon formed during cyclisation
• Anomers


  The α and β isomers formed during cyclisation
• Mutarotation

  The interconversion of anomers via the open-chain form
• Pyranose

  The ring structure formed by aldohexoses
• Furanose

  The ring structure formed by ketohexoses
• Glucose is predominantly in a ring form, the proportion in the open-chain form is less than 1%
• Optical rotation

  The rotation of plane polarised light passing through a solution of a monosaccharide
• β-D-glucopyranose rotates light +18.7 degrees, the α anomer rotates +112 degrees = optical rotation
• When a pure sample of either anomer is dissolved in water, the specific rotation changes with time until an equilibrium value of +52.7 degrees is attained, corresponding to a mixture of 1/3 α-anomer and 2/3 β-anomer
• Monosaccharide rings

  • Although commonly drawn as flat, they have a tetrahedral geometry
  • Pyranose rings adopt chair and boat conformations
  • Furanose rings adopt envelope conformations
• Aldehydes and ketones react with alcohol to form hemiacetal and hemiketal, important to cyclisation
• What is the anomeric carbon for ketoses?

  C-2
• Which monosaccharide can form both furanose and pyranose?

  ribose so anomeric carbon is C1 irrespective of furanose or pyranose is formed because it is an aldose
• Formation of glycosidic bonds

  Condensation reaction between anomeric carbon of one sugar and hydroxyl group of another
• Directionality of polysaccharides

  Difference between reducing and non-reducing ends
• Monosaccharide derivatives

  • Amino (-NH2)
  • N-acetyl (-NHCOCH3)
  • Phosphate (-OPO32-)
  • Sulphate (-OSO3-)
  • Carboxylate (-COO-)
  • Hydrogen (Deoxy -H)
• Disaccharides

  • Maltose (Glcα(1-4)Glc)
  • Sucrose (Glcα(1-2)βFru)
  • Lactose (Galβ(1-4)Glc)
• Polysaccharides

  Storage:

  • Mannans (Polymers of Mannose)
  • Starch (Polymer of Glc, Amylose α(1-4), Amylopectin α(1-4) and α(1-6)) - broken down by amylase
  • Glycogen (Polymer of Glc, α(1-4) and α(1-6) linked) - broken down by glycogen phosphorylase

  Structural:

  • Acid mucopolysaccharides (Polymers of uronic acids and amino sugars)
  • Peptidoglycan (Polymer of β(1-4) GlcNAc and MurNAc with peptide side chains)
  • Chitin (Polymer of β(1-4) GlcNAc)
  • Cellulose (Polymer of β(1-4) Glc)
• Glycolysis

  1. Converts Glucose (6C) into 2x Pyruvate (3C)

  2. Releases some chemical energy as ATP (net 2 ATP per Glucose)
• Conversion of Pyruvate to Acetyl-CoA

  Occurs in mitochondrial matrix before entering citric acid cycle
• Citric acid cycle

  1. Oxidises Acetyl-CoA (derived from Pyruvate)

  2. Produces 1 ATP (GTP), 3 NADH, 1 FADH2 per turn
• The citric acid cycle is essential to learn as it is fundamental to much of biochemistry and bioscience
• Open-chain forms of aldoses and ketoses can reduce indicators such as Cu2+ to Cu+
• Non-reducing end:

  Anomeric end is in a glycosidic bond so cannot mutarotate