Biochemistry I Fall Term, 2000

Lecture 28: Carbohydrates

Assigned reading in Campbell: Chapter 12.1-12.3

Take the Review Quiz on Lecture 28 concepts.

Structures of Sugars: Chime images of the building blocks.
Boat to Chair Conformers of Glucose: Chime animations of the boat to chair conversion of glucose.
D-glucose: from Fischer projection to 3D hemi-acetal: Animations of the linear to the ring forms of glucose. (Chime & ChemSymphony, Java Applets are used.)
  11.3.00: A 1/3-speed version of the above Chime page by Dr. Hens Borkent might display better, depending on your computer.

Carbohydrates are:

1. The primary energy reservoir in biosphere.
2. Used for energy storage and distribution.
3. Biosynthetic precursors to amino acids and nucleic acids.
4. Used on glycoproteins as addresses for intracellular traffic.
5. Antigenic (e.g. the ABO blood group specificity determinates are carbohydrates).
6. Structural and mechanical components.
   • Cell walls in plants
   • Cell walls in bacteria
   • Cartilage

Structural Hierarchy:

1. Monosaccharides: cannot be hydrolyzed to simpler sugars.
2. Oligosaccharides: 'a few' covalently linked monosaccharides.
3. Polysaccharides: 'many' covalently linked monosaccharides

The simplest monosaccharides contain three carbons:

• Aldehyde or ketone group.
• All carbons are 'hydrated'.

Important aldehydes to remember are:

1. Glyceraldehyde (C3)
2. Ribose (C5)
3. Glucose (C6)

Oxidation/Reduction of Saccharides

Oxidation: Conversion of aldehyde to carboxylate group. Can be used to test for the presence of a free aldehyde group, e.g. in disaccharides.

Reduction: Conversion of aldehyde to alcohol (e.g. glyceraldehyde to give glycerol, a major component of lipids)

Conformation of Monosaccharides in solution

Short (<C5) saccharides are linear chains

Long (>C4) saccharides form rings.

C6 - Glucose

1. Six membered ring created by forming a bond between C1 and O5.
2. This form is called pyranose, i.e. glucopyranose after the organic compound, pyran:
   
3. The C1 carbon becomes chiral and is called the anomeric carbon
4. The new OH group (on C1) can exist in:
   • a-form, pointing 'down'.
   • b-form, pointing 'up'.
5. The a and b forms can readily inter-convert via the linear intermediate.
6. Rings can exist in three forms:
   • Chair form A.
   • Chair Form B.
   • Boat (causes steric crowding).

   

The boat form is approximately 17 kJ/mol (4 kcal/mol) less stable than the chair form. Interchanging between the two chair forms interconverts axial and equatorial groups. Depending on the nature of the groups, one chair form is more stable than the other. Note that interconversion between the different chair forms does not affect the configuration of the anomeric carbon.

In the case of glucose, one of the chair forms presents all of the OH groups as equatorial. This conformation reduces steric crowding and provides ready access to the OH groups. This may explain, in part, why glucose is one of the more common C6 sugars.
View these features in the Boat to Chair Conformers of Glucose animation, linked above.

C5: Ribose

1. Formation of a 5 membered ring can also occur by forming a bond between C1 and O4.
2. This is called a furanose (i.e. ribofuranose) after the organic compound, furan:
   
3. Two different conformations of the anomeric carbon (a or b) are possible.
4. 'Boat' and 'chair' conformations do not exist due to the small size of the ring.

Modified Sugars:

1. Deoxy-sugars: loss of the 2'-OH group: e.g.DNA versus RNA
2. Amino-sugars:
   D-glucosamine
   N-acetyl glucosamine (NAG, found in cartilage and bacterial cell walls)
   Sulfo-D-Glucose amine (found in heparin, a natural anti-blood clotting agent)

Disaccharides

Linkage of the anomeric carbon of one monosaccharide to the OH of another monosaccharide. The bond is termed a glycosidic bond.

Sucrose: O-a -D-glucopyranosyl-(1-2)-b -D-fructofuranoside (a.k.a. table sugar).

The anomeric carbon of glucose forms a bridge to the anomeric carbon of fructose. This sugar cannot be reduced with silver.

Lactose: b (1-4) linkage between galactose and glucose. The anomeric carbon of galactose is bonded to O4 of glucose. This sugar can be reduced with silver.

• Lactose is the major sugar in mammalian milk.
• Infants produce lactase to hydrolyze the disaccharide to monosaccharides.
• Some adults have low levels of lactase. This leads to lactose intolerance. The ingested lactose is not absorbed in the small intestine, but instead is fermented by bacteria in the large intestine, producing uncomfortable volumes of CO₂.
  (This is where E. coli actually uses its b-galactosidase to its own survival advantage, rather than 'in service' to molecular biologists! A related speculative question is, whether the ability of E. coli to metabolize lactose has evolved since the (relatively recent) emergence of mammals, or whether the prior existence of the lac Z gene allowed E. coli to become the principal flora of our lower tract.)

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