Chapter 9

Carbohydrates

Please mail me with any comments, suggestions or reviews.

If you are an instructor and wish to order a desk copy of Biochemistry 2nd edition by Mathews and van Holde simply click the image of the text above.

Introduction

Carbohydrates or saccharides are molecules of general formula (CH2O)n that may be either monomeric, monosaccharides, composed of a few monomeric units linked together, oligosaccharides, or long polynermers, polysaccharides such as starch amylose. Carbohydrates function as :

Monosaccharides

These are simple monomeric sugars of formula (CH2O)n where:

Monosaccharides exhibit a number of important features.

Stereoisomerism

As with amino acids monomeric sugars contain chiral carbon centers. Where n is the number of CH2O units there are 2 to the power of n stereoisomers possible for that structure. For Glyceraldehyde, where n=1, there two possible enantiomers, the D and L forms.
For the tetroses, where n=2, there are four possible stereoisomers: D and L Erythrose and D and L Threose. As can be seen from the following graphic D and L Erythrose are enantiomers, but D Erythrose and D Threose are diastereomers.

Ring Structures

With pentoses and hexoses a new feature arises, i.e. the possibility of ring structures. The following imagemap for the ring closure of D-Ribose illustrates a variety of features :

  1. the representations used in the graphic are termed Haworth projections, designed to give a sense of the 3-D structure. For a better 3-D representation of the ring structures click the image oval above each of the ring structures.
  2. to manipulate these structures in real time click the coordinate oval above each of the ring structures.
  3. the open-chain form comprises less than 1% of the structure, with the pyranose, i.e. six-membered ring being more than 75% and the furanose, i.e. five-membered ring comprising 24%.
  4. each ring can exist in two anomeric forms, alpha and beta, as shown below.


Polysaccharides

Polysaccharides serve a wide variety of functions :

Unlike what is observed for nucleic acids and proteins polysaccharides often have simple primary structures with the polymer composed of one or a few very monomer types. The secondary structure is most often a simple regular pattern but ,unlike what is found for most proteins, the polysaccharide chain can be indeterminate in length.

Structural Polysaccharides

Click here to view a capsular polysaccharide composed of simple helical chain of four monomers : alpha-D Mannose, beta-D-glucose, beta-D-glucuronate and alpha-D-galactose. This polymer serves as a structural component of E. Coli and the view represents a single unit cell of a chain of indeterminate length.

allows for an interactive display of this structural polysaccharide.

Iota Carrageenan is an alternating copolymer of alpha-D and beta-D galactose sulfate. It is a mjor component of the connective tissue of some plants. The secondary structure is a double helix, with the above image reflecting a view down the helical axis. Again this structure represents a single unit cell of a chain of indeterminate length.

allows for an interactive display of this structure.

Glycosaminoglycans are polymers of repeating disaccharide units where one of the monomers is either N-acetylgalactosamine or N-acetyl glucosamine. These polymers serve important roles in the formation of skin and connective tissue.
Chondroitin sulfate is a polymer that binds covalently, through serine residues to collagen and thus helps hold the collagen fibers in a tight, strong network. This is an example of a protein-carbohydrate, or proteoglycan complex.

allows for an interactive display of chonroitin sulfate.

Heparin is another such glycosaminoglycan, which serves as a natural anti-coagulant by binding strongly to certain blood proteins with the resulting complex inhibiting the blood-clotting process.

allows for an interactive display of heparin.