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NEWSLETTER 8 - Extension 1 : The Great Variety of Saccharides (Updated September 2005)

Introduction
Prototype
Properties

Introduction

Many forward-thinking professionals in nutrition medicine still discuss dietary saccharides only in terms of energy sources.

There is much more to say about saccharides.

Saccharides play an important role in cell and organ structure, in cell communication, and in immunity, as is demonstrated in the following chapter Saccharides in Our Biochemistry .

To better follow the demonstration, let us first have a close look at the structure of a saccharide.

As a prototype of a saccharide we shall use the molecule of glucose.

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A Prototype for the Saccharide Molecule

Saccharides, also known as carbohydrates, are organic molecules containing carbon, oxygen, and hydrogen atoms. Saccharides have a sweet taste.

Our prototype of a saccharide -- the glucose molecule -- contains six atoms of carbon, six of oxygen and twelve of hydrogen.

To illustrate molecules, chemists use different formulations.

The Empirical Formulation

The empirical formulation of a molecule only indicates the atoms involved and their numbers. The empirical formulation of a glucose molecule comes like this : C6O6H12

The Projection Formulation

Another way to illustrate a molecule -- the projection formulation -- simulates the orientation of the atomic constituents of the molecule in space. Since the three dimensional orientation of a molecule governs its chemical properties, the projection formulation contains more information about the molecule.

The projection formulation is rather hard to decipher, and we shall use a more visual presentation of the prototype saccharide molecule for our demonstration.

A Visual Presentation

The spine of a glucose molecule is formed of 6 carbon atoms -- illustrated here as blue-black spheres -- in a vertical straight row. (Saccharides share their central carbon row with proteins and lipids. Human life and life on earth are organized around a carbon spine)

The glucose molecule also contains 6 atoms of oxygen -- red -- and 12 atoms of hydrogen -- white.

A biochemist would immediately react to this presentation by telling you that the carbon atoms in a saccharide molecule do not come in a straight line, but in a broken line with well defined angles between each of them.

The chemist may add that saccharides not always come in a (broken) line, that they also exist in a closed (circular) line where the first carbon is linked -- with an oxygen atom between them -- to another carbon of the molecule. Both remarks are correct.

We shall overlook the broken line and the closed configuration because we don't need it for our demonstration.

Glucose vs. Mannose

Look at the above illustration of the glucose molecule and compare its structure to that of another saccharide molecule -- the mannose molecule -- which is illustrated to the right of this text.

As you can see, the two molecules have an identical composition of 6 carbon (blue-black), 6 oxygen (red), and 12 hydrogen (white) atoms.

At closer look the difference between the two molecules appears. The mannose molecule has its second atom of carbon -- starting from the top of the molecule -- and the adjacent hydrogen and oxygen atoms rotated 180 degrees as compared to their orientation in the glucose molecule.

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Properties : Diversity, Terminology, and Complexity of Saccharide Molecules

Diversity

The difference between the glucose and the mannose molecule is the orientation of one of the asymmetrical structures the molecules contain. There are four such asymmetrical structures in glucose and in mannose.

A consequence of it is that there are theoretically as much as sixteen different combinations of asymmetric group orientations possible. Each of these 16 combinations is a molecule with different chemical properties.

Now consider that a typical saccharide has up to four sites (handles) to grasp another saccahride. The number of possible combination of saccharides is much larger than the number of combination of amino-acids. Indeed, an amino acids has only two handles to form proteins (Table 1).

Number of Molecules Number of Combinations for Amino-acids Number of Combinations for Saccharides
2 4 16
3 8 64
16 64 256

Table 1: The number of combinations for a saccharide is greater than for an amino acid.

Terminology

Saccharides with six carbons are termed hexose (from the Greek word "hex" for six). Glucose and mannose are hexoses. There are saccharides with less than six carbons. A saccharide with five carbons is a pentose (Greek penta = five), with four it is a tetrose (tetra = four), with three a triose (tres). Pyruvic acid is a triose. Pentoses form the spine of our chromosomes.

Complexity

Before going any further, we have to become familiar with four new terms: "monosaccharide", "disaccharide", "oligosaccharide", and "polysaccharide".

A monosaccharide is a molecule with six or less carbon atoms.

A disaccharide contains two monosaccharides.

An oligosaccharide contains more than two and up to six monosaccharides.

A polysaccharide is a more complex molecule formed from more than six monosaccharides.

Monosaccharides form polysaccharides by attaching to each other in long sequences. That attachement is termed polymerization and the obtained complex molecule is a polymer. Polymerization is an enzymatic process.

In animals -- humans included -- such polymerization of the monosaccharide glucose produces glycogen. Glycogen is an ramified polymer. Glycogen is the storage form of glucose.

Chondroitine -- the basic substance of bones and cartilage -- is a polymer of glucose and glucosamine. (Glucosamine is a molecule of glucose with a nitrogen atom added)

In plants the polymerization of saccharides produces cellulose, in insects, crustacea and other organisms the polymerization of saccharides results in the production of chitin. Chitin is a polymer of glucosamine.

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