Structure of hyaluronan


Hyaluronan has an identical chemical structure, irrespective of the tissue or species it is derived from. It is a glycosaminoglycan (GAG), a polysaccharide made up from repeating disaccharide units. The only sugar units found in hyaluronan are alternating N-acetyl glucosamine and D-glucuronic acid. These sugars are linked by a glycosidic bond to form the basic disaccharide unit found in hyaluronan. Each disaccharide unit is twisted through 180 degrees (inverted) compared with those following and behind it in the chain.

   The name GAG comes from the fact that one of the two monosaccharide units is always a sugar with an amine group in it.    


The repeating disaccharide unit of the sodium salt of hyaluronan


Theoretically, the absolute uniformity in the chemical structure of hyaluronan, whatever its biological source, should make it an ideal material for use in human medicine because it is essentially non-allergenic.
However, with hyaluronan extracted from animal sources there is always a small risk of contamination with proteins, which might be allergenic. This risk is eliminated with hyaluronan produced by fermentation. 


The structure of hyaluronan can be represented as a very stable, unbranched linear chain.


3D structure of hyaluronan

However, in a physiological solution, the backbone of a hyaluronan molecule is stiffened by a combination of the chemical structure of the disaccharide, internal hydrogen bonds and interactions with the solvent (the medium in which the hyaluronan is dissolved). The hyaluronan molecule is mainly hydrophilic (i.e. it has a strong affinity for water).

However, it has been shown that hyaluronan also has an extensive hydrophobic patch (which has little affinity for water). The axial hydrogen atoms (shown in the next figure) form the non-polar, relatively hydrophobic face while the equatorial side chains form the more polar, hydrophilic face. Thus, hyaluronan has the properties of a highly hydrophilic material, but also possesses hydrophobic patches characteristic of lipids. For this reason, the hyaluronan molecule is described as an amphiphilic molecule.


H axial hydrogens contribute to the hydrophobic face


The arrangement of the hydrogen atoms, combined with the alternately inverted disaccharide units, creates a twisting ribbon structure that is stabilised by water. Repulsion between the hydrophilic and hydrophobic faces of the molecule leads to an
expanded, random coil structure in physiological solutions, so that hyaluronan occupies a very large three-dimensional space (domain). The hyaluronan coils are so large that they occupy the entire volume of the solution at concentrations of about 1 mg/ml (0.1%). At higher concentrations, the hyaluronan coils intertwine and entangle, forming a flexible molecular network.


Model of the hyaluronan ribbon in a 3-dimensional domain

The light blue box represents the domain of the molecule in solution. The alternating blue and red strands 
represent the ribbon structure, with blue (hydrophilic) and red (hydrophobic) faces.

 

The domain structure of hyaluronan has important consequences. Small molecules such as water, electrolytes and nutrients can freely diffuse through the solvent within the domain. However, large molecules such as proteins will be partially excluded from the domain because of their volume in solution. The diffusion-retarding effects of a solution of hyaluronan are different to those of a filter. A filter prevents the passage of all molecules or particles above a certain size; a solution of hyaluronan slows the diffusion of large molecules proportionate to their size - the larger the molecule, the more slowly it diffuses through the hyaluronan solution.

 

Diffusion-retarding effects of a solution of hyaluronan

 

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