Hyaluronic acid

 

In the previous article we talked about Glycosaminoglycans (GAGs). We discussed 6 types of GAGs and out of the 6, Hyaluronic acid is one which deserves a separate article. 

Hyaluronic acid is the largest out of the 6 GAGs that is composed of the highest number of repeating disaccharide units. The disaccharide unit is composed of ,as any other GAG, a Uronic acid and an amino sugar. 

The Uronic acid is a Glucuronic acid and the Amino sugar is a N-acetylglucosamine compound. From the image below you can get an idea as to the structure of these two compounds,

 

The 2 compounds are bonded together by a 𝛃 (1-3) Glycosidic linkage and each unit is bonded together by 𝛃 (1-4) Glycosidic linkages forming a long chained heteropolysaccharide. These chains are visible in an electron micrograph of a proteoglycan aggregate extracted from a cartilage specimen.

Now, what is a Proteoglycan aggregate?

Hyaluronic acid is known to be a GAG which doesn't bond with a Core protein to form a Proteoglycan. GAGs such as Chondroitin Sulfate (CS) and Keratan Sulfate (KS) bond to Core proteins covalently forming a Proteoglycan monomer. ( Several chains of CS and KS are involved in forming the proteoglycan monomers)

These individual Proteoglycan monomers then bond non covalently through a link protein to the long hyaluronic chain. The structure that is formed ,which resembles a bottle brush used to clean test tubes in your chemistry lab, is called the Proteoglycan Aggregate. The below is only a rendition of the proteoglycan aggregate😁😁,

The proteoglycan aggregates are mainly found in the connective tissue of animals such as cartilage tissue. Cartilage tissue that are specially found around the bones of articulating joints possess special properties that renders it suitable for bones that bear large loads such as the weight of our body.

These cartilages are lined by synovial membranes which encloses a fluid called the Synovial fluid. The Synovial fluid consist of Hyaluronic acid molecules. 

The carboxylate group in the Glucuronic acid renders the disaccharide unit negative as a result the Hyaluronic acid molecule has a high  negative charge density. The high negativity attracts cations and polycations which result in the increase of osmolarity within the tissue environment. Therefore water is absorbed osmotically to reduce the osmolarity. 

Consequently the environment within the tissue is highly hydrated and therefore the ground substance has a gel like property to it. This grants the tissue special properties.  

1. Shear Dependent - when there is a shear force acting on the fluid of the cartilage ,in this case the synovial fluid of synovial joints, the hyaluronic acid (HA) molecules in the fluid will act according to the intensity of that force. That is when the Shear force is low, the HA molecules tangle together forming a mass which obstruct the flow of fluid. When the shear force is high the HA molecules line up parallel to the lines of force and gives less resistance to the flow of fluid.
2. Shock absorber
3. Lubricant

Another place where you can find HA molecules showing the same property is the Vitreous humour of the eye. Here, HA molecules are responsible in maintaining the intralocular pressure of the eye, thereby preventing the collapse of the eye.