|MadSci Network: Biochemistry|
Hyaluronic acid is a carbohydrate; more specifically, it is a polysaccharide. Polysaccharides are long polymer chains of sugars that can be used for storing sugar (like starch and glycogen) or as structural supports for cells and tissues (like cellulose in plants and chitin in animal shells). Hyaluronic acid is in a subset of the polysaccharides called the glycosaminoglycans (GAG's), because one of the sugars used in its structure is modified with an amino group (-NH2). The GAG's are a diverse group of polysaccharides, found almost exclusively in animals, that are used primarily to give structural integrity to the tissues, in which they are found. The simplest GAG is chitin - the major component of insect and crustacean exoskeletons - which is composed entirely of N-acetyl-glucosamine: glucose with an amino group is called glucosamine, and the glucosamine in chitin has an acetyl group (CH3-CO-) attached to the nitrogen of the amino group; hence, N-acetyl-glucosamine. (For more on names of carbohydrates visit the IUPAC Nomenclature of Carbohydrates page.)
Hyaluronic acid is closely related to chitin, insofar as half of its sugars are N-acetyl-glucosamines. The other half of its sugars are also modified forms of glucose, in which the alcohol group (-CH2 OH) at the end of glucose has been replaced with carboxylic acid (- COOH). Sugars that are modified in this way are called uronic acids, so this modified glucose is called glucuronic acid. Hyaluronic Acid is defined as "Alternating copoly(ß-D-glucuronic acid-[1Æ3]-ß-D-N-acetyl-glucosamine-[1Æ4])" (pictured below).
Hyaluronic acid can be several thousands of sugars long, making it the longest of the GAG's. When not bound to other molecules, the larger hyaluronic acid's lengths and chemical properties give it a stiff, gelatinous quality in aqueous (water-based) solutions, similar to agar - a "Jello" made from seaweed starch. The smaller hyaluronic acids are less stiff, and have excellent lubricative properties.
These properties are important for hyaluronic acid's main biological function: it is the major component of cartilage. Cartilage comes in a few different varieties, the most familiar of which is the hyaline cartilage , that is used throughout the skeleton for joints and in place of bones in some tissues. In fact, the cartilaginous fish, e.g. sharks and rays, have skeletons composed entirely of hyaline cartilage. As you've probably guessed, hyaluronic acid is short for hyaline uronic acid, since it is found in hyaline cartilage and contains large amounts of glucuronic acid. Beside acting along, hyaluronic acid can be complexed with other large molecules called aggrecans, which attach to linear hyaluronic acid like the bristles of a bottle brush. Each of these bristles is a proteoglycan: a long, linear protein molecule decorated with smaller GAG's, mostly chondroitin sulfate, which is a slight variation on hyaluronic acid that uses N-acetyl-galactosamine-4-sulfate in place of the N-acetyl- glucosamine.
Because you are asking about hyaluronic acid, and because of the recent addition of GAG's to pharmacy shelves, I think it's important to address a few points about the uses of these compounds therapeutically. On the up side, hyaluronic acid is the same from whatever source it is isolated, and it is so common in the body that it can be used as a hypoallergenic surgical material for reconstructing and building up tissues that are lost. So hyaluronic acid has become a widely used substance among plastic and reconstructive surgeons. On the down side, many companies have begun selling hyaluronic acid (as well as chondroitin sulfate and glucosamine) as a vastly overpriced, nutritional supplement that is supposed to have curative properties against arthritis, cancer, and other ailments. The idea is that the supplements will help your body replace worn out cartilage. The problems with this supposition are: first, our digestive tracts are very good at breaking large molecules down into their subunits, and tend to absorb only the subunits preferentially; second, hyaline cartilage is an avascular tissue, that is, it doesn't get its nutrients directly from the bloodstream; and third, a functional extracellular matrix must be synthesized by the cells to have the right properties, and cartilage cells prefer to use unmodified sugars - like glucose and galactose - to synthesize its own precursors. If you are a consumer, I would suggest saving your money for something else; if you're looking to make money by selling hyaluronic acid, it can be isolated easily and cheaply from any cartilage from any animal and sold at a sufficient mark-up to make you rich.
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