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A glycosaminoglycan is a long chain of polymerized sugars that is essential to life and plays an important role in connective tissue. Any glycosaminoglycan that is sulfated is referred to as keratan sulfate. Keratan sulfates are usually found in tissues such as cartilage or bone and in the cornea. Produced in the central nervous system, these types of glycosaminoglycans play an important role in development and can act as shock absorbers in joints. This is primarily because they are highly hydrated large molecules.
Keratan sulfates are similar to other glycosaminoglycans in that they are a linear polymer of repeating sugar units. They are attached to proteoglycans, which are core proteins that occur in connective tissue and have glycosaminoglycans such as keratan sulfates attached to them. These keratan sulfates are negatively charged and attach themselves to proteins found in the extracellular matrix. Two such proteins are keratocan and lumican.
There are two types of this glycosaminglycan: keratan sulfate I (KSI) and keratan sulfate II (KSII). The location of these two types was the first key factor in differentiating whether KSI or KSII was isolated from the connective tissue within the body. The keratan sulfate found in corneal tissue was classified as KSI, whereas if it was found in the skeletal tissue, it was classified as KSII. The highest quantity of keratan sulfate is present in corneal tissue — about 10 times more than that of cartilage tissue. Aside from cartilage tissue, it is about three times more likely that keratan sulfate will be present in corneal tissue than in other tissues within the body.
The major differences between the KSI and KSII types have very little to do with their composition but rather with the way they are linked via protein linkages to the extracellular proteins. Thanks to advancements in science, it has been proven that KSI and KSII types can both be found in corneal and skeletal tissue. This rejected the notion of classifying the type of keratan sulfate based on its location. Now, the type of keratan sulfate present in connective tissue is determined by the type of linkages that occur with its core protein rather than its location.
Keratan sulfates play a role in the overall health of an individual. The abnormal production or lack of production of this glycosaminoglycan can result in a condition known as macular corneal dystrophy, a rare recessive disease that affects the cornea and results in a significant reduction in vision over time. Overproduction of the glycosaminoglycan can result in Morquio syndrome, a type of dwarfism that results in serious health effects.