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A glycosaminoglycan is a long, unbranched polysaccharide built from repeating two-sugar units containing an amino sugar derivative. The partner is commonly a uronic acid; keratan sulfate instead contains galactose. Many glycosaminoglycans carry sulfate groups and occur as carbohydrate chains of proteoglycans. A proteoglycan is the larger protein–carbohydrate assembly, while a glycosaminoglycan is the sugar chain.^{[ref] [ref]}

Hyaluronic acid is a useful comparison: it alternates glucuronic acid with N-acetylglucosamine, is nonsulfated, and is not covalently attached to a protein core. Thus neither “sulfated” nor “covalently protein-bound” defines every member of the family. At physiological pH its carboxylate groups give the chain negative charge, helping explain its interaction with water and surrounding ions. ^[ref]

The distinction matters in experiments on the endothelial glycocalyx. Inhibition or removal of a particular sugar-chain component can change the environment through which blood cells move without removing the entire endothelial cell membrane. Heparinase treatment in microvessels altered capillary hematocrit, supporting a role for the surface carbohydrate layer in blood-cell distribution; it does not establish that every glycosaminoglycan produces the same effect.^[ref]

I. Eronen is the first author of a 1985 study isolating and characterizing keratan sulfate from human brain. The researchers identified a sulfated carbohydrate fraction with repeating galactose and N-acetylglucosamine units. Its susceptibility to a keratan-sulfate-cleaving enzyme supplied a second test of the proposed chemical identity.^[ref]

The fraction also spanned a range of molecular sizes in size-exclusion chromatography. A material can therefore share a characteristic backbone while containing chains of different lengths. Combining chemical composition, size separation and a cleavage test makes the identification more informative than simply detecting a negatively charged substance in a tissue sample.^[ref]

Keratan sulfate is a sulfated glycosaminoglycan with a backbone of alternating galactose and N-acetylglucosamine residues. Unlike hyaluronic acid and many other glycosaminoglycans, its characteristic two-sugar repeat does not contain a uronic acid. Backbone identity and the extent of sulfation are separate structural questions.^[ref]

An ideal backbone segment with 30 complete repeats has 30 galactose residues and 30 N-acetylglucosamine residues, for 60 sugar residues altogether. This count does not fix its number of sulfate groups or the molecular mass of an actual preparation.

A 1985 study isolated a sulfated fraction from human brain and combined chemical analysis with susceptibility to a keratan-sulfate-cleaving enzyme. Agreement between the repeat structure and cleavage behavior supported its identification as keratan sulfate; size measurements also showed heterogeneity. Several different observations make identification stronger than reliance on a sulfated material's charge alone.^[ref]