Keratan sulfate (KS) is a sulfated glycosaminoglycan characterized by its poly-N-acetyllactosamine backbone, predominantly present in the corneal stroma, cartilage, and brain tissues. Unlike other glycosaminoglycans, KS lacks uronic acids and instead relies on sulfation to confer its anionic nature, contributing to tissue hydration and structural integrity.
Chemical Structure
KS is composed of repeating disaccharide units of β1-4-linked D-galactose (Gal) and N-acetyl-D-glucosamine (GlcNAc). Sulfation occurs mainly at the 6-position of GlcNAc and variably on Gal. The chains show regional heterogeneity, including non-sulfated poly-N-acetyllactosamine stretches, mono-sulfated domains, and highly sulfated regions, often capped with sialic acid or fucose. KS typically has a molecular weight of 10–20 kDa per chain and attaches to core proteins such as lumican, keratocan, and aggrecan through N- or O-linkages.
Physicochemical Properties
Sulfate groups impart negative charge, enhancing hydration and viscoelasticity, though KS is less acidic than glycosaminoglycans containing uronic acids. In solution, it forms extended, hydrated coils and resists enzymatic degradation due to capping residues. KS interacts electrostatically with cations and proteins. Tissue-specific isoforms include KS-I (corneal; N-glycosylation) and KS-II (cartilage; O-linkage), reflecting distinct biosynthetic pathways.
Biosynthesis and Distribution
KS biosynthesis begins in the Golgi apparatus with the transfer of GlcNAc to asparagine (KS-I) or to serine/threonine residues (KS-II). Subsequent steps involve galactosylation, chain elongation via β1-4GalT and β1-3GlcNAcT enzymes, and sulfation catalyzed by GlcNAc6ST and Gal6ST. KS is abundant in the cornea—representing up to 90% of proteoglycans—and is also found in intervertebral discs, brain tissue, and articular cartilage, where it contributes to extracellular matrix organization.
Biological Functions
In the cornea, KS preserves transparency and regulates collagen fibril spacing through its association with small leucine-rich proteoglycans. In cartilage and brain tissues, it provides mechanical cushioning, modulates growth factor interactions (e.g., FGF, TGF-β), and supports neuronal development and plasticity. Sulfation patterns encode molecular specificity for cell signaling, extracellular matrix structuring, and regulation of inflammation.