chondroitin sulfate

STRUCTURE - Repeating disaccharide: D-glucuronic acid + N-acetylgalactosamine (GalNAc), with sulfate groups at C4 or C6 of GalNAc (chondroitin-4-sulfate, chondroitin-6-sulfate). - Negatively charged (carboxylate and sulfate groups), so it attracts cations (Na+) and water. This creates a hydrated, gel-like matrix that resists compression. - Covalently attached to a core protein through a tetrasaccharide linker (xylose-galactose-galactose-glucuronic acid) on a serine residue, forming a proteoglycan. Aggrecan is the key example. Aggrecan binds hyaluronic acid via link protein, forming huge aggregates in cartilage.

SYNTHESIS AND DEGRADATION - Synthesized in the Golgi. Sulfation uses PAPS (3'-phosphoadenosine-5'-phosphosulfate) as the sulfate donor. GAG chains (except hyaluronic acid) are elongated on the core protein in the Golgi. - Degraded in lysosomes by lysosomal hydrolases (e.g., iduronidase, iduronate sulfatase, galactosamine sulfatases, β-glucuronidase, hexosaminidase). Enzyme deficiency causes mucopolysaccharidoses (MPS), with accumulation of GAGs (heparan sulfate, dermatan sulfate, keratan sulfate, chondroitin sulfate) in lysosomes. - Hurler (MPS I, α-L-iduronidase): corneal clouding, coarse facies, hepatosplenomegaly, developmental delay. Accumulates heparan and dermatan sulfate. - Hunter (MPS II, iduronate sulfatase): X-linked recessive, no corneal clouding, aggressive behavior, milder than Hurler. - Maroteaux-Lamy (MPS VI, arylsulfatase B) involves dermatan sulfate. - Sly syndrome (MPS VII, β-glucuronidase) can involve chondroitin sulfate, dermatan sulfate, and heparan sulfate.

LOCATION AND FUNCTION - Cartilage (hyaline, articular), bone, tendons, ligaments, cornea, aorta and other blood vessel walls, and intervertebral discs. - Provides compressive strength and shock absorption in cartilage. The sulfated GAG content gives cartilage its resilience, and aggrecan loss is part of osteoarthritis. - In the CNS, chondroitin sulfate proteoglycans (CSPGs) are a major component of the glial scar and perineuronal nets. They inhibit axonal regeneration after injury. Chondroitinase ABC has been studied experimentally to promote regeneration.

CLINICAL SIGNIFICANCE - Osteoarthritis: chondroitin sulfate (often with glucosamine) is a popular supplement. Evidence for symptomatic benefit is inconsistent and modest at best, and it is not routinely recommended by major guidelines (e.g., ACR 2019 conditionally recommends against it). - MPS: see above. Diagnosis is by urine GAG excretion and enzyme assay. Treatment includes enzyme replacement therapy for some types, and hematopoietic stem cell transplant for some (e.g., Hurler). - Does not require vitamin K or any other particular cofactor, and unlike heparin it is not a clinically useful anticoagulant. Heparin and heparan sulfate carry the highest sulfate density and have anticoagulant activity via antithrombin.

HIGH-YIELD COMPARISON OF GAGs - Hyaluronic acid: no sulfate, not attached to a core protein, in synovial fluid, vitreous humor, and ECM. - Chondroitin sulfate: cartilage, bone, cornea. - Dermatan sulfate: skin, blood vessels, heart valves. - Keratan sulfate: cornea, cartilage, and the only GAG without uronic acid (galactose instead of glucuronic acid). - Heparan sulfate: basement membranes (e.g., glomerular basement membrane), cell surfaces. - Heparin: mast cell granules, anticoagulant.

BOARD PEARLS - GAGs are long, unbranched, negatively charged polysaccharides with repeating disaccharide units (an amino sugar plus a uronic acid, except keratan sulfate). - Sulfate donor is PAPS, and mutations affecting sulfation can cause skeletal dysplasias (e.g., diastrophic dysplasia, a defect in the DTDST sulfate transporter). - Think "chondro" = cartilage.