Proteoglycan

Proteoglycans are a class of glycoconjugates consisting of a core protein covalently attached to one or more glycosaminoglycan (GAG) chains. The GAG chains are long, unbranched polysaccharides that are highly sulfated or otherwise negatively charged, giving proteoglycans a high density of negative charge and an ability to attract water and ions.

In synthesis, GAG chains are assembled in the Golgi apparatus and covalently linked to serine residues on the core protein via a common tetrasaccharide linkage region. The types of GAGs vary, including chondroitin sulfate, dermatan sulfate, heparan sulfate, and keratan sulfate. The specific sulfation pattern and chain length influence interactions with proteins and other molecules.

Proteoglycans occur in the extracellular matrix, where they form aggregates such as aggrecan with hyaluronan and link protein in cartilage. They also include small leucine-rich proteoglycans like decorin and biglycan, which regulate collagen fibrillogenesis. Some proteoglycans are membrane-associated (syndecans and glypicans) or basement-membrane–associated (perlecan).

Functions of proteoglycans include providing structural support and resisting compression in cartilage, shaping and protecting the extracellular matrix, regulating diffusion of molecules, and modulating signaling by binding growth factors and morphogens. They interact with other matrix components and cell-surface receptors to influence adhesion, migration, proliferation, and differentiation.

Disorders of GAG catabolism cause mucopolysaccharidoses. Altered proteoglycan expression or sulfation is linked to aging, arthritis, and cancer progression, reflecting their important roles in development and disease.