BIOCHEMISTRY - L. Stryer - 1984

VOLUME 1

PART I. CONFORMATION AND DYNAMICS

CHAPTER 9. CONNECTIVE TISSUE PROTEINS: COLLAGEN, ELASTIN, AND PROTEOGLYCANS

9.16. Proteoglycans form the ground substance of connective tissue

Connective Tissue is also rich in Proteoglycans, which consist of polysaccharide (about 95%) and protein (about 5%) units. These very large polyanions bind Water and cations, forming the ground substance of connective tissue. Proteoglycans determine the viscoelastic properties of joints and other anatomical structures subjected to mechanical deformation. The polysaccharide chains of proteoglycans, called glycosaminoglycans, are built of repeating disaccharide units containing an amino sugar derivative—glucosamine or galactosamine. At least one of the sugars in the disaccharide has a negatively charged carboxyl or sulfate group. The most important glycosaminoglycans are hyaluronate, chondroitin sulfate, keratan sulfate, heparan sulfate, and heparin (Fig. 9.26). Heparan sulfate is similar to heparin, except that it contains fewer N- and O-sulfate groups and more N-acetyl groups.

Class="center">Fig. 9.26. Structural formulas of repeating disaccharide units in several major glycosaminoglycans. Negatively charged groups are shown in red

In Cartilage proteoglycans (Fig. 9.27), keratan sulfate and chondroitin sulfate are covalently attached to a polypeptide chain, referred to in this case as the core protein. Approximately 140 subunits formed in this way are, in turn, noncovalently attached at 300 Å intervals to a long strand of hyaluronic acid. A small link protein is involved in the assembly of this complex. The molecular mass of the entire complex is approximately 10⁵ kDa, and its length is several microns. Proteoglycans of other Tissues may have different structures. This suggests that this class of very large Biomolecules performs several Functions that are still unknown.

Fig. 9.27. A—Electron micrograph of a proteoglycan aggregate from bovine embryonic epiphyseal cartilage. Proteoglycan monomers are arranged at regular intervals along both sides of an elongated central hyaluronate strand