Hyaluronan, also called hyaluronic acid (HA) or hyaluronate, is a nonsulfated GAG (see Figure 20-29a) made by a plasma-membrane-bound enzyme called HA synthase and is secreted directly into the extracellular space as it is synthesized. (A similar approach is used by plant cells to make the ECM component cellulose.) Hyaluronan is a major component of the ECM that surrounds migrating and proliferating cells, particularly in embryonic tissues. In addition, it forms the backbone of complex proteoglycan aggregates found in many ECMs, particularly cartilage. Because of its remarkable physical properties, hyaluronan imparts stiffness and resilience as well as a lubricating quality to many types of connective tissue such as joints.

Hyaluronan molecules range in length from a few disaccharide repeats to about 25,000. The typical hyaluronan in joints such as the elbow has 10,000 repeats for a total mass of 4 × 10⁶ Da and a length of 10 µm (about the diameter of a small cell). Individual segments of a hyaluronan molecule fold into a rodlike conformation because of the β glycosidic linkages between the sugars and extensive intrachain hydrogen bonding. Mutual repulsion between negatively charged carboxylate groups that protrude outward at regular intervals also contributes to these locally rigid structures. Overall, however, hyaluronan is not a long, rigid rod like fibrillar collagen; rather, it is very flexible in solution, bending and twisting into many conformations, forming a random coil.

Because of the large number of anionic residues on its surface, the typical hyaluronan molecule binds a large amount of water and behaves as if it were a large hydrated sphere with a diameter of about 500 nm. As the concentration of hyaluronan increases, the long chains begin to entangle, forming a viscous gel. Even at low concentrations, hyaluronan forms a hydrated gel; when placed in a confining space, such as that between two cells, the long hyaluronan molecules tend to push outward. This outward pushing creates a swelling, or turgor pressure, within the extracellular space. In addition, the binding of cations by carboxylate (COO⁻) groups on the surface of hyaluronan increases the concentration of ions and thus the osmotic pressure in the gel. As a result, large amounts of water are taken up, contributing to the turgor pressure. These swelling forces give connective tissues their ability to resist compression forces, in contrast to collagen fibers, which are best able to resist stretching forces. Other highly charged GAG chains are similarly hydrated.

Hyaluronan is bound to the surface of many migrating cells by a number of adhesion receptors, such as the receptor called CD44, which contains hyaluronan-binding domains, each with a similar three-dimensional conformation. Because of its loose, hydrated, porous nature, the hyaluronan “coat” bound to cells appears to keep them apart from one another, giving them the freedom to move about and proliferate. The cessation of cell movement and the initiation of cell-cell attachments are frequently correlated with a decrease in hyaluronan, a decrease in hyaluronan receptors, and an increase in the extracellular enzyme hyaluronidase, which degrades hyaluronan in the matrix. These alterations of hyaluronan are particularly important during the many cell migrations that facilitate differentiation and in the release of a mammalian egg cell from its surrounding cells after ovulation.

The predominant proteoglycan in cartilage, called aggrecan, assembles with hyaluronan into very large aggregates, illustrative of the complex structures that proteoglycans sometimes form. The backbone of this proteoglycan aggregate is a long molecule of hyaluronan to which multiple aggrecan molecules are bound tightly but noncovalently (Figure 20-32). A single hyaluronan-aggrecan aggregate, one of the largest macromolecular complexes known, can be more than 4 µm long and have a volume larger than that of a bacterial cell. These aggregates give cartilage its unique gel-like properties and its resistance to deformation, essential for distributing the load in weight-bearing joints.

The aggrecan core protein (~250,000 MW) has one N-terminal globular domain that binds with high affinity to a specific decasaccharide sequence within hyaluronan. This specific sequence is generated by covalent modification of some of the repeating disaccharides in the hyaluronan chain. The interaction between aggrecan and hyaluronan is facilitated by a link protein that binds to both the aggrecan core protein and hyaluronan (Figure 20-32b). Aggrecan and the link protein have in common a “link” domain, about 100 amino acids long, that is found in numerous ECM and cell-surface hyaluronan-binding proteins in both cartilaginous and noncartilaginous tissues. These proteins almost certainly arose in the course of evolution from a single ancestral gene that encoded just this domain.