 Hyaluronic acid

Hyaluronic acid, also called hyaluronan, is a linear glycosaminoglycan composed of alternating glucuronic acid and N-acetylglucosamine residues. It is nonsulfated, and unlike the sugar chains of conventional proteoglycans, it does not require covalent attachment to a protein core. Its carboxyl groups are largely ionized in physiological conditions, so the polymer is often encountered as hyaluronate rather than as the fully protonated acid. ^[ref]

The repeating sequence makes a useful bookkeeping example. A chain segment with $n$ complete two-sugar repeats contains $n$ glucuronic acid residues and $n$ N-acetylglucosamine residues. Its length can change enormously without changing that alternating composition. Composition alone, however, does not determine viscosity: chain length, concentration and solution conditions also matter.

Synthesis tests the two-building-block description experimentally. P. L. DeAngelis characterized the hyaluronic acid synthase from Pasteurella multocida using activated sugar donors, UDP-glucuronic acid and UDP-N-acetylglucosamine. The enzyme incorporates the two distinct sugars into the growing chain; activity also depends on divalent ions. This turns an abstract repeating formula into a laboratory reaction whose substrates and conditions can be varied.^[ref]

Hyaluronic acid is therefore both a molecular species defined by its repeat chemistry and a material whose bulk behavior depends on its molecular population. This distinction helps explain why identifying the sugar repeat does not by itself specify the properties of every preparation.