Chemical Structure and Molecular Charge
From a biochemical perspective, hyaluronic acid is the largest glycosaminoglycan. Its molecular weight reaches colossal values ranging from $10^5$ to $10^7$ Daltons. Notably, it is strictly classified as a glycosaminoglycan rather than a proteoglycan because its structure completely lacks a protein core (axial protein).
The structure of this biopolymer is linear and consists of numerous repeating disaccharide units. Each structural unit comprises two components:
- D-glucuronic acid;
- N-acetyl-D-glucosamine.
Within a single disaccharide, these monosaccharides are linked by a $\beta(1\to3)$ glycosidic bond. In turn, the disaccharide units themselves are connected via $\beta(1\to4)$ bonds, forming a long polymer chain that does not undergo sulfation.
A distinctive feature of the molecule is its pronounced negative charge. Each disaccharide unit contains a carboxyl group belonging to glucuronic acid. Given the immense chain length and giant number of monomers, the molecule as a whole acquires the properties of a powerful polyanion.
Biological Functions in the Extracellular Matrix
The chemical structure directly determines the physical properties and functions of hyaluronic acid in body tissues.
- Binding colossal volumes of water. Due to its polyanionic nature, the molecule exhibits extremely high hydrophilicity. Negatively charged groups attract and tightly hold $H_2O$ molecules.
- Formation of a gel-like matrix. By becoming saturated with water, hyaluronic acid gives the intercellular space a viscous, gel-like structural framework that maintains tissue shape.
- Binding cations. Like all glycosaminoglycans, hyaluronan actively interacts with positively charged ions. Specifically, it plays a key role in sequestering sodium ions ($Na^+$) in the extracellular environment.
Role of Hyaluronidase in the Spread of Infections
Normally, the extracellular matrix acts as a reliable barrier against the spread of foreign agents. However, many pathogenic microorganisms have evolved mechanisms to overcome this defense during evolution. Causative agents of purulent infections and gas gangrene secrete a specific enzyme known as hyaluronidase.
The substrate for this enzyme is hyaluronic acid itself. Hyaluronidase catalyzes hydrolysis, specifically cleaving $\beta$-glycosidic bonds between disaccharide residues. This process is called depolymerization.
Consequences of enzymatic cleavage:
- Rapid destruction of the normal gel-like structure of the tissue matrix.
- A sharp and significant decrease in extracellular fluid viscosity.
- Increased permeability of tissue barriers.
For this reason, hyaluronidase is often referred to as a "spreading factor" or permeability factor. As a result of its action, pathogenic bacteria and their toxins can freely and rapidly invade new tissue areas, worsening the severity of the infection.