Molecular Structure of Fibronectin
The fibronectin molecule is a large glycoprotein. Its basic structure is formed by precisely two polypeptide chains. To ensure spatial stability, these chains are bound together by strong disulfide bonds (S-S bonds). According to structural models, these covalent bonds are localized near the C-terminal regions of the molecule.
A key feature of its molecular structure is a clearly defined domain organization. Each of the two polypeptide chains is divided into 7–8 functional domains. The presence of these isolated regions (sites) determines the molecule's unique ability to simultaneously interact with a vast number of various cell structures and extracellular matrix components, acting as a universal linking element.
Specific Binding Sites
The uniqueness of fibronectin lies in its multiple specific binding sites. These highly specific regions, located on the domains of the polypeptide chains, allow the molecule to act as a multifunctional linking hub. The glycoprotein is capable of selectively and tightly binding the following components of the extracellular environment:
- Cell surface: interaction occurs via specific cell receptors called integrins.
- Collagen fibers: the molecule possesses a separate, strictly determined collagen-binding site.
- Proteoglycans and heparan sulfates: binding sites for these are distributed on both polypeptide chains of fibronectin.
- Hyaluronic acid: interaction occurs through a specialized binding site.
- Carbohydrate components: fibronectin actively binds to carbohydrates on the plasma membranes of cells.
- Enzymes: binding to a specific enzyme—transglutaminase—has been proven.
Biological Functions in Tissues
Synthesized and secreted into the intercellular space by a vast variety of cell types, fibronectin undertakes two fundamental biological tasks:
- Integrating function. The molecule takes an active part in the global organization of the intercellular substance. Due to its multiple domains, it binds disparate matrix elements together, creating a unified spatial network.
- Adhesive function. The glycoprotein acts as an effective molecular anchor. It significantly enhances and promotes cell adhesion—the reliable attachment of cells to the surrounding structures of the extracellular space.
Role of Fibronectin in Oncology (Metastasis)
The study of fibronectin is critically important for clinical medicine, primarily for understanding pathological processes in oncology. During malignant cell transformation, profound changes occur in the molecular composition of the cell surface. Specifically, a sharp decrease in the amount of this glycoprotein is registered on the plasma membrane of tumor cells.
A direct consequence of fibronectin deficiency is the loss of stable intercellular contacts. Malignant cells become significantly less tightly bound to each other and to the surrounding extracellular matrix. Such weakening of adhesive properties critically facilitates their detachment from the primary tumor site and subsequent migration through the body. This molecular defect underlies metastasis—the key and most dangerous stage of cancer progression.