Collagen as a structural protein of connective tissue
Collagen is the main structural protein of the extracellular matrix in connective tissue. It is synthesized by connective tissue cells (primarily fibroblasts). The collagen polypeptide chain contains about 1000 amino acid residues, with every third residue being glycine. About 20% consists of proline and hydroxyproline, and the remaining 40% accounts for other amino acids.
Amino acid composition feature: cysteine and tryptophan are practically absent in collagen, and histidine content is low. The primary structure is conventionally written as a repeating triplet (Gly — X — Y), where position X is frequently proline, and Y is hydroxyproline. This sequence allows the chain to fold into a characteristic left-handed α-helix, more extended than the α-helix of globular proteins: there are only three amino acid residues per turn. Three such chains then intertwine into a right-handed superhelix (tropocollagen).
Procollagen biosynthesis and the role of the ER
Collagen polypeptide chains are synthesized on the ribosomes of the rough endoplasmic reticulum as a precursor, preprocollagen. A signal peptide is located at the N-terminus of the molecule, ensuring the orientation of the growing chain into the ER lumen. After translocation, the signal peptide is cleaved off, forming procollagen, which undergoes numerous post-translational modifications.
Main post-translational modifications:
- hydroxylation of proline and lysine residues;
- glycosylation of hydroxylysine residues;
- formation of the triple helix from three pro-α-chains;
- secretion of the molecule into the intercellular space;
- cleavage of N- and C-terminal peptides (containing cysteine residues — which is why mature collagen is virtually devoid of cysteine).
Proline hydroxylation and the role of vitamin C
The key reaction in collagen maturation is proline hydroxylation, catalyzed by the enzyme prolyl hydroxylase. An iron atom (Fe²⁺) is located in the active site of the enzyme. To maintain iron in its reduced (+2) state, ascorbic acid (vitamin C) is required: it acts as a cofactor, reducing the oxidized iron.
Molecular oxygen and α-ketoglutarate participate in the reaction. One oxygen atom is added to proline to form hydroxyproline, while the second is used for the oxidative decarboxylation of α-ketoglutarate to form succinate and CO₂. Lysine is hydroxylated in a similar manner to form hydroxylysine.
Biological significance of hydroxylation: the introduced OH group forms additional hydrogen bonds that stabilize the collagen triple helix. In vitamin C deficiency, hydroxylation is impaired, the triple helix becomes unstable, which clinically manifests as scurvy: bleeding gums, loose teeth, impaired wound healing, and vascular fragility.
Assembly of collagen fibrils and fibers
After the formation of the triple helix, procollagen is secreted from the cell. In the extracellular space, specific proteases cleave the N- and C-terminal registration peptides, converting procollagen into tropocollagen. Tropocollagen molecules spontaneously associate into ordered structures with a staggered offset (~1/4 of their length), forming:
- microfibrils — bundles of tropocollagen molecules;
- collagen fibrils — aggregates of microfibrils;
- collagen fibers — bundles of fibrils visible under a light microscope.
Additional structural stabilization is provided by covalent cross-links formed with the participation of hydroxylysine residues.
Hyaluronic acid in the extracellular matrix
Hyaluronic acid is a non-sulfated glycosaminoglycan, a crucial component of the intercellular substance. It consists of repeating disaccharide units of glucuronic acid and N-acetylglucosamine. Its main biological property is the ability to bind large amounts of water, giving the intercellular substance the character of a gel-like matrix.
Functions of hyaluronic acid:
- water retention and maintenance of tissue turgor;
- binding of sodium and calcium ions, participation in water-salt metabolism regulation;
- ensuring lubrication of articular surfaces and shock absorption;
- participation in cell migration and reparative processes.
A number of pathogenic bacteria (for example, the causative agents of gas gangrene — clostridia) synthesize the enzyme hyaluronidase, which breaks down hyaluronic acid. Matrix degradation disrupts the gel-like state of the intercellular substance and facilitates the spread of infection in tissues.
Key points
- Every third amino acid in collagen is glycine; the characteristic triplet is (Gly—X—Y) with proline and hydroxyproline.
- The collagen triple helix consists of three left-handed α-chains forming a right-handed superhelix.
- Biosynthesis occurs in the ER via preprocollagen with the cleavage of the signal peptide.
- Prolyl hydroxylase requires Fe²⁺, α-ketoglutarate, O₂, and ascorbic acid as a cofactor.
- Vitamin C deficiency impairs hydroxylation and leads to scurvy.
- Mature collagen fibers form in a stepwise manner: tropocollagen → microfibrils → fibrils → fibers.
- Hyaluronic acid is a glycosaminoglycan that retains water and forms a gel-like matrix.
- Bacterial hyaluronidase destroys the matrix and promotes the development of infections, including gas gangrene.