Collagen Type I

Type I collagen is the most abundant protein in the mammalian body and the principal tensile element of tendon, ligament, bone matrix and dermis. Each molecule is a heterotrimer of two alpha-1 chains encoded by COL1A1 and one alpha-2 chain from COL1A2, wound into a triple helix that requires glycine at every third residue, since only glycine fits at the crowded helix axis. Proline and hydroxyproline stabilise the fold, and the hydroxylation step is ascorbate-dependent, which is the biochemistry behind scurvy. Fibrils are then cross-linked by lysyl oxidase into fibres that resist both stretch and enzymatic attack.

Tendon is roughly two thirds to four fifths type I collagen by dry mass. Mutations in COL1A1 or COL1A2 cause osteogenesis imperfecta, where the defect is collagen quality rather than mineral quantity. The bone formation marker P1NP is literally the N-terminal propeptide clipped from type I procollagen as it is deposited, which is why it tracks matrix synthesis rather than mineralisation.

Quantity and quality are different claims. Healing tendon and skin lay down abundant collagen, but scar collagen has smaller, more uniform fibril diameters, disordered alignment and different cross-link chemistry, and repaired tendon plateaus well below its original material strength. A study reporting more collagen has measured deposition, not restored architecture.

Two errors recur. The first is reading total hydroxyproline or a generic collagen stain as type I, when newly deposited matrix in early healing is disproportionately type III. The second is the oral collagen argument, which treats ingested collagen as though it were routed to tendon; it is digested, and while some hydroxyproline-containing dipeptides do appear in plasma, the human outcome evidence in tendon remains small, short and largely industry-funded.