telopeptide

A telopeptide is the non-helical terminal region of a collagen molecule. Each molecule of type I collagen, for example, has an N-terminal telopeptide and a C-terminal telopeptide that flank the central triple helix. These segments are not part of the collagen’s helical core, but they contribute to fibril formation, cross-linking, and interactions with other matrix components. During tissue remodeling, the telopeptides can be cleaved by proteolytic enzymes, releasing fragments that enter extracellular fluids.

Structure and function: Telopeptides contain residues such as lysine and hydroxylysine that participate in covalent cross-links between collagen molecules, helping to stabilize mature fibrils. They are also prime sites for enzymatic cleavage by collagenases and other proteases during turnover. In some research contexts, telopeptides can be removed by limited proteolysis to yield atelopeptide, or atelocollagen, which is used in biomaterials and studies of collagen properties.

Clinical significance: Fragments derived from the telopeptide regions, particularly the C-terminal telopeptide of type I collagen (CTX-I) and the N-terminal telopeptide of type I collagen (NTX), are measured in blood or urine as biomarkers of bone resorption and overall collagen turnover. Elevated levels generally indicate increased bone resorption and can aid in diagnosing osteoporosis, monitoring treatment response, and assessing fracture risk. Immunoassays are commonly used for these measurements, with results influenced by age, sex, circadian rhythm, and renal function.

Other contexts: The term telopeptide can also refer more broadly to non-helical ends in other collagens. In addition, pepsin digestion can remove telopeptides to produce atelocollagen, a form frequently used in research and biomaterials.