Model conformations of the carboxyl telopeptides in vivo based on type I collagen fibral banding patterns

Densitometric scans of high resolution banding patterns of uranyl acetate-stained, type I collagen fibrils teased from rat tail tendon were correlated with sequence derived model scans. The best model for the alpha 1 carboxyl telopeptide has the first 15 residues in a "S fold" or "random coil" with a net axial displacement of 7 residues (a contracted region) and the last 10 residues extended along the fibril axis. The correlation of both models with experimental scans is significantly improved when two 4D-staggered intermolecular crosslinks are modeled by eliminating four charges in band a3: the two lysines in the alpha 1 carboxyl telopeptides and two hydroxylysines at residues 87 and 88 in the triple helix. The alpha 2 carboxyl telopeptide is modeled as being folded back on the triple helix. Also, as previously found for type I segment long spacing crystallites, the highest correlation of models with the experimental data is obtained when the alpha 2 chain is in the A or reference position of the triple helix (alpha 2, alpha 1, alpha 1). It is further hypothesized that unfolding of the "S fold" or "random coil" in the alpha 1 carboxyl telopeptide may be a reversible mechanism for low strain energy dissipation in tendon and other connective tissues.