Differences between the thermal stabilities of the three triple-helical domains of type IX collagen

Fibre-forming collagens in dilute solution show highly co-operative helix-coil transitions at temperatures that are remarkably close to the body temperature of the animal from which the collagen was extracted. This close correlation holds across animal Phyla and the transition temperatures, which range from 5 degrees C to 40 degrees C, are adjusted to suit by changing the primary structure, especially the concentration of the water-bridge-enhancing hydroxyproline residue. Fibril-forming collagens are thermally stabilised by fibrillogenesis, which causes a loss of random coil configurational entropy by intermolecular and intramolecular cross-linking and by spacial confinement of the molecule within the lattice of the fibre. But this mechanism cannot apply to the full length of the type IX collagen molecule, since its COL3 arm, according to current models, projects out from the stabilising influence of the type II fibre. In this paper we examine the thermal stability of the type IX collagen molecule and its three triple-helical domains, thereby demonstrating that the COL3 arm is much more stable than the rest of the molecule. At a scanning rate of 60 deg. C/h COL3 exhibited an unfolding endotherm with a tmax at 49.0 degrees C, well above body temperature. Corresponding peak maxima for COL1 and COL2 were seen at 40.6 degrees C and 39.6 degrees C, respectively. The sizes of the thermally labile units of COL1, COL2 and COL3, calculated from the measured activation enthalpies, were 24, 28 and 28 residues, respectively, much smaller than type I (65 residues) because of the relatively short lengths of triple helix to be unfolded. However, unlike type I collagen, no regions of the required size were found completely devoid of hydroxyproline. Consequently, the intrinsic stabilities of these thermally labile units were higher than that of type I with DeltaH updownarrow DeltaS updownarrow for COL1, COL2 and COL3 being, respectively, 385 K, 371 K and 384 K, contrasting with the much lower 349 K of type I collagen. We therefore speculate that the increased thermal stability of the thermally labile units was caused by the presence of the water-bridge-enhancing residue, hydroxyproline. Finally the stabilisation of type IX collagen tissue is considered and an alternative structural organisation of the type IX molecule on the type II fibre is proposed.