Hysteresis in the triple helix-coil transition of type III collagen.

A hysteresis is observed in the triple helix-coil transition of type III collagen. This hysteresis appears in the thermal transition, where it is not solvent-dependent, and also in the transition induced by guanidinium hydrochloride. The transitions are observed both by optical rotatory dispersion and by trypsin digestion. Experiments with various fragments of type III collagen, which, like the intact protein, contain a set of disulfide bonds at the carboxyl-terminal end, which functions as an effective nucleus for refolding, indicate that the hysteresis depends on the amino acid sequence, rather than being a general property of the triple helix. The carboxyl-terminal one-quarter fragment of type III collagen does not show a hysteresis, whereas a mutant type III collagen missing residues 595-1008 shows a hysteresis of the same magnitude as normal type III collagen. Several fragments were produced that are missing parts of the amino-terminal end, and these fragments show no hysteresis. The amino-terminal part of the triple helix of type III collagen consists of a stable region containing six Gly-Pro-Hyp tripeptide units within the first 16 tripeptide units. This region forms an amino-terminal clamp for the triple helix and, together with the set of disulfide bonds at the carboxyl-terminal end, allows for highly cooperative unfolding of the triple helix. The presence of this amino-terminal region is a prerequisite for the observed hysteresis.