Probability of internal unwinding in the triple helix of collagen

The energy of formation of internal loops in triple-helical collagen polypeptides

The sequence-dependent local destabilization in the interior of the collagen triple helix has been evaluated by means of conformational energy computations. Using a model poly(Gly-Pro-Pro) triple helix as the reference state, a method was developed for generating local loops, i.e., internal deformations, and analyzing their conformations. A seven-residue Gly-Pro-Pro- Gly-Pro-Pro-Gly fragment was replaced by the Gly-Pro-Ala-Gly-Ala-Ala-Gly sequence in one, two, or all three of the strands of the loop region. A set of loop conformations was generated in which the ends of the loop were initially fixed in the triple-helical structure. The potential energy of the entire deformed triple helix was then minimized, resulting in a variety of structures that contained deformed loops. The conformations of the triple helices at the two ends of the loops remained essentially unchanged in many of the low-energy conformations. In numerous high-energy conformations, however, the triple-helical segments were also partially or totally disrupted. The minimum-energy conformations of the whole structures were compared in terms of rms deviations of atomic coordinates with respect to the original triple helix, and of the shapes of the loops (using a distance function derived from differential geometry). Three new geometrical parameters-stretch S, kink K, and unwinding U-were defined to describe the changes in the overall orientation of the triple helices at the two ends of the loop. It is shown that, when the number of Pro residues in a short fragment is reduced, the triple helical structure can accomodate internal loops (i.e., distortions) within a 5 kcal/mol cutoff from the essentially unperturbed triple helical structure. For structures with a Gly-Pro-Ala-Gly-Ala-Ala-Gly sequence in all three strands, the probability of finding conformations with internal loops is small, i.e., 0.06. Internal loops affect the overall orientation of these structures, as measured by the helix-distortion parameters S, K, and U.

Procollagen is more stable in cellulo than in vitro

The thermal denaturation of both intracellular and freshly secreted chick embryo tendon type I procollagen was investigated using susceptibility to proteolysis by trypsin and chymotrypsin as a probe for triple-helical conformation. Freshly secreted procollagen from the medium of matrix-free tendon cells in suspension or procollagen within the cells and in the pericellular environment melted at 45 degrees C. In contrast, if freshly secreted procollagen was subjected to the melting procedure after dialysis of the medium against 0.4 M NaCl, 0.1 M Tris HCl, pH 7.4 the protein melted at 42 degrees C, the melting temperature of purified procollagen dissolved in the same buffer. In each of these cases, the thermal denaturation profile was narrow, with a width of 1.0-1.5 degrees C. These results demonstrate that, in situ, procollagen is more stable toward thermal denaturation than was previously thought. This extra margin of thermal stability partially resolves the dilemma of how tissues are able to assemble triple-helical procollagen molecules at body temperatures that closely approach the melting temperature of the purified protein.