Formation of the triple helix of type I procollagen in cellulo. Temperature-dependent kinetics support a model based on cis in equilibrium trans isomerization of peptide bonds.

The kinetics of triple-helix formation in type I procollagen at 37 degrees C in cellulo have been found to agree with predictions from the following model: triple-helix formation is initiated after completion of the synthesis of the procollagen polypeptide chains and after the chains associate to form interchain disulfide bonds within the C-propeptide; triple-helix formation propagates from this single nucleation site toward the N terminus of the molecule, interrupted by the random occurrence of peptide bonds in the cis configuration; cis-trans isomerization controls the rate of triple-helix formation. This model predicts that the activation energy of the rate-limiting process should be strongly positive. However, studies of triple-helix formation in vitro using thermally denatured material have shown only a low, or even negative, dependence of the rate on temperature in the physiological range. Here we report the temperature dependence of the rate of triple-helix formation in cellulo and a novel procedure for analyzing the resulting data to give an estimate of the Arrhenius activation energy of the rate-controlling process. It was found that this rate showed a strong, positive dependence on temperature, as expected, and that the activation energy was in satisfactory agreement with independent direct determinations of this parameter for cis-trans isomerizations. These findings lend further support to the model of triple-helix formation described above.