Collagen triple-helix formation in all-trans chains proceeds by a nucleation/growth mechanism with a purely entropic barrier

Collagen consists of repetitive Gly-Xaa-Yaa tripepticle units with proline and hydroxyproline frequently found in the Xaa and Yaa position, respectively. This sequence motif allows the formation of a highly regular triple helix that is stabilized by steric (entropic) restrictions in the constituent polyproline-II-helices and backbone hydrogen bonds between the three strands. Concentration-dependent association reactions and slow prolyl isomerization steps have been identified as major rate-limiting processes during collagen folding. To gain information on the dynamics of triple-helix formation in the absence of these slow reactions, we performed stopped-flow double-jump experiments on cross-linked fragments derived from human type III collagen. This technique allowed us to measure concentration-independent folding kinetics starting from unfolded chains with all pepticle bonds in the trans conformation. The results show that triple-helix formation occurs with a rate constant of 113 +/- 20 s(-1) at 3.7 degrees C and is virtually independent of temperature, indicating a purely entropic barrier. Comparison of the effect of guaniclinium chloride on folding kinetics and stability reveals that the rate-limiting step is represented by bringing 10 consecutive tripepticle units (3.3 per strand) into a triple-helical conformation. The following addition of tripepticle units occurs on a much faster time scale and cannot be observed experimentally. These results support an entropy-controlled zipper-like nucleation/growth mechanism for collagen triple-helix formation.