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

Bachmann A, Kiefhaber T, Boudko S, Engel J, B├Ąchinger HP
Proc Natl Acad Sci U S A. 2005 102 (39): 13897-902

PMID: 16172389 · PMCID: PMC1236557 · DOI:10.1073/pnas.0505141102

Collagen consists of repetitive Gly-Xaa-Yaa tripeptide 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 peptide 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 guanidinium chloride on folding kinetics and stability reveals that the rate-limiting step is represented by bringing 10 consecutive tripeptide units (3.3 per strand) into a triple-helical conformation. The following addition of tripeptide 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.

MeSH Terms (7)

Collagen Type III Disulfides Entropy Guanidine Humans Protein Folding Temperature

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