Sequence dependence of the folding of collagen-like peptides. Single amino acids affect the rate of triple-helix nucleation

The refolding of thermally denatured model collagen-like peptides was studied for a set of 21 guest triplets embedded in a common host framework: acetyl-(Gly-Pro-Hyp)3-Gly-Xaa-Yaa-(Gly-Pro-Hyp)4-Gly-Gly-amide. The results show a strong dependence of the folding rate on the identity of the guest Gly-Xaa-Yaa triplet, with the half-times for refolding varying from 6 to 110 min (concentration = 1 mg/ml). All triplets of the form Gly-Xaa-Hyp promoted rapid folding, with the rate only marginally dependent on the residue in the Xaa position. In contrast, triplets of the form Gly-Pro-Yaa and Gly-Xaa-Yaa were slower and showed a wide range of half-times, varying with the identity of the residues in the triplet. At low concentrations, the folding can be described by third-order kinetics, suggesting nucleation is rate-limiting. Data on the relative nucleation ability of different Gly-Xaa-Yaa triplets support the favorable nature of imino acids, the importance of hydroxyproline, the varying effects of the same residue in the Xaa position versus the Yaa position, and the difficulties encountered when leucine or aspartic acid are in the Yaa position. Information on the relative propensities of different tripeptide sequences to promote nucleation of the triple-helix in peptides will aid in identification of nucleation sites in collagen sequences.