Thermodynamic and kinetic consequences of substituting glycine at different positions in a Pro-Hyp-Gly repeat collagen model peptide.

A glycine occurs at every third residue in the X-Y-Gly repeat of natural collagen. Replacing Gly residues destabilizes collagen and is often associated with many diseases. We present a comprehensive study on the thermodynamic and kinetic consequences of replacing Gly residues at different sites in collagen. For this, we prepared a series of peptides that contain a single substitution of Gly with L-Ala, D-Ala, β-Ala, or sarcosine (Sar), at different positions in a host peptide (Pro-Hyp-Gly)(8) . Circular dichroism measurements showed that peptides with the mutation site near the C-terminus (C-terminal mutations) form a more stable collagen triple helix than those with the substitution near the N-terminus (N-terminal mutations), which is consistent with the known in vivo folding mechanism of collagen, from the C to the N-terminus. Thermodynamic analysis indicated that the destabilization in C-terminal mutations is due to entropic effects, while that in N-terminal mutations is mainly from enthalpic effects. The destabilization order is L-Ala < Sar < β-Ala < D-Ala substitution in both the N and C-terminal mutations, suggesting that residues with normal torsion angles are less destabilizing at either position. Moreover, Sar was shown to be a better substituent than the other three amino acids at the central site of collagen strands. Kinetic studies further demonstrated that steric strains imposed by the side chains may be the most critical factor affecting the folding rate of collagen. Our data provide valuable insights into how backbone conformation, side chains, and interstrand hydrogen bonds affect the collagen triple helix at different positions.