The Peptides Acetyl-(Gly-3(S)Hyp-4(R)Hyp)10-NH2 and Acetyl-(Gly-Pro-3(S)Hyp)10-NH2 Do Not Form a Collagen Triple Helix

Abstract

Hydroxylation of proline residues in the Yaa position of the Gly-Xaa-Yaa repeated sequence to 4(R)-hydroxyproline is essential for the formation of the collagen triple helix. A small number of 3(S)-hydroyxyproline residues are present in most collagens in the Xaa position. Neither the structural nor a biological role is known for 3(S)-hydroxyproline. To characterize the structural role of 3(S)-hydroxyproline, the peptide Ac-(Gly-3(S)Hyp-4(R)Hyp) 10-NH2 was synthesized and analyzed by circular dichroism spectroscopy, analytical ultracentrifugation, and 1H nuclear magnetic resonance spectroscopy. At 4 °C in water the circular dichroism spectrum indicates that this peptide was in a polyproline-II-like secondary structure with a positive peak at 225 nm similar to Ac-(Gly-Pro-4(R)Hyp) 10-NH2. The positive peak at 225 nm almost linearly decreases with increasing temperature to 95 °C without an obvious transition. Although the peptide Ac-(Gly-Pro-4(R)Hyp)10-NH 2 forms a trimer at 10 °C, sedimentation equilibrium experiments indicate that Ac-(Gly-3(S)Hyp-4(R)Hyp)10-NH2 is a monomer in water at 7 °C. To study the role of 3(S)-hydroxyproline in the Yaa position, we synthesized Ac-(Gly-Pro-3(S)Hyp)10-NH2. This peptide also does not form a triple helix in water. 1H Nuclear magnetic resonance spectroscopy data (including line widths and nuclear Overhauser effects) are entirely consistent, with neither Ac-(Gly-3(S)Hyp-4(R)Hyp)10-NH2 nor Ac-(Gly-Pro-3(S)Hyp)10-NH2 forming a triple helix in water. Therefore 3(S)-hydroxyproline destabilizes the collagen triple helix in either position. In contrast, when 3(S)-hydroxyproline is inserted as a guest in the highly stable-Gly-Pro-4(R)Hyp-repeated host sequence, Ac-(Gly-Pro-4(R)Hyp)3-Gly-3(S)Hyp-4(R)Hyp-(Gly- Pro-4(R)Hyp) 4-Gly-Gly-NH2 forms as stable a trimer (Tm = 49.6 °C) as Ac-(Gly-Pro-4(R)Hyp)3-Gly-Gly-NH2 (T m = 48.9 °C). Given that Ac-(Gly-Pro-4(R)Hyp) 3-Gly-4(R)Hyp-Pro-(Gly-Pro-4(R)Hyp)4-Gly-Gly-NH 2 forms a triple helix nearly as stable as the above two peptides (Tm = 45.0 °C) and the knowledge that Ac-(Gly-4(R)Hyp-Pro) 10-NH2 does not form a triple helix, we conclude that the host environment dominates the structure of host-guest peptides and that these peptides are not necessarily accurate predictors of triple helical stability.