Effect of 3-Hydroxyproline Residues on Collagen Stability

Collagen is an integral part of many types of connective tissue in animals, especially skin, bones,
cartilage, and basement membranes. A fibrous protein, collagen has a triple-helical structure, which is
comprised of strands with a repeating Xaa-Yaa-Gly sequence. l-Proline (Pro) and 4(R)-hydroxy-l-proline
(4-Hyp) residues occur most often in the Xaa and Yaa positions. The 4-Hyp residue is known to increase
markedly the conformational stability of a collagen triple helix. In natural collagen, a 3(S)-hydroxy-l-proline
(3-Hyp) residue occurs in the sequence:  3-Hyp-4-Hyp-Gly. Its effect on collagen stability is unknown. Here,
two host−guest peptides containing 3-Hyp are synthesized:  (Pro-4-Hyp-Gly)3-3-Hyp-4-Hyp-Gly-(Pro-4-Hyp-Gly)3 (peptide 1) and (Pro-4-Hyp-Gly)3-Pro-3-Hyp-Gly-(Pro-4-Hyp-Gly)3 (peptide 2). The 3-Hyp residues
in these two peptides diminish triple-helical stability in comparison to Pro. This destabilization is small
when 3-Hyp is in the natural Xaa position (peptide 1). There, the inductive effect of its 3-hydroxyl group
diminishes slightly the strength of the interstrand 3-HypCO···HNGly hydrogen bond. The destabilization
is large when 3-Hyp is in the nonnatural Yaa position (peptide 2). There, its pyrrolidine ring pucker leads
to inappropriate mainchain dihedral angles and interstrand steric clashes. Thus, the natural regioisomeric
residues 3-Hyp and 4-Hyp have distinct effects on the conformational stability of the collagen triple helix.