Amino acid propensities for the collagen triple-helix.

Determination of the tendencies of amino acids to form alpha-helical and beta-sheet structures has been important in clarifying stabilizing interactions, protein design, and the protein folding problem. In this study, we have determined for the first time a complete scale of amino acid propensities for another important protein motif: the collagen triple-helix conformation with its Gly-X-Y repeating sequence. Guest triplets of the form Gly-X-Hyp and Gly-Pro-Y are used to quantitate the conformational propensities of all 20 amino acids for the X and Y positions in the context of a (Gly-Pro-Hyp)(8) host peptide. The rankings for both the X and Y positions show the highly stabilizing nature of imino acids and the destabilizing effects of Gly and aromatic residues. Many residues show differing propensities in the X versus Y position, related to the nonequivalence of these positions in terms of interchain interactions and solvent exposure. The propensity of amino acids to adopt a polyproline II-like conformation plays a role in their triple-helix rankings, as shown by a moderate correlation of triple-helix propensity with frequency of occurrence in polyproline II-like regions. The high propensity of ionizable residues in the X position suggests the importance of interchain hydrogen bonding directly or through water to backbone carbonyls or hydroxyprolines. The low propensity of side chains with branching at the C(delta) in the Y position supports models suggesting these groups block solvent access to backbone C=O groups. These data provide a first step in defining sequence-dependent variations in local triple-helix stability and binding, and are important for a general understanding of side chain interactions in all proteins.

Collagen model peptides: Sequence dependence of triple-helix stability.

The triple helix is a specialized protein motif, found in all collagens as well as in noncollagenous proteins involved in host defense. Peptides will adopt a triple-helical conformation if the sequence contains its characteristic features of Gly as every third residue and a high content of Pro and Hyp residues. Such model peptides have proved amenable to structural studies by x-ray crystallography and NMR spectroscopy, suitable for thermodynamic and kinetic analysis, and a valuable tool in characterizing the binding activities of the collagen triple helix. A systematic approach to understanding the amino acid sequence dependence of the collagen triple helix has been initiated, based on a set of host-guest peptides of the form, (Gly-Pro-Hyp)(3)-Gly-X-Y-(Gly-Pro-Hyp)(4). Comparison of their thermal stabilities has led to a propensity scale for the X and Y positions, and the additivity of contributions of individual residues is now under investigation. The local and global stability of the collagen triple helix is normally modulated by the residues in the X and Y positions, with every third position occupied by Gly in fibril-forming collagens. However, in collagen diseases, such as osteogenesis imperfecta, a single Gly may be substituted by another residue. Host-guest studies where the Gly is replaced by various amino acids suggest that the identity of the residue in the Gly position affects the degree of destabilization and the clinical severity of the disease.