POSITIONAL PREFERENCES OF IONIZABLE RESIDUES IN GLY-X-Y TRIPLETS OF THE COLLAGEN TRIPLE-HELIX

Collagens contain a high amount of charged residues involved in triple -helix stability, fibril formation, and ligand binding. The contributi on of charged residues to stability was analyzed utilizing a host-gues t peptide system with a single Gly-X-Y triplet embedded within -Pro-Hy p)(3)-Gly-X-Y-(Gly-Pro-Hyp)(4)-Gly-Gly-NH2. The ionizable residues Arg , Lys, Glu, and Asp were incorporated into the X position of Gly-X-Hyp ; in the Y position of Gly-Pro-Y; or as pairs of oppositely charged re sidues occupying X and Y positions, The Gly-X-Hyp peptides had similar thermal stabilities, only marginally less stable than Gly-Pro-Hyp, wh ereas Gly-Pro-Y peptides showed a wide thermal stability range (T-m = 30-45 degrees C). The stability of peptides with oppositely charged re sidues in the X and Y positions appears to reflect simple additivity o f the individual residues, except when X is occupied by a basic residu e and Y = Asp, The side chains of Glu, Lys, and Arg have the potential to form hydrogen bonds with available peptide backbone carbonyl group s within the triple-helix, whereas the shorter Asp side chain does not , This may relate to the unique involvement of Asp residues in energet ically favorable ion pair formation. These studies clarify the depende nce of triple-helix stability on the identity, position, and ionizatio n state of charged residues.