X-RAY CRYSTALLOGRAPHIC DETERMINATION OF A COLLAGEN-LIKE PEPTIDE WITH THE REPEATING SEQUENCE (PRO-PRO-GLY)
PEPTIDE WAS CRYSTALLIZED FROM 4.0 MG/ML PEPTIDE IN 10% ACETIC ACID, 0.1% SODIUM AZIDE, AND 3.0% PEG400.
HYDROGEN BONDS BETWEEN PEPTIDE CHAINS FOLLOW THE RICH AND CRICK MODEL II FOR COLLAGEN.
FOR EACH CHAIN, RESIDUE NUMBERING CORRESPONDS TO THE ENTIRE MOLECULE RATHER THAN THE SHORTER ASYMMETRIC UNIT.
DUE TO THE QUASI-INFINITE, AVERAGED NATURE OF THE TRIPLE HELIX, DURING REFINEMENT COVALENT BONDS ARE NECESSARY TO JOIN THE MOLECULE WITH ITS SYMMETRY MATES BOTH ABOVE IT AND BELOW IT ALONG THE HELICAL AXIS AND TIGHT REFINEMENT CONSTRAINTS WERE MAINTAINED. THE UNIT CELL AXES WERE CHOSEN TO COINCIDE WITH A PREVIOUS STRUCTURE DETERMINATION (OKUYAMA 1981) OF THIS PEPTIDE.
THE 21 RESIDUE ASYMMETRIC UNIT CORRESPONDS TO ONE TRIPLE-HELICAL REPEAT AND IS SMALLER THAN THE ENTIRE 90 RESIDUE PEPTIDE DUE TO TRANSLATIONAL DISORDER ALONG THE HELICAL AXIS. THE RESULT IS A POLYMER-LIKE STRUCTURE WITH NO DEFINED ENDS. THE POLYMER STRUCTURE IS FORMED BY CONTINUATION OF THE CHAINS USING THE SYMMETRY-RELATED MOLECULES ALONG THE HELICAL AXIS. THE TVECT RECORD BELOW PRESENTS THE TRANSLATION THAT WILL GENERATE THE POLYMER. NOTE: THEREFORE, CLOSE CONTACTS BETWEEN SYMMETRY-RELATED MOLECULES ARE INTENTIONAL AND NECESSARY. INTERCHAIN HYDROGEN BONDING AT THE END OF CHAINS ALSO UTILIZES SYMMETRY-RELATED MOLECULES. THE ENTIRE 30 RESIDUE LONG PEPTIDE CAN BE GENERATED FROM THE SUBMITTED ASYMMETRIC UNIT BY APPLYING THE FOLLOWING TRANSLATIONS (USING FRACTIONAL COORDINATES): CHAIN A: TRANSLATE RESIDUES 1 - 9 BY (0 0 1), (0 0 2), AND (0 0 3) AND RESIDUES 7 - 9 BY (0 0 4). CHAIN B: TRANSLATE RESIDUES 31 - 36 BY (0 0 1), (0 0 2), AND (0 0 3). CHAIN C: TRANSLATE RESIDUES 61 - 66 BY (0 0 1), (0 0 2), AND (0 0 3) AND RESIDUES 64 - 66 BY (004). THIS WILL RESULT IN A MOLECULE WITH A TOTAL OF 90 RESIDUES, 30 IN EACH CHAIN.