title: Helix formation by the phospholipase A2 38-59 fragment: influence of chain shortening and dimerization monitored by nmr chemical shifts.
abstract: The solution structure of a peptide fragment corresponding to the 38-59 region of porcine phospholipase A2 has been investigated using CD, nmr chemical shifts, and nuclear overhauser effects (NOEs). This isolated fragment of phospholipase forms an alpha-helix spanning residues 38-55, very similar to the one found in the native protein, except for residues 56-58, which were helical in the crystal but found random in solution. Addition of triflouroethanol (TFE) merely increased helix population but it did not redefine helix limits. To investigate how the folding information, in particular that concerning eventual helix start and stop signals, was coded in this particular amino acid sequence, the helices formed by synthetic peptides reproducing sections of this phospholipase 38-59 fragment, namely 40-59, 42-59, 38-50, and 45-57, were characterized using NOEs and helix populations quantitatively evaluated on different peptide chain segments using nmr chemical shifts in two solvents (H2O and 30% TFE/H2O). A set of nmr spectra was also recorded and assigned under denaturing conditions (6M urea) to obtain reliable values for the chemical shifts of each peptide in the random state. Based on chemical shift data, it was concluded that the helix formed by the phospholipase 38-59 fragment was not abruptly, but progressively, destabilized all along its length by successive elimination of residues at the N end, while the removal of residues at the C end affected helix stability more locally and to a lesser extent. These results are consistent with the idea that there are not single residues responsible for helix initiation or helix stability, and they also evidence an asymmetry for contributions to helix stability by residues located at the two chain ends. The restriction of molecular mobility caused by linking with a disulphide bridge at Cys 51 two identical 38-59 peptide chains did not increase helix stability. The helix formed by the covalently formed homodimer was very similar in length and population to that formed by the monomer.
authors.affiliation: Instituto de Estructura de la Materia, CSIC, Madrid, Spain.
authors.forename: M A
authors.initials: MA
authors.lastname: Jiménez
authors.forename: C
authors.initials: C
authors.lastname: Carreño
authors.forename: D
authors.initials: D
authors.lastname: Andreu
authors.forename: F J
authors.initials: FJ
authors.lastname: Blanco
authors.forename: J
authors.initials: J
authors.lastname: Herranz
authors.forename: M
authors.initials: M
authors.lastname: Rico
authors.forename: J L
authors.initials: JL
authors.lastname: Nieto
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D009682:Magnetic Resonance Spectroscopy; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000737:chemistry; D010741:Phospholipases A / Q000737:chemistry*; D054467:Phospholipases A2; D017433:Protein Structure, Secondary; D013552:Swine
chemical_list: D010446:Peptide Fragments; D010741:Phospholipases A; D054467:Phospholipases A2