title: Bicyclic peptides as models of calcium binding sites: synthesis and conformation of a homodetic undecapeptide.
abstract: A bicyclic undecapeptide of sequence cyclo-(Ala(1)-Pro(2)-Asp(3)-Glu(4)-Lys(5)-Ala(6)-Pro(7)-Asp(8)-Ser(9) -Glu(10))-cyclo-(10gamma --> 5varepsilon)-Gly(11), designed to mimic the calcium coordination site I of Calmodulin, has been synthesized and its conformation and calcium binding properties have been investigated by means of CD and nmr spectroscopy. The nmr analysis of the free peptide, carried out in DMSO and in TFE/H(2)O at different pH values, shows the presence in solution of one stable conformer, exhibiting trans configuration around both Proline residues. The nmr results in both solvents suggest for the molecule a rectangular shape constituted by two antiparallel beta-strands connected by two beta-turns. Interproton distances, evaluated by NOE contacts, have been used to obtain feasible models by means of Restrained Molecular Dynamic (RMD). The average models from RMD calculations, for both solvents, exhibit good analogies with Calmodulin site I. The model system, when compared with the reference system (Asp(20)-Glu(31) segment in CaM), shows similar dimensions and an effective superimposition of the respective sequence segments Ala(1)-Glu(4) and Thr(28)-Glu(31). The remaining segments of the model peptide exhibit a bending that is intermediate between that of the free and Ca(2+)-coordinated site I. CD spectra, recorded in TFE solutions, point to a 1:1 stoichiometry for the Ca(2+)-peptide complex, with an association constant of at least 1 x 10(5) M(-1).
authors.affiliation: Dipartimento di Chimica, Universitâ di Napoli "Federico II", Naples 80134, Italy.
authors.forename: R
authors.initials: R
authors.lastname: Oliva
authors.forename: L
authors.initials: L
authors.lastname: Falcigno
authors.forename: G
authors.initials: G
authors.lastname: D'Auria
authors.forename: M
authors.initials: M
authors.lastname: Saviano
authors.forename: L
authors.initials: L
authors.lastname: Paolillo
authors.forename: G
authors.initials: G
authors.lastname: Ansanelli
authors.forename: G
authors.initials: G
authors.lastname: Zanotti
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D002147:Calmodulin / Q000737:chemistry*; D002942:Circular Dichroism; D007202:Indicators and Reagents; D008958:Models, Molecular; D019906:Nuclear Magnetic Resonance, Biomolecular; D009842:Oligopeptides / Q000138:chemical synthesis / Q000737:chemistry*; D010456:Peptides, Cyclic / Q000138:chemical synthesis / Q000737:chemistry*; D011487:Protein Conformation
chemical_list: D002147:Calmodulin; D007202:Indicators and Reagents; D009842:Oligopeptides; D010456:Peptides, Cyclic; D002118:Calcium