Gly-Trp-Gly

The prion protein is usually pictured as globular structured C-terminal domain that is linked to an extended flexible N-terminal tail. However, in its physiological form, it is a glycoprotein tethered to the cell surface via a C-terminal GPI anchor. The low solubility of PrP even without GPI anchor and its strong tendency for aggregation has forced most structural investigations to be performed at low pH and mostly with N-terminally truncated variants. In the present study, we have used a synthetic peptide related to the PrP tetra-octarepeat region, i.e., the sequence (Pro-His-Gly-Gly-Gly-Trp-Gly-Gln)(4), for NMR structural analysis of its preferred conformation in DPC micelles as membrane mimic. Well-defined and identical loops are observed between the four octarepeats that are linked by flexible Gly-Gly-Gly sequences. Interaction with the micelles is mainly through the tryptophan residues that appear to act as anchors. Copper binding to the peptide in the presence of DPC micelles revealed marked conformational rearrangements although binding to the micelles is preserved. Interestingly, titration experiments point to cooperative effects for the four binding sites. A destabilization of the DPC micelles by the peptide parallels the destabilizing effect of the prion protein on membranes so that the octarepeat region appears to be very membrane-active. How the physico-chemical properties reported here are linked to the function and significance of the prion protein remains a puzzle as long as the functional mechanism of the prion protein is not precisely elucidated. Nevertheless, our results emphasize the strong influence of the (membrane) environment on the PrP properties.

Cerein 7B is a new bacteriocin produced simultaneously with cerein 7A by Bacillus cereus Bc7 in liquid brain heart infusion cultures. Both bacteriocins are not synergistic. The two peptides have been purified to homogeneity by hydrophobic interaction, cation exchange and reverse-phase liquid chromatography. They can be distinguished by their N-terminal amino acid sequences N-Gly-Trp-Gly-Asp-Val-Leu (7A) and N-Gly-Trp-Trp-Asn-Ser-Trp-Gly-Lys (7B). Pre-cerein 7B is 74 amino acids long and contains an 18 aminoacid double-glycine type leader sequence that is removed to produce the mature bacteriocin. The leader peptide sequence is related to that of sec-independent secretion signals suggesting that cerein 7B belongs to class II sec-independent bacteriocins.

It is well-known that tryptophan residues are efficiently oxidized by peroxyl radicals, generating kynurenine, and N-formyl kynurenine as well as hydroperoxide derivatives as products. In the present work we studied the kinetic of such reaction employing free and peptide-included tryptophan. Two azocompounds were used to produce peroxyl radicals: AAPH (2,2'-Azobis(2-methylpropionamidine) dihydrochloride) and ABCVA (4,4'-Azobis(4-cyanovaleric acid)), which generate cationic and anionic peroxyl radicals, respectively. Tryptophan consumption was assessed by fluorescence spectroscopy and the reactions were carried out in phosphate buffer (75mM, pH 7.4) at 45°C. Only a slight effect of the peroxyl radical charge was evidenced on the consumption of free tryptophan and the dipeptide Gly-Trp. Employing AAPH as peroxyl radical source, at low free tryptophan concentrations (1-10µM) near 0.3 mol of tryptophan were consumed per each mol of peroxyl radicals introduced into the system. However, at high free tryptophan concentrations (100µM-1mM) such stoichiometry increased in a tryptophan concentration-way. At 1mM three moles of tryptophan were consumed per mol of AAPH-derived peroxyl radicals, evidencing the presence of chain reactions. A similar behavior was observed when di and tri-peptides (Gly-Trp, Trp-Gly, Gly-Trp-Gly, Trp-Ala, Ala-Trp-Ala) were studied. Nonetheless, at low initial concentration (5µM), the initial consumption rate of tryptophan included in the peptides was two times higher than free tryptophan. In contrast, at high concentration (1mM) free and peptide-included tryptophan showed similar initial consumption rates. These results could be explained considering a disproportionation process of tryptophanyl radicals at low free tryptophan concentrations, a process that would be inhibited when tryptophan is included in peptides.