The role of acidic pH in the conversion of human prion protein to the pathogenic isoform is investigated by means of molecular dynamics simulations, focusing the attention on the effect of protonation of histidine residues on the conformational behavior of human PrPC globular domain. Our simulations reveal a significant loss of alpha-helix content under mildly acidic conditions, due to destructuration of the C-terminal part of HB (thus suggesting a possible involvement of HB into the conformational transition leading to the pathogenic isoform) and a transient lengthening of the native beta-sheet. Protonation of His-187 and His-155 seems to be crucial for the onset of the conformational rearrangement. This finding can be related to the existence of a pathogenic mutation, H187R, which is associated with GSS syndrome. Finally, the relevance of our results for the location of a Cu2+-binding pocket in the C-terminal part of the prion is discussed.

Acylpeptide hydrolase (APEH), one of the four members of the prolyl oligopeptidase class, catalyses the removal of N-acylated amino acids from acetylated peptides and it has been postulated to play a key role in protein degradation machinery. Disruption of protein turnover has been established as an effective strategy to down-regulate the ubiquitin-proteasome system (UPS) and as a promising approach in anticancer therapy.Here, we illustrate a new pathway modulating UPS and proteasome activity through inhibition of APEH. To find novel molecules able to down-regulate APEH activity, we screened a set of synthetic peptides, reproducing the reactive-site loop of a known archaeal inhibitor of APEH (SsCEI), and the conjugated linoleic acid (CLA) isomers. A 12-mer SsCEI peptide and the trans10-cis12 isomer of CLA, were identified as specific APEH inhibitors and their effects on cell-based assays were paralleled by a dose-dependent reduction of proteasome activity and the activation of the pro-apoptotic caspase cascade. Moreover, cell treatment with the individual compounds increased the cytoplasm levels of several classic hallmarks of proteasome inhibition, such as NFkappaB, p21, and misfolded or polyubiquitinylated proteins, and additive effects were observed in cells exposed to a combination of both inhibitors without any cytotoxicity. Remarkably, transfection of human bronchial epithelial cells with APEH siRNA, promoted a marked accumulation of a mutant of the cystic fibrosis transmembrane conductance regulator (CFTR), herein used as a model of misfolded protein typically degraded by UPS. Finally, molecular modeling studies, to gain insights into the APEH inhibition by the trans10-cis12 CLA isomer, were performed.Our study supports a previously unrecognized role of APEH as a negative effector of proteasome activity by an unknown mechanism and opens new perspectives for the development of strategies aimed at modulation of cancer progression.

A thorough study of the acid-base behavior of the four histidines and the other titratable residues of the structured domain of human prion protein (125-228) is presented. By using multi-tautomer electrostatic calculations, average titration curves have been built for all titratable residues, using the whole bundles of NMR structures determined at pH 4.5 and 7.0. According to our results, (1) only histidine residues are likely to be involved in the first steps of the pH-driven conformational transition of prion protein; (2) the pK(a)'s of His140 and His177 are approximately 7.0, whereas those of His155 and His187 are < 5.5. 10-ns long molecular dynamics simulations have been performed on five different models, corresponding to the most significant combinations of histidine protonation states. A critical comparison between the available NMR structures and our computational results (1) confirms that His155 and His187 are the residues whose protonation is involved in the conformational rearrangement of huPrP in mildly acidic condition, and (2) shows how their protonation leads to the destructuration of the C-terminal part of HB and to the loss of the last turn of HA that represent the crucial microscopic steps of the rearrangement.

No abstract

The conformational and spectroscopic properties of the tyrosyl radical dipeptide analogue (T(R)DA) are investigated both in gas phase and in aqueous solution by means of density functional calculations. Electronic interactions between backbone and side chain, determining the relative stability of the different energy minimums, depend on the electronic state of the phenoxy substituent. As a consequence, (i) the conformational behavior of T(R)DA is quite different from that of the tyrosine dipeptide analogue, and (ii) the energy required for the homolytic breaking of the OH bond depends on the adopted conformation. The calculated hyperfine coupling constants are in good agreement with the available experimental results. Side-chain-backbone interactions cause an asymmetrization of the magnetic properties of the phenoxy ring and deviations from McConnell relationship. Solvent effects, taken into account by means of a combined discrete/continuum model, significantly affect both the conformational and the magnetic behavior of T(R)DA.

Carbonic anhydrases (CAs) III and VII are two cytosolic isoforms of the α-CA family which catalyze the physiological reaction of carbon dioxide hydration to bicarbonate and proton. Despite these two enzymes share a 49% sequence identity and present a very similar three-dimensional structure, they show profound differences when comparing the specific activity for CO2 hydration reaction, with CA VII being much more active than CA III. Recently, CA III and CA VII have been proposed to play a new role as scavenger enzymes in cells where oxidative damage occurs. Here, we will examine functional and structural features of these two isoforms giving insights into their newly proposed protective role against oxidative stress.

Carboxylates are the least investigated class of inhibitors of carbonic anhydrases (CAs). Here we explain the versatility of binding of these molecules to CAs by examining a new adduct of hCA II with N-carboxymethyl-saccharin.

Prosystemin, originally isolated from Lycopersicon esculentum, is a tomato pro-hormone of 200 aminoacid residues which releases a bioactive peptide of 18 aminoacids called Systemin. This signaling peptide is involved in the activation of defense genes in solanaceous plants in response to herbivore feeding damage. Using biochemical, biophysical and bioinformatics approaches we characterized Prosystemin, showing that it is an intrinsically disordered protein possessing a few secondary structure elements within the sequence. Plant treatment with recombinant Prosystemin promotes early and late plant defense genes, which limit the development and survival of Spodoptera littoralis larvae fed with treated plants.