Miguel Machuqueiro

Zn-Proline catalyzed direct aldol reaction in aqueous mediaElectronic supplementary information (ESI) available: experimental details. See http://www.rsc.org/suppdata/cc/b3/b301117h/

Zn complexes of proline, lysine and arginine are efficient catalysts for the aldol addition of p-nitrobenzaldehyde and acetone in aqueous medium, giving quantitative yields and enantiomeric excesses up to 56% with 5 mol% of the catalysts at room temperature.

Constant-pH molecular dynamics simulations reveal a β-rich form of the human prion protein.

The misfolding of the prion protein (PrP) into a pathogenic β-rich form (PrP(Sc)) has been suggested to occur in the endocytic pathway, triggered by low pH. In this work we performed several constant-pH molecular dynamics simulations of human PrP 90-231 in the pH range 2-7, totaling more than 2 μs. We observed a strong conformational pH dependence where on average the helix content decreased and the β content increased toward acidic pH. Unlike some proposed models, the flexible N-terminus region did not gain stable structure at low pH. Rather, the main structural changes occurred on the helix-rich C-terminus core, as proposed in other models, namely, in the regions around 135-155 and 185-200. The protonation of His187 is found to be associated with a loss of interaction between two PrP subdomains, potentially playing a major role in the misfolding process. In one of the simulations at pH 2, a stable β-rich structure was formed that may be an intermediate of PrP(Sc) formation, indicating that misfolding may precede dimerization.

Acidic range titration of HEWL using a constant‐pH molecular dynamics method

In this work, we present the first application to a protein of the stochastic constant‐pH molecular dynamics (MD) method with the inclusion of proton tautomerism. The acidic titration of HEWL was performed under different conditions. Both generalized reaction field (GRF) and particle mesh Ewald (PME) methods were used in the treatment of the long range electrostatics and, even though the PME simulations revealed to be more stable, the better results were obtained using GRF (pKa RMSD of 0.82 for GRF and 1.13 for PME). The results using PME at different dielectric constants (2, 4, and 8) also revealed that there was no significant improvement in pKas prediction upon increasing the dielectric constant. The secondary structure analysis of HEWL revealed a remarkably stable protein in the acidic pH range. The β‐sheet strands (unlike the α‐helices) seem to be destabilized upon pH decrease, suggesting that the β‐domain is less stable than the α‐domain. The four principal α‐helices were also ordered according to their stability in the acidic pH range and the results (4 < 1 < 2 ≈ 3) were consistent with the ones obtained in thermal denaturation studies. Proteins 2008.

Zinc–proline catalyzed pathway for the formation of sugars

Zn-proline catalyzes the aldolisation of unprotected glycolaldehyde in water to give tetroses and hexoses; threose (33% of the product mixture) was formed with 10% enantiomeric excess of the D-isomer.

Molecular Dynamics at Constant pH and Reduction Potential: Application to Cytochrome c3

Here we present a new implementation and extension of the stochastic titration method which makes it possible to perform MD simulations at constant pH and reduction potential. The method was applied to the redox titration of cytochrome c(3) from Desulfovibrio vulgaris Hildenborough, and a major finding of this study was that the method showed a better performance when the protein region is assigned a high dielectric constant. This dependence on the value of the protein dielectric constant was not found in previous constant-pH MD simulations and is attributed to excessively high heme-heme interactions at low dielectric constants. The simulations revealed strong coupling between hemes in close proximity, and we also showed how these couplings can be used to estimate the sensibility of the heme reductions to small pH changes.

Is the prediction of pKa values by constant‐pH molecular dynamics being hindered by inherited problems?

In this study, we investigate two factors that can hinder the performance of constant‐pH molecular dynamics methods in predicting protein pKa values, using hen egg white lysozyme as a test system. The first factor is related to the molecular definition and pKa value of model compounds in the Poisson‐Boltzmann framework. We address this by defining the model compound as a molecular fragment with an associated pKa value that is calibrated against experimental data, which results in a decrease of 0.12 units in pKa errors. The second addressed factor is the possibility that detrimental structural distortions are being introduced in the simulations by the underlying molecular mechanics force field. This issue is investigated by analyzing how the gradual structural rearrangements affect the predicted pKa values. The two GROMOS force fields studied here (43A1 and 53A6) yield good pKa predictions, although a time‐dependent performance is observed: 43A1 performs better after a few nanoseconds of structural reorganization (pKa errors of ∼0.45), while 53A6 gives the best prediction right at the first nanosecond (pKa errors of 0.42). These results suggest that the good performance of constant‐pH molecular dynamics methods could be further improved if these force field limitations were overcome. Proteins 2011;.

Reversibility of prion misfolding: insights from constant-pH molecular dynamics simulations.

The prion protein (PrP) is the cause of a group of diseases known as transmissible spongiform encephalopathies (TSEs). Creutzfeldt-Jakob disease and bovine spongiform encephalopathy are examples of TSEs. Although the normal form of PrP (PrP(C)) is monomeric and soluble, it can misfold into a pathogenic form (PrP(Sc)) that has a high content of β-structure and can aggregate forming amyloid fibrils. The mechanism of conversion of PrP(C) into PrP(Sc) is not known but different triggers have been proposed. It can be catalyzed by a PrP(Sc) sample, or it can be induced by an external factor, such as low pH. The pH effect on the structure of PrP was recently studied by computational methods [Campos et al. J. Phys. Chem. B 2010, 114, 12692-12700], and an evident trend of loss of helical structure was observed with pH decrease, together with a gain of β-structures. In particular, one simulation at pH 2 showed an evident misfolding transition. The main goal of the present work was to study the effects of a change in pH to 7 in several transient conformations of this simulation, in order to draw some conclusions about the reversibility of PrP misfolding. Although the most significant effect caused by the change of pH to 7 was a global stabilization of the protein structure, we could also observe that some conformational transitions induced by pH 2 were reversible in many of our simulations, namely those started from the early moments of the misfolding transition. This observation is in good agreement with experiments showing that, even at pH as low as 1.7, it is possible to revert the misfolding process [Bjorndahl et al. Biochemistry 2011, 50, 1162-1173].

A Simulated Intermediate State for Folding and Aggregation Provides Insights into ΔN6 β2-Microglobulin Amyloidogenic Behavior

A major component of ex vivo amyloid plaques of patients with dialysis-related amyloidosis (DRA) is a cleaved variant of β2-microglobulin (ΔN6) lacking the first six N-terminal residues. Here we perform a computational study on ΔN6, which provides clues to understand the amyloidogenicity of the full-length β2-microglobulin. Contrary to the wild-type form, ΔN6 is able to efficiently nucleate fibrillogenesis in vitro at physiological pH. This behavior is enhanced by a mild acidification of the medium such as that occurring in the synovial fluid of DRA patients. Results reported in this work, based on molecular simulations, indicate that deletion of the N-terminal hexapeptide triggers the formation of an intermediate state for folding and aggregation with an unstructured strand A and a native-like core. Strand A plays a pivotal role in aggregation by acting as a sticky hook in dimer assembly. This study further predicts that the detachment of strand A from the core is maximized at pH 6.2 resulting into higher aggregation efficiency. The structural mapping of the dimerization interface suggests that Tyr10, His13, Phe30 and His84 are hot-spot residues in ΔN6 amyloidogenesis.

Conformational Study of GSH and GSSG Using Constant-pH Molecular Dynamics Simulations

Glutathione is a small peptide with a crucial role in living organisms. This molecule is found in Nature in both reduced (GSH) and oxidized (GSSG) forms and a high GSH/GSSG ratio is essential to the cell. Glutathione is also present in several enzymatic reactions and can be found in many protein structures. As small peptides, these molecules do not have a defined structure in solution and are able to sample a broad conformational space. In addition, both molecules have several titration sites (four in GSH and six in GSSG) and their conformational space is inevitably influenced by pH. Here, we present a detailed conformational study of GSH and GSSG in a range of pH values, together with a full pH titration of these molecules. We performed constant-pH MD simulations of GSH and GSSG at 24 pH values in a total of 14.4 μs (300 ns per pH value). We obtained the two titration curves and the pKa values for all titrable groups with good agreement with experimental data. We also observed that GSH and GSSG have a large conformational variability in solution and their structural preferences are not significantly affected upon binding to proteins. Some exceptions were found and investigated in detail.

Protonation of DMPC in a Bilayer Environment Using a Linear Response Approximation

pH is a very important property, influencing all important biomolecules such as proteins, nucleic acids, and lipids. The effect of pH on proteins has been the subject of many computational works in recent years. However, the same has not been done for lipids, especially in their most biologically relevant environment: the bilayer. A reason for this is the inherent technical difficulty in dealing with this type of periodic systems. Here, we tackle this problem by developing a Poisson-Boltzmann-based method that takes in consideration the periodic boundary conditions of lipid bilayer patches. We used this approach with a linear response approximation to calculate the pKa value of a DMPC molecule when diluted in zwitterionic lipids. Our results show that DMPC protonation only becomes relevant at quite low pH values (2-3). However, when it happens, it has a strong impact on lipid conformations, leading to significant heterogeneity in the membrane.