Methionine Oxidation Changes the Mechanism of Aβ Peptide Binding to the DMPC Bilayer

Methionine Alkylation as an Approach to Quantify Methionine Oxidation Using Mass Spectrometry

Post-translational oxidation of methionine residues can destabilize proteins or modify their functions. Although levels of methionine oxidation can provide important information regarding the structural integrity and regulation of proteins, their quantitation is often challenging as analytical procedures in and of themselves can artifactually oxidize methionines. Here, we develop a mass-spectrometry-based method called Methionine Oxidation by Blocking with Alkylation (MObBa) that quantifies methionine oxidation by selectively alkylating and blocking unoxidized methionines. Thus, alkylated methionines can be used as a stable proxy for unoxidized methionines. Using proof of concept experiments, we demonstrate that MObBa can be used to measure methionine oxidation levels within individual synthetic peptides and on proteome-wide scales. MObBa may provide a straightforward experimental strategy for mass spectrometric quantitation of methionine oxidation.

Replica Exchange with Hybrid Tempering Efficiently Samples PGLa Peptide Binding to Anionic Bilayer

We evaluated the utility of a variant of the replica exchange method, a replica exchange with hybrid tempering (REHT), for all-atom explicit water biomolecular simulations and compared it with a more traditional replica exchange with the solute tempering (REST) algorithm. As a test system, we selected a 21-mer antimicrobial peptide PGLa binding to an anionic DMPC/DMPG lipid bilayer. Application of REHT revealed the following binding mechanism. Due to the strong hydrophobic moment, the bound PGLa adopts an extensive helical structure. The binding free energy landscape identifies two major bound states, a metastable surface bound state and a dominant inserted state. In both states, positively charged PGLa amino acids maintain electrostatic interactions with anionic phosphate groups by rotating the PGLa helix around its axis. PGLa binding causes an influx of anionic DMPG and an efflux of zwitterionic DMPC lipids from the peptide proximity. PGLa thins the bilayer and disorders the adjacent fatty acid tails. Deep invasion of water wires into the bilayer hydrophobic core is detected in the inserted peptide state. The analysis of charge density distributions indicated that peptide positive charges are nearly compensated for by lipid negative charges and water dipole ordering, whereas ions play no role in peptide binding. Thus, electrostatic interactions are the key energetic factor in binding cationic PGLa to an anionic DMPC/DMPG bilayer. Comparison of REHT and REST shows that due to exclusion of lipids from tempered partition, REST lags behind REHT in peptide equilibration, particularly, with respect to peptide insertion and helix acquisition. As a result, REST struggles to provide accurate details of PGLa binding, although it still qualitatively maps the bimodal binding mechanism. Importantly, REHT not only equilibrates PGLa in the bilayer faster than REST, but also with less computational effort. We conclude that REHT is a preferable choice for studying interfacial biomolecular systems.

Lysine Acetylation Changes the Mechanism of Aβ25-35 Peptide Binding and Dimerization in the DMPC Bilayer

The impact of Lys28 acetylation on Alzheimer's Aβ peptide binding to the lipid bilayer has not been previously studied, either experimentally or computationally. To probe this common post-translational modification, we performed all-atom replica exchange molecular dynamics simulations targeting binding and aggregation of acetylated acAβ25-35 peptide within the DMPC bilayer. Using the unmodified Aβ25-35 studied previously as a reference, our results can be summarized as follows. First, Lys28 acetylation strengthens the Aβ25-35 hydrophobic moment and consequently promotes the helical structure across the peptide extending it into the N-terminus. Second, because Lys28 acetylation disrupts electrostatic contact between Lys28 and lipid phosphate groups, it reduces the binding affinity of acAβ25-35 peptides to the DMPC bilayer. Accordingly, although acetylation preserves the bimodal binding featuring a preferred inserted state and a less probable surface bound state, it decreases the stability of the former. Third, acetylation promotes acAβ25-35 aggregation and eliminates monomers as thermodynamically viable species. More importantly, acAβ25-35 retains as the most thermodynamically stable the inserted dimer with unique head-to-tail helical aggregation interface. However, due to enhanced helix structure, this dimer state becomes less stable and is less likely to propagate into higher order aggregates. Thus, acetylation is predicted to facilitate the formation of low-molecular-weight oligomers. Other post-translational modifications, including phosphorylation and oxidation, reduce helical propensity and have divergent impact on aggregation. Consequently, acetylation, when considered in its totality, has distinct consequences on Aβ25-35 binding and aggregation in the lipid bilayer.

Met35 Oxidation Hinders Aβ25-35 Peptide Aggregation within the Dimyristoylphosphatidylcholine Bilayer

Using all-atom explicit solvent replica exchange molecular dynamics simulations, we studied the aggregation of oxidized (ox) Aβ25-35 peptides into dimers mediated by the zwitterionic dimyristoylphosphatidylcholine (DMPC) lipid bilayer. By comparing oxAβ25-35 aggregation with that observed for reduced and phosphorylated Aβ25-35 peptides, we elucidated plausible impact of post-translational modifications on cytotoxicity of Aβ peptides involved in Alzheimer's disease. We found that Met35 oxidation reduces helical propensity in oxAβ25-35 peptides bound to the lipid bilayer and enhances backbone fluctuations. These factors destabilize the wild-type head-to-tail dimer interface and lower the aggregation propensity. Met35 oxidation diversifies aggregation pathways by adding monomeric species to the bound conformational ensemble. The oxAβ25-35 dimer becomes partially expelled from the DMPC bilayer and as a result inflicts limited disruption to the bilayer structure compared to wild-type Aβ25-35. Interestingly, the effect of Ser26 phosphorylation is largely opposite, as it preserves the wild-type head-to-tail aggregation interface and strengthens, not weakens, aggregation propensity. The differing effects can be attributed to the sequence locations of these post-translational modifications, since in contrast to Ser26 phosphorylation, Met35 oxidation directly affects the wild-type C-terminal aggregation interface. A comparison with experimental data is provided.

A Fungal Secretome Adapted for Stress Enabled a Radical Wood Decay Mechanism

Brown rot fungi release massive amounts of carbon from forest deadwood, particularly at high latitudes. These fungi degrade wood by generating small reactive oxygen species (ROS) to loosen lignocellulose, to then selectively remove carbohydrates. The ROS mechanism has long been considered the key adaptation defining brown rot wood decomposition, but recently, we found preliminary evidence that fungal glycoside hydrolases (GHs) implicated in early cell wall loosening might have been adapted to tolerate ROS stress and to synergize with ROS to loosen woody lignocellulose. In the current study, we found more specifically that side chain hemicellulases that help in the early deconstruction of the lignocellulosic complex are significantly more tolerant of ROS in the brown rot fungus Rhodonia placenta than in a white rot fungus (Trametes versicolor) and a soft rot fungus (Trichoderma reesei). Using proteomics to understand the extent of tolerance, we found that significant oxidation of secreted R. placenta proteins exposed to ROS was less than half of the oxidation observed for T. versicolor or T. reesei. The principal oxidative modifications observed in all cases were monooxidation and dioxidation/trioxidation (mainly in methionine and tryptophan residues), some of which were critical for enzyme activity. At the peptide level, we found that GHs in R. placenta were the least ROS affected among our tested fungi. These results confirm and describe underlying mechanisms of tolerance in early-secreted brown rot fungal hemicellulases. These enzymatic adaptations may have been as important as nonenzymatic ROS pathway adaptations in brown rot fungal evolution.

Peripheral Methionine Residues Impact Flavin Photoreduction and Protonation in an Engineered LOV Domain Light Sensor

Proton-coupled electron transfer reactions play critical roles in many aspects of sensory phototransduction. In the case of flavoprotein light sensors, reductive quenching of flavin excited states initiates chemical and conformational changes that ultimately transmit light signals to downstream targets. These reactions generally require neighboring aromatic residues and proton-donating side chains for rapid and coordinated electron and proton transfer to flavin. Although photoreduction of flavoproteins can produce either the anionic (ASQ) or neutral semiquinone (NSQ), the factors that favor one over the other are not well understood. Here we employ a biologically active variant of the light-oxygen-voltage (LOV) domain protein VVD devoid of the adduct-forming Cys residue (VVD-III) to probe the mechanism of flavin photoreduction and protonation. A series of isosteric and conservative residue replacements studied by rate measurements, fluorescence quantum yields, FTIR difference spectroscopy, and molecular dynamics simulations indicate that tyrosine residues facilitate charge recombination reactions that limit sustained flavin reduction, whereas methionine residues facilitate radical propagation and quenching and also gate solvent access for flavin protonation. Replacement of a single surface Met residue with Leu favors formation of the ASQ over the NSQ and desensitizes photoreduction to oxidants. In contrast, increasing site hydrophilicity by Gln substitution promotes rapid NSQ formation and weakens the influence of the redox environment. Overall, the photoreactivity of VVD-III can be understood in terms of redundant electron donors, internal hole quenching, and coupled proton transfer reactions that all depend upon protein conformation, dynamics, and solvent penetration.

Partitioning of Aβ Peptide Fragments into Blood-Brain Barrier Mimetic Bilayer

We used all-atom replica-exchange umbrella sampling molecular dynamics simulations to investigate the partitioning of the charged tetrapeptide KLVF and its neutral apolar counterpart VVIA into the blood-brain barrier (BBB)-mimetic bilayer. Our findings allowed us to reconstruct the partitioning mechanism for these two Aβ peptide fragments. Despite dissimilar sequences, their permeation shares significant common features. Computations of free energies and permeabilities show that partitioning of both peptides is highly unfavorable, ruling out passive transport. The peptides experience multiple rotational transitions within the bilayer and typically cause considerable lipid disorder and bilayer thinning. Near the bilayer midplane, they lose almost entirely their solvation shell and the interactions with the lipid headgroups. The peptides cause complex reorganization within the proximal bilayer region. Upon insertion, they induce striking cholesterol influx reversed by its depletion and the influx of DMPC when the peptides reach the midplane. The differences in partitioning mechanisms are due to the much higher polarity of KLVF peptide, the permeation of which is more unfavorable and which exclusively assumes vertical orientations within the bilayer. In contrast, VVIA positions itself flat between the leaflets, causing minor disorder and even thickening of the BBB-mimetic bilayer. Due to the high density of the cholesterol-rich BBB bilayer, the unfavorable work associated with the peptide insertion provides a significant, but not dominant, contribution to the partition free energy, which is still governed by dehydration and loss of peptide-headgroup interactions. Comparison with experiments indicates that KLVF and VVIA permeation is similar to that of proline tetrapeptide, mannitol, or cimetidine, all of which exhibit no passive transport.

Partitioning of Benzoic Acid into 1,2-Dimyristoyl-sn-glycero-3-phosphocholine and Blood-Brain Barrier Mimetic Bilayers

Using an all-atom explicit water model and replica exchange umbrella sampling simulations, we investigated the molecular mechanisms of benzoic acid partitioning into two model lipid bilayers. The first was formed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipids, whereas the second was composed of an equimolar mixture of DMPC, 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine, palmitoylsphingomyelin, and cholesterol to constitute a blood-brain barrier (BBB) mimetic bilayer. Comparative analysis of benzoic acid partitioning into the two bilayers has revealed qualitative similarities. Partitioning into the DMPC and BBB bilayers is thermodynamically favorable although insertion into the former lowers the free energy of benzoic acid by approximately an additional 1 kcal mol^-1. The partitioning energetics for the two bilayers is also largely similar based on the balance of benzoic acid interactions with apolar fatty acid tails, polar lipid headgroups, and water. In both bilayers, benzoic acid retains a considerable number of residual water molecules until reaching the bilayer midplane where it experiences nearly complete dehydration. Upon insertion into the bilayers, benzoic acid undergoes several rotations primarily determined by the interactions with the lipid headgroups. Nonetheless, in addition to the depth of the free energy minimum, the BBB bilayer differs from the DMPC counterpart by a much deeper location of the free energy minimum and the appearance of a high free energy barrier and positioning of benzoic acid near the midplane. Furthermore, DMPC and BBB bilayers exhibit different structural responses to benzoic acid insertion. Taken together, the BBB mimetic bilayer is preferable for an accurate description of benzoic acid partitioning.

Autoinhibition by Iodide Ion in the Methionine-Iodine Reaction

The methionine–iodine reaction was reinvestigated spectrophotometrically in detail monitoring the absorbance belonging to the isosbestic point of iodine at 468 nm, at T = 25.0 ± 0.1 °C, and at 0.5 M ionic strength in buffered acidic medium. The stoichiometric ratio of the reactants was determined to be 1:1 producing methionine sulfoxide as the lone sulfur-containing product. The direct reaction between methionine and iodine was found to be relatively rapid in the absence of initially added iodide ion, and it can conveniently be followed by the stopped-flow technique. Reduction of iodine eventually leads to the formation of iodide ion that inhibits the reaction making the whole system autoinhibitory with respect to the halide ion. We have also shown that this inhibitory effect appears quite prominently, and addition of iodide ion in the millimole concentration range may result in a rate law where the formal kinetic order of this species becomes −2. In contrast to this, hydrogen ion has just a mildly inhibitory effect giving rise to the fact that iodine is the kinetically active species in the system but not hypoiodous acid. The surprisingly complex kinetics of this simple reaction may readily be interpreted via the initiating rapidly established iodonium-transfer process between the reactants followed by the subsequent hydrolytic decomposition of the short-lived iodinated methionine. A seven-step kinetic model to be able to describe the most important characteristics of the measured kinetic curves is established and discussed in detail.

Do Cholesterol and Sphingomyelin Change the Mechanism of Aβ25-35 Peptide Binding to Zwitterionic Bilayer?

Using replica exchange with solute tempering all-atom molecular dynamics, we studied the equilibrium binding of Aβ_25-35 peptide to the ternary bilayer composed of an equimolar mixture of dimyristoylphosphatidylcholine (DMPC), N-palmitoylsphingomyelin (PSM), and cholesterol. Binding of the same peptide to the pure DMPC bilayer served as a control. Due to significant C-terminal hydrophobic moment, binding to the ternary and DMPC bilayers promotes helical structure in the peptide. For both bilayers a polarized binding profile is observed, in which the N-terminus anchors to the bilayer surface, whereas the C-terminus alternates between unbound and inserted states. Both ternary and DMPC bilayers feature two Aβ_25-35 bound states, surface bound, S, and inserted, I, separated by modest free energy barriers. Experimental data are in agreement with our results but indicate that cholesterol impact is Aβ fragment dependent. For Aβ_25-35, we predict that its binding mechanism is independent of the inclusion of PSM and cholesterol into the bilayer.