Redox Potentials of Protein Disulfide Bonds from Free-Energy Calculations

Oxidative Folding Catalysts of Conotoxins Derived from the Venom Duct Transcriptome of C. frigidus and C. amadis

Two novel redox conopeptides with proline residues outside and within the active site disulfide loop were derived from the venom duct transcriptome of the marine cone snails Conus frigidus and Conus amadis. Mature peptides with possible post-translational modification of 4-trans-hydroxylation of proline, namely, Fr874, Fr890[P1O], Fr890[P2O], Fr906, Am1038, and Am1054, have been chemically synthesized and characterized using mass spectrometry. The estimated reduction potential of cysteine disulfides of synthetic peptides varied from -298 to -328 mV, similar to the active site cysteine disulfide motifs of the redox family of proteins. Fr906/Am1054 exhibited pronounced catalytic activity and assisted in improving the yields of natively folded globular form α-conotoxin ImI. Three-dimensional (3D) structures of the redox conopeptides were optimized using computational methods and verified by 2D-ROESY NMR spectroscopy: C. frigidus peptides adopt an N-terminal helical fold and C. amadis peptides adopt distinct structures based on the Phe4-Pro/Hyp5 peptide bond configuration. The shift in the cis-trans configuration of the Phe4-Pro/Hyp5 peptide bond of Am1038/Am1054 was observed between reduced free thiol and oxidized disulfide forms of the optimized peptides. The report confirms the position-specific effect of hydroxyproline on the oxidative folding of conotoxins and sequence diversity of redox conopeptides in the venom duct of cone snails.

Assessing the Performance of Non-Equilibrium Thermodynamic Integration in Flavodoxin Redox Potential Estimation

Flavodoxins are enzymes that contain the redox-active flavin mononucleotide (FMN) cofactor and play a crucial role in numerous biological processes, including energy conversion and electron transfer. Since the redox characteristics of flavodoxins are significantly impacted by the molecular environment of the FMN cofactor, the evaluation of the interplay between the redox properties of the flavin cofactor and its molecular surroundings in flavoproteins is a critical area of investigation for both fundamental research and technological advancements, as the electrochemical tuning of flavoproteins is necessary for optimal interaction with redox acceptor or donor molecules. In order to facilitate the rational design of biomolecular devices, it is imperative to have access to computational tools that can accurately predict the redox potential of both natural and artificial flavoproteins. In this study, we have investigated the feasibility of using non-equilibrium thermodynamic integration protocols to reliably predict the redox potential of flavodoxins. Using as a test set the wild-type flavodoxin from Clostridium Beijerinckii and eight experimentally characterized single-point mutants, we have computed their redox potential. Our results show that 75% (6 out of 8) of the calculated reaction free energies are within 1 kcal/mol of the experimental values, and none exceed an error of 2 kcal/mol, confirming that non-equilibrium thermodynamic integration is a trustworthy tool for the quantitative estimation of the redox potential of this biologically and technologically significant class of enzymes.

Cysteine Oxidation in Proteins: Structure, Biophysics, and Simulation

Cysteine side chains can exist in distinct oxidation states depending on the pH and redox potential of the environment, and cysteine oxidation plays important yet complex regulatory roles. Compared with the effects of post-translational modifications such as phosphorylation, the effects of oxidation of cysteine to sulfenic, sulfinic, and sulfonic acid on protein structure and function remain relatively poorly characterized. We present an analysis of the role of cysteine reactivity as a regulatory factor in proteins, emphasizing the interplay between electrostatics and redox potential as key determinants of the resulting oxidation state. A review of current computational approaches suggests underdeveloped areas of research for studying cysteine reactivity through molecular simulations.

Identification of the Shigella flexneri Wzy Domain Modulating WzzpHS-2 Interaction and Detection of the Wzy/Wzz/Oag Complex

Shigella flexneri implements the Wzy-dependent pathway to biosynthesize the O antigen (Oag) component of its surface lipopolysaccharide. The inner membrane polymerase Wzy_SF catalyzes the repeat addition of undecaprenol-diphosphate-linked Oag (Und-PP-RUs) to produce a polysaccharide, the length of which is tightly regulated by two competing copolymerase proteins, Wzz_SF (short-type Oag; 10 to 17 RUs) and Wzz_pHS-2 (very-long-type Oag; >90 RUs). The nature of the interaction between Wzy_SF and Wzz_SF/Wzz_pHS-2 in Oag polymerization remains poorly characterized, with the majority of the literature characterizing the individual protein constituents of the Wzy-dependent pathway. Here, we report instead a major investigation into the specific binding interactions of Wzy_SF with its copolymerase counterparts. For the first time, a region of Wzy_SF that forms a unique binding site for Wzz_pHS-2 has been identified. Specifically, this work has elucidated key Wzy_SF moieties at the N- and C-terminal domains (NTD and CTD) that form an intramolecular pocket modulating the Wzz_pHS-2 interaction. Novel copurification data highlight that disruption of residues within this NTD-CTD pocket impairs the interaction with Wzz_pHS-2 without affecting Wzz_SF binding, thereby specifically disrupting polymerization of longer polysaccharide chains. This study provides a novel understanding of the molecular interaction of Wzy_SF with Wzz_SF/Wzz_pHS-2 in the Wzy-dependent pathway and, furthermore, detects the Wzy/Wzz/Und-PP-Oag complex for the first time. Beyond S. flexneri, this work may be extended to provide insight into the interactions between protein homologues expressed by related species, especially members of Enterobacteriaceae, that produce dual Oag chain length determinants. IMPORTANCE Shigella flexneri is a pathogen causing significant morbidity and mortality, predominantly devastating the pediatric age group in developing countries. A major virulence factor contributing to S. flexneri pathogenesis is its surface lipopolysaccharide, which is comprised of three domains: lipid A, core oligosaccharide, and O antigen (Oag). The Wzy-dependent pathway is the most common biosynthetic mechanism implemented for Oag biosynthesis by Gram-negative bacteria, including S. flexneri. The nature of the interaction between the polymerase, Wzy_SF, and the polysaccharide copolymerases, Wzz_SF and Wzz_pHS-2, in Oag polymerization is poorly characterized. This study investigates the molecular interplay between Wzy_SF and its copolymerases, deciphering key interactions in the Wzy-dependent pathway that may be extended beyond S. flexneri, providing insight into Oag biosynthesis in Gram-negative bacteria.

Molecular interpretation of single-molecule force spectroscopy experiments with computational approaches

Single molecule force-spectroscopy techniques have granted access to unprecedented molecular-scale details about biochemical and biological mechanisms. However, the interpretation of the experimental data is often challenging. Computational and simulation approaches (all-atom steered MD simulations in particular) are key to provide molecular details about the associated mechanisms, to help test different hypotheses and to predict experimental results. In this review, particular recent efforts directed towards the molecular interpretation of single-molecule force spectroscopy experiments on proteins and protein-related systems (often in close collaboration with experimental groups) will be presented. These results will be discussed in the broader context of the field, highlighting the recent achievements and the ongoing challenges for computational biophysicists and biochemists. In particular, I will focus on the input gained from molecular simulations approaches to rationalize the origin of the unfolded protein elasticity and the protein conformational behavior under force, to understand how force denaturation differs from chemical, thermal or shear unfolding, and to unravel the molecular details of unfolding events for a variety of systems. I will also discuss the use of models based on Langevin dynamics on a 1-D free-energy surface to understand the effect of protein segmentation on the work exerted by a force, or, at the other end of the spectrum of computational techniques, how quantum calculations can help to understand the reactivity of disulfide bridges exposed to force.

Detection of a disulphide bond and conformational changes in Shigella flexneri Wzy, and the role of cysteine residues in polymerase activity

Shigella flexneri utilises the Wzy-dependent pathway for the production of a plethora of complex polysaccharides, including the lipopolysaccharide O-antigen (Oag) component. The inner membrane protein Wzy_SF polymerises Oag repeat units, whilst two co-polymerase proteins, Wzz_SF and Wzz_pHS-2, together interact with Wzy_SF to regulate production of short- (S-Oag) and very long- (VL-Oag) Oag modal lengths, respectively. The 2D arrangement of Wzy_SF transmembrane and soluble regions has been previously deciphered, however, attaining information on the 3D structural and conformational arrangement of Wzy_SF, or any homologue, has proven difficult. For the first time, the current study detected insights into the in situ Wzy_SF arrangement. In vitro assays using thiol-reactive PEG-maleimide were used to probe Wzy_SF conformation, which additionally detected novel, unique conformational changes in response to interaction with intrinsic factors, including Wzz_SF and Wzz_pHS-2, and extrinsic factors, such as temperature. Site-directed mutagenesis of Wzy_SF cysteine residues revealed the presence of a putative intramolecular disulphide bond, between cysteine moieties 13 and 60. Subsequent analyses highlighted both the structural and functional importance of Wzy_SF cysteines. Substitution of Wzy_SF cysteine residues significantly decreased biosynthesis of the VL-Oag modal length, without disruption to S-Oag production. This phenotype was corroborated in the absence of co-polymerase competition for Wzy_SF interaction. These data suggest Wzy_SF cysteine substitutions directly impair the interaction between Wzy/Wzz_pHS-2, without altering the Wzy/Wzz_SF interplay, and in combination with structural data, we propose that the N- and C-termini of Wzy_SF are arranged in close proximity, and together may form the unique Wzz_pHS-2 interaction site.

Evaluation of Cyc1 protein stability in Acidithiobacillus ferrooxidans bacterium after E121D mutation by molecular dynamics simulation to improve electron transfer

Cyc₁ (Cytochrome c₅₅₂) is a protein in the electron transport chain of the Acidithiobacillus ferrooxidans (Af) bacteria which obtain their energy from oxidation Fe²⁺ to Fe³⁺. The electrons are directed through Cyc₂, RCY (rusticyanin), Cyc₁ and Cox aa₃ proteins to O₂. Cyc₁ protein consists of two chains, A and B. In the present study, a novel mutation (E121D) in the A chain of Cyc₁ protein was selected due to electron receiving from Histidine 143 of RCY. Then, the changes performed in the E121D mutant were evaluated by MD simulations analyzes. Cyc₁ and RCY proteins were docked by a Patchdock server. By E121D mutation, the connection between Zn 1388 of chain B and aspartate 121 of chain A weaken. Asp 121 gets farther from Zn 1388. Therefore, the aspartate gets closer to Cu 1156 of the RCY leading to the higher stability of the RCY/Cyc₁ complex. Further, an acidic residue (Glu121) becomes a more acidic residue (Asp121) and improves the electron transfer to Cyc₁ protein. The results of RMSF analysis showed further ligand flexibility in mutation. This leads to fluctuation of the active site and increases redox potential at the mutation point and the speed of electron transfer. This study also predicts that in all respiratory chain proteins, electrons probably enter the first active site via glutamate and exit histidine in the second active site of each respiratory chain protein.

Human Vitamin K Epoxide Reductase as a Target of Its Redox Protein

Human vitamin K epoxide reductase (hVKORC1) enzymatic activity requires an initial activation by a specific redox protein, a less studied step in the hVKORC1 vital cycle. Significant steric conditions must be met by enzymes, being that to adapt their configurations is mandatory for hVKORC1 activation. We studied, by molecular dynamics (MD) simulations, the folding and conformational plasticity of hVKORC1 in its inactive (fully oxidised) state using available structures, crystallographic and from de novo modelling. According to the obtained results, hVKORC1 is a modular protein composed of the stable transmembrane domain (TMD) and intrinsically disordered luminal (L) loop, possessing the great plasticity/adaptability required to perform various steps of the activation process. The docking (HADDOCK) of Protein Disulfide Isomerase (PDI) onto different hVKORC1 conformations clearly indicated that the most interpretable solutions were found on the target closed L-loop form, a prevalent conformation of hVKORC1’s oxidised state. We also suggest that the cleaved L-loop is an appropriate entity to study hVKORC1 recognition/activation by its redox protein. Additionally, the application of hVKORC1 (membrane protein) in aqueous solution is likely to prove to be very useful in practice in either in silico studies or in vitro experiments.

The Redox-Active Conopeptide Derived from the Venom Duct Transcriptome of Conus lividus Assists in the Oxidative Folding of Conotoxin

The tetrapeptides Li504 and Li520, differing in the modification of the 4-trans-hydroxylation of proline, are novel conopeptides derived from the venom duct transcriptome of the marine cone snail Conus lividus. These predicted mature peptides are homologous to the active site motif of oxidoreductases that catalyze the oxidation, reduction, and rearrangement of disulfide bonds in peptides and proteins. The estimated reduction potential of the disulfide of Li504 and Li520 is within the range of disulfide reduction potentials of oxidoreductases, indicating that they may catalyze the oxidative folding of conotoxins. Conformational features of Li504 and Li520 include the trans configuration of the Cys1-Pro2/Hyp2 peptide bond with a type 1 turn that is similar to the active site motif of glutaredoxin that regulates the oxidation of cysteine thiols to disulfides. Li504- and Li520-assisted oxidative folding of α-conotoxin ImI confirms that Li520 improves the yield of the natively folded peptide by concomitantly decreasing the yield of the non-native disulfide isomer and thus acts as a miniature disulfide isomerase. The geometry of the Cys1-Hyp2 peptide bond of Li520 shifts between the trans and cis configurations in the disulfide form and thiol/thiolate form, which regulates the deprotonation of the N-terminal cysteine residue. Hydrogen bonding of the hydroxyl group of 4-trans-hydroxyproline with the interpeptide chain unit in the mixed disulfide form may play a vital role in shifting the geometry of the Cys1-Hyp2 peptide bond from cis to trans configuration. The Li520 conopeptide together with similar peptides derived from other species may constitute a new family of "redox-active" conopeptides that are integral components of the oxidative folding machinery of conotoxins.

Investigation of Cyc1 protein structure stability after H53I mutation using computational approaches to improve redox potential

Acidithiobacillus ferrooxidans (Af) is an acidophilic bacterium that grows in rigid surroundings and gets its own energy from the oxidation of Fe²⁺ to Fe³⁺. These bacteria are involved in the bioleaching process. Cyc₁ is a periplasmic protein with a crucial role in electron transportation in the respiratory chain. His53 of the Cyc₁ protein, involved in electron transfer to CoxB, was selected for mutation and bioinformatics studies. His53 was substituted by Ile using PyMol software. Molecular dynamics simulations were performed for wild and mutant types of Cyc₁ protein. The conformational changes of mutated protein were studied by analyzing RMSD, RMSF, SASA, Rg, H Bond, and DSSP. The results of the RMSF analysis indicated an increase in the flexibility of the ligand in the mutant. Finally, active site instability leads to an increase in the value of E₀ at the mutation point and improving electron transfer. On the other, His53 in Cyc₁ is interconnected to Glu126 in CoxB through the water molecule (W76) and hydrogen bonding. In the H53I mutation, there was a decrease in the distance between H2O 2030, 2033, and isoleucine 53, and subsequently, the distance to the water molecule 76 between the two proteins was reduced and strengthens the hydrogen bond between Cyc₁ and CoxB, finally improves electron transfer and the bioleaching process.

Redox Potentials of Disulfide Bonds in LOXL2 Studied by Nonequilibrium Alchemical Simulation

Lysyl oxidase-like 2 (LOXL2) is a metalloenzyme that catalyzes the oxidative deamination ε-amino group of lysine. It is found that LOXL2 is a promotor for the metastasis and invasion of cancer cells. Disulfide bonds are important components in LOXL2, and they play a stabilizing role for protein structure or a functional role for regulating protein bioactivity. The redox potential of disulfide bond is one important property to determine the functional role of disulfide bond. In this study, we have calculated the reduction potential of all the disulfide bonds in LOXL2 by non-equilibrium alchemical simulations. Our results show that seven of seventeen disulfide bonds have high redox potentials between -182 and -298 mV and could have a functional role, viz., Cys573-Cys625, Cys579-Cys695, Cys657-Cys673, and Cys663-Cys685 in the catalytic domain, Cys351-Cys414, Cys464-Cys530, and Cys477-Cys543 in the scavenger receptor cysteine-rich (SRCR) domains. The disulfide bond of Cys351-Cys414 is predicted to play an allosteric function role, which could affect the metastasis and invasion of cancer cells. Other functional bonds have a catalytic role related to enzyme activity. The rest of disulfide bonds are predicted to play a structural role. Our study provides an important insight for the classification of disulfide bonds in LOXL2 and can be utilized for the drug design that targets the cysteine residues in LOXL2.

Computing Proton-Coupled Redox Potentials of Fluorotyrosines in a Protein Environment

The oxidation of tyrosine to form the neutral tyrosine radical via proton-coupled electron transfer is essential for a wide range of biological processes. The precise measurement of the proton-coupled redox potentials of tyrosine (Y) in complex protein environments is challenging mainly because of the highly oxidizing and reactive nature of the radical state. Herein, a computational strategy is presented for predicting proton-coupled redox potentials in a protein environment. In this strategy, both the reduced Y-OH and oxidized Y-O^• forms of tyrosine are sampled with molecular dynamics using a molecular mechanical force field. For a large number of conformations, a quantum mechanical/molecular mechanical (QM/MM) electrostatic embedding scheme is used to compute the free-energy differences between the reduced and oxidized forms, including the zero-point energy and entropic contributions as well as the impact of the protein electrostatic environment. This strategy is applied to a series of fluorinated tyrosine derivatives embedded in a de novo α-helical protein denoted as α₃Y. The force fields for both the reduced and oxidized forms of these noncanonical fluorinated tyrosine residues are parameterized for general use. The calculated relative proton-coupled redox potentials agree with experimentally measured values with a mean unsigned error of 24 mV. Analysis of the simulations illustrates that hydrogen-bonding interactions between tyrosine and water increase the redox potentials by ∼100-250 mV, with significant variations because of the fluctuating protein environment. This QM/MM approach enables the calculation of proton-coupled redox potentials of tyrosine and other residues such as tryptophan in a variety of protein systems.

Allosteric disulphide bonds as reversible mechano-sensitive switches that control protein functions in the vasculature

Disulphide bonds are covalent linkages of two cysteine residues (R-S-S-R') in proteins. Unlike peptide bonds, disulphide bonds are reversible in nature allowing cleaved bonds to reform. Disulphide bonds are important structural elements that stabilise protein conformation. They can be of catalytic function found in enzymes that facilitate redox reactions in the cleavage/formation of disulphide bonds in their substrates. Emerging evidence also indicates that disulphide bonds can be of regulatory function which alter protein activity when they are cleaved or formed. This class of regulatory disulphide bonds is known as allosteric disulphide bonds. Allosteric disulphide bonds are mechano-sensitive, and stretching or twisting the sulphur-sulphur bond by mechanical force can make it easier or harder to be cleaved. This makes allosteric disulphide bonds an ideal type of mechano-sensitive switches for regulating protein functions in the vasculature where cells are continuously subjected to fluid shear force. This review will discuss the chemistry and biophysical properties of allosteric disulphide bonds and how they emerge to be mechano-sensitive switches in regulating platelet function and clot formation.

Determining the Redox Potential of a Protein Disulphide Bond

The redox potential of a protein disulphide bond is one of the most important factors for determining the role of a disulphide bond. Disulphide bonds can have a stabilizing role for the structure of a protein or they can play a functional role which can regulate protein bioactivity. Determining the redox potential of disulphides can help distinguish the functional from the structural disulphide bonds. In this chapter, two methods for determining the redox potential of a protein disulphide bond are described. The first method uses maleimide-biotin labeling of free cysteine thiols and western blot densitometry to determine the fraction of reduced disulphide bond under various redox-buffering conditions. The second method uses differential cysteine labeling and tandem mass spectrometry to determine the redox potential.

Studying Functional Disulphide Bonds by Computer Simulations

Biochemical and structural data reveal important aspects of the properties and function of a protein disulphide bond. Molecular dynamics simulations can complement this experimental data and can yield valuable insights into the dynamical behavior of the disulphide bond within the protein environment. Due to the increasing accuracy of the underlying energetic description and the increasing computational power at hand, such simulations have now reached a level, at which they can also make quantitative and experimentally testable predictions. We here give an overview of the computational methods used to predict functional aspects of protein disulphides, including the prestress, protein allosteric effects upon thiol/disulphide exchange, and disulphide redox potentials. We then outline in detail the use of free-energy perturbation methods to calculate the redox potential of a protein disulphide bond of interest. In a step-by-step protocol, we describe the workflow within the MD suite Gromacs, including practical advice on the simulation setup and choice of parameters. For other disulphide-related simulation methods, we refer to resources available online.

Accessibility explains preferred thiol-disulfide isomerization in a protein domain

Disulfide bonds are key stabilizing and yet potentially labile cross-links in proteins. While spontaneous disulfide rearrangement through thiol-disulfide exchange is increasingly recognized to play an important physiological role, its molecular determinants are still largely unknown. Here, we used a novel hybrid Monte Carlo and Molecular Dynamics scheme to elucidate the molecular principles of thiol-disulfide exchange in proteins, for a mutated immunoglobulin domain as a model system. Unexpectedly, using simple proximity as the criterion for thiol-disulfide exchange, our method correctly predicts the experimentally observed regiospecificity and selectivity of the cysteine-rich protein. While redox reactivity has been examined primarily on the level of transition states and activation barriers, our results argue for accessibility of the disulfide by the attacking thiol given the highly dynamic and sterically demanding protein as a major bottleneck of thiol-disulfide exchange. This scenario may be similarly at play in other proteins with or without an evolutionarily designed active site.

Prediction of reversible disulfide based on features from local structural signatures

Background

Disulfide bonds are traditionally considered to play only structural roles. In recent years, increasing evidence suggests that the disulfide proteome is made up of structural disulfides and reversible disulfides. Unlike structural disulfides, reversible disulfides are usually of important functional roles and may serve as redox switches. Interestingly, only specific disulfide bonds are reversible while others are not. However, whether reversible disulfides can be predicted based on structural information remains largely unknown.

Methods

In this study, two datasets with both types of disulfides were compiled using independent approaches. By comparison of various features extracted from the local structural signatures, we identified several features that differ significantly between reversible and structural disulfides, including disulfide bond length, along with the number, amino acid composition, secondary structure and physical-chemical properties of surrounding amino acids. A SVM-based classifier was developed for predicting reversible disulfides.

Results

By 10-fold cross-validation, the model achieved accuracy of 0.750, sensitivity of 0.352, specificity of 0.953, MCC of 0.405 and AUC of 0.751 using the RevSS_PDB dataset. The robustness was further validated by using RevSS_RedoxDB as independent testing dataset. This model was applied to proteins with known structures in the PDB database. The results show that one third of the predicted reversible disulfide containing proteins are well-known redox enzymes, while the remaining are non-enzyme proteins. Given that reversible disulfides are frequently reported from functionally important non-enzyme proteins such as transcription factors, the predictions may provide valuable candidates of novel reversible disulfides for further experimental investigation.

Conclusions

This study provides the first comparative analysis between the reversible and the structural disulfides. Distinct features remarkably different between these two groups of disulfides were identified, and a SVM-based classifier for predicting reversible disulfides was developed accordingly. A web server named RevssPred can be accessed freely from: http://biocomputer.bio.cuhk.edu.hk/RevssPred.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3668-8) contains supplementary material, which is available to authorized users.

Lid mobility in lipase SMG1 validated using a thiol/disulfide redox potential probe

Most lipases possess a lid domain above the catalytic site that is responsible for their activation. Lipase SMG1 from Malassezia globose CBS 7966 (Malassezia globosa LIP1), is a mono‐ and diacylglycerol lipase with an atypical loop‐like lid domain. Activation of SMG1 was proposed to be solely through a gating mechanism involving two residues (F278 and N102). However, through disulfide bond cross‐linking of the lid, this study shows that full activation also requires mobility of the lid domain, contrary to a previous proposal. The newly introduced disulfide bond makes lipase SMG1 eligible as a ratiometric thiol/disulfide redox potential probe, when it is coupled with chromogenic substrates. This redox‐switch lipase could also be of potential use in cascade biocatalysis.