Effects of Proline Substitutions on the Thermostable LOV Domain from Chloroflexus aggregans

Improving the thermostability and substrate affinity of IsPETase through the S209H mutation, and exploring the structural role of the N-terminal: a computational study

The current in silico investigation aimed to increase the thermostability of IsPETase for more efficient PET degradation. N-Truncated and S209H mutants were designed to improve the thermostability of IsPETase. The deletion of the first seven N-terminal residues in PETase (N-truncated mutant) disrupts structural integrity, as Arg5, a crucial residue, maintains stability by forming a hydrogen bond network with Pro2, Thr48, Lys230, and Arg238. This links the N-terminal to the C-terminal, while its absence increases RMSF values in this region. The S209H mutation, located in the catalytic loop of IsPETase, enhances thermostability by introducing a new hydrophobic interaction with residue W130. MD simulations at 353.15 K have demonstrated this improvement, showing reduced structural flexibility and compactness in the S209H mutant compared to the WT. Specifically, the overall RMSD, Cα RMSD, SASA, and Rg values decreased from 3.36249 ± 0.853 Å, 1.321843 ± 0.0953 Å, 10,057.73 ± 135.11 Å2, and 17.09687 ± 1.387 Å in the WT to 3.184878 ± 0.786 Å, 0.969998 ± 0.119 Å, 9894.527 ± 118.53 Å2, and 16.962 ± 1.265 Å in the S209H mutant, respectively. Molecular docking revealed binding energies of -4.9 kcal/mol for WT, -5.1 kcal/mol for the S209H mutant, and -4.8 kcal/mol for the N-truncated mutant. MM-PBSA analysis using YASARA showed that the S209H mutation increased binding energy from -17.3606 kJ/mol (WT) to -7.82077 kJ/mol, enhancing binding affinity, while the N-truncated mutant reduced binding energy to -23.5032 kJ/mol, lowering binding affinity. In conclusion, this study has demonstrated that the S209H mutation enhanced the thermostability and the PET affinity of IsPETase by introducing the hydrophobic interactions. The N-truncated mutant reduced both thermostability and PET affinity, highlighting the critical role of the N-terminal region in maintaining the stability and activity of IsPETase.

In Silico Structural Insights into a Glucanase from Clostridium perfringens and Prediction of Structural Stability Improvement Through Hydrophobic Interaction Network and Aromatic Interaction

Glucanases are widely applied in industrial applications such as brewing, biomass conversion, food, and animal feed. Glucanases catalyze the hydrolysis of glucan to produce the sugar hemiacetal through hydrolytic cleavage of glycosidic bonds. Current study aimed to investigate structural insights of a glucanase from Clostridium perfringens through blind molecular docking, site-specific molecular docking, molecular dynamics (MD) simulation, and binding energy calculation. Furthermore, we aimed to enhance structural stabilization through formation of hydrophobic interaction network. The molecular docking results illustrated that residues Glu222 and Asp187 may act as nucleophile acid/base catalyst. Moreover, the MM/PBSA results illustrated a high binding affinity of 108.71 ± 8.5 kJ/mol between glucanase and barely glucan during 100 ns simulation. The RMSF analysis illustrated a high flexible surface loop with the highest mobility at position D130. Therefore, the structural engineering was carried out through introducing a double-mutant S125Y/D130P, and the structural stability was improved by forming the hydrophobic interaction network and one π-π aromatic interaction. The spatial distance between the mutation sites and the catalytic pocket attenuates their direct impact on binding interactions within the catalytic pocket.

Engineering of LOV-domains for their use as protein tags

Light-Oxygen-Voltage (LOV) domains are the protein-based light switches used in nature to trigger and regulate various processes. They allow light signals to be converted into metabolic signaling cascades. Various LOV-domain proteins have been characterized in the last few decades and have been used to develop light-sensitive tools in cell biology research. LOV-based applications exploit the light-driven regulation of effector elements to activate signaling pathways, activate genes, or locate proteins within cells. A relatively new application of an engineered small LOV-domain protein called miniSOG (mini singlet oxygen generator) is based on the light-induced formation of reactive oxygen species (ROS). The first miniSOG was engineered from a LOV domain from Arabidopsis thaliana. This engineered 14 kDa light-responsive flavin-containing protein can be exploited as protein tag for the light-triggered localized production of ROS. Such tunable ROS production by miniSOG or similarly redesigned LOV-domains can be of use in studies focused on subcellular phenomena but may also allow new light-fueled catalytic processes. This review provides an overview of the discovery of LOV domains and their development into tools for cell biology. It also highlights recent advancements in engineering LOV domains for various biotechnological applications and cell biology studies.

Capturing the blue-light activated state of the Phot-LOV1 domain from Chlamydomonas reinhardtii using time-resolved serial synchrotron crystallography

Light-oxygen-voltage (LOV) domains are small photosensory flavoprotein modules that allow the conversion of external stimuli (sunlight) into intracellular signals responsible for various cell behaviors (e.g. phototropism and chloroplast relocation). This ability relies on the light-induced formation of a covalent thioether adduct between a flavin chromophore and a reactive cysteine from the protein environment, which triggers a cascade of structural changes that result in the activation of a serine/threonine (Ser/Thr) kinase. Recent developments in time-resolved crystallography may allow the activation cascade of the LOV domain to be observed in real time, which has been elusive. In this study, we report a robust protocol for the production and stable delivery of microcrystals of the LOV domain of phototropin Phot-1 from Chlamydomonas reinhardtii (CrPhotLOV1) with a high-viscosity injector for time-resolved serial synchrotron crystallography (TR-SSX). The detailed process covers all aspects, from sample optimization to data collection, which may serve as a guide for soluble protein preparation for TR-SSX. In addition, we show that the crystals obtained preserve the photoreactivity using infrared spectroscopy. Furthermore, the results of the TR-SSX experiment provide high-resolution insights into structural alterations of CrPhotLOV1 from Δt = 2.5 ms up to Δt = 95 ms post-photoactivation, including resolving the geometry of the thioether adduct and the C-terminal region implicated in the signal transduction process.

Heating-mediated purification of active FGF21 and structure-based design of its variant with enhanced potency

Fibroblast growth factor 21 (FGF21) has pharmaceutical potential against obesity-related metabolic disorders, including non-alcoholic fatty liver disease. Since thermal stability is a desirable factor for therapeutic proteins, we investigated the thermal behavior of human FGF21. FGF21 remained soluble after heating; thus, we examined its temperature-induced structural changes using circular dichroism (CD). FGF21 showed inter-convertible temperature-specific CD spectra. The CD spectrum at 100 °C returned to that at 20 °C when the heated FGF21 solution was cooled. Through loop swapping, the connecting loop between β10 and β12 in FGF21 was revealed to be associated with the unique thermal behavior of FGF21. According to surface plasmon resonance (SPR) experiments, in vitro cell-based assays, and model high-fat diet (HFD)-induced obesity studies, heated FGF21 maintained biological activities that were comparable to those of non-heated and commercial FGF21s. Based on sequence comparison and structural analysis, five point-mutations were introduced into FGF21. Compared with the wild type, the heated FGF21 variant displayed improved therapeutic potential in terms of body weight loss, the levels of hepatic triglycerides and lipids, and the degree of vacuolization of liver in HFD-fed mice.

Thermostability engineering of industrial enzymes through structure modification

Thermostability is an essential requirement of enzymes in the industrial processes to catalyze the reactions at high temperatures; thus, enzyme engineering through directed evolution, semi-rational design and rational design are commonly employed to construct desired thermostable mutants. Several strategies are implemented to fulfill enzymes' thermostability demand including decreasing the entropy of the unfolded state through substitutions Gly → Xxx or Xxx → Pro, hydrogen bond, salt bridge, introducing two different simultaneous interactions through single mutant, hydrophobic interaction, filling the hydrophobic cavity core, decreasing surface hydrophobicity, truncating loop, aromatic-aromatic interaction and introducing positively charged residues to enzyme surface. In the current review, horizons about compatibility between secondary structures and substitutions at preferable structural positions to generate the most desirable thermostability in industrial enzymes are broadened. KEY POINTS: • Protein engineering is a powerful tool for generating thermostable industrial enzymes. • Directed evolution and rational design are practical approaches in enzyme engineering. • Substitutions in preferable structural positions can increase thermostability.

Extreme dependence of Chloroflexus aggregans LOV domain thermo- and photostability on the bound flavin species

Light-oxygen-voltage (LOV) domains are common photosensory modules that found many applications in fluorescence microscopy and optogenetics. Here, we show that the Chloroflexus aggregans LOV domain can bind different flavin species (lumichrome, LC; riboflavin, RF; flavin mononucleotide, FMN; flavin adenine dinucleotide, FAD) during heterologous expression and that its physicochemical properties depend strongly on the nature of the bound flavin. We show that whereas the dissociation constants for different chromophores are similar, the melting temperature of the protein reconstituted with single flavin species varies from ~ 60 °C for LC to ~ 81 °C for FMN, and photobleaching half-times vary almost 100-fold. These observations serve as a caution for future studies of LOV domains in non-native conditions yet raise the possibility of fine-tuning various properties of LOV-based fluorescent probes and optogenetic tools by manipulating the chromophore composition.

Unlocking the Stereoselectivity and Substrate Acceptance of Enzymes: Proline-Induced Loop Engineering Test

Protein stability and evolvability influence each other. Although protein dynamics play essential roles in various catalytically important properties, their high flexibility and diversity makes it difficult to incorporate such properties into rational engineering. Therefore, how to unlock the potential evolvability in a user-friendly rational design process remains a challenge. In this endeavor, we describe a method for engineering an enantioselective alcohol dehydrogenase. It enables synthetically important substrate acceptance for 4-chlorophenyl pyridine-2-yl ketone, and perfect stereocontrol of both (S)- and (R)-configured products. Thermodynamic analysis unveiled the subtle interaction between enzyme stability and evolvability, while computational studies provided insights into the origin of selectivity and substrate recognition. Preparative-scale synthesis of the (S)-product (73 % yield; >99 % ee) was performed on a gram-scale. This proof-of-principle study demonstrates that interfaced proline residues can be rationally engineered to unlock evolvability and thus provide access to new biocatalysts with highly improved catalytic performance.

Tolerance to Glutaraldehyde in Escherichia coli Mediated by Overexpression of the Aldehyde Reductase YqhD by YqhC

Glutaraldehyde is a widely used biocide on the market for about 50 years. Despite its broad application, several reports on the emergence of bacterial resistance, and occasional outbreaks caused by poorly disinfection, there is a gap of knowledge on the bacterial adaptation, tolerance, and resistance mechanisms to glutaraldehyde. Here, we analyze the effects of the independent selection of mutations in the transcriptional regulator yqhC for biological replicates of Escherichia coli cells subjected to adaptive laboratory evolution (ALE) in the presence of glutaraldehyde. The evolved strains showed improved survival in the biocide (11-26% increase in fitness) as a result of mutations in the activator yqhC, which led to the overexpression of the yqhD aldehyde reductase gene by 8 to over 30-fold (3.1-5.2 log2FC range). The protective effect was exclusive to yqhD as other aldehyde reductase genes of E. coli, such as yahK, ybbO, yghA, and ahr did not offer protection against the biocide. We describe a novel mechanism of tolerance to glutaraldehyde based on the activation of the aldehyde reductase YqhD by YqhC and bring attention to the potential for the selection of such tolerance mechanism outside the laboratory, given the existence of YqhD homologs in various pathogenic and opportunistic bacterial species.

High-resolution structure of a naturally red-shifted LOV domain

LOV domains are widespread photosensory modules that have also found applications in fluorescence microscopy, optogenetics, and light-driven generation of reactive oxygen species. Many of these applications require stable proteins with altered spectra. Here, we report a flavin-based fluorescent protein CisFbFP derived from Chloroflexus islandicus LOV domain-containing protein. We show that CisFbFP is thermostable, and its absorption and fluorescence spectra are red-shifted for ∼6 nm, which has not been observed for other cysteine-substituted natural LOV domains. We also provide a crystallographic structure of CisFbFP at the resolution of 1.2 Å that reveals alterations in the active site due to replacement of conservative asparagine with a serine. Finally, we discuss the possible effects of presence of cis-proline in the Aβ-Bβ loop on the protein's structure and stability. The findings provide the basis for engineering and color tuning of LOV-based tools for molecular biology.

Scaffold Hopping Transformations Using Auxiliary Restraints for Calculating Accurate Relative Binding Free Energies

In silico screening of drug-target interactions is a key part of the drug discovery process. Changes in the drug scaffold via contraction or expansion of rings, the breaking of rings, and the introduction of cyclic structures from acyclic structures are commonly applied by medicinal chemists to improve binding affinity and enhance favorable properties of candidate compounds. These processes, commonly referred to as scaffold hopping, are challenging to model computationally. Although relative binding free energy (RBFE) calculations have shown success in predicting binding affinity changes caused by perturbing R-groups attached to a common scaffold, applications of RBFE calculations to modeling scaffold hopping are relatively limited. Scaffold hopping inevitably involves breaking and forming bond interactions of quadratic functional forms, which is highly challenging. A novel method for handling ring opening/closure/contraction/expansion and linker contraction/expansion is presented here. To the best of our knowledge, RBFE calculations on linker contraction/expansion have not been previously reported. The method uses auxiliary restraints to hold the atoms at the ends of a bond in place during the breaking and forming of the bonds. The broad applicability of the method was demonstrated by examining perturbations involving small-molecule macrocycles and mutations of proline in proteins. High accuracy was obtained using the method for most of the perturbations studied. The rigor of the method was isolated from the force field by validating the method using relative and absolute hydration free energy calculations compared to standard simulation results. Unlike other methods that rely on λ-dependent functional forms for bond interactions, the method presented here can be employed using modern molecular dynamics software without modification of codes or force field functions.

Insights into the mechanisms of light-oxygen-voltage domain color tuning from a set of high-resolution X-ray structures

Light-oxygen-voltage (LOV) domains are widespread photosensory modules that can be used in fluorescence microscopy, optogenetics and controlled production of reactive oxygen species. All of the currently known LOV domains have absorption maxima in the range of ~440 to ~450 nm, and it is not clear whether they can be shifted significantly using mutations. Here, we have generated a panel of LOV domain variants by mutating the key chromophore-proximal glutamine aminoacid of a thermostable flavin based fluorescent protein CagFbFP (Gln148) to asparagine, aspartate, glutamate, histidine, lysine and arginine. Absorption spectra of all of the mutants are blue-shifted, with the maximal shift of 8 nm observed for the Q148H variant. While CagFbFP and its Q148N/D/E variants are not sensitive to pH, Q148H/K/R reveal a moderate red shift induced byacidic pH. To gain further insight, we determined high resolution crystal structures of all of the mutants studied at the resolutions from 1.07 Å for Q148D to 1.63 Å for Q148R. Whereas in some of the variants, the aminoacid 148 remains in the vicinity of the flavin, in Q148K, Q148R and partially Q148D, the C-terminus of the protein unlatches and the side chain of the residue 148 is reoriented away from the chromophore. Our results explain the absence of color shifts from replacing Gln148 with charged aminoacids and pave the way for rational design of color-shifted flavin based fluorescent proteins.

Rational Design of a Split Flavin-Based Fluorescent Reporter

Protein-fragment complementation assays are used ubiquitously for probing protein-protein interactions. Most commonly, the reporter protein is split in two parts, which are then fused to the proteins of interest and can reassemble and provide a readout if the proteins of interest interact with each other. The currently known split fluorescent proteins either can be used only in aerobic conditions and assemble irreversibly, or require addition of exogenous chromophores, which complicates the design of experiments. In recent years, light-oxygen-voltage (LOV) domains of several photoreceptor proteins have been developed into flavin-based fluorescent proteins (FbFPs) that, under some circumstances, can outperform commonly used fluorescent proteins such as GFP. Here, we show that CagFbFP, a small thermostable FbFP based on a LOV domain-containing protein from Chloroflexus aggregans, can serve as a split fluorescent reporter. We use the available genetic and structural information to identify three loops between the conserved secondary structure elements, Aβ-Bβ, Eα-Fα, and Hβ-Iβ, that tolerate insertion of flexible poly-Gly/Ser segments and eventually splitting. We demonstrate that the designed split pairs, when fused to interacting proteins, are fluorescent in vivo in E. coli and human cells and have low background fluorescence. Our results enable probing protein-protein interactions in anaerobic conditions without using exogenous fluorophores and provide a basis for further development of LOV and PAS (Per-Arnt-Sim) domain-based fluorescent reporters and optogenetic tools.