Stable Tertiary Structure Research Articles

Drude2019IDPC polarizable force field reveals structure-function relationship of insulin

Intrinsically disordered proteins (IDPs) lack stable tertiary structures under physiological conditions, yet play key roles in biological processes and associated with human complex diseases. Their conformational characteristics and high content of charged residues make the use of polarizable force fields an advantageous for simulating IDPs. The Drude2019IDP polarizable force field, previously introduced, has demonstrated comprehensive enhancements and improvements in dipeptides, short peptides, and IDPs, achieving a balanced sampling between IDPs and structured proteins. However, the performance in simulating 5 dipeptides was found to be underestimate. Therefore, we individually performed reweighting and grid-based energy correction map (CMAP) optimization for these 5 dipeptides, resulting in the enhanced Drude2019IDPC force field. The performance of Drude2019IDPC was evaluated with 5 dipeptides, 5 disordered short peptides, and a representative IDP. The results demonstrated a marked improvement comparing with original Drude2019IDP. To further substantiate the capabilities of Drude2019IDPC, MD simulation and Markov state model (MSM) were applied to wild type and mutant for insulin, to elucidate the difference of conformational characteristics and transition path. The findings reveal that mutation can maintain the monomorphic characteristics, providing insights for engineered insulin development. These results indicate that Drude2019IDPC could be used to reveal the structure-function relationship for other proteins.

Influence of low voltage programmed thawing on the quality of frozen pork and its myofibrillar protein

This study explored the effects of low voltage programmed thawing (LVPT) on thawing period, water status, current density as well as physicochemical properties of porcine longissimus dorsi muscle, considering air thawing (AT), water immersion thawing (WT) and cold storage thawing (CST) for the comparison, additionally making fresh meat (FM) as the control. The thawing process used current limiting to gradually reduce the maximum output current to 500, 200 and 100 mA, respectively, by applying a safe voltage of 35 V in a home defrosting apparatus. Using two pairs of foil plate, the thawing (LVPT-2) with dual current loop accelerated the melting of ice crystals than the process with one pair of polar plate (LVPT-1) with one current loop. From −18 °C to −1 °C, the thawing period of LVPT-2 was reduced by 14.54 %, 11.72 %, and 15.95 % compared with that of LVPT-1 for samples within the thickness of 2, 3 and 4 cm, respectively. During the process, the intensity and density of the current loaded on the frozen meat gradually increased. LVPT-1 retained water and reduced its migration inside the meat, and protein solubility was better maintained. As the thickness increased, the effect of LVPT-2 on the moisture distribution and protein solubility of the sample decreased. LVPT-1 treatment resulted in less fat oxidation as well as stable secondary and tertiary structures of myofibrillar proteins compared to other treatments. At low voltage, the ability of LVPT −2 to maintain meat quality was reduced due to the increase in current density. In addition, LVPT-1 reduced muscle damage because of rapid melting of ice crystals. In contrast, LVPT-2 was conductive the formation of hot spots due to dual current loop passed through the pork in final phase of thawing, which would adversely influence the quality of thawed meat. Finally, LVPT could improve the rate of thawing, water binding capacity and meat quality, especially, there was no overheating effect on the meat because of current limiting.

Multivalency drives interactions of alpha-synuclein fibrils with tau.

Age-related neurodegenerative disorders like Alzheimer's disease (AD) and Parkinson's disease (PD) are characterized by deposits of protein aggregates, or amyloid, in various regions of the brain. Historically, aggregation of a single protein was observed to be correlated with these different pathologies: tau in AD and α-synuclein (αS) in PD. However, there is increasing evidence that the pathologies of these two diseases overlap, and the individual proteins may even promote each other's aggregation. Both tau and αS are intrinsically disordered proteins (IDPs), lacking stable secondary and tertiary structure under physiological conditions. In this study we used a combination of biochemical and biophysical techniques to interrogate the interaction of tau with both soluble and fibrillar αS. Fluorescence correlation spectroscopy (FCS) was used to assess the interactions of specific domains of fluorescently labeled tau with full length and C-terminally truncated αS in both monomer and fibrillar forms. We found that full-length tau as well as individual tau domains interact with monomer αS weakly, but this interaction is much more pronounced with αS aggregates. αS aggregates also mildly slow the rate of tau aggregation, although not the final degree of aggregation. Our findings suggest that co-occurrence of tau and αS in disease are more likely to occur through monomer-fiber binding interactions, rather than monomer-monomer or co-aggregation.

Formerly degenerate seventh zinc finger domain from transcription factor ZNF711 rehabilitated by experimental NMR structure.

Domain Z7 of nuclear transcription factor ZNF711 has the consensus last metal-ligand H23 found in odd-numbered zinc fingers of this protein replaced by a phenylalanine. Ever since the discovery of ZNF711, it has been thought that Z7 is probably non-functional because of the H23F substitution. The presence of H26 three positions downstream prompted us to examine if this histidine could substitute as the last metal-ligand. The Z7 domain adopts a stable tertiary structure upon metal-binding. The NMR structure of Zn2+-bound Z7 shows the classical ββα-fold of CCHH zinc fingers. Mutagenesis and pH titration experiments indicate that H26 is not involved in metal binding and that Z7 has a tridentate metal-binding site comprised of only residues C3, C6, and H19. By contrast, an F23H mutation that introduces a histidine in the consensus position forms a tetradentate ligand. The structure of the WT Z7 is stable causing restricted ring-flipping of phenylalanines 10 and 23. Dynamics are increased with either the H26A or F23H substitutions and aromatic ring rotation is no longer hindered in the two mutants. The mutations have only small effects on the Kd values for Zn2+ and Co2+ and retain the high thermal stability of the WT domain above 80°C. Like two previously reported designed zinc fingers with the last ligand replaced by water, the WT Z7 domain is catalytically active, hydrolyzing 4-nitrophenyl acetate. We discuss the implications of naturally occurring tridentate zinc fingers for cancer mutations and drug targeting of notoriously undruggable transcription factors.

Predicting Conformational Ensembles of Intrinsically Disordered Proteins: From Molecular Dynamics to Machine Learning.

Intrinsically disordered proteins and regions (IDP/IDRs) are ubiquitous across all domains of life. Characterized by a lack of a stable tertiary structure, IDP/IDRs populate a diverse set of transiently formed structural states that can promiscuously adapt upon binding with specific interaction partners and/or certain alterations in environmental conditions. This malleability is foundational for their role as tunable interaction hubs in core cellular processes such as signaling, transcription, and translation. Tracing the conformational ensemble of an IDP/IDR and its perturbation in response to regulatory cues is thus paramount for illuminating its function. However, the conformational heterogeneity of IDP/IDRs poses several challenges. Here, we review experimental and computational methods devised to disentangle the conformational landscape of IDP/IDRs, highlighting recent computational advances that permit proteome-wide scans of IDP/IDRs conformations. We briefly evaluate selected computational methods using the disordered N-terminal of the human copper transporter 1 as a test case and outline further challenges in IDP/IDRs ensemble prediction.

O-glycosylation of intrinsically disordered regions regulates homeostasis of membrane proteins in streptococci.

Proteins harboring intrinsically disordered regions (IDRs) lacking stable secondary or tertiary structures are abundant across the three domains of life. These regions have not been systematically studied in prokaryotes. Our genome-wide analysis identifies extracytoplasmic serine/threonine-rich IDRs in several biologically important membrane proteins in streptococci. We demonstrate that these IDRs are O-glycosylated with glucose by glycosyltransferases GtrB and PgtC2 in Streptococcus pyogenes and Streptococcus pneumoniae, and with N-acetylgalactosamine by a Pgf-dependent mechanism in Streptococcus mutans. Absence of glycosylation leads to a defect in biofilm formation under ethanol-stressed conditions in S. mutans. We link this phenotype to the C-terminal IDR of a post-translocation secretion chaperone PrsA. O-glycosylation of the IDR protects this region from proteolytic degradation. The IDR length attenuates the efficiency of glycosylation and, consequently, the expression level of PrsA. Taken together, our data reveal that O-glycosylation of IDRs functions as a dynamic switch of protein homeostasis in streptococci.

Structural domain in the Titin N2B-us region binds to FHL2 in a force-activation dependent manner.

Titin N2B unique sequence (N2B-us) is a 572 amino acid sequence that acts as an elastic spring to regulate muscle passive elasticity. It is thought to lack stable tertiary structures and is a force-bearing region that is regulated by mechanical stretching. In this study, the conformation of N2B-us and its interaction with four-and-a-half LIM domain protein 2 (FHL2) are investigated using AlphaFold2 predictions and single-molecule experimental validation. Surprisingly, a stable alpha/beta structural domain is predicted and confirmed in N2B-us that can be mechanically unfolded at forces of a few piconewtons. Additionally, more than twenty FHL2 LIM domain binding sites are predicted to spread throughout N2B-us. Single-molecule manipulation experiments reveals the force-dependent binding of FHL2 to the N2B-us structural domain. These findings provide insights into the mechano-sensing functions of N2B-us and its interactions with FHL2.

Plasma activated water (PAW), Ozonated water (OW) and Slightly acidic electrolytic water (SAEW) treated tilapia fillets during chilled storage: A comparative study on quality changes and related mechanisms

Changes in protein degradation and lipid oxidation in tilapia fillets treated with plasma-activated water (PAW), ozonated water (OW), and slightly acidic electrolyzed water (SAEW) were investigated during refrigeration (12 d) at 4 °C. Compared to the control group, PAW, OW, and SAEW effectively delayed the formation of trichloroacetic acid (TCA)-soluble peptides and free amino acids, and significantly reduced the drip loss rate by 1.52–3.60% after 12 d (P < 0.05). However, the total content of unsaturated fatty acids in all treatment groups decreased after 12 d. Compared to PAW and OW, SAEW better-maintained protein secondary and tertiary structure stability and preserved the most polyunsaturated fatty acids (27.32%). UHPLC-Q-TOF/MS analysis showed that accumulation of dipeptides/tripeptides (such as Trp-Trp-Pro, Tyr-Thr-Trp, Leu-Gln-Phe) enhanced the bioavailability and potential bioactivity of the proteins in all treatment groups. The 4-hydroxy-octanoic acid and 4-amino-pentanoic acid from fatty acid oxidation were significantly (P < 0.05) down-regulated in PAW and SAEW. Overall, PAW, OW, and SAEW may be feasible techniques for improving the quality of tilapia fillets during cold storage.

6‐Strand to Stable 10/12 Helix Conformational Switch by Incorporating Flexible β‐hGly in the Homooligomers of Camphor Derived β‐Amino Acid: NMR and X‐Ray Crystallographic Evidence

AbstractRational design of unnatural amino acid building blocks capable of stabilizing predictable secondary structures similar to protein fragments is pivotal for foldamer chemistry/catalysis. Here, we introduce novel β‐amino acid building blocks: [1S,2R,4R]exoCDA and [1S,2S,4R]endoCDA, derived from the abundantly available R(+)‐camphor, which is traditionally known for its medicinal value. Further, we demonstrate that the homooligomers of exoCDA adopt 6‐strand conformation, which switches to a robust 10/12‐helix simply by inserting flexible β‐hGly spacer at alternate positions (1 : 1 β‐hGly/exoCDA heterooligomers), as evident by DFT‐calculations, solution‐state NMR spectroscopy and X‐ray crystallography. To the best of our knowledge, this is the first example of crystalline‐state structure of left‐handed 10/12‐mixed helix, that is free from the conventional approach of employing β‐amino acids of either alternate chirality or alternate β2/β3 substitutions, to access the 10/12‐helix. The results also show that the homooligomers of heterochiral exoCDA don′t adopt helical fold, instead exhibit banana‐shaped strands, whereas the homodimers of the other diastereomer endoCDA, nucleate 8‐membered turns. Furthermore, the homo‐exoCDA and hetero‐[β‐hGly‐exoCDA] oligomers are found to exhibit self‐association properties with distinct morphological features. Overall, the results offer new possibilties of constructing discrete stable secondary and tertiary structures based on CDAs, which can accommodate flexible residues with desired side‐chain substitutions.

6-Strand to Stable 10/12 Helix Conformational Switch by Incorporating Flexible β-hGly in the Homooligomers of Camphor Derived β-Amino Acid: NMR and X-Ray Crystallographic Evidence.

Rational design of unnatural amino acid building blocks capable of stabilizing predictable secondary structures similar to protein fragments is pivotal for foldamer chemistry/catalysis. Here, we introduce novel β-amino acid building blocks: [1S,2R,4R]exoCDA and [1S,2S,4R]endoCDA, derived from the abundantly available R(+)-camphor, which is traditionally known for its medicinal value. Further, we demonstrate that the homooligomers of exoCDA adopt 6-strand conformation, which switches to a robust 10/12-helix simply by inserting flexible β-hGly spacer at alternate positions (1 : 1 β-hGly/exoCDA heterooligomers), as evident by DFT-calculations, solution-state NMR spectroscopy and X-ray crystallography. To the best of our knowledge, this is the first example of crystalline-state structure of left-handed 10/12-mixed helix, that is free from the conventional approach of employing β-amino acids of either alternate chirality or alternate β2/β3 substitutions, to access the 10/12-helix. The results also show that the homooligomers of heterochiral exoCDA don't adopt helical fold, instead exhibit banana-shaped strands, whereas the homodimers of the other diastereomer endoCDA, nucleate 8-membered turns. Furthermore, the homo-exoCDA and hetero-[β-hGly-exoCDA] oligomers are found to exhibit self-association properties with distinct morphological features. Overall, the results offer new possibilties of constructing discrete stable secondary and tertiary structures based on CDAs, which can accommodate flexible residues with desired side-chain substitutions.

Never Fold to Fold Continuously: A Conundrum in Ubiquitin–Proteasome System (UPS)-Mediated Protein Quality Control (PQC)

In the last few decades, the traditional paradigm of teleonomy, in which the amino acid sequence of a protein is tightly associated with its structure and, in turn, with its function, has been partially undermined. The idea of a protein as a two-state object has been superseded by that of understanding it as a multistate object. Indeed, some proteins, or portions of a protein, display intrinsically disordered regions (IDRs), which means that they lack stable secondary or tertiary structures. While we are aware that IDRs are present in almost half of the total human proteins, we are still quite far away from understanding their contextual-specific functions and figuring out how they mechanistically work. In the present perspective article, we will attempt to summarize the role/s of IDRs in ubiquitin–proteasome system (UPS)-mediated protein quality control (PQC) at different levels, ranging from ubiquitination to protein degradation through the proteasome machinery up to their role in decoding the complex ubiquitin code. Ultimately, we will critically discuss the future challenges we are facing to gain insights into the role of IDRs in regulating UPS-mediated PQC.

Integrated ultra-high pressure and salt addition to improve the in vitro digestibility of myofibrillar proteins from scallop mantle (Patinopecten yessoensis)

Myofibrillar protein (MP) is susceptible to the effect of ionic strength and ultra-high pressure (UHP) treatment, respectively. However, the impact of UHP combined with ionic strength on the structure and in vitro digestibility of MP from scallop mantle (Patinopecten yessoensis) is not yet clear. Therefore, it is particularly important to analyze the structural properties and enhance the in vitro digestibility of MP by NaCl and UHP treatment. The findings demonstrated that as ionic strength increased, the α-helix and β-sheet gradually transformed into β-turn and random coil. The decrease of endogenous fluorescence intensity indicated the formation of a more stable tertiary structure. Additionally, the exposure of internal sulfhydryl groups increased the amount of total sulfhydryl content, and reactive sulfhydryl groups gradually transformed into disulfide bonds. Moreover, it reduces aggregation through increased solubility, decreased turbidity, particle sizes, and a relatively dense and uniform microstructure. When MP from the scallop mantle was treated with 0.5 mol/L ionic strength and 200 MPa UHP treatment, it had the highest solubility (90.75 ± 0.13%) and the lowest turbidity (0.41 ± 0.03). The scallop mantle MP with NaCl of 0.3 mol/L and UHP treatment had optimal in vitro digestibility (95.14 ± 2.01%). The findings may offer a fresh perspectives for developing functional foods for patients with dyspepsia and a theoretical foundation for the comprehensive utilization of scallop mantle by-products with low concentrations of NaCl.

Coupling of zinc and GTP binding drives G-domain folding in Acinetobacter baumannii ZigA

COG0523 proteins, also known as nucleotide-dependent metallochaperones, are a poorly understood class of small P-loop G3E GTPases. Multiple family members play critical roles in bacterial pathogen survival during an infection as part of the adaptive response to host-mediated “nutritional immunity.” Our understanding of the structure, dynamics, and molecular-level function of COG0523 proteins, apart from the eukaryotic homolog, Zng1, remains in its infancy. Here, we use X-ray absorption spectroscopy to establish that Acinetobacter baumannii (Ab) ZigA coordinates ZnII using all three cysteines derived from the invariant CXCC motif to form an S3(N/O) coordination complex, a feature inconsistent with the ZnII-bound crystal structure of a distantly related COG0523 protein of unknown function from Escherichia coli, EcYjiA. The binding of ZnII and guanine nucleotides is thermodynamically linked in AbZigA, and this linkage is more favorable for the substrate GTP relative to the product GDP. Part of this coupling originates with nucleotide-induced stabilization of the G-domain tertiary structure as revealed by global thermodynamics measurements and hydrogen-deuterium exchange mass spectrometry (HDX-MS). HDX-MS also reveals that the HDX behavior of the G2 (switch 1) loop is highly sensitive to nucleotide status and becomes more exchange labile in the GDP (product)-bound state. Significant long-range perturbation of local stability in both the G-domain and the C-terminal domain define a candidate binding pocket for a client protein that appears sensitive to nucleotide status (GDP versus GTP). We place these new insights into the structure, dynamics, and energetics of intermolecular metal transfer into the context of a model for AbZigA metallochaperone function.

Tertiary folds of the SL5 RNA from the 5′ proximal region of SARS-CoV-2 and related coronaviruses

Coronavirus genomes sequester their start codons within stem-loop 5 (SL5), a structured, 5' genomic RNA element. In most alpha- and betacoronaviruses, the secondary structure of SL5 is predicted to contain a four-way junction of helical stems, some of which are capped with UUYYGU hexaloops. Here, using cryogenic electron microscopy (cryo-EM) and computational modeling with biochemically determined secondary structures, we present three-dimensional structures of SL5 from six coronaviruses. The SL5 domain of betacoronavirus severe-acute-respiratory-syndrome-related coronavirus 2 (SARS-CoV-2), resolved at 4.7 Å resolution, exhibits a T-shaped structure, with its UUYYGU hexaloops at opposing ends of a coaxial stack, the T's "arms." Further analysis of SL5 domains from SARS-CoV-1 and MERS (7.1 and 6.4 to 6.9 Å resolution, respectively) indicate that the junction geometry and inter-hexaloop distances are conserved features across these human-infecting betacoronaviruses. The MERS SL5 domain displays an additional tertiary interaction, which is also observed in the non-human-infecting betacoronavirus BtCoV-HKU5 (5.9 to 8.0 Å resolution). SL5s from human-infecting alphacoronaviruses, HCoV-229E and HCoV-NL63 (6.5 and 8.4 to 9.0 Å resolution, respectively), exhibit the same coaxial stacks, including the UUYYGU-capped arms, but with a phylogenetically distinct crossing angle, an X-shape. As such, all SL5 domains studied herein fold into stable tertiary structures with cross-genus similarities and notable differences, with implications for potential protein-binding modes and therapeutic targets.

Are TaNAC Transcription Factors Involved in Promoting Wheat Yield by cis-Regulation of TaCKX Gene Family?

NAC transcription factors (TFs) are one of the largest TF families in plants, and TaNACs have been known to participate in the regulation of the transcription of many yield-regulating genes in bread wheat. The TaCKX gene family members (GFMs) have already been shown to regulate yield-related traits, including grain mass and number, leaf senescence, and root growth. The genes encode cytokinin (CK) degrading enzymes (CKXs) and are specifically expressed in different parts of developing wheat plants. The aim of the study was to identify and characterize TaNACs involved in the cis-regulation of TaCKX GFMs. After analysis of the initial transcription factor data in 1.5 Kb cis-regulatory sequences of a total of 35 homologues of TaCKX GFMs, we selected five of them, namely TaCKX1-3A, TaCKX22.1-3B, TaCKX5-3D, TaCKX9-1B, and TaCKX10, and identified five TaNAC genes: TaNACJ-1, TaNAC13a, TaNAC94, TaNACBr-1, and TaNAC6D, which are potentially involved in the cis-regulation of selected TaCKX genes, respectively. Protein feature analysis revealed that all of the selected TaNACs have a conserved NAC domain and showed a stable tertiary structure model. The expression profile of the selected TaNACs was studied in 5 day-old seedling roots, 5-6 cm inflorescences, 0, 4, 7, and 14 days-after-pollination (DAP) spikes, and the accompanying flag leaves. The expression pattern showed that all of the selected TaNACs were preferentially expressed in seedling roots, 7 and 14 DAP spikes, and flag leaves compared to 5-6 cm inflorescence and 0 and 4 DAP spikes and flag leaves in Kontesa and Ostka spring wheat cultivars (cvs.). In conclusion, the results of this study highlight the potential role of the selected TaNACs in the regulation of grain productivity, leaf senescence, root growth, and response to various stresses.

Effects of nano freezing-thawing on myofibrillar protein of Atlantic salmon fillets: Protein structure and label-free proteomics

This study investigated the impact of magnetic nanoparticles (MNPs) -assisted cryogenic freezing integrated with MNPs combined microwave thawing (NNMT) on the structural integrity of myofibrillar proteins and alterations in protein profiles in salmon fillets. The NNMT showed the lowest myofibrillar fragmentation index (MFI) value (2.73 ± 0.31) among the four freezing-thawing groups. The myofibrillar structure exhibited the highest level of integrity, while the myofibrillar proteins demonstrated minimal aggregation and displayed the most stable secondary and tertiary structures in response to NNMT treatment. Compared with the other three treatments, NNMT exhibited a high abundance of ionic and hydrogen bonds, resulting in stronger interactions between the proteins and water molecules. The label-free proteomics analysis revealed that different freezing-thawing methods primarily affected the cytoskeletal proteins, with collagen and myosin being down-regulated due to degradation caused by cold stress and recrystallization. Additionally, NNMT demonstrated a superior capability in stabilizing salmon cytoskeletal proteins.

Structural Flexibility of Tau in Its Interaction with Microtubules as Viewed by Site-Directed Spin Labeling EPR Spectroscopy.

Tau is a microtubule-associated protein that belongs to the Intrinsically Disordered Proteins (IDPs) family. IDPs or Intrinsically Disordered Regions (IDRs) play key roles in protein interaction networks and their dysfunctions are often related to severe diseases. Defined by their lack of stable secondary and tertiary structures in physiological conditions while being functional, these proteins use their inherent structural flexibility to adapt to and interact with various binding partners. Knowledges on the structural dynamics of IDPs and their different conformers are crucial to finely decipher fundamental biological processes controlled by mechanisms such as conformational adaptations or switches, induced fit, or conformational selection events. Different mechanisms of binding have been proposed: among them, the so-called folding-upon-binding in which the IDP adopts a certain conformation upon interacting with a partner protein, or the formation of a "fuzzy" complex in which the IDP partly keeps its dynamical character at the surface of its partner. The dynamical nature and physicochemical properties of unbound as well as bound IDPs make this class of proteins particularly difficult to characterize by classical bio-structural techniques and require specific approaches for the fine description of their inherent dynamics.Among other techniques, Site-Directed Spin Labeling combined with Electron Paramagnetic Resonance (SDSL-EPR) spectroscopy has gained much interest in this last decade for the study of IDPs. SDSL-EPR consists in grafting a paramagnetic label (mainly a nitroxide radical) at selected site(s) of the macromolecule under interest followed by its observation using and/or combining different EPR strategies. These nitroxide spin labels detected by continuous wave (cw) EPR spectroscopy are used as perfect reporters or "spy spins" of their local environment, being able to reveal structural transitions, folding/unfolding events, etc. Another approach is based on the measurement of inter-label distance distributions in the 1.5-8.0nm range using pulsed dipolar EPR experiments, such as Double Electron-Electron Resonance (DEER) spectroscopy. The technique is then particularly well suited to study the behavior of Tau in its interaction with its physiological partner: microtubules (MTs). In this chapter we provide a detailed experimental protocol for the labeling of Tau protein and its EPR study while interacting with preformed (Paclitaxel-stabilized) MTs, or using Tau as MT inducer. We show how the choice of nitroxide label can be crucial to obtain functional information on Tau/tubulin complexes.

Conserved Residues Lys64 and Glu78 at the Subunit Surface of Tau Glutathione Transferase in Rice Affect Structure and Enzymatic Properties.

Glutathione transferases (GSTs) are a superfamily of dimeric proteins associated with the detoxification of various reactive electrophiles and responsive to a multitude of stressors. We individually substituted Lys64 and Glu78 with Ala using site-directed mutagenesis to understand the role of subunit interactions in the structure and enzymatic properties of a rice GST (OsGSTU17). The wild-type OsGSTU17 lost the conserved hydrogen bond between subunits in tau class GSTs due to conserved Tyr92 replaced with Phe92, but still exhibited high substrate activities, and thermal stability remained in its dimeric structure. The significant decrease in thermal stability and obvious changes in the structure of mutant K64A implied that conserved Lys64 might play an essential role in the structural stability of tau class GSTs. The mutant E78A, supposed to be deprived of hydrogen and salt bonds between subunits, appeared in the soluble form of dimers, even though its tertiary structure altered and stability declined dramatically. These results suggest that the hydrogen and ionic bonds provided by conserved residues are not as important for OsGSTU17 dimerization and enzymatic properties. These results further supplement our understanding of the relationship between the structure and function of GSTs and provide a theoretical basis for improving crop resistance through targeted modification of GSTs.

Effects of konjac glucomannan as a freeze-denaturation inhibitor or binder on the physiochemical properties of heat-induced gel of freeze-dried duck blood.

During freeze-drying, the degradation or eutectic melting of duck blood proteins can reduce the quality of duck blood gels. However, the interaction between proteins and polysaccharides during drying can improve protein-based gel quality. Therefore, here, we investigated the physicochemical properties of heat-induced gels of freeze-dried duck blood (FDB) and FDB with different proportions of the polysaccharide konjac glucomannan (KG), which serves as a freeze-denaturation inhibitor agent (FDA) or binder (BG). The pH and water-holding capacity (WHC) of FDB + KG gels were higher than those of FDB gel without KG (control). Especially, the WHC increased from 11.00% for control to 55.65% for FDB gel with 1% KG as a BG. Consequently, cooking loss and texture parameters of FDB + KG gels decreased. The hardness of control was 2.14kg, which significantly reduced to 0.12-0.87kg with KG addition. The highest carbonyl content was observed in control gel, and the thiobarbituric acid reactive substance content was reduced by the addition of 1% KG as an FDA (T1) or 0.8% KG as an FDA with 0.2% KG as a BG (T2) (p<0.05). These changes might be induced by the alteration of tertiary structure and thermodynamic stability of gels. In conclusion, 1% KG can be used as an FDA to improve the quality and physicochemical properties of heat-induced gels of FDB. Optimized FDB gels with KG can be used as an innovative food ingredient to fortify nutrition and develop special dietary purposes.

Influence of pulsed electric field on thawing of frozen pork: Physical properties, fat oxidation and protein structure

This study explored the effects of constant-voltage pulsed electric field thawing (CV-T) and constant-current pulsed electric field thawing (CC-T) on electrical characteristics, temperature distribution and qualities of porcine longissimus dorsi muscle, considering air thawing (AT) and water immersion thawing (WT) for the comparison, additionally making fresh meat (FM) as the control. The results demonstrated that the impedance of pork decreased rapidly from -20 to -8 °C, while the changing trend slowed down from -8 to 0 °C; higher frequency was conducive to lowering its impedance and to the thawing. In terms of the effect, pulsed electric field thawing reduced the duration by 33% to 85%, avoiding the loss significantly (P<0.05); however, there was no significant difference in cooking loss (P>0.05). CC-T indicated better temperature uniformity than CV-T, and consistently maintained the surface temperature difference at 0.3 - 9 °C. Therefore, CC-T significantly reduced water migration, muscle damage as well as protein and fat oxidation during the process (P<0.05), meanwhile maintaining colour, water retention and texture properties of the muscle. Circular dichroism and fluorescence spectrometry indicated that CC-T significantly reduced cross-linking, aggregation and sedimentation of myofibrillar proteins (P<0.05) with more stable secondary and tertiary structures than other treatments. In addition, microstructural observations showed that CC-T reduced muscle damage owing to ice crystal rapid melting. Therefore, CC-T could improve the thawing speed, temperature rise uniformity, and quality of pork, which demonstrates the potential of the CC-T method for application.