Electrolytic reduction: modification of proteins occurring in isoelectric focusing electrophoresis and in electrolytic reactions in the presence of high salts

A facile double moving redox boundary model for visual electrophoresis titration of ascorbic acid

In this work, we proposed a double moving redox boundary (MROB) model to realize the colorless analyte electrophoresis titration (ET) by the two steps of the redox reaction. Single MROB has been proposed for the development of ET sensing (Analyst, 2013, 138, 1137. ACS Sensor, 2019, 4, 126.), and faces great challenges in detecting the analyte without color change during redox reaction. Herein, a novel model of double-MROB electrophoresis, including its mechanisms, equations, and procedures, was developed for titration by using ascorbic acid as a model analyte. The first MROB was created with ferric iron (Fe3+) and iodide ion (I-) in which Fe3+ was reduced as Fe2+ and I- was oxidized as molecular iodine (I2) used as an indicator of visible MROB due to blue starch-iodine complex. The second boundary was then formed between the molecular iodine and model analyte of ascorbic acid. Under given conditions, there was a quantitative relationship between velocity of MROB (VMROB(ii)) and ascorbic acid concentration (CVit C) in the double-MROB system (1/VMROB(ii) = 0.6502CVit C + 4.5165, and R = 0.9939). The relevant relative standard deviation values of intraday and inter-day were less than ∼5.55% and ∼6.64%, respectively. Finally, the titration of ascorbic acid in chewable vitamin C tablets was performed by the developed method, the titration results agreed with those via the classic iodometric titration. All the results briefly demonstrated the validity of the double MROB model, in which Vit C was used as a model analyte. The developed method had potential use in quantitative analysis of redox-active species in biomedical samples.

"Yellow" laccase from Sclerotinia sclerotiorum is a blue laccase that enhances its substrate affinity by forming a reversible tyrosyl-product adduct

Yellow laccases lack the typical blue type 1 Cu absorption band around 600 nm; however, multi-copper oxidases with laccase properties have been reported. We provide the first evidence that the yellow laccase isolated from Sclerotinia sclerotiorum is obtained from a blue form by covalent, but nevertheless reversible modification with a phenolic product. After separating the phenolics from the extracellular medium, a typical blue laccase is obtained. With ABTS as model substrate for this blue enzyme, a non-natural purple adduct is formed with a spectrum nearly identical to that of the 1:1 adduct of an ABTS radical and Tyr. This modification significantly increases the stability and substrate affinity of the enzyme, not by acting primarily as bound mediator, but by structural changes that also alters the type 1 Cu site. The HPLC-MS analyses of the ABTS adduct trypsin digests revealed a distinct tyrosine within a unique loop as site involved in the modification of the blue laccase form. Thus, S. sclerotiorum yellow laccase seems to be an intrinsically blue multi-copper oxidase that boosts its activity and stability with a radical-forming aromatic substrate. This particular case could, at least in part, explain the enigma of the yellow laccases.

A platform for chemical modification of mandelate racemase: characterization of the C92S/C264S and γ-thialysine 166 variants

Mandelate racemase (MR) serves as a paradigm for our understanding of enzyme-catalyzed deprotonation of a carbon acid substrate. To facilitate structure-function studies on MR using non-natural amino acid substitutions, we engineered the Cys92Ser/Cys264Ser variant (dmMR) as a platform for introducing Cys residues at specific locations for subsequent covalent modification. While the highly reactive thiol of Cys furnishes a site for chemical modification, site-specificity requires that other Cys residues be non-reactive or replaced by a non-reactive amino acid, especially if chemical modification is conducted under denaturing conditions. The catalytic efficiency of dmMR is reduced only ~2-fold relative to wild-type MR, making dmMR a viable platform for the site-specific introduction of Cys. As an example, the inactive Lys166Cys variant of dmMR was treated with ethylenimine under denaturing conditions to replace the Brønsted acid-base catalyst Lys 166 with the non-natural amino acid γ-thialysine. Comparison of the pH-activity profiles of dmMR and the active γ-thialysine variant revealed a reduction in the pKa for the side chain amino group of ~0.4 units for the latter variant. Unlike wild-type MR for which diffusion is partially rate-limiting, dmMR and the γ-thialysine variant showed no dependence on the solvent viscosity suggesting that the chemical step is fully rate-limiting.

An Improved 2-Dimensional Gel Electrophoresis Method for Resolving Human Erythrocyte Membrane Proteins

The 2-dimensional gel electrophoresis (2-DE) technique is widely used for the analysis of complex protein mixtures extracted from biological samples. It is one of the most commonly used analytical techniques in proteomics to study qualitative and quantitative protein changes between different states of a cell or an organism (eg, healthy and diseased), conditionally expressed proteins, posttranslational modifications, and so on. The 2-DE technique is used for its unparalleled ability to separate thousands of proteins simultaneously. The resolution of the proteins by 2-DE largely depends on the quality of sample prepared during protein extraction which increases results in terms of reproducibility and minimizes protein modifications that may result in artifactual spots on 2-DE gels. The buffer used for the extraction and solubilization of proteins influences the quality and reproducibility of the resolution of proteins on 2-DE gel. The purification by cleanup kit is another powerful process to prevent horizontal streaking which occurs during isoelectric focusing due to the presence of contaminants such as salts, lipids, nucleic acids, and detergents. Erythrocyte membrane proteins serve as prototypes for multifunctional proteins in various erythroid and nonerythroid cells. In this study, we therefore optimized the selected major conditions of 2-DE for resolving various proteins of human erythrocyte membrane. The modification included the optimization of conditions for sample preparation, cleanup of protein sample, isoelectric focusing, equilibration, and storage of immobilized pH gradient strips, which were further carefully examined to achieve optimum conditions for improving the quality of protein spots on 2-DE gels. The present improved 2-DE analysis method enabled better detection of protein spots with higher quality and reproducibility. Therefore, the conditions established in this study may be used for the 2-DE analysis of erythrocyte membrane proteins for different diseases, which may help to identify the proteins that may serve as markers for diagnostics as well as targets for development of new therapeutic potential.

Methylglyoxal in cells elicits a negative feedback loop entailing transglutaminase 2 and glyoxalase 1

Glyoxalase 1 (GlxI) is the key enzyme that converts the highly reactive α-oxo-aldehydes into the corresponding α-hydroxy acids using l-glutathione as a cofactor. In our preliminary data, GlxI was identified as a substrate of transglutaminase 2 (TG2), a ubiquitous enzyme with multiple functions. According to the catalytic properties of TG2, protein cross-linking, polyamine conjugation, and/or deamidation are potential post-translational modifications. In this article, we have demonstrated that TG2 catalyzes either polyamine conjugation or deamidation to GlxI depending on the presence of polyamines or not. Deamidation leads to activation of GlxI while polyamine conjugation results in activation of GlxI as well as stabilization of GlxI against denaturation treatment. In cultured HeLa cells, methylglyoxal challenge causes increase in intracellular levels of reactive oxygen species (ROS) and calcium leading to TG2 activation and subsequent transamidation and activation of GlxI. The inhibition of TG2 significantly weakens the cell resistance to the methylglyoxal challenge. Thus, GlxI is a novel substrate of TG2 and is activated by TG2 in vitro and in cellulo. Exposure to methylglyoxal elicits a negative feedback loop entailing ROS, calcium, TG2 and GlxI, thus leading to attenuation of the increase in the methylglyoxal level. The results imply that cancer cells highly express TG2 or GlxI can endure the oxidative stress derived from higher glycolytic flux and may gain extra growth advantage from the aerobic glycolysis.

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We have demonstrated novel modifications of glyoxalase I by transglutaminase 2.

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The modifications mediated by transglutaminse 2 modulate the glyoxalase I activities.

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Methylglyoxal treatment in cells induces increases in the levels of endogenous reactive oxygen species and activation transglutaminase 2 and glyoxalase I.

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Cells dispose the accumulated intracellular methylglyoxal by a negative feedback loop consisting of reactive oxygen species, calcium, transglutaminase 2 and glyoxalase I.

Isoelectric focusing management: an investigation for salt interference and an algorithm for optimization

Two-dimensional electrophoresis (2-DE) has evolved into a robust separation technique in proteomic research. However, one of the major challenges in 2-DE experiments, the reproducibility of the first dimensional electrophoresis (IEF), has remained unsolved. It is well-known that the quality of IEF experiments is significantly affected by the salt interference. Nevertheless, the interference mechanisms of salts in IEF have never been systematically investigated. In this study, we comprehensively investigated the interference effects in IEF due to various kinds of simple and buffer salts in protein samples. Two interference schemes were proposed accordingly to elucidate the interference mechanisms of salts in IEF. Furthermore, to increase the reproducibility of IEF, we proposed that conductivity measurement is a feasible method to assess the salt content of 2-DE samples and developed an algorithm to predict the optimal total volt-hours (Vh) required for protein focusing in IEF. The developed algorithm had been evaluated under various IEF conditions for a variety of 2-DE samples and proven to be a reliable guide. In sum, information disclosed in this study should be of use for increasing the reproducibility and thus the applicability of 2-DE in current proteomics.

Development of a membrane-less dynamic field gradient focusing device for the separation of low-molecular-weight molecules

Dynamic field gradient focusing uses an electric field gradient generated by controlling the voltage profile of an electrode array to separate and concentrate charged analytes according to their individual electrophoretic mobilities. This study describes a new instrument in which the electrodes have been placed within the separation channel. The major challenge faced with this device is that when applied voltages to the electrodes are larger than the redox potential of water, electrolysis will occur, producing hydrogen ions (H+) plus oxygen gas on the anodes and hydroxide (OH(-)) plus hydrogen gas on the cathodes. The resulting gas bubbles and pH excursions can cause problems with system performance and reproducibility. An on-column, degassing system that can remove gas bubbles "on-the-fly" is described. In addition, the use of a high capacity, low-conductivity buffer to address the problem of the pH shift that occurs due to the production of H+ on the anodes is illustrated. Finally, the successful separation of three, low-molecular-weight dyes (amaranth, bromophenol blue and methyl red) is described.