Catalysis of Hydrogen Evolution by Polylysine, Polyarginine and Polyhistidine at Mercury Electrodes

Electrochemistry in sensing of molecular interactions of proteins and their behavior in an electric field

Electrochemical methods can be used not only for the sensitive analysis of proteins but also for deeper research into their structure, transport functions (transfer of electrons and protons), and sensing their interactions with soft and solid surfaces. Last but not least, electrochemical tools are useful for investigating the effect of an electric field on protein structure, the direct application of electrochemical methods for controlling protein function, or the micromanipulation of supramolecular protein structures. There are many experimental arrangements (modalities), from the classic configuration that works with an electrochemical cell to miniaturized electrochemical sensors and microchip platforms. The support of computational chemistry methods which appropriately complement the interpretation framework of experimental results is also important. This text describes recent directions in electrochemical methods for the determination of proteins and briefly summarizes available methodologies for the selective labeling of proteins using redox-active probes. Attention is also paid to the theoretical aspects of electron transport and the effect of an external electric field on the structure of selected proteins. Instead of providing a comprehensive overview, we aim to highlight areas of interest that have not been summarized recently, but, at the same time, represent current trends in the field.

AGR2-AGR3 hetero-oligomeric complexes: Identification and characterization

In this paper we compare electrochemical behavior of two homolog proteins, namely anterior gradient 2 (AGR2) and anterior gradient 3 (AGR3), playing an important role in cancer cell biology. The slight variation in their protein structures has an impact on protein adsorption and orientation at charged surface and also enables AGR2 and AGR3 to form heterocomplexes. We confirm interaction between AGR2 and AGR3 (i) in vitro by immunochemical and constant current chronopotentiometric stripping (CPS) analysis and (ii) in vivo by bioluminescence resonance energy transfer (BRET) assay. Mutation of AGR2 in dimerization domain (E60A) prevents development of wild type AGR2 dimers and also negatively affects interaction with wild type AGR3 as shown by CPS analysis. Beside new information about AGR2 and AGR3 protein including their joint interaction, our work introduces possible applications of CPS in bioanalysis of protein complexes, including those relatively unstable, but important in the cancer research.

Bovine Serum Albumin Catalysed Hydrogen and Deuterium Evolution at Mercury Electrodes

The hydrogen evolution reaction (HER), catalysed by proteins at mercury electrodes and reflected in chronopotentiometric stripping peak H, provides a label-free and reagentless analytical technique that is sensitive to protein structure. Here we show how the kinetic isotope effect affected the HER catalysed by the protein bovine serum albumin (BSA). We found that the deuteron bond, which is stronger than that of a proton, contributed to less effective transport of deuterons mediated by BSA at the Hg|D₂ O interface, and enhanced structural stability of the surface-attached native BSA in D₂ O solution. A structural transition was also observed in the surface-attached urea-denatured BSA, and is probably due to the destabilisation of some secondary structural remnants retained by the 17 SS-bonds. Because the catalytically active groups involved in proton or deuteron transfer in native proteins are often exposed towards solutions and their protons exchange almost instantly, no signs of H/D exchange were observed in native BSA using peak H under the given conditions.

Influence of Protein Modification and Glycosylation in the Catalytic Hydrogen Evolution Reaction of Avidin and Neutravidin: An Electrochemical Analysis

To investigate glycans' influence on the behavior of glycoproteins on charged surfaces, avidin and its nonglycosylated and neutralized version neutravidin were studied by label-free chronopotentiometric stripping (CPS) analysis and alternating current voltammetry combined with a mercury electrode. Despite neutravidin's and avidin's similar size and structure, their CPS responses differed due to the different amounts of catalytically active free amino groups of lysine and arginine residues. Acetylation of the proteins resulted in the suppression of their CPS responses by almost four times for avidin and by about 50 % for neutravidin, respectively. On the other hand, the presence of glycans in the acetylated avidin induced about 30 % higher chronopotentiometric response compared to the acetylated neutravidin. We suggest that the presence, size and composition of the glycans influenced the CPS signal due to differences in the orientation at a charged surface. The obtained results can be utilized in glycoprotein research.

Chronopotentiometric sensing of specific interactions between lysozyme and the DNA aptamer

Specific DNA-protein interactions are vital for cellular life maintenance processes, such as transcriptional regulation, chromosome maintenance, replication and DNA repair, and their monitoring gives valuable information on molecular-level organization of those processes. Here, we propose a new method of label-free electrochemical sensing of sequence specific binding between the lysozyme protein and a single stranded DNA aptamer specific for lysozyme (DNA_apta) that exploits the constant current chronopotentiometric stripping (CPS) analysis at modified mercury electrodes. Specific lysozyme-DNA_apta binding was distinguished from nonspecific lysozyme-DNA interactions at thioglycolic acid-modified mercury electrodes, but not at the dithiothreitol-modified or bare mercury electrodes. Stability of the surface-attached lysozyme-DNA_apta layer depended on the stripping current (I_str) intensity, suggesting that the integrity of the layer critically depends on the time of its exposure to negative potentials. Stabilities of different lysozyme-DNA complexes at the negatively polarized electrode surface were tested, and it was shown that structural transitions of the specific lysozyme-DNA_apta complexes occur in the I_str ranges different from those observed for assemblies of lysozyme with DNA sequences capable of only nonspecific lysozyme-DNA interactions. Thus, the CPS allows distinct discrimination between specific and non-specific protein-DNA binding and provides valuable information on stability of the nucleic acid-protein interactions at the polarized interfaces.

Electrocatalytic monitoring of peptidic proton-wires

The transfer of protons or proton donor/acceptor abilities is an important phenomenon in many biomolecular systems. One example is the recently proposed peptidic proton-wires (H-wires), but the ability of these His-containing peptides to transfer protons has only been studied at the theoretical level so far. Here, for the first time the proton transfer ability of peptidic H-wires is examined experimentally in an adsorbed state using an approach based on a label-free electrocatalytic reaction. The experimental findings are complemented by theoretical calculations at the ab initio level in a vacuum and in an implicit solvent. Experimental and theoretical results indicated Ala3(His-Ala2)6 to be a high proton-affinity peptidic H-wire model. The methodology presented here could be used for the further investigation of the proton-exchange chemistry of other biologically or technologically important macromolecules.

Investigation of protein FTT1103 electroactivity using carbon and mercury electrodes. Surface-inhibition approach for disulfide oxidoreductases using silver amalgam powder

Recently, it was shown that electrochemical methods can be used for analysis of poorly water-soluble proteins and for study of their structural changes and intermolecular (protein-ligand) interactions. In this study, we focused on complex electrochemical investigation of recombinant protein FTT1103, a disulfide oxidoreductase with structural similarity to well described DsbA proteins. This thioredoxin-like periplasmic lipoprotein plays an important role in virulence of bacteria Francisella tularensis. For electrochemical analyses, adsorptive transfer (ex situ) square-wave voltammetry with pyrolytic graphite electrode, and alternating-current voltammetry and constant-current chronopotentiometric stripping analysis with mercury electrodes, including silver solid amalgam electrode (AgSAE) were used. AgSAE was used in poorly water-soluble protein analysis for the first time. In addition to basic redox, electrocatalytic and adsorption/desorption characterization of FTT1103, electrochemical methods were also used for sensitive determination of the protein at nanomolar level and study of its interaction with surface of AgSA microparticles. Proposed electrochemical protocol and AgSA surface-inhibition approach presented here could be used in future for biochemical studies focused on proteins associated with membranes as well as on those with disulfide oxidoreductase activity.