Redox-induced conformational changes in myoglobin and hemoglobin: electrochemistry and ultraviolet-visible and Fourier transform infrared difference spectroscopy at surface-modified gold electrodes in an ultra-thin-layer spectroelectrochemical cell

Effects of Ionic Liquids on Metalloproteins

In the past decade, innovative protein therapies and bio-similar industries have grown rapidly. Additionally, ionic liquids (ILs) have been an area of great interest and rapid development in industrial processes over a similar timeline. Therefore, there is a pressing need to understand the structure and function of proteins in novel environments with ILs. Understanding the short-term and long-term stability of protein molecules in IL formulations will be key to using ILs for protein technologies. Similarly, ILs have been investigated as part of therapeutic delivery systems and implicated in numerous studies in which ILs impact the activity and/or stability of protein molecules. Notably, many of the proteins used in industrial applications are involved in redox chemistry, and thus often contain metal ions or metal-associated cofactors. In this review article, we focus on the current understanding of protein structure-function relationship in the presence of ILs, specifically focusing on the effect of ILs on metal containing proteins.

Protonation of the Cysteine Axial Ligand Investigated in His/Cys c-Type Cytochrome by UV-Vis and Mid- and Far-IR Spectroscopy

His/Cys coordination was recently found in several c-type cytochromes, which could act as sensors, in electron transport or in regulation. Toward a better understanding of Cys function and reactivity in these cytochromes, we compare cytochrome c6 (c6wt) from the cyanobacterium Nostoc PCC 7120 with its Met58Cys mutant. We probe the axial ligands and heme properties by combining visible and mid- to far-FTIR difference spectroscopies. Cys58 determines the strong negative redox potential and pH dependence of M58C (E_mM58C = -375 mV, versus E_mc6wt = +339 mV). Mid-IR (notably Cys ν(SH), His ν(C₅N₁), heme δ(CmH)) and far-IR (ν(Fe(II)-His), ν(His-Fe(III)-Cys)) markers of the heme and ligands show that Cys58 remains a strong thiolate ligand of reduced Met58Cys at alkaline pH, while it is protonated at pH 7.5, is stabilized by a strong hydrogen bonding interaction, and weakly interacts with Fe(II). These data provide a benchmark for further analysis of c-type cytochromes with natural His/Cys coordination.

Structural Origin of the Large Redox-Linked Reorganization in the 2-Oxoglutarate Dependent Oxygenase, TauD

2-Oxoglutarate (2OG)-dependent oxygenases catalyze a wide range of chemical transformations via C-H bond activation. Prior studies raised the question of whether substrate hydroxylation by these enzymes occurs via a hydroxyl rebound or alkoxide mechanism and highlighted the need to understand the thermodynamic properties of transient intermediates. A recent spectroelectrochemical investigation of the 2OG-dependent oxygenase, taurine hydroxylase (TauD), revealed a strong link between the redox potential of the Fe(II)/Fe(III) couple and conformational changes of the enzyme. In this study, we show that the redox potential of wild-type TauD varies by 468 mV between the reduction of 2OG-Fe(III)-TauD (-272 mV) and oxidation of 2OG-Fe(II)-TauD (+196 mV). We use active site variants to investigate the structural origin of the redox-linked reorganization and the contributions of the metal-bound residues to the dynamic tuning of the redox potential of TauD. Time-dependent redox titrations show that reorganization occurs as a multistep process. Transient optical absorption and infrared spectroelectrochemistry show that substitution of any metal ligand alters the kinetics and thermodynamics of the reorganization. The H99A variant shows the largest net redox change relative to the wild-type protein, suggesting that redox-coupled protonation of H99 is required for high redox potentials of the metal. The D101Q and H255Q variants also suppress the conformational change, supporting their involvement in the structural rearrangement. Similar redox-linked conformational changes are observed in another 2OG dependent oxygenase, ethylene-forming enzyme, indicating that dynamic structural flexibility and the associated thermodynamic tuning may be a common phenomenon in this family of enzymes.

Fourier Transform Infrared Spectrovoltammetry and Quantitative Modeling of Analytes in Kinetically Constrained Redox Mixtures

Redox-active analytes that do not support direct electron transfer on the electrode, such as proteins with buried redox centers, pose challenges to characterization of their structural and thermodynamic properties. Investigations of indirect transitions in analytes supported by complex redox mixtures require a careful balance between kinetic limitations and spectral interference from the mediators. Using methylene green and thionine acetate as redox mediators and myoglobin as the analyte, we demonstrate that normal pulse spectrovoltammetry (NPSV) with Fourier transform infrared (FT-IR) detection and subsequent global spectral regression analysis can resolve structural and thermodynamic properties simultaneously with little a priori information. Both the E_1/2 and unbiased redox difference FT-IR spectra of the Fe(II)/Fe(III) redox couple of myoglobin in reduction and oxidation NPSV modes were in good agreement with those reported earlier by independent techniques. The thermodynamic and kinetic limitations of mediators/analyte interactions were investigated using comprehensive semiempirical kinetic simulation models. This modeling effort yielded a flexible computational tool capable of quantitatively predicting the redox response in mediated electrochemical studies and defining its limitations, thus greatly expanding the range and precision of the formal mediator/analyte concentration ratio rule.

Effect of Crotalus basiliscus snake venom on the redox reaction of myoglobin

In this work, we have studied the effect of Crotalus basiliscus snake venom on the redox reaction of myoglobin (Mb), and by means of electrochemical techniques, we have shown that this reaction is undoubtedly affected following the interaction with the venom. Surface plasmon resonance, electrophoresis, UV-Vis, and circular dichroism showed that the interaction involves the attachment of some constituent of the venom to the protein, although not affecting its first and secondary structures. Mass spectra support this suggestion by showing the appearance of signals assigned to the Mb dimer and to a new species resulting from the interaction between Mb and the venom proteins. In addition, the mass spectra suggest the aromatic amino acids of myoglobin, mainly tryptophan and phenylalanine, are more exposed to the solvent medium upon the exposure to the venom solution. The results altogether indicate that the harmful effects of the venom of Crotalus basiliscus snake are likely connected to the blocking of the redox site of Mb.

An amperometric H2O2 biosensor based on hemoglobin nanoparticles immobilized on to a gold electrode

The nanoparticles (NPs) of hemoglobin (Hb) were prepared by desolvation method and characterized by transmission electron microscopy (TEM), UV spectroscopy and Fourier-transform IR (FTIR) spectroscopy. An amperometric H₂O₂ biosensor was constructed by immobilizing HbNPs covalently on to a polycrystalline Au electrode (AuE). HbNPs/AuE were characterized by scanning electron microscopy (SEM), cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) before and after immobilization of HbNPs. The HbNPs/AuE showed optimum response within 2.5 s at pH 6.5 in 0.1 M sodium phosphate buffer (PB) containing 100 μM H₂O₂ at 30°C, when operated at –0.2 V against Ag/AgCl. The HbNPs/AuE exhibited V_max of 5.161 ± 0.1 μA cm⁻² with apparent Michaelis-Menten constant (K_m) of 0.1 ± 0.01 mM. The biosensor showed lower detection limit (1.0 μM), high sensitivity (129 ± 0.25 μA cm⁻² mM⁻¹) and wider linear range (1.0–1200 μM) for H₂O₂ as compared with earlier biosensors. The analytical recoveries of added H₂O₂ in serum (0.5 and 1.0 μM) were 97.77 and 98.01% respectively, within and between batch coefficients of variation (CV) were 3.16 and 3.36% respectively. There was a good correlation between sera H₂O₂ values obtained by standard enzymic colorimetric method and the present biosensor (correlation coefficient, R² =0.99). The biosensor measured H₂O₂ level in sera of apparently healthy subjects and persons suffering from diabetes type II. The HbNPs/AuE lost 10% of its initial activity after 90 days of regular use, when stored dry at 4°C.

Hemoglobin-Conjugated Gelatin Microsphere as a Smart Oxygen Releasing Biomaterial

In this study, a novel micrometric biomaterial acting as a cyclic oxygen releasing system is designed. Human hemoglobin (Hb) is conjugated to the surface of gelatin microspheres (GM) to produce gelatin hemoglobin oxygen depot (G-HbOD). G-HbOD is obtained by means of two different conjugation strategies. The degree of conjugation of GM surfaces in terms of free amino groups by using HPLC is first evaluated. By following the strategy A (G-HbOD_A), Hb is conjugated to GM by means of the formation of a polyurethane linker. In the strategy B (G-HbOD_B) the conjugation occurs via amide bound formation. Physical and morphological differences between G-HbOD_A and G-HbOD_B are investigated by means of Fourier Transform Infrared Spectroscopy (FTIR), Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). Differences in oxygen uptake/release kinetics are found depending on the conjugation strategy and it is proved that G-HbOD works under repeated cycles in microfluidic chip. Moreover, G-HbOD is also able to work as oxygen depot in the early stages of 3D cell cultures.

Surface Enhanced Resonance Raman Spectroscopy Reveals Potential Induced Redox and Conformational Changes of Cytochrome c Oxidase on Electrodes

Immobilization of Cytochrome c oxidase (CcO) on electrodes makes voltage-driven reduction of oxygen to water possible. Efficient catalytic turnover in CcO/electrode systems is, however, often observed at large overpotentials that cannot be rationalized by the redox properties of the enzyme itself. To understand the structural basis for this observation, CcO was electrostatically adsorbed on amino-functionalized Ag electrodes, and the redox transitions of heme a and a3 were monitored via surface enhanced resonance Raman spectroscopy (SERRS) as a function of applied potential. Under completely anaerobic conditions, the reduction of heme a3 could be seen at potentials close to those measured in solution indicating an intact catalytic center. However, in the immobilized state, a new non-native heme species was observed that exhibited a redox potential much more negative than measured for the native hemes. Analysis of the high and low frequency SERR spectra indicated that this new species is formed from heme a upon axial loss of one histidine ligand. It is concluded that the formation of the non-native heme a species alters the potential-dependent electron supply to the catalytic reaction and, thus, can have a impact on the applicability of this enzyme in biofuel cells.

Investigation on the conformation change of hemoglobin immobilized on MPA-modified electrode by electrochemical method

The conformation change of bovine hemoglobin (Hb) during the unfolding process induced by urea and acid was investigated by an electrochemical method. Hb unfolding induced by urea of different concentrations was realized by bonding Hb onto a 3-mercaptopropionic acid (MPA) modified gold electrode. The difference in unfolding percentage showed that the Hb unfolding induced by urea was a two-step, three-state transition process, while the unfolding induced by acid was a two-state transition process. The results obtained by the electrochemical method coincided closely with those obtained by UV-vis spectroscopy and fluorescence spectroscopy. Some thermodynamic parameters during the conformational change were also calculated to study the intermediate state during the Hb unfolding process. The present work may lead to an easy and effective way to study metalloproteins unfolding, and holds great promise for the design of novel sensitive biosensors.

Developing SyrinOX total antioxidant capacity assay for measuring antioxidants in humans

Accurate monitoring of the antioxidant status or of oxidative stress in patients is still a big challenge in clinical laboratories. This study investigates the possibility of applying a newly developed total antioxidant capacity assay method based on laccase or peroxidase oxidized syringaldazine [Tetramethoxy azobismethylene quinone (TMAMQ)] which is referred to here as SyrinOX, as a diagnostic tool for monitoring both oxidative stress and antioxidant status in patients. Attempts to adapt the Randox total antioxidant procedure [simultaneous incubation of the radical generating system (metmyoglobin and H(2) O(2) ) and antioxidant sample] for SyrinOX were abandoned after it was discovered that the H(2) O(2) reacted with enzymatically generated TMAMQ and ABTS radicals at a rate of 6.4 × 10(-2) /μM/s and 5.7 × 10(-3) /μM/s respectively. Thus this study for the first time demonstrates the negative effects of H(2) O(2) in the Randox system. This leads to erroneous results because the total antioxidant values obtained are the sum of radicals reduced by antioxidants plus those reacting with the radical generating system. Therefore they should be avoided not only for this particular method but also when using other similar methods. Consequently, SyrinOX is best applied using a three-step approach involving, production of TMAMQ, recovery and purification (free from enzyme and other impurities) and then using TMAMQ for measuring the total antioxidant capacity of samples. Using this approach, the reaction conditions for application of SyrinOX when measuring the total antioxidant capacity of plasma sample were determined to be 50% (v/v) ethanol/50 mM sodium succinate buffer pH 5.5, between 20 and 25 °C for at least 1 h.

Equilibrium and kinetic behavior of Fe(CN)6(3-/4-) and cytochrome c in direct electrochemistry using a film electrode thin-layer transmission cell

We report on the design and performance of a thin-layer electrochemical cell optimized for use with optically transparent film electrodes in combination with UV/vis and IR transmission spectroscopic measurements. The cell allows for measurements under both aerobic and anaerobic conditions. The direct, unmediated electron transfer, as assessed by the current transient, and the corresponding optical response observed for the Fe(CN)(6)(3-/4-) couple were in good agreement with theoretical predictions for voltammetry and optical absorption by an analyte confined in a thin layer. Chronoamperometric and spectroscopic measurements of Fe(CN)(6)(3-/4-) on gold mesh electrode revealed fast kinetics strongly influenced by the electrolyte concentration. Maximal apparent rates exceeding 2 s(-1) in 1 M KCl were observed optically. The direct kinetic and thermodynamic behavior of cytochrome c was compared with several electrode materials using the cell. The results showed heme ligand-dependent changes in the protein-electrode interactions. Mid-UV/visible spectral changes upon redox transitions in native cytochrome c and its cyanide derivative, as well as dissociation of the ferrous cytochrome c-CN complex, are reported.

Amperometric hydrogen peroxide biosensor based on immobilization of hemoglobin on a glassy carbon electrode modified with fe(3)o(4)/chitosan core-shell microspheres

Novel magnetic Fe₃O₄/chitosan (CS) microspheres were prepared using magnetic Fe₃O₄ nanoparticles and the natural macromolecule chitosan. Then, using an easy and effective hemoglobin (Hb) immobilization method, an innovative biosensor with a Fe₃O₄/CS-Hb-Fe₃O₄/CS “sandwich” configuration was constructed. This biosensor had a fast (less than 10 s) response to H₂O₂ and excellent linear relationships were obtained in the concentration range of 5.0 × 10⁻⁵ to 1.8 × 10⁻³ M and 1.8 × 10⁻³ to 6.8 × 10⁻³ M with a detection limit of 4.0 × 10⁻⁶ M (s/n = 3) under the optimum conditions. The apparent Michaelis-Menten constant K_m was 0.29 mM and it showed the excellent biological activity of the fixed Hb. Moreover, the biosensor had long-time stability and good reproducibility. The method was used to determine H₂O₂ concentration in real samples.

Time-resolved flow-flash FT-IR difference spectroscopy: the kinetics of CO photodissociation from myoglobin revisited

Fourier-transform infrared (FT-IR) difference spectroscopy has been proven to be a significant tool in biospectroscopy. In particular, the step-scan technique monitors structural and electronic changes at time resolutions down to a few nanoseconds retaining the multiplex advantage of FT-IR. For the elucidation of the functional mechanisms of proteins, this technique is currently limited to repetitive systems undergoing a rapid photocycle. To overcome this obstacle, we developed a flow-flash experiment in a miniaturised flow channel which was integrated into a step-scan FT-IR spectroscopic setup. As a proof of principle, we studied the rebinding reaction of CO to myoglobin after photodissociation. The use of microfluidics reduced the sample consumption drastically such that a typical step-scan experiment takes only a few 10 ml of a millimolar sample solution, making this method particularly interesting for the investigation of biological samples that are only available in small quantities. Moreover, the flow cell provides the unique opportunity to assess the reaction mechanism of proteins that cycle slowly or react irreversibly. We infer that this novel approach will help in the elucidation of molecular reactions as complex as those of vectorial ion transfer in membrane proteins. The potential application to the oxygen splitting reaction of cytochrome c oxidase is discussed.

Porous nanosheet-based ZnO microspheres for the construction of direct electrochemical biosensors

Nanosheet-based ZnO microsphere with porous nanostructures was synthesized by a facile chemical bath deposition method followed by thermal treatment, which was explored for the construction of electrochemical biosensors. Spectroscopic and electrochemical researches revealed the ZnO-based composite was a biocompatible immobilization matrix for enzymes with good enzymatic stability and bioactivity. With advantages of nanostructured inorganic-organic hybrid materials, a pair of stable and well-defined quasi-reversible redox peaks of hemoglobin was obtained with a formal potential of -0.345 V (vs. Ag/AgCl) in pH 7.0 buffer. Facilitated direct electron transfer of the metalloenzymes with an apparent heterogeneous electron transfer rate constant (k(s)) of 3.2s(-1) was achieved on the ZnO-based enzyme electrode. Comparative studies demonstrated the nanosheet-based ZnO microspheres were more effective in facilitating the electron transfer of immobilized enzyme than solid ZnO microspheres, which may result from the unique nanostructures and larger surface area of the porous ZnO. The prepared biosensor displayed good performance for the detection of H(2)O(2) and NaNO(2) with a wide linear range of 1-410 and 10-2700 microM, respectively. The entrapped hemoglobin exhibits high peroxidase-like activity for the catalytic reduction of H(2)O(2) with an apparent Michaelis-Menten constant (K(M)(app)) of 143 microM. The nanosheet-based ZnO could be a promising matrix for the fabrication of direct electrochemical biosensors, and may find wide potential applications in biomedical detection and environmental analysis.

Methods to probe protein transitions with ATR infrared spectroscopy

We describe techniques that can be used in conjunction with modern attenuated total reflection (ATR) infrared micro-prisms to allow proteins to be manipulated cyclically between different states whilst simultaneously monitoring both mid-IR and UV/visible/near IR changes. These methods provide increased flexibility of the types of changes that can be induced in proteins in comparison to transmission methods. Quantitative measurements can be made of vibrational changes associated with conversion between stable catalytic reaction intermediates, ligand binding and oxidation-reduction. Both hydrophobic and soluble proteins can be analysed and the ability to induce transitions repetitively allows IR difference spectra to be acquired at a signal/noise sufficient to resolve changes due to specific cofactors or amino acids. Such spectra can often be interpreted at the atomic level by standard IR methods of comparisons with model compounds, by isotope and mutation effects and, increasingly, by ab initio simulations. Combination of such analyses with atomic 3D structural models derived from X-ray and NMR studies can lead to a deeper understanding of molecular mechanisms of enzymatic reactions.

Redox infrared markers of the heme and axial ligands in microperoxidase: bases for the analysis of c-type cytochromes

Structural changes accompanying the change in the redox state of microperoxidase-8 (MP8), the heme-octapeptide obtained from cytochrome c, and its complexes with (methyl)imidazole ligands were studied by electrochemically induced Fourier transform IR (FTIR) difference spectroscopy. To correlate with confidence IR modes with a specific electronic state of the iron, we used UV-vis and electron paramagnetic resonance spectroscopy to define precisely the heme spin state in the samples at the millimolar concentration of MP8 required for FTIR difference spectroscopy. We identified four intense redox-sensitive IR heme markers, nu38 at 1,569 cm(-1) (ox)/1,554 cm(-1) (red), nu42 at 1,264 cm(-1) (ox)/1,242 cm(-1) (red), nu43 at 1,146 cm(-1) (ox), and nu44 at 1,124-1,128 cm(-1) (ox). The intensity of nu42 and nu43 was clearly enhanced for low-spin imidazole-MP8 complexes, while that of nu44 increased for high-spin MP8. These modes can thus be used as IR markers of the iron spin state in MP8 and related c-type cytochromes. Moreover, one redox-sensitive band at 1,044 cm(-1) (red) is attributed to an IR marker specific of c-type hemes, possibly the delta(CbH3)(2,4) heme mode. Other redox-sensitive IR bands were assigned to the MP8 peptide backbone and to the fifth and sixth axial heme ligands. The distinct IR frequencies for imidazole (1,075 cm(-1)) and histidine (1,105 cm(-1)) side chains in the imidazole-MP8 complex allowed us to provide the first direct determination of their pKa at pH 9 and 12, respectively.

Effect of nano cadmium sulfide on the electron transfer reactivity and peroxidase activity of hemoglobin

Hemoglobin (Hb) is immobilized with cadmium sulfide (CdS) nanoparticles (NPs) on pyrolytic graphite (PG) electrode to characterize the electrochemical reactivity and peroxidase activity of the protein. The result demonstrates that fine redox waves of Hb can be achieved after this protein is entrapped in CdS NPs. Meanwhile, the protein can exhibit nice catalytic activity towards hydrogen peroxide (H2O2). Linear relationship between the reductive peak current and the H2O2 concentration has been obtained from 5.0 x 10(-6) to 4.0 x 10(-4) mol/L, on the basis of which a new kind of H2O2 biosensor might be developed in the future.

Direct electrochemistry and Raman spectroscopy of sol-gel-encapsulated myoglobin

The direct electrochemistry of myoglobin (Mb) has been observed at a glassy carbon (GC) electrode coated with silica sol-gel-encapsulated Mb film. A well-behaved cyclic voltammogram is observed with a midpoint potential (E(1/2)) of -0.25 V vs Ag/AgCl in a pH 7.0 phosphate buffer. This potential, which is pH-dependent, is 70-90 mV more negative than the formal potential values obtained by using the spectroeletrochemical titration method at the same pH. Square wave voltametry (SWV) also shows a peak potential of -0.25 V for the reduction of Mb under the same experimental conditions. Both cathodic and anodic peak currents have a linear relationship with the scan rate. The midpoint potential decreases with pH, having a slope of -30 mV/pH. UV-vis and resonance Raman spectroscopic studies reveal that the sol-gel provides a bio-compatible environment where Mb retains a structure similar to its solution form, a 6-coordinated aquomet myoglobin. These results suggest that the silica sol-gel is a useful matrix for studying direct electrochemistry of other heme proteins.

Studies on direct electron transfer and biocatalytic properties of heme proteins in lecithin film

Myoglobin (Mb), hemoglobin (Hb) and horseradish peroxidase (HRP) were incorporated in lecithin (PC) film on glassy carbon (GC) electrode by the method of vesicle-fusion. A pair of well-defined and quasi-reversible cyclic voltammetric peaks was obtained, which reflected the direct electron transfer of heme proteins. UV-Vis and reflectance absorption infrared (RAIR) spectroscopy showed that proteins in PC films remained at their secondary structure similar to their native states. Scanning electron microscopy (SEM) demonstrated the interaction between the proteins and PC would make the morphology of protein-PC films very different from the PC films alone. The immobilized proteins retained their biocatalytic activity to the reduction of NO and hydrogen peroxide, which provide the perspective to be the third generation sensors.

Heme protein films with polyamidoamine dendrimer: direct electrochemistry and electrocatalysis

Biocompatible nanosized polyamidoamine (PAMAM) dendrimer films provided a suitable microenvironment for heme proteins to transfer electron directly with underlying pyrolytic graphite (PG) electrodes. Hemoglobin (Hb), myoglobin (Mb), horseradish peroxidase (HRP), and catalase (Cat) incorporated in PAMAM films exhibited a pair of well-defined, quasi-reversible cyclic voltammetric peaks, respectively, characteristic of the protein heme Fe(III)/Fe(II) redox couples. While Hb-, Mb-, and HRP-PAMAM films showed the cyclic voltammetry (CV) peaks at about -0.34 V vs. saturated calomel electrode (SCE) in pH 7.0 buffers, Cat-PAMAM films displayed the peak pair at a more negative potential of -0.47 V. The protein-PAMAM films demonstrated a surface-confined or thin-layer voltammetric behavior. The electrochemical parameters such as apparent heterogeneous electron transfer rate constants (k(s)) and formal potentials (E (degrees ')) were estimated by square wave voltammetry with nonlinear regression analysis. UV-vis and IR spectroscopy showed that the proteins retained their near-native secondary structures in PAMAM films. Oxygen, hydrogen peroxide, and nitrite were catalytically reduced at the protein-PAMAM film electrodes, showing the potential applicability of the films as the new type of biosensors or bioreactors based on direct electrochemistry of the proteins.

Enhanced Peroxidase Activity of Hemoglobin in a DNA Membrane and Its Application to an Unmediated Hydrogen Peroxide Biosensor

Hemoglobin can exhibit not only a direct electron transfer reacting after being entrapped in a DNA membrane, but also a greatly enhanced peroxidase activity toward the reduction of hydrogen peroxide. Based on the direct electrochemical property and nice enzymatic activity of the protein in a DNA membrane, a reagentless hydrogen peroxide biosensor was prepared. The peak current related to hydrogen peroxide was linearly proportional to its concentration in the range of 1.9 x 10(-6)-6.8 x 10(-4) mol L(-1). The detection limit was 1 x 10(-6) mol/L.

Electrochemical biosensors utilising electron transfer in heme proteins immobilised on Fe3O4nanoparticles

Fe3O4 nanoparticles cast on pyrolytic graphite (PG) electrodes were used to immobilize hemoglobin (Hb), myoglobin (Mb) and horseradish peroxidase (HRP). The Fe3O4 nanoparticles provided a favorable microenvironment for the proteins to directly transfer electrons with electrodes. The protein-Fe3O4 films were used to electrochemically catalyze the reduction of oxygen, trichloroacetic acid, nitrite and hydrogen peroxide, and showed a potential applicability in fabricating biosensors. Transmission electron microscopy (TEM), UV-visible absorption and reflectance absorption infrared (RAIR) spectroscopy, and cyclic and square wave voltammetry, were used to characterize the films.

Inhibition of Human Hemoglobin Autoxidaiton by Sodium n-Dodecyl Sulphate

The effect of sodium n-dodecyl sulphate (SDS) on hemoglobin autoxidation was studied in the presence of a 100 mM phosphate buffer (pH 7.0) by different methods. These included spectrophotometry, fluorescence technique, cyclic voltametry, differential scanning calorimetry, and densitometry. Spectroscopic studies showed that SDS concentrations up to 1 mM increased deoxy-, decreases oxy-, and had no significant effect on the met- conformation of hemoglobin. Therefore, a SDS concentration up to 1 mM increased the deoxy form of hemoglobin as the folded, compact state and decreases the oxy conformation. The turbidity measurements and differential scanning calorimetry techniques indicated a more stable conformation for hemoglobin in the presence of SDS up to 1 mM. Electrochemical studies also confirmed a more difficult oxidation under these conditions. The induction of the deoxy form in the presence of SDS was confirmed by densitometry techniques. The compact structure of deoxyhemoglobin blocks the formation of met-conformation in low SDS concentrations.

Structural electrochemical study of hemoglobin by in situ circular dichroism thin layer spectroelectrochemistry

Secondary and tertiary or quaternary structural changes in hemoglobin (HB) during an electroreduction process were studied by in situ circular dichroism (CD) spectroelectrochemistry with a long optical path thin-layer cell. By means of singular value decomposition least-squares analysis, CD spectra in the far-UV region give two similar alpha components with different CD intensity, indicating slight denaturation in the secondary structures due to the electric field effect. CD spectra in the Soret band show a R-->T transition of two quaternary structural components induced by electroreduction of the heme, which changes the redox states of the center ion from Fe3+ to Fe2+ and the co-ordination number from 6 to 5. The double logarithmic analysis shows that electroreduction of hemoglobin follows a chemical reaction with R-->T transition. Some parameters in the electrochemical process were obtained: formal potential, E0'=-0.167 V; electrochemical kinetic overpotential, deltaE0=-0.32 V; standard electrochemical reaction rate constant, k0=1.79 x 10(-5) cm s(-1); product of electron transfer coefficient and electron number, alphan=0.14; and the equilibrium constant of R-->T transition, Kc=9.0.

Electron-transfer reactivity and enzymatic activity of hemoglobin in a SP Sephadex membrane

Hemoglobin can exhibit a direct electron-transfer reaction after being entrapped in a SP Sephadex membrane. A pair of stable and well-defined redox waves are obtained at a hemoglobin-SP sephadex modified pyrolytic graphite electrode. The anodic and cathodic peak potentials are located at -0.244 and -0.336 V (vs SCE), respectively. On the other hand, the peroxidase activity of the protein in the membrane is also greatly enhanced. The apparent Michaelis-Menten constant is calculated to be 1.9 mM, which shows a large catalytic activity of hemoglobin in the SP Sephadex membrane toward hydrogen peroxide (H2O2). According to the direct electron-transfer property and enhanced peroxidase activity of Hb in the membrane, a Hb/SP Sephadex membrane-based H2O2 biosensor is prepared, with a linear range approximately 5.0 x 10(-6) to 1.6 x 10(-4) mol/L.

Lanthanide ions induce hydrolysis of hemoglobin-bound 2,3-diphosphoglycerate (2,3-DPG), conformational changes of globin and bidirectional changes of 2,3-DPG-hemoglobin's oxygen affinity

The changes in structure and function of 2,3-diphosphoglycerate-hemoglobin (2,3-DPG-Hb) induced by Ln(3+) binding were studied by spectroscopic methods. The binding of lanthanide cations to 2,3-DPG is prior to that to Hb. Ln(3+) binding causes the hydrolysis of either one from the two phosphomonoester bonds in 2,3-DPG non-specifically. The results using the ultrafiltration method indicate that Ln(3+) binding sites for Hb can be classified into three categories: i.e. positive cooperative sites (N(I)), non-cooperative strong sites (N(S)) and non-cooperative weak sites (N(W)) with binding constants in decreasing order: K(I)>K(S)>K(W). The total number of binding sites amounts to about 65 per Hb tetramer. Information on reaction kinetics was obtained from the change of intrinsic fluorescence in Hb monitored by stopped-flow fluorometry. Fluctuation of fluorescence dependent on Ln(3+) concentration and temperature was observed and can be attributed to the successive conformational changes induced by Ln(3+) binding. The results also reveal the bidirectional changes of the oxygen affinity of Hb in the dependence on Ln(3+) concentration. At the range of [Ln(3+)]/[Hb]<2, the marked increase of oxygen affinity (P(50) decrease) with the Ln(3+) concentration can be attributed to the hydrolysis of 2,3-DPG, while the slight rebound of oxygen affinity in higher Ln(3+) concentration can be interpreted by the transition to the T-state of the Hb tetramer induced by Ln(3+) binding. This was indicated by the changes in secondary structure characterized by the decrease of alpha-helix content.

Insight into protein structure and protein-ligand recognition by Fourier transform infrared spectroscopy

An overview of the application of Fourier transform infrared spectroscopy for the analysis of the structure of proteins and protein-ligand recognition is given. The principle of the technique and of the spectra analysis is demonstrated. Spectral signal assignments to vibrational modes of the peptide chromophore, amino acid side chains, cofactors and metal ligands are summarized. Several examples for protein-ligand recognition are discussed. A particular focus is heme proteins and, as an example, studies of cytochrome P450 are reviewed. Fourier transform infrared spectroscopy in combination with the various techniques such as time-resolved and low-temperature methods, site-directed mutagenesis and isotope labeling is a helpful approach to studying protein-ligand recognition.

Temperature dependence of the formal reduction potential of putidaredoxin

Putidaredoxin (Pdx), a [2Fe-2S] redox protein of size M(r) 11,600, transfers two electrons in two separate steps from the flavin containing putidaredoxin reductase to the heme protein, cytochrome CYP101 in the P450cam catalytic cycle. It has recently come to light, through NMR measurements, that there can be appreciable differences in the Pdx conformational dynamics between its reduced and oxidized states. The redox reaction entropy, deltaS(0')rc = (S(0')Pdx(r)-S(0')Pdx(0)), as determined from measurements of the variation in formal potential with temperature, E0'(T), provides a measure of the strength of this influence on Pdx function. We designed a spectroelectrochemical cell using optically transparent tin oxide electrodes, without fixed or diffusible mediators, to measure E0'(T) over the temperature range 0-40 degrees C. The results indicate that the redox reaction entropy for Pdx is biphasic, decreasing from -213 +/- 27 J mol(-1) K(-1) over 0-27 degrees C, to -582 +/- 150 J mol(-1) K (-1) over 27-40 degrees C. These redox reaction entropy changes are significantly more negative than the changes reported for most cytochromes, although our measurement over the temperature interval 0-27 degrees C is in the range reported for other iron-sulfur proteins. This suggests that Pdx (and other ferredoxins) is a less rigid system than monohemes, and that redox-linked changes in conformation, and/or conformational dynamics, impart to these proteins the ability to interact with a number of redox partners.

Orally administrated cerium chloride induces the conformational changes of rat hemoglobin, the hydrolysis of 2,3-DPG and the oxidation of heme-Fe(II), leading to changes of oxygen affinity

The structure and oxygen affinity of hemoglobin from erythrocytes of CeCl(3) fed Wistar rats in the dose range of 0.2-20.0 mg/kg body weight/day were investigated by means of various spectroscopic methods. The changes in oxygen saturation curves of hemoglobin are dependent upon both feeding dose and feeding time. After 40 days feeding with 20 mg CeCl(3)/kg body weight/day, the curve changed to a double sigmoid shape and the oxygen affinity in low oxygen pressure increases. It regained the sigmoid form after 80 days feeding, but the degree of oxygen saturation in higher oxygen pressure became higher than that in the control. These results indicate that CeCl(3) can increase the oxygen affinity of hemoglobin of rat erythrocytes. This effect is further demonstrated by the analysis of Mössbauer spectra of erythrocytes. Increase of hemoglobin content in erythrocytes was found in rats fed with CeCl(3). It might be the offset response to the poor oxygen-releasing capability of the hemoglobin. CD and FT-IR deconvoluted spectra indicate that secondary structures of hemoglobin have remarkable changes, characterized by a gradual decrease of alpha-helix content, in a dose- and feeding time-dependent fashion. Meanwhile, the 31P NMR spectra demonstrate that the level of 2,3-diphosphoglyceric acid (2,3-DPG) in erythrocytes, an allosteric regulator of oxygen release from hemoglobin, decreases due to its hydrolysis. In addition, the Mössbauer and ESR spectra show clearly that a fraction of the heme-iron changes from Fe (II) to Fe (III) in CeCl(3) fed rats. The results indicate that the oral administration of CeCl(3) leads to a microenvironment changes of heme in intracellular hemoglobin. Oxygen affinity changes might be attributed to a series of events triggered by the binding of Ce (III) to hemoglobin and 2,3-DPG, including conformational changes of hemoglobin and 2,3-DPG hydrolysis, respectively and also the partial transformation from heme-Fe (II) to heme-Fe (III).