New Insights into Horseradish Peroxidase Function in Benzene from Resonance Raman Spectroscopy

Effect of high hydrostatic pressure on activity, thermal stability and structure of horseradish peroxidase

The mechanism of the high hydrostatic pressure (HHP) effect on horseradish peroxidase (HRP) is still unclear. The activity, thermal stability and structural changes of HRP after HHP treatments were studied in this work. Compared with the untreated sample, the enzyme activity reduces by 36% after 800 MPa processing. The results indicated that the conformation of the enzyme active center changes under pressure. Furthermore, HHP also changes the conformation of disulfide bonds and some secondary structures in HRP. These structural and conformational changes induce decreased activity. In addition, differential thermal scanning (DSC) results showed that the thermal denaturation temperature decreased from 103.74 °C to 85.78 °C after pressure treatment, suggesting HRP molecules formed large aggregates after pressure treatment. In this study, the interaction mechanism between pressure and enzyme was studied as well, and the results can provide some guidance for the application of HHP technology in fruit and vegetable products processing.

Using spectroscopic tools to probe porphyrin deformation and porphyrin-protein interactions

The reactivity and functionality of heme proteins are to a significant extent determined by the conformation of their functional heme groups and by the interaction of axial ligands with their protein environment. This review focuses on experimental methods and theoretical concepts for elucidating symmetry lowering perturbations of the heme induced by the protein environment of the heme pocket. First, we discuss a variety of methods which can be used to probe the electric field at the heme, including spectral hole burning as well as low temperature absorption and room temperature circular dichroism spectroscopy. Second, we show how heme deformations can be described as superposition of deformations along normal coordinates, thereby using the irreducible representations of the D4h point group as a classification tool. Finally, resonance Raman spectroscopy is introduced as a tool to probe the deformations of metalloprophyrins in solution and in protein matrices by measuring and comparing intensities and depolarization properties rather than wavenumber positions.

Spectroscopic Characterization and Electrochemistry of Poly(ethylene oxide)-Modified Myoglobin in Organic Solvents

Myoglobin (Mb) was chemically modified with activated poly(ethylene oxide) (PEO) (average molecular weight of 2000) to solubilize it in various organic solvents. UV-vis, circular dichroism, and Raman spectroscopy were used to characterize the structure correlated with the electron-transfer reactions of PEO-modified Mb (PEO-Mb). Spectroscopic data indicated changes in heme coordination geometry for PEO-Mb in various organic solvents that are different from that in water. The Raman spectrum showed the characteristics of PEO-Mb in PEO oligomer (average MW of 200) in the five-coordinate high-spin state, which facilitates fast electron-transfer reactions between protein and the glassy carbon electrode. These results suggest heme environment effects on the properties of proteins in organic solvents.

Structure-function relationship of reduced cytochrome c probed by complete solution structure determination in 30% acetonitrile/water solution

The complete solution structure of ferrocytochrome c in 30% acetonitrile/70% water has been determined using high-field 1D and 2D (1)H NMR methods and deposited in the Protein Data Bank with codes 1LC1 and 1LC2. This is the first time a complete solution protein structure has been determined for a protein in nonaqueous media. Ferrocyt c retains a native protein secondary structure (five alpha-helices and two omega loops) in 30% acetonitrile. H18 and M80 residues are the axial heme ligands, as in aqueous solution. Residues believed to be axial heme ligands in the alkaline-like conformers of ferricyt c, specifically H33 and K72, are positioned close to the heme iron. The orientations of both heme propionates are markedly different in 30% acetonitrile/70% water. Comparative structural analysis of reduced cyt c in 30% acetonitrile/70% water solution with cyt c in different environments has given new insight into the cyt c folding mechanism, the electron transfer pathway, and cell apoptosis.

Heme structural perturbation of PEG-modified horseradish peroxidase C in aromatic organic solvents probed by optical absorption and resonance Raman dispersion spectroscopy

The heme structure perturbation of poly(ethylene glycol)-modified horseradish peroxidase (HRP-PEG) dissolved in benzene and toluene has been probed by resonance Raman dispersion spectroscopy. Analysis of the depolarization ratio dispersion of several Raman bands revealed an increase of rhombic B(1g) distortion with respect to native HRP in water. This finding strongly supports the notion that a solvent molecule has moved into the heme pocket where it stays in close proximity to one of the heme's pyrrole rings. The interactions between the solvent molecule, the heme, and the heme cavity slightly stabilize the hexacoordinate high spin state without eliminating the pentacoordinate quantum mixed spin state that is dominant in the resting enzyme. On the contrary, the model substrate benzohydroxamic acid strongly favors the hexacoordinate quantum mixed spin state and induces a B(2g)-type distortion owing to its position close to one of the heme methine bridges. These results strongly suggest that substrate binding must have an influence on the heme geometry of HRP and that the heme structure of the enzyme-substrate complex (as opposed to the resting state) must be the key to understanding the chemical reactivity of HRP.

Structure of poly(ethylene glycol)-modified horseradish peroxidase in organic solvents: infrared amide I spectral changes upon protein dehydration are largely caused by protein structural changes and not by water removal per se

Fourier transform infrared (FTIR) spectroscopy has emerged as a powerful tool to guide the development of stable lyophilized protein formulations by providing information on the structure of proteins in amorphous solids. The underlying assumption is that IR spectral changes in the amide I and III region upon protein dehydration are caused by protein structural changes. However, it has been claimed that amide I IR spectral changes could be the result of water removal per se. Here, we investigated whether such claims hold true. The structure of horseradish peroxidase (HRP) and poly(ethylene glycol)-modified HRP (HRP-PEG) has been investigated under various conditions (in aqueous solution, the amorphous dehydrated state, and dissolved/suspended in toluene and benzene) by UV-visible (UV-Vis), FTIR, and resonance Raman spectroscopy. The resonance Raman and UV-Vis spectra of dehydrated HRP-PEG dissolved in neat toluene or benzene were very similar to that of HRP in aqueous buffer, and thus the heme environment (heme iron spin, coordination, and redox state) was essentially the same under both conditions. Therefore, the three-dimensional structure of HRP-PEG dissolved in benzene and toluene was similar to that in aqueous solution. The amide I IR spectra of HRP-PEG in aqueous buffer and of dehydrated HRP-PEG dissolved in neat benzene and toluene were also very similar, and the secondary structure compositions (percentages of alpha-helices and beta-sheets) were within the standard error the same. These results are irreconcilable with recent claims that water removal per se could cause substantial amide I IR spectral changes (M. van de Weert, P.I. Haris, W.E. Hennink, and D.J. Crommelin. 2001. Anal. Biochem. 297:160-169). On the contrary, amide I IR spectral changes upon protein dehydration are caused by perturbations in the secondary structure.

Spectroscopic study on structure of horseradish peroxidase in water and dimethyl sulfoxide mixture

The structure of horseradish peroxidase (HRP) in phosphate buffered saline (PBS)/dimethyl sulfoxide (DMSO) mixed solvents at different compositions is investigated by IR, electronic absorption, and fluorescence spectroscopies. The fluorescence spectra and the amide I spectra of ferric HRP [HRP(Fe3+)] show that overall structural changes are relatively small up to 60% DMSO. Although the amide I band of HRP(Fe3+) shows a gradual change in the secondary structure and a decrease in the contents of a helices, its fluorescence spectra indicate that the distance between the heme and Trp173 is almost constant. In contrast, the changes in the positions of the Soret bands for resting HRP(Fe3+) and catalytic intermediates (compounds I and II) and the IR spectra at the C-O stretching vibration mode of carbonyl ferrous HRP [HRP(Fe2+)-CO] show that the microenvironment in the distal heme pocket is altered, even with low DMSO contents. The large reduction of the catalytic activity of HRP even at low DMSO contents can be attributed to the structural transition in the distal heme pocket. In PBS/DMSO mixtures containing more than 70 vol % DMSO, HRP undergoes large structural changes, including a large loss of the secondary structure and a dissociation of the heme from the apoprotein. The presence of the components of the amide I band that can be assigned to strongly hydrogen bonding amide C=O groups at 1616 and 1684 cm(-1) suggests that the denatured HRP may aggregate through strong hydrogen bonds.

Raman evidence that the lyoprotectant poly(ethylene glycol) does not restore nativity to the heme active site of horseradish peroxidase suspended in organic solvents

Resonance Raman spectroscopy was used to interrogate the heme active site of horseradish peroxidase (HRP) lyophilized in the presence and absence of the lyoprotectant poly(ethylene glycol) (PEG; FW 5000; 0-80% w/w) suspended in acetone, chloroform, or acetonitrile. In aqueous solution, Fe(3+)HRP is characterized by a five-coordinate high-spin (5-c HS) heme system. The structure of the heme-active site of HRP in all solvents is perturbed by co-lyophilization of HRP with PEG. Heme active site structural changes are consistent with coordination of water in the distal axial coordination site of the ferric heme iron and disruption of the hydrogen-bond network when the protein is lyophilized in the presence of PEG (>or=60% w/w) in all of the solvent systems studied. Similar active site structural changes were previously observed for HRP in benzene and attributed to a change in the reaction mechanism for HRP in benzene. (Mabrouk, P. A.; Spiro, T. G. J. Am. Chem. Soc. 1998, 120, 10303-10309.) Thus, PEG is proposed to increase the catalytic activity of HRP in nonaqueous media by locking the heme active site into a structure that functions through an alternative catalytic pathway in nonaqueous media.

Structural change and catalytic activity of horseradish peroxidase in oxidative polymerization of phenol

The effects of solvent and reaction conditions on the catalytic activity of horseradish peroxidase (HRP) were investigated for oxidative polymerization of phenol in water/organic mixtures using hydrogen peroxide as an oxidant. Also, the structural changes of HRP were investigated by CD and absorption spectroscopy in these solvents. The results suggest that the yield of phenol polymer (the conversion of phenol to polymer) is strongly affected by the reaction conditions due to the structural changes of HRP, that is, the changes in higher structure of the apo-protein and dissociation or decomposition of the prosthetic heme. Optimum solvent compositions for phenol polymerization depend on the nature of the organic solvents owing to different effects of the solvents on HRP structure. In addition to initial rapid changes, slower changes of HRP structure occur in water/organic solvents especially at high concentrations of organic solvents. In parallel with these structural changes, catalytic activity of HRP decreases with time in these solvents. At higher reaction temperatures, the yield of the polymer decreases, which is also ascribed to modification of HRP structure. It is known that hydrogen peroxide is an inhibitor of HRP, and the yield of phenol polymer is strongly dependent on the manner of addition of hydrogen peroxide to the reaction solutions. The polymer yield decreases significantly when hydrogen peroxide was added to the reaction solution in a large amount at once. This is probably due to inactivation of HRP by excess hydrogen peroxide. From the CD and absorption spectra, it is suggested that excess hydrogen peroxide causes not only decomposition of the prosthetic heme but also modification of the higher structure of HRP.

Biocatalysis in low-water media: understanding effects of reaction conditions

The key role played by counter-ions with enzymes in low-water systems has become better appreciated with, for example, large effects on enantioselectivity. In low-dielectric media, counter-ions will associate strongly with charges in the protein or its substrates. Studies of temperature dependence have shown that hard-to-model entropies have a significant effect on behaviour, including enantioselectivity. Evidence has been presented that the supramolecular organisation of enzyme molecules can have important effects on behaviour, for example collapse of microstructure in cross-linked crystals.