Electrochemistry of heme–thiolate proteins

Surface charge-controlled electron transfer and catalytic behavior of immobilized cytochrome P450 BM3 inside dendritic mesoporous silica nanoparticles

Understanding the influencing factors on the reaction kinetics of P450 BM3 within confined spaces is essential for developing efficient P450 BM3 bioreactors. Herein, two dendritic mesoporous silica nanoparticles (OH-DMSNs and NH₂-DMSNs) with similar pore size but opposite surface charge have been prepared and served as the vehicle to immobilize P450 BM3. With the help of the film-forming material of chitosan, P450 BM3/OH-DMSN and P450 BM3/NH₂-DMSN composites were immobilized on GC electrode and characterized with electrochemical measurements. Compared with P450 BM3/OH-DMSNs/GCE, P450 BM3/NH₂-DMSNs/GCE showed higher electron transfer efficiency with higher current charge and lower k_s value. Besides, the generated catalytic current towards testosterone on P450 BM3/NH₂-DMSNs/GCE was 1.81 times larger than P450 BM3/OH-DMSNs/GCE. Furthermore, P450 BM3 inside NH₂-DMSNs displayed higher affinity towards testosterone with the lower K_m^app value of 244.82 μM. These results are attributed to the positively charged internal walls of NH₂-DMSNs so that P450 BM3 adapts to an orientation favorable for electron exchange with electrodes and substrate binding with the active sites. The present study provides fundamentals for regulating the surface charge to optimize redox process and catalytic behavior in CYP bioreactors through electrostatic interactions.

[Methods for determining of cytochrome P450 isozymes functional activity]

The review is dedicated to modern methods and technologies for determining of cytochrome P450 isozymes functional activity, such as absorbance and fluorescent spectroscopy, electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), Raman, Mossbauer, and X-ray spectroscopy, surface plasmon resonance (SPR), atomic force microscopy (AFM). Methods of molecular genetic analysis were reviewed from personalized medicine point of view. The use of chromate-mass-spectrometric methods for cytochrome P450-dependent catalytic reactions' products was discussed. The review covers modern electrochemical systems based on cytochrome P450 isozymes for their catalytic activity analysis, their use in practice and further development perspectives for experimental pharmacology, biotechnology and translational medicine.

Directed evolution of P450cin for mediated electron transfer

Directed evolution is a powerful method to optimize enzyme properties for application demands. Interesting targets are P450 monooxygenases which catalyze the stereo- and regiospecific hydroxylation of chemically inert C-H bonds. Synthesis employing P450s under cell-free reaction conditions is limited by low total turnover numbers, enzyme instability, low product yields and the requirement of the expensive co-factor NADPH. Bioelectrocatalysis is an alternative to replace NADPH in cell-free P450-catalyzed reactions. However, natural enzymes are often not suitable for using non-natural electron delivery systems. Here we report the directed evolution of a previously engineered P450 CinA-10aa-CinC fusion protein (named P450cin-ADD-CinC) to use zinc/cobalt(III)sepulchrate as electron delivery system for an increased hydroxylation activity of 1,8-cineole. Two rounds of Sequence Saturation Mutagenesis (SeSaM) each followed by one round of multiple site-saturation mutagenesis of the P450 CinA-10aa-CinC fusion protein generated a variant (Gln385His, Val386Ser, Thr77Asn, Leu88Arg; named KB8) with a 3.8-fold increase in catalytic efficiency (28 µM^-1 min^-1) compared to P450cin-ADD-CinC (7 µM^-1 min^-1). Furthermore, variant KB8 exhibited a 1.5-fold higher product formation (500 µM µM^-1 P450) compared to the equimolar mixture of CinA, CinC and Fpr using NADPH as co-factor (315 µM µM^-1 P450). In addition, electrochemical experiments with the electron delivery system platinum/cobalt(III)sepulchrate showed that the KB8 variant had a 4-fold higher product formation rate (0.16 nmol (nmol) P450^-1 min^-1 cm^-2) than the P450cin-ADD-CinC (0.04 nmol (nmol) P450^-1 min^-1 cm^-2). In summary, the current work shows prospects of using directed evolution to generate P450 enzymes suitable for use with alternative electron delivery systems.

Electrocatalytic cycle of P450 cytochromes: the protective and stimulating roles of antioxidants

This study reports the investigation of the catalytic activity of isolated cytochromes from the cytochrome P450 superfamily. Electrochemically driven CYP reactions may have practical relevance, providing a useful tool for drug assay studies.

Novel oxazolinyl derivatives of pregna-5,17(20)-diene as 17α-hydroxylase/17,20-lyase (CYP17A1) inhibitors

New oxazolinyl derivatives of [17(20)E]-pregna-5,17(20)-diene: 2'-{[(E)-3β-hydroxyandrost-5-en-17-ylidene]methyl}-4',5'-dihydro-1',3'-oxazole 1 and 2'-{[(E)-3β-hydroxyandrost-5-en-17-ylidene]methyl}-4',4'-dimethyl-4',5'-dihydro-1',3'-oxazole 2 were evaluated as potential CYP17A1 inhibitors in comparison with 17-(pyridin-3-yl)androsta-5,16-dien-3β-ol 3 (abiraterone). Differential absorption spectra of human recombinant CYP17A1 in the presence of compound 1 (λmax=422 nm, λmin=386 nm) and compound 2 (λmax=416 nm) indicated significant differences in enzyme/inhibitors complexes. CYP17A1 activity was measured using electrochemical methods. Inhibitory activity of compound 1 was comparable with abiraterone 3 (IC50=0.9±0.1 μM, and IC50=1.3±0.1 μM, for compounds 1 and 3, respectively), while compound 2 was found to be weaker inhibitor (IC50=13±1 μM). Docking of aforementioned compounds to CYP17A1 revealed that steroid fragments of compound 1 and abiraterone 3 occupied close positions; oxazoline cycle of compound 1 was coordinated with heme iron similarly to pyridine cycle of abiraterone 3. Configuration of substituents at 17(20) double bond in preferred docked position corresponded to Z-isomers of compounds 1 and 2. Presence of 4'-substituents in oxazoline ring of compound 2 prevents coordination of oxazoline nitrogen with heme iron and worsens its docking score in comparison with compound 1. These data indicate that oxazolinyl derivative of [17(20)E]-pregna-5,17(20)-diene 1 (rather than 4',4'-dimethyl derivative 2) may be considered as potential CYP17A1 inhibitor and template for development of new compounds affecting growth and proliferation of prostate cancer cells.

Thiol, disulfide, and trisulfide complexes of Ru porphyrins: potential models for iron-sulfur bonds in heme proteins

Thirty-two Ru(porp)L(2) complexes have been synthesized, where porp = the dianion of meso-tetramesitylporphyrin (TMP) or meso-tetrakis(4-methylphenyl)porphyrin (H(2)T-pMe-PP), and L = a thiol, a sulfide, a disulfide, or a trisulfide. Species studied were with RSH [R = Me, Et, (n)Pr, (i)Pr, (t)Bu, Bn (benzyl), and Ph], RSR (R = Me, Bn), RSSR (R = Me, Et, (n)Pr, Bn) and MeSS(t)Bu, and RSSSR (R = Me, Bn). All the species except two, which were the isolated Ru(T-pMe-PP)((t)BuSH)(2) and Ru(TMP)(MeSSMe)(2), were characterized in situ. The disulfide complex was characterized by X-ray analysis. (1)H NMR data for the coordinated thiols are the first reported within metalloporphyrin systems, and are especially informative because of the upfield shifts of the axial sulfur-containing ligands due to the porphyrin π-ring current effect, which is also present in the di- and trisulfide species. The disulfide in the solid state structure of Ru(TMP)(MeSSMe)(2) is η(1)(end-on) coordinated, the first example of such bonding in a nontethered, acyclic dialkyl disulfide; (1)H-(1)H EXSY NMR data in solution show that the species undergoes 1,2-S-metallotropic shifts. Stepwise formation of the bis(disulfide) complex from Ru(TMP)(MeCN)(2) in solution occurs with a cooperativity effect, resembling behavior of Fe(II)-porphyrin systems where crystal field effects dominate, but ligand trans-effects are more likely in the Ru system. The η(1)(end-on) coordination mode is also favored for the trisulfide ligand. Discussed also are the remarkable linear correlations that exist between the ring-current shielding shifts for the axial ligand C(1) protons of Ru(porp)(RS(x)R)(2) and x (the number of S atoms). The Introduction briefly reviews literature on Ru- and Fe porphyrins (including heme proteins) with sulfur-containing ligands or substrates, and relationships between our findings and this literature are discussed throughout the paper.

[Screening of potential substrates or inhibitors of cytochrome P450 17A1 (CYP17A1) by electrochemical methods]

The electrochemical reduction of the recombinant form of human cytochrome P450 17A1 (CYP17A1) was investigated. Hemeprotein was immobilized on electrode modified with biocompatable nanocomposite material based on the membrane-like synthetic surfactant didodecyldimethylammonium bromide (DDAB) and gold nanoparticles. Analytical characteristics of DDAB/Au/CYP17A1 electrodes were investigated with cyclic voltammetry, square wave voltammetry, and differential pulse voltammetry. Analysis of electrochemical behaviour of cytochrome P450 17A1 was conducted in the presence of substrate pregnenolone (1), inhibitor ketoconazole (2), and in the presence of synthetic derivatives of pregnenolone: acetylpregnenolone (3), cyclopregnenolone (4), and tetrabrompregnenolone (5). Ketoconazole, azole inhibitor of cytochromes P450, blocked catalytic current in the presence of substrate pregnenolone (1). Compounds 3-5 did not demonstrate substrate properties towards electrode/CYP17A1 system. Compound 3 did not block catalytic activity towards pregnenolone, but compounds 4 and 5 inhibited such activity. Electrochemical reduction of CYP17A1 may serve as an adequate substitution of the reconstituted system which requires additional redox partners - for the exhibition of catalytic activity of hemoproteins of the cytochrome P450 superfamily.

Bioelectronic delivery of electrons to cytochrome P450 enzymes

Cytochrome P450s (cyt P450s) are the major oxidative enzymes in human oxidative metabolism of drugs and xenobiotic chemicals. In nature, the iron heme cyt P450s utilize oxygen and electrons delivered from NADPH by a reductase enzyme to oxidize substrates stereo- and regioselectively. Significant research has been directed toward achieving these events electrochemically. This Feature Article discusses the direct electrochemistry of cyt P450s in thin films and the utilization of such films for electrochemically driven biocatalysis. Maintaining and confirming structural integrity and catalytic activity of cyt P450s in films is an essential feature of these efforts. We highlight here our efforts to elucidate the influence of iron heme spin state and secondary structure of human cyt P450s on voltammetric and biocatalytic properties, using methodologies to quantitatively describe the dynamics of these processes in thin films. We also describe the first cyt P450/reductase films that accurately mimic the natural biocatalytic pathway and show how they can be used with voltammetry to elucidate key mechanistic features. Such bioelectronic cyt P450 systems have high value for future drug development, toxicity screening, fundamental investigations, and chemical synthesis systems.

2D-SEIRA spectroscopy to highlight conformational changes of the cytochrome c oxidase induced by direct electron transfer

Potentiometric titrations of the cytochrome c oxidase (CcO) immobilized in a biomimetic membrane system were followed by two-dimensional surface-enhanced IR absorption spectroscopy (2D SEIRAS) in the ATR-mode. Direct electron transfer was employed to vary the redox state of the enzyme. The CcO was shown to undergo a conformational transition from a non-activated to an activated state after it was allowed to turnover in the presence of oxygen. Differences between the non-activated and activated state were revealed by 2D SEIRA spectra recorded as a function of potential. The activated state was characterized by a higher number of correlated transitions as well as a higher number of amino acids associated with electron transfer.

DiGeorge critical region 8 (DGCR8) is a double-cysteine-ligated heme protein

All known heme-thiolate proteins ligate the heme iron using one cysteine side chain. We previously found that DiGeorge Critical Region 8 (DGCR8), an essential microRNA processing factor, associates with heme of unknown redox state when overexpressed in Escherichia coli. On the basis of the similarity of the 450-nm Soret absorption peak of the DGCR8-heme complex to that of cytochrome P450 containing ferrous heme with CO bound, we identified cysteine 352 as a probable axial ligand in DGCR8. Here we further characterize the DGCR8-heme interaction using biochemical and spectroscopic methods. The DGCR8-heme complex is highly stable, with a half-life exceeding 4 days. Mutation of the conserved proline 351 to an alanine increases the rate of heme dissociation and allows the DGCR8-heme complex to be reconstituted biochemically. Surprisingly, DGCR8 binds ferric heme without CO to generate a hyperporphyrin spectrum. The electronic absorption, magnetic circular dichroism, and electron paramagnetic resonance spectra of the DGCR8-heme complex suggest a ferric heme bearing two cysteine ligands. This model was further confirmed using selenomethionine-substituted DGCR8 and mercury titration. DGCR8 is the first example of a heme-binding protein with two endogenous cysteine side chains serving as axial ligands. We further show that native DGCR8 binds heme when expressed in eukaryotic cells. This study provides a chemical basis for understanding the function of the DGCR8-heme interaction in microRNA maturation.

[Sensor systems for medical application based on hemoproteins and nanocomposite materials]

Recent advances in nanotechnologies stimulate the development of sensor systems based on nanocomposite materials. This review discusses the prospects and challenges of sensors coupled with functionally important for medicine hemoproteins and nanoscale materials. Authors summarized their own experimental results and literature data on hemoprotein-based sensor systems. Mechanisms and the main function principles of electrochemical nanosensors are also discussed.

Control of electrochemical and ferryloxy formation kinetics of cyt P450s in polyion films by heme iron spin state and secondary structure

Voltammetry of cytochrome P450 (cyt P450) enzymes in ultrathin films with polyions was related for the first time to electronic and secondary structure. Heterogeneous electron transfer (hET) rate constants for reduction of the cyt P450s depended on heme iron spin state, with low spin cyt P450cam giving a value 40-fold larger than high spin human cyt P450 1A2, with mixed spin human P450 cyt 2E1 at an intermediate value. Asymmetric reduction-oxidation peak separations with increasing scan rates were explained by simulations featuring faster oxidation than reduction. Results are consistent with a square scheme in which oxidized and reduced forms of cyt P450s each participate in rapid conformational equilibria. Rate constants for oxidation of ferric cyt P450s in films by t-butyl hydroperoxide to active ferryloxy cyt P450s from rotating disk voltammetry suggested a weaker dependence on spin state, but in the reverse order of the observed hET reduction rates. Oxidation and reduction rates of cyt P450s in the films are also likely to depend on protein secondary structure around the heme iron.

Electrochemical reduction of sterol-14α-demethylase from Mycobacterium tuberculosis (CYP51b1)

The electrochemical reduction of the heme protein sterol-14alpha-demethylase from Mycobacterium tuberculosis (CYP51b1, or further CYP51) was investigated. Direct electron transfer was demonstrated between CYP51 and graphite screen-printed electrodes modified with gold nanoparticles and with the membrane-like synthetic surfactant didodecyl dimethylammonium bromide. The formal potential of the Fe3+/Fe2+ pair, E(1/2), is equal to -273 mV (vs. Ag/AgCl). The cathodic current corresponding to the reduction of oxygen by immobilized heme protein was registered in the presence of oxygen. Addition of lanosterol, one of the substrates of the CYP51 family, to the oxygenated solution caused a concentration-dependent increase in the reduction current in voltammetric and amperometric experiments. Ketoconazole, an inhibitor of CYP51, inhibited the catalytic cathodic current in the presence of lanosterol. Electrochemical reduction of CYP51 may serve as an adequate alternative to the reconstituted system, which requires additional redox partners for the exhibition of catalytic activity of heme proteins of the cytochrome P450 superfamily.

Electrochemical properties of cytochroms P450 using nanostructured electrodes: Direct electron transfer and electro catalysis

The present study demonstrates direct electron transfer between cytochromes P450 2B4 (CYP2B4), P450 1A2 (CYP1A2), sterol 14alpha-demethylase (CYP51b1) on the one hand and screen-printed graphite electrodes, modified with gold nanoparticles and didodecyldimethylammonium bromide (DDAB) on the other. Electro detection of heme proteins was possible when 2-200 pmol P450/electrode were adsorbed on the surface of nanostructured electrochemical interfaces. Electron transfer, direct electrochemical reduction and interaction with P450 substrates (oxygen, benzphetamine, and lanosterol) and with P450 inhibitor (ketoconazole) were analyzed using cyclic voltammetry (CV), square wave voltammetry (SWV) differential pulse voltammetry (DPV), and amperometry.

Preparative use of isolated CYP102 monooxygenases—A critical appraisal

Isolated P450 monooxygenases have for long been neglected catalysts in enzyme technology. This is surprising as they display a remarkable substrate specificity catalyzing reactions, which represent a challenge for classic organic chemistry. On the other hand, many P450 monooxygenases are membrane bound, depend on rather complicated electron transfer systems and require expensive cofactors such as NAD(P)H. Their activities are low, and stability leaves much to be desired. The use of bacterial P450 monooxygenases from CYP102 family allows overcoming some of these handicaps. They are soluble and their turnovers are high, presumably because their N-terminal heme monooxygenase and their C-terminal diflavin reductase domain are covalently linked. In recent years, protein engineering approaches have been successfully used to turn CYP102 monooxgenases into powerful biocatalysts.

Recent progress in cytochrome P450 enzyme electrochemistry

Cytochrome P450 (CYP) enzymes perform crucial functions in humans, including the metabolism of drugs and hormone synthesis. The catalytic reactions performed by these enzymes (typically monoxygenation) require the transfer of electrons. Thermodynamic and mechanistic detail of the electron transfer component of these catalytic processes has been obtained traditionally from potentiometric titrations. More recently, voltammetric approaches (that are inherently simpler and require less sample) have been used. This has been made possible by the creation of biocompatible electrode surfaces at which the P450 enzyme is confined and able to undergo physiologically relevant electron transfer processes. The continuing challenge has been to obtain an in vivo-like enzyme response, and to provide the basis for the creation of an artificial bioprocess in vitro. A powerful instrumental electrochemical method, employing Fourier-transformed large-amplitude ac voltammetry, offers the potential for greater insight and new opportunities to understand the nuances of the electron transfer process. This review highlights several recent advances in the electrochemistry of P450 enzymes rather than providing a comprehensive review of P450 electrochemistry.

Improving catalytic properties of P450 BM3 haem domain electrodes by molecular Lego

In this work the catalytic properties of a cytochrome P450 immobilised onto an electrode surface are improved by means of the molecular Lego approach.