Recent progress in cytochrome P450 enzyme electrochemistry

Electrochemical transformations catalyzed by cytochrome P450s and peroxidases

Cytochrome P450s (Cyt P450s) and peroxidases are enzymes featuring iron heme cofactors that have wide applicability as biocatalysts in chemical syntheses. Cyt P450s are a family of monooxygenases that oxidize fatty acids, steroids, and xenobiotics, synthesize hormones, and convert drugs and other chemicals to metabolites. Peroxidases are involved in breaking down hydrogen peroxide and can oxidize organic compounds during this process. Both heme-containing enzymes utilize active Fe^IVO intermediates to oxidize reactants. By incorporating these enzymes in stable thin films on electrodes, Cyt P450s and peroxidases can accept electrons from an electrode, albeit by different mechanisms, and catalyze organic transformations in a feasible and cost-effective way. This is an advantageous approach, often called bioelectrocatalysis, compared to their biological pathways in solution that require expensive biochemical reductants such as NADPH or additional enzymes to recycle NADPH for Cyt P450s. Bioelectrocatalysis also serves as an ex situ platform to investigate metabolism of drugs and bio-relevant chemicals. In this paper we review biocatalytic electrochemical reactions using Cyt P450s including C-H activation, S-oxidation, epoxidation, N-hydroxylation, and oxidative N-, and O-dealkylation; as well as reactions catalyzed by peroxidases including synthetically important oxidations of organic compounds. Design aspects of these bioelectrocatalytic reactions are presented and discussed, including enzyme film formation on electrodes, temperature, pH, solvents, and activation of the enzymes. Finally, we discuss challenges and future perspective of these two important bioelectrocatalytic systems.

Protein bioelectronics: a review of what we do and do not know

We review the status of protein-based molecular electronics. First, we define and discuss fundamental concepts of electron transfer and transport in and across proteins and proposed mechanisms for these processes. We then describe the immobilization of proteins to solid-state surfaces in both nanoscale and macroscopic approaches, and highlight how different methodologies can alter protein electronic properties. Because immobilizing proteins while retaining biological activity is crucial to the successful development of bioelectronic devices, we discuss this process at length. We briefly discuss computational predictions and their connection to experimental results. We then summarize how the biological activity of immobilized proteins is beneficial for bioelectronic devices, and how conductance measurements can shed light on protein properties. Finally, we consider how the research to date could influence the development of future bioelectronic devices.

Direct enzymatic bioelectrocatalysis: differentiating between myth and reality

Enzymatic bioelectrocatalysis is being increasingly exploited to better understand oxidoreductase enzymes, to develop minimalistic yet specific biosensor platforms, and to develop alternative energy conversion devices and bioelectrosynthetic devices for the production of energy and/or important chemical commodities. In some cases, these enzymes are able to electronically communicate with an appropriately designed electrode surface without the requirement of an electron mediator to shuttle electrons between the enzyme and electrode. This phenomenon has been termed direct electron transfer or direct bioelectrocatalysis. While many thorough studies have extensively investigated this fascinating feat, it is sometimes difficult to differentiate desirable enzymatic bioelectrocatalysis from electrocatalysis deriving from inactivated enzyme that may have also released its catalytic cofactor. This article will review direct bioelectrocatalysis of several oxidoreductases, with an emphasis on experiments that provide support for direct bioelectrocatalysis versus denatured enzyme or dissociated cofactor. Finally, this review will conclude with a series of proposed control experiments that could be adopted to discern successful direct electronic communication of an enzyme from its denatured counterpart.

Nanodiscs in Membrane Biochemistry and Biophysics

Membrane proteins play a most important part in metabolism, signaling, cell motility, transport, development, and many other biochemical and biophysical processes which constitute fundamentals of life on the molecular level. Detailed understanding of these processes is necessary for the progress of life sciences and biomedical applications. Nanodiscs provide a new and powerful tool for a broad spectrum of biochemical and biophysical studies of membrane proteins and are commonly acknowledged as an optimal membrane mimetic system that provides control over size, composition, and specific functional modifications on the nanometer scale. In this review we attempted to combine a comprehensive list of various applications of nanodisc technology with systematic analysis of the most attractive features of this system and advantages provided by nanodiscs for structural and mechanistic studies of membrane proteins.

Electrochemistry of cytochrome P450 17α-hydroxylase/17,20-lyase (P450c17)

Within the superfamily of cytochrome P450 enzymes (P450s), there is a small class which is functionally employed for steroid biosynthesis. The enzymes in this class appear to have a small active site to accommodate the steroid substrates specifically and snuggly, prior to the redox transformation or hydroxylation to form a product. Cytochrome P450c17 is one of these and is also a multi-functional P450, with two activities, the first 17α-hydroxylation of pregnenolone is followed by a subsequent 17,20-lyase transformation to dehydroepiandrosterone (DHEA) as the dominant pathways to cortisol precursors or androgens in humans, respectively. How P450c17 regulates these two redox reactions is of special interest. There is a paucity of direct electrochemical studies on steroidogenic P450s, and in this mini-review we provide an overview of these studies with P450c17. Historical consideration as to the difficulties in obtaining reliable electrochemistry due to issues of handling proteins on an electrode, together with advances in the electrochemical techniques are addressed. Recent work using Fourier transformed alternating current voltammetry is highlighted as this technique can provide both catalytic information simultaneously with the underlying redox transfer with the P450 haem.

The Oxygen Dilemma: A Severe Challenge for the Application of Monooxygenases?

Monooxygenases are promising catalysts because they in principle enable the organic chemist to perform highly selective oxyfunctionalisation reactions that are otherwise difficult to achieve. For this, monooxygenases require reducing equivalents, to allow reductive activation of molecular oxygen at the enzymes' active sites. However, these reducing equivalents are often delivered to O2 either directly or via a reduced intermediate (uncoupling), yielding hazardous reactive oxygen species and wasting valuable reducing equivalents. The oxygen dilemma arises from monooxygenases' dependency on O2 and the undesired uncoupling reaction. With this contribution we hope to generate a general awareness of the oxygen dilemma and to discuss its nature and some promising solutions.

Application of nanodisc technology for direct electrochemical investigation of plant cytochrome P450s and their NADPH P450 oxidoreductase

Direct electrochemistry of cytochrome P450 containing systems has primarily focused on investigating enzymes from microbes and animals for bio-sensing applications. Plant P450s receive electrons from NADPH P450 oxidoreductase (POR) to orchestrate the bio-synthesis of a plethora of commercially valuable compounds. In this report, full length CYP79A1, CYP71E1 and POR of the dhurrin pathway in Sorghum bicolor were reconstituted individually in nanoscale lipid patches, "nanodiscs", and directly immobilized on unmodified gold electrodes. Cyclic voltammograms of CYP79A1 and CYP71E1 revealed reversible redox peaks with average midpoint potentials of 80 ± 5 mV and 72 ± 5 mV vs. Ag/AgCl, respectively. POR yielded two pairs of redox peaks with midpoint potentials of 90 ± 5 mV and -300 ± 10 mV, respectively. The average heterogeneous electron transfer rate constant was calculated to be ~1.5 s(-1). POR was electro-catalytically active while the P450s generated hydrogen peroxide (H2O2). These nanodisc-based investigations lay the prospects and guidelines for construction of a simplified platform to perform mediator-free, direct electrochemistry of non-engineered cytochromes P450 under native-like conditions. It is also a prelude for driving plant P450 systems electronically for simplified and cost-effective screening of potential substrates/inhibitors and fabrication of nano-bioreactors for synthesis of high value natural products.

Electrochemiluminescence Arrays for Studies of Metabolite-related Toxicity

This article reviews recent progress from our laboratory in electrochemiluminescence (ECL) arrays designed for screening toxicity-related chemistry of chemical and drug candidates. Cytochrome P450s and metabolic bioconjugation enzymes convert lipophilic chemicals in our bodies by oxidation and bioconjugation that can lead to toxic metabolites. DNA can be used as an easily measurable toxicity-related endpoint, targeting DNA oxidation and addcut formation with metabolites. ECL using guanosines in the DNA strands as co-reactants have been used in high throughput arrays utilizing DNA-enzyme films fabricated layer-by-layer. This review describes approaches developed to provide new high throughput ECL arrays to aid in toxicity assessment for drug and chemical product development.

Nanoscale electron transport measurements of immobilized cytochrome P450 proteins

Gold nanopillars, functionalized with an organic self-assembled monolayer, can be used to measure the electrical conductance properties of immobilized proteins without aggregation. Measurements of the conductance of nanopillars with cytochrome P450 2C9 (CYP2C9) proteins using conducting probe atomic force microscopy demonstrate that a correlation exists between the energy barrier height between hopping sites and CYP2C9 metabolic activity. Measurements performed as a function of tip force indicate that, when subjected to a large force, the protein is more stable in the presence of a substrate. This agrees with the hypothesis that substrate entry into the active site helps to stabilize the enzyme. The relative distance between hopping sites also increases with increasing force, possibly because protein functional groups responsible for electron transport (ETp) depend on the structure of the protein. The inhibitor sulfaphenazole, in addition to the previously studied aniline, increased the barrier height for electron transfer and thereby makes CYP2C9 reduction more difficult and inhibits metabolism. This suggests that P450 Type II binders may decrease the ease of ETp processes in the enzyme, in addition to occupying the active site.

Thin multicomponent films for functional enzyme devices and bioreactor particles

Complex functional films containing enzymes and other biomolecules are easily fabricated in nm-scale thicknesses by using layer-by-layer (LbL) methodologies first popularized by Lvov and Decher. In this review, we highlight the high level functional capabilities possible with LbL films of biomolecules based on our own research experiences. We first describe the basics of enzyme film fabrication by LbL alternate electrostatic adsorption, then discuss how to make functional enzyme-polyion films of remarkably high stability. Focusing on cytochrome P450s, we discuss films developed to electrochemically activate the natural catalytic cycle of these key metabolic enzymes. We then describe multifunctional, multicomponent DNA/enzyme/polyion films on arrays and particle surfaces for high throughput metabolic toxicity screening using electrochemiluminescence and LC-MS/MS. Using multicomponent LbL films, complex functionality for bioanalytical and biochemical purposes can be achieved that is difficult or impossible using conventional approaches.

Cytochromes P450 as promising catalysts for biotechnological application: chances and limitations

Cytochromes P450 (CYPs) belong to the superfamily of heme b containing monooxygenases with currently more than 21,000 members. These enzymes accept a vast range of organic molecules and catalyze diverse reactions. These extraordinary capabilities of CYP systems that are unmet by other enzymes make them attractive for biotechnology. However, the complexity of these systems due to the need of electron transfer from nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) via redox partner proteins for the initial hydroxylation step limits a broader technical implementation of CYP enzymes. There have been several reviews during the past years tackling the potential CYPs for synthetic application. The aim of this review is to give a critical overview about possibilities and chances for application of these interesting catalysts as well as to discuss drawbacks and problems related to their use. Solutions to overcome these limitations will be demonstrated, and several selected examples of successful CYP applications under industrial conditions will be reviewed.

Measurement of electron transfer through cytochrome P450 protein on nanopillars and the effect of bound substrates

Electron transfer in cytochrome P450 enzymes is a fundamental process for activity. It is difficult to measure electron transfer in these enzymes because under the conditions typically used they exist in a variety of states. Using nanotechnology-based techniques, gold conducting nanopillars were constructed in an indexed array. The P450 enzyme CYP2C9 was attached to each of these nanopillars, and conductivity measurements made using conducting probe atomic force microscopy under constant force conditions. The conductivity measurements were made on CYP2C9 alone and with bound substrates, a bound substrate-effector pair, and a bound inhibitor. Fitting of the data with the Poole-Frenkel model indicates a correlation between the barrier height for electron transfer and the ease of CYP2C9-mediated metabolism of the bound substrates, though the spin state of iron is not well correlated. The approach described here should have broad application to the measurement of electron transfer in P450 enzymes and other metalloenzymes.

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.

Enzyme-based amperometric platform to determine the polymorphic response in drug metabolism by cytochromes P450

"Personalized medicine" is a new concept in health care, one aspect of which defines the specificity and dosage of drugs according to effectiveness and safety for each patient. Dosage strongly depends from the rate of metabolism which is primarily regulated by the activity of cytochrome P450. In addition to the need for a genetic characterization of the patients, there is also the necessity to determine the drug-clearance properties of the polymorphic P450 enzyme. To address this issue, human P450 2D6 and 2C9 were engineered and covalently linked to an electrode surface allowing fast, accurate, and reliable measurements of the kinetic parameters of these phase-1 drug metabolizing polymorphic enzymes. In particular, the catalytic activity of P450 2C9 on the electrode surface was found to be improved when expressed from a gene-fusion with flavodoxin from Desulfovibrio vulgaris (2C9/FLD). The results are validated using marker drugs for these enzymes, bufuralol for 2D6, and warfarin for 2C9/FLD. The platform is able to measure the same small differences in K(M), and it allows a fast and reproducible mean to generated the product identified by HPLC from which the k(cat) is calculated.

Efficient bioelectronic actuation of the natural catalytic pathway of human metabolic cytochrome P450s

Cytochrome (cyt) P450s comprise the enzyme superfamily responsible for human oxidative metabolism of a majority of drugs and xenobiotics. Electronic delivery of electrons to cyt P450s could be used to drive the natural catalytic cycle for fundamental investigations, stereo- and regioselective synthesis, and biosensors. We describe herein 30 nm nanometer-thick films on electrodes featuring excess human cyt P450s and cyt P450 reductase (CPR) microsomes that efficiently mimic the natural catalytic pathway for the first time. Redox potentials, electron-transfer rates, CO-binding, and substrate conversion rates confirmed that electrons are delivered from the electrode to CPR, which transfers them to cyt P450. The film system enabled electrochemical probing of the interaction between cyt P450 and CPR for the first time. Agreement of film voltammetry data with theoretical simulations supports a pathway featuring a key equilibrium redox reaction in the natural catalytic pathway between reduced CPR and cyt P450 occurring within a CPR-cyt P450 complex uniquely poised for substrate conversion.

Effects of sampling rate on the interpretation of cellular transport measurements

Electrochemical sensing techniques are increasingly used to study biological processes by monitoring concentration changes of the molecule of interest close to cells. The measured concentration is the result of cellular transport across the cell membrane and diffusion of the released molecules from the cells to the sensing electrode. The objective of such experiments is to understand the cellular processes underlying the observed changes in concentration. Thus, the influence of mass transport on the measured concentration trace has to be removed. This is done by deconvolution of the impulse response function of diffusion from the concentration data. We have recently observed that measuring concentration at a sampling rate that satisfies the Nyquist criterion for the observed concentration dynamics may not be sufficient to correctly reconstruct cellular flux. This is because the impulse response function of diffusion also has to be represented with sufficient temporal resolution. We discuss this problem here using the example of monitoring drug efflux from a monolayer of cancer cells with microvoltammetry, and chloride secretion from an epithelial cell monolayer monitored with an ion-selective electrode.