Rapid kinetic methods to dissect steroidogenic cytochrome P450 reaction mechanisms

Synthesis of 6β-hydroxy androgens from a 3,5-diene steroid precursor to test for cytochrome P450 3A4-catalyzed hydroxylation of androstenedione

6β-Hydroxytestosterone is a biomarker for the activity of human cytochrome P450 3A4 (P450 3A4), the major drug metabolizing cytochrome P450 enzyme. Despite its significance, efficient routes for the chemical synthesis of 6β-hydroxytestosterone are rare. In this study, 6β-hydroxytestosterone was synthesized through the oxidation of a 3,5-diene precursor under the Uemura-Doyle reaction conditions using a dirhodium catalyst in the presence of tert-butylhydroperoxide. Mechanistic studies showed that some oxygen is incorporated from molecular oxygen and CH abstraction is partially rate-limiting. This reaction was used to synthesize 6β-hydroxyandrostenedione, which was used as a standard to test the hypothesis of whether P450 3A4 catalyzes the hydroxylation of androstenedione. Upon incubation of P450 3A4 with androstenedione, a hydroxylated product was formed, which matched the retention time of synthetic 6β-hydroxyandrostenedione. This reaction can be exploited to study other biochemical processes involving compounds with a 6 β -hydroxy-3-keto-Δ4 steroid backbone.

An electron transfer competent structural ensemble of membrane-bound cytochrome P450 1A1 and cytochrome P450 oxidoreductase

Cytochrome P450 (CYP) heme monooxygenases require two electrons for their catalytic cycle. For mammalian microsomal CYPs, key enzymes for xenobiotic metabolism and steroidogenesis and important drug targets and biocatalysts, the electrons are transferred by NADPH-cytochrome P450 oxidoreductase (CPR). No structure of a mammalian CYP-CPR complex has been solved experimentally, hindering understanding of the determinants of electron transfer (ET), which is often rate-limiting for CYP reactions. Here, we investigated the interactions between membrane-bound CYP 1A1, an antitumor drug target, and CPR by a multiresolution computational approach. We find that upon binding to CPR, the CYP 1A1 catalytic domain becomes less embedded in the membrane and reorients, indicating that CPR may affect ligand passage to the CYP active site. Despite the constraints imposed by membrane binding, we identify several arrangements of CPR around CYP 1A1 that are compatible with ET. In the complexes, the interactions of the CPR FMN domain with the proximal side of CYP 1A1 are supplemented by more transient interactions of the CPR NADP domain with the distal side of CYP 1A1. Computed ET rates and pathways agree well with available experimental data and suggest why the CYP-CPR ET rates are low compared to those of soluble bacterial CYPs.

From electrochemistry to enzyme kinetics of cytochrome P450

This review is an attempt to describe advancements in the electrochemistry of cytochrome P450 enzymes (EC 1.14.14.1) and to study molecular aspects and catalytic behavior of enzymatic electrocatalysis. Electroanalysis of cytochrome P450 demonstrates how to translate theoretical laws and equations of classical electrochemistry for the calculation of the kinetic parameters of enzymatic reactions and then translation of kinetic parameters to interpretation of drug-drug interactions. The functional significance of cytochrome P450s (CYPs) includes the metabolism of drugs, foreign chemicals, and endogenic compounds. The pharmaceutical industry needs sensitive and cost-effective systems for screening new drugs and investigation of drug-drug interactions. The development of different types of CYP-based biosensors is now in great demand. This review also highlights the characteristics of electrode processes and electrode properties for optimization of the cytochrome P450 electroanalysis. Electrochemical cytochrome P450-biosensors are the most studied. In this review, we analyzed electrode/cytochrome P450 systems in terms of the mechanisms underlying P450-catalyzed reactions. Screening of potential substrates or inhibitors of cytochromes P450 by means of electrodes were described.

Spectroscopic studies of the cytochrome P450 reaction mechanisms

The cytochrome P450 monooxygenases (P450s) are thiolate heme proteins that can, often under physiological conditions, catalyze many distinct oxidative transformations on a wide variety of molecules, including relatively simple alkanes or fatty acids, as well as more complex compounds such as steroids and exogenous pollutants. They perform such impressive chemistry utilizing a sophisticated catalytic cycle that involves a series of consecutive chemical transformations of heme prosthetic group. Each of these steps provides a unique spectral signature that reflects changes in oxidation or spin states, deformation of the porphyrin ring or alteration of dioxygen moieties. For a long time, the focus of cytochrome P450 research was to understand the underlying reaction mechanism of each enzymatic step, with the biggest challenge being identification and characterization of the powerful oxidizing intermediates. Spectroscopic methods, such as electronic absorption (UV-Vis), electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), electron nuclear double resonance (ENDOR), Mössbauer, X-ray absorption (XAS), and resonance Raman (rR), have been useful tools in providing multifaceted and detailed mechanistic insights into the biophysics and biochemistry of these fascinating enzymes. The combination of spectroscopic techniques with novel approaches, such as cryoreduction and Nanodisc technology, allowed for generation, trapping and characterizing long sought transient intermediates, a task that has been difficult to achieve using other methods. Results obtained from the UV-Vis, rR and EPR spectroscopies are the main focus of this review, while the remaining spectroscopic techniques are briefly summarized. This article is part of a Special Issue entitled: Cytochrome P450 biodiversity and biotechnology, edited by Erika Plettner, Gianfranco Gilardi, Luet Wong, Vlada Urlacher, Jared Goldstone.

Bystander signaling via oxidative metabolism

The radiation-induced bystander effect (RIBE) is the initiation of biological end points in cells (bystander cells) that are not directly traversed by an incident-radiation track, but are in close proximity to cells that are receiving the radiation. RIBE has been indicted of causing DNA damage via oxidative stress, besides causing direct damage, inducing tumorigenesis, producing micronuclei, and causing apoptosis. RIBE is regulated by signaling proteins that are either endogenous or secreted by cells as a means of communication between cells, and can activate intracellular or intercellular oxidative metabolism that can further trigger signaling pathways of inflammation. Bystander signals can pass through gap junctions in attached cell lines, while the suspended cell lines transmit these signals via hormones and soluble proteins. This review provides the background information on how reactive oxygen species (ROS) act as bystander signals. Although ROS have a very short half-life and have a nanometer-scale sphere of influence, the wide variety of ROS produced via various sources can exert a cumulative effect, not only in forming DNA adducts but also setting up signaling pathways of inflammation, apoptosis, cell-cycle arrest, aging, and even tumorigenesis. This review outlines the sources of the bystander effect linked to ROS in a cell, and provides methods of investigation for researchers who would like to pursue this field of science.