1
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Collin GB, Won J, Krebs MP, Hicks WJ, Charette JR, Naggert JK, Nishina PM. Disruption in murine Eml1 perturbs retinal lamination during early development. Sci Rep 2020; 10:5647. [PMID: 32221352 PMCID: PMC7101416 DOI: 10.1038/s41598-020-62373-5] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2019] [Accepted: 03/09/2020] [Indexed: 11/23/2022] Open
Abstract
During mammalian development, establishing functional neural networks in stratified tissues of the mammalian central nervous system depends upon the proper migration and positioning of neurons, a process known as lamination. In particular, the pseudostratified neuroepithelia of the retina and cerebrocortical ventricular zones provide a platform for progenitor cell proliferation and migration. Lamination defects in these tissues lead to mispositioned neurons, disrupted neuronal connections, and abnormal function. The molecular mechanisms necessary for proper lamination in these tissues are incompletely understood. Here, we identified a nonsense mutation in the Eml1 gene in a novel murine model, tvrm360, displaying subcortical heterotopia, hydrocephalus and disorganization of retinal architecture. In the retina, Eml1 disruption caused abnormal positioning of photoreceptor cell nuclei early in development. Upon maturation, these ectopic photoreceptors possessed cilia and formed synapses but failed to produce robust outer segments, implying a late defect in photoreceptor differentiation secondary to mislocalization. In addition, abnormal positioning of Müller cell bodies and bipolar cells was evident throughout the inner neuroblastic layer. Basal displacement of mitotic nuclei in the retinal neuroepithelium was observed in tvrm360 mice at postnatal day 0. The abnormal positioning of retinal progenitor cells at birth and ectopic presence of photoreceptors and secondary neurons upon maturation suggest that EML1 functions early in eye development and is crucial for proper retinal lamination during cellular proliferation and development.
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Affiliation(s)
- G B Collin
- The Jackson Laboratory, 600 Main Street, Bar Harbor, Maine, 04609, USA
| | - J Won
- The Jackson Laboratory, 600 Main Street, Bar Harbor, Maine, 04609, USA
| | - M P Krebs
- The Jackson Laboratory, 600 Main Street, Bar Harbor, Maine, 04609, USA
| | - W J Hicks
- The Jackson Laboratory, 600 Main Street, Bar Harbor, Maine, 04609, USA
| | - J R Charette
- The Jackson Laboratory, 600 Main Street, Bar Harbor, Maine, 04609, USA
| | - J K Naggert
- The Jackson Laboratory, 600 Main Street, Bar Harbor, Maine, 04609, USA
| | - P M Nishina
- The Jackson Laboratory, 600 Main Street, Bar Harbor, Maine, 04609, USA.
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2
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Mak PJ, Denisov IG. Spectroscopic studies of the cytochrome P450 reaction mechanisms. BIOCHIMICA ET BIOPHYSICA ACTA. PROTEINS AND PROTEOMICS 2018; 1866:178-204. [PMID: 28668640 PMCID: PMC5709052 DOI: 10.1016/j.bbapap.2017.06.021] [Citation(s) in RCA: 44] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/02/2017] [Accepted: 06/22/2017] [Indexed: 10/19/2022]
Abstract
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.
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Affiliation(s)
- Piotr J Mak
- Department of Chemistry, Saint Louis University, St. Louis, MO, United States.
| | - Ilia G Denisov
- Department of Biochemistry, University of Illinois Urbana-Champaign, Urbana, IL, United States.
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3
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Yoshimoto FK, Auchus RJ. Rapid kinetic methods to dissect steroidogenic cytochrome P450 reaction mechanisms. J Steroid Biochem Mol Biol 2016; 161:13-23. [PMID: 26472553 PMCID: PMC4841756 DOI: 10.1016/j.jsbmb.2015.10.005] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/10/2015] [Revised: 08/12/2015] [Accepted: 10/07/2015] [Indexed: 01/03/2023]
Abstract
All cytochrome P450 enzyme reactions involve a catalytic cycle with several discreet physical or chemical steps. This cycle ends with the formation of the reactive heme iron-oxygen complex, which oxygenates substrate. While the steps might be very similar for each P450 enzyme, the rates of each step varies tremendously for each enzyme and sometimes even for different reactions catalyzed by the same enzyme. For example, the rate-limiting step for most bacterial P450 enzymes, with turnover numbers over 1000s(-1), is the second electron transfer. In contrast, steroidogenic P450s from eukaryotes catalyze much slower reactions, with turnover numbers of ∼5-250min(-1); therefore, assumptions about kinetic properties for the mammalian P450 enzymes based on the bacterial enzymes are tenuous. In order to dissect the rates for individual steps, special techniques that isolate individual steps and/or single turnovers are required. This article will review the theoretical principles and practical considerations for several of these techniques, with illustrative published examples. The reader should gain an appreciation for the appropriate methods used to interrogate particular steps in the P450 reaction cycle.
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Affiliation(s)
- Francis K Yoshimoto
- Department of Biochemistry and Center in Molecular Toxicology, Vanderbilt University School of Medicine, Nashville, TN 37232-0146, USA
| | - Richard J Auchus
- Division of Metabolism, Endocrinology, and Diabetes, Department of Internal Medicine, Ann Arbor, MI 48019, USA; Department of Pharmacology, University of Michigan, Ann Arbor, MI 48019, USA.
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4
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van Lier JE, Mast N, Pikuleva IA. Cholesterol Hydroperoxides as Substrates for Cholesterol-Metabolizing Cytochrome P450 Enzymes and Alternative Sources of 25-Hydroxycholesterol and other Oxysterols. Angew Chem Int Ed Engl 2015. [DOI: 10.1002/ange.201505002] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
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van Lier JE, Mast N, Pikuleva IA. Cholesterol hydroperoxides as substrates for cholesterol-metabolizing cytochrome P450 enzymes and alternative sources of 25-hydroxycholesterol and other oxysterols. Angew Chem Int Ed Engl 2015; 54:11138-42. [PMID: 26230055 PMCID: PMC4578806 DOI: 10.1002/anie.201505002] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/02/2015] [Indexed: 12/20/2022]
Abstract
The interaction of the primary autoxidation products of cholesterol, namely 25- and 20ξ-hydroperoxides, with the four principal cholesterol-metabolizing cytochrome P450 enzymes is reported. Addition of cholesterol 25-hydroperoxide to the enzymes CYP27A1 and CYP11A1 induced well-defined spectral changes while generating 25-hydroxycholesterol as the major product. The 20ξ-hydroperoxides induced spectral shifts in CYP27A1 and CYP11A1 but glycol metabolites were detected only with CYP11A1. CYP7A1 and CYP46A1 failed to give metabolites with any of the hydroperoxides. A P450 hydroperoxide-shunt reaction is proposed, where the hydroperoxides serve as both donor for reduced oxygen and substrate. CYP27A1 was shown to mediate the reduction of cholesterol 25-hydroperoxide to 25-hydroxycholesterol, a role of potential significance for cholesterol-rich tissues with high oxidative stress. CYP27A1 may participate in the removal of harmful autoxidation products in these tissues, while providing a complementary source of 25-hydroxycholesterol, a modulator of immune cell function and mediator of viral cell entry.
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Affiliation(s)
- Johan E van Lier
- Department of Nuclear Medicine and Radiobiology, Faculty of Medicine and Health Sciences, Université de Sherbrooke, Sherbrooke, QC, J1H 5N4 (Canada).
| | - Natalia Mast
- Department of Ophthalmology and Visual Sciences, Case Western Reserve University, Cleveland, OH 44106 (USA)[*]Corresponding authors
| | - Irina A Pikuleva
- Department of Ophthalmology and Visual Sciences, Case Western Reserve University, Cleveland, OH 44106 (USA)[*]Corresponding authors.
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6
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Kamada H, Nonaka I, Takenouchi N, Amari M. Effects of selenium supplementation on plasma progesterone concentrations in pregnant heifers. Anim Sci J 2013; 85:241-6. [PMID: 24206213 DOI: 10.1111/asj.12139] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/07/2013] [Accepted: 07/26/2013] [Indexed: 11/26/2022]
Abstract
It is known that selenium (Se) has various functions in animals. Many investigations on the biochemical and physiological effects of Se have been previously reported; however, the detailed function of Se in reproduction is not yet clear. We proposed the possibility that Se plays a notable role in progesterone production. The aim of this study was to clarify the effects of Se supplementation on progesterone levels of pregnant Holstein heifers. Eight Holstein heifers (-Se) were fed basal diet (containing 0.022 ppm of Se) throughout the experiment. While a 0.3 ppm diet of Se (sodium selenite) was fed to another seven animals (+Se) with basal diet. Blood sampling was carried out every week. Plasma Se concentrations were higher in Se-supplemented cows compared with controls (-Se) (P < 0.01) throughout the experiment. Se supplementation increased plasma progesterone in the 29-39 weeks of pregnancy from 4.98 ± 0.64 to 6.86 ± 0.49 ng/mL on average (P < 0.05). The present findings suggest that Se contributes to maintaining the function of the corpus luteum and/or placenta in the latter period of pregnancy.
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Affiliation(s)
- Hachiro Kamada
- NARO Institute of Livestock and Grassland Science, Tsukuba, Japan
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7
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Hrycay EG, Bandiera SM. The monooxygenase, peroxidase, and peroxygenase properties of cytochrome P450. Arch Biochem Biophys 2012; 522:71-89. [DOI: 10.1016/j.abb.2012.01.003] [Citation(s) in RCA: 83] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/24/2011] [Revised: 12/22/2011] [Accepted: 01/04/2012] [Indexed: 12/30/2022]
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8
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Luthra A, Denisov IG, Sligar SG. Spectroscopic features of cytochrome P450 reaction intermediates. Arch Biochem Biophys 2010; 507:26-35. [PMID: 21167809 DOI: 10.1016/j.abb.2010.12.008] [Citation(s) in RCA: 108] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2010] [Revised: 12/06/2010] [Accepted: 12/07/2010] [Indexed: 11/24/2022]
Abstract
Cytochromes P450 constitute a broad class of heme monooxygenase enzymes with more than 11,500 isozymes which have been identified in organisms from all biological kingdoms [1]. These enzymes are responsible for catalyzing dozens chemical oxidative transformations such as hydroxylation, epoxidation, N-demethylation, etc., with very broad range of substrates [2,3]. Historically these enzymes received their name from 'pigment 450' due to the unusual position of the Soret band in UV-vis absorption spectra of the reduced CO-saturated state [4,5]. Despite detailed biochemical characterization of many isozymes, as well as later discoveries of other 'P450-like heme enzymes' such as nitric oxide synthase and chloroperoxidase, the phenomenological term 'cytochrome P450' is still commonly used as indicating an essential spectroscopic feature of the functionally active protein which is now known to be due to the presence of a thiolate ligand to the heme iron [6]. Heme proteins with an imidazole ligand such as myoglobin and hemoglobin as well as an inactive form of P450 are characterized by Soret maxima at 420nm [7]. This historical perspective highlights the importance of spectroscopic methods for biochemical studies in general, and especially for heme enzymes, where the presence of the heme iron and porphyrin macrocycle provides rich variety of specific spectroscopic markers available for monitoring chemical transformations and transitions between active intermediates of catalytic cycle.
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Affiliation(s)
- Abhinav Luthra
- Department of Biochemistry, School of Molecular and Cellular Biology, University of Illinois, Urbana, IL 61801, USA
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9
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Hersleth HP, Ryde U, Rydberg P, Görbitz CH, Andersson KK. Structures of the high-valent metal-ion haem–oxygen intermediates in peroxidases, oxygenases and catalases. J Inorg Biochem 2006; 100:460-76. [PMID: 16510192 DOI: 10.1016/j.jinorgbio.2006.01.018] [Citation(s) in RCA: 126] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/07/2005] [Revised: 01/09/2006] [Accepted: 01/09/2006] [Indexed: 11/22/2022]
Abstract
Peroxidases, oxygenases and catalases have similar high-valent metal-ion intermediates in their respective reaction cycles. In this review, haem-based examples will be discussed. The intermediates of the haem-containing enzymes have been extensively studied for many years by different spectroscopic methods like UV-Vis, EPR (electron paramagnetic resonance), resonance Raman, Mössbauer and MCD (magnetic circular dichroism). The first crystal structure of one of these high-valent intermediates was on cytochrome c peroxidase in 1987. Since then, structures have appeared for catalases in 1996, 2002, 2003, putatively for cytochrome P450 in 2000, for myoglobin in 2002, for horseradish peroxidase in 2002 and for cytochrome c peroxidase again in 1994 and 2003. This review will focus on the most recent structural investigations for the different intermediates of these proteins. The structures of these intermediates will also be viewed in light of quantum mechanical (QM) calculations on haem models. In particular quantum refinement, which is a combination of QM calculations and crystallography, will be discussed. Only small structural changes accompany the generation of these intermediates. The crystal structures show that the compound I state, with a so called pi-cation radical on the haem group, has a relatively short iron-oxygen bond (1.67-1.76A) in agreement with a double-bond character, while the compound II state or the compound I state with a radical on an amino acid residue have a relatively long iron-oxygen bond (1.86-1.92A) in agreement with a single-bond character where the oxygen-atom is protonated.
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Affiliation(s)
- Hans-Petter Hersleth
- Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway
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10
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Jung C, Schünemann V, Lendzian F. Freeze-quenched iron-oxo intermediates in cytochromes P450. Biochem Biophys Res Commun 2005; 338:355-64. [PMID: 16143295 DOI: 10.1016/j.bbrc.2005.08.166] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/15/2005] [Accepted: 08/15/2005] [Indexed: 11/29/2022]
Abstract
Since the discovery of cytochromes P450 and their assignment to heme proteins a reactive iron-oxo intermediate as the hydroxylating species has been discussed. It is believed that the electronic structure of this intermediate corresponds to an iron(IV)-porphyrin-pi-cation radical system (Compound I). To trap this intermediate the reaction of P450 with oxidants (shunt pathway) has been used. The common approaches are stopped-flow experiments with UV-visible spectroscopic detection or rapid-mixing/freeze-quench studies with EPR and Mössbauer spectroscopic characterization of the trapped intermediate. Surprisingly, the two approaches seem to give conflicting results. While the stopped-flow data indicate the formation of a porphyrin-pi-cation radical, no such species is seen by EPR spectroscopy, although the Mössbauer data indicate iron(IV) for P450cam (CYP101) and P450BMP (CYP102). Instead, radicals on tyrosine and tryptophan residues are observed. These findings are reviewed and discussed with respect to intramolecular electron transfer from aromatic amino acids to a presumably transiently formed porphyrin-pi-cation radical.
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Affiliation(s)
- Christiane Jung
- Max-Delbrück-Center for Molecular Medicine, 13125 Berlin, Germany.
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11
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Gorren ACF, Sørlie M, Andersson KK, Marchal S, Lange R, Mayer B. Tetrahydrobiopterin as combined electron/proton donor in nitric oxide biosynthesis: cryogenic UV-Vis and EPR detection of reaction intermediates. Methods Enzymol 2005; 396:456-66. [PMID: 16291253 DOI: 10.1016/s0076-6879(05)96038-2] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 04/07/2023]
Abstract
The role of tetrahydrobiopterin (BH4) as a cofactor in nitric oxide synthase (NOS) has been the object of intense research in the last few years. It was found that in addition to its established effects on the NOS heme spin state, substrate affinity, and enzyme dimerization, BH4 is required as a one-electron donor to oxyferrous [Fe(II).O2] heme that is formed as an intermediate in the catalytic cycle. Cryogenic spectroscopic techniques proved particularly useful in the identification of this role of BH4 in NO synthesis. With these methods, the mechanism of fast reactions, such as the reaction of ferrous NOS with O2, can be unraveled by lowering the reaction temperature to subzero values. This may not only reduce the rate to such an extent that the reaction can be followed on a time scale from seconds to minutes, but intermediates may be observed that do not accumulate at higher temperatures. Cryogenic ultraviolet-visible (UV-vis) and electron paramagnetic resonance spectroscopy have been applied to clarify why the BH4 analogue 4-amino-tetrahydrobiopterin (4-amino-BH4) is unable to support NO synthesis. In the course of these studies, evidence was gathered supporting a role for BH4 as an obligate proton and electron donor. It is believed that the inhibitory action of 4-amino-BH4 derives from an inability to serve as a proton donor, even though it is perfectly able to serve as an electron donor. In this chapter, the suitability, drawbacks, and advantages of cryogenic methods are discussed.
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Affiliation(s)
- Antonius C F Gorren
- Department of Pharmacology and Toxicology, Karl-Franzens-University Graz, Austria
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12
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Lange R. My passion for extreme conditions to solve the cytochrome P450 and NO synthase reaction mechanisms. Biochem Biophys Res Commun 2003; 312:103-7. [PMID: 14630026 DOI: 10.1016/j.bbrc.2003.10.083] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 04/27/2023]
Abstract
Cytochrome P450, and especially its reaction mechanism, has always been a major research interest of Professor I.C. Gunsalus. The reaction cycle of this enzyme is complex, containing many elementary steps and intermediates, and constitutes a challenge for the scientific community. During his repeated stays in our laboratories in Paris and in Montpellier, he contributed decisively to our study of the P450 reaction mechanism under extreme conditions, i.e., at subzero temperatures and at high pressure. From this initial impulse, we continued the work with different forms of cytochrome P450 and later on with nitric oxide synthase. This paper gives an overview of the insights into these enzymes gained by the use of extreme conditions. These exciting achievements were initiated by numerous discussions with Professor Gunsalus and also reflect a long-lasting collaboration and friendship.
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Affiliation(s)
- Reinhard Lange
- INSERM U128, IFR122, 1919 route de Mende, 34293 Montpellier Cédex 5, France.
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13
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Wei CC, Wang ZQ, Stuehr DJ. Nitric oxide synthase: use of stopped-flow spectroscopy and rapid-quench methods in single-turnover conditions to examine formation and reactions of heme-O2 intermediate in early catalysis. Methods Enzymol 2003; 354:320-38. [PMID: 12418237 DOI: 10.1016/s0076-6879(02)54026-x] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/27/2023]
Affiliation(s)
- Chin-Chuan Wei
- Department of Immunology, Lerner Research Institute, Cleveland Clinic, Cleveland, Ohio 44195, USA
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14
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Gorren ACF, Bec N, Lange R, Mayer B. Redox role for tetrahydrobiopterin in nitric oxide synthase catalysis: low-temperature optical absorption spectral detection. Methods Enzymol 2002; 353:114-21. [PMID: 12078487 DOI: 10.1016/s0076-6879(02)53041-x] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/25/2023]
Affiliation(s)
- Antonius C F Gorren
- Institute for Pharmacology and Toxicology, Karl-Franzens-Universität Graz, A-8010 Graz, Austria
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15
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Abstract
The essay examines the evidence upon which the presently accepted version of the mechanism of the cytochrome P450(scc)-catalyzed-cleavage of the sidechain of cholesterol is based. This analysis indicates that the generally held view of the process (two consecutive hydroxylations, followed by cleavage of the resulting glycol) most likely does not describe the true mechanism. The available evidence can not be used to support this traditional notion. Two alternative hypotheses are proposed.
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Affiliation(s)
- S Lieberman
- Department of Obstetrics and Gynecology, Institute for Health Sciences, St. Luke's-Roosevelt Hospital Center, 432W 58th Street, New York, NY 10019, USA.
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Bec N, Gorren AC, Voelker C, Mayer B, Lange R. Reaction of neuronal nitric-oxide synthase with oxygen at low temperature. Evidence for reductive activation of the oxy-ferrous complex by tetrahydrobiopterin. J Biol Chem 1998; 273:13502-8. [PMID: 9593685 DOI: 10.1074/jbc.273.22.13502] [Citation(s) in RCA: 132] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The reaction of reduced NO synthase (NOS) with molecular oxygen was studied at -30 degreesC. In the absence of substrate, the complex formed between ferrous NOS and O2 was sufficiently long lived for a precise spectroscopic characterization. This complex displayed similar spectral characteristics as the oxyferrous complex of cytochrome P450 (lambda max = 416.5 nm). It then decomposed to the ferric state. The oxidation of the flavin components was much slower and could be observed only at temperatures higher than -20 degreesC. In the presence of substrate (L-arginine), another, 12-nm blue-shifted, intermediate spectrum was formed. The breakdown of the latter species resulted in the production of Nomega-hydroxy-L-arginine in a stoichiometry of maximally 52% per NOS heme. This product formation took place also in the absence of the reductase domain of NOS. Both formation of the blue-shifted intermediate and of Nomega-hydroxy-L-arginine required the presence of tetrahydrobiopterin (BH4). We propose that the blue-shifted intermediate is the result of reductive activation of the oxygenated complex, and the electron is provided by BH4. These observations suggest that the reduction of the oxyferroheme complex may be the main function of BH4 in NOS catalysis.
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Affiliation(s)
- N Bec
- Institut National de la Santé et de la Recherche Scientifique, U 128, Institut Fédératif de Recherche 24, 34293 Montpellier, France
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17
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Abstract
1. Experimental data previously used to support an electron/proton transfer mechanism for oxidative dealkylation of amines by P450 are critically analysed with the conclusion that the mechanistic evidence is indecisive. 2. A new mechanistic criterion recently proposed to distinguish between electron/proton transfer and hydrogen atom transfer mechanisms is discussed. It is based on isotope effect profiles determined for the deprotonation of a series of para-substituted N-methyl-N-trideuteriomethyl)aniline cation radicals by pyridine and for hydrogen atom abstraction from the corresponding neutral amines by the tert-butoxyl radical. These reactions model the steps proposed in the two P450 mechanisms. 3. Isotope effect profiles measured for the demethylation of substituted NN-bis(dideuteriomethyl)anilines by four different forms of P450 were found to be experimentally indistinguishable from the hydrogen atom transfer profile, and distinctly different from the cation radical deprotonation profile. This provides strong evidence that P450 oxidatively dealkylates the amines by a hydrogen atom transfer mechanism and not by an electron/proton transfer mechanism.
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Affiliation(s)
- S B Karki
- Department of Chemistry, University of Rochester, NY 14627-0216, USA
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Hildebrandt P, Heibel G, Anzenbacher P, Lange R, Krüger V, Stier A. Conformational analysis of mitochondrial and microsomal cytochrome P-450 by resonance Raman spectroscopy. Biochemistry 1994; 33:12920-9. [PMID: 7947698 DOI: 10.1021/bi00209a024] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
Mitochondrial and microsomal cytochromes P-450SCC and P-450LM2 in the ferric substrate-free and substrate-bound states were studied by resonance Raman spectroscopy. In the spectra of cytochrome P-450SCC two conformational states (A and B) were detected, each of them constituting an equilibrium between a six-coordinated low-spin and a high-spin form. Both the conformational and the spin equilibria are pH- and temperature-dependent, which is in line with previously published results [Lange, R., Larroque, C., & Anzenbacher, P. (1992) Eur. J. Biochem. 207, 69-73)]. On the basis of well-resolved resonance Raman spectra, measured at different pH and temperatures, these equilibria were analyzed quantitatively. Both low-spin configurations of A and B exhibit different band patterns in the spin state marker band region, indicating differences in the active-site structures. While in the high-spin configuration of state A the heme iron remains weakly bound by a sixth ligand, the high-spin form of state B is five-coordinated. Binding of cholesterol to cytochrome P-450SCC causes a significant population of the high-spin forms, particularly of state A (62%). On the other hand, binding of 22R-hydroxycholesterol to the substrate-free enzyme leaves the overall spin equilibrium largely unchanged, i.e., six-coordinated low spin (76% A and 24% B). In both substrate-bound complexes, interactions between the substrate and the heme lead to small but distinct differences in the resonance Raman spectra of the low-spin form of state A. In contrast to cytochrome P-450SCC, the resonance Raman spectra of microsomal cytochrome P-450LM2 provide no indications for multiple conformers at 22 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)
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Affiliation(s)
- P Hildebrandt
- Max-Planck-Institut für Strahlenchemie, Mülheim, Federal Republic of Germany
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19
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