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Compound I is the reactive intermediate in the first monooxygenation step during conversion of cholesterol to pregnenolone by cytochrome P450scc: EPR/ENDOR/cryoreduction/annealing studies.

ABSTRACT: Cytochrome P450scc (CYP11A1) catalyzes conversion of cholesterol (CH) to pregnenolone, the precursor to all steroid hormones. This process proceeds via three sequential monooxygenation reactions: two stereospecific hydroxylations with formation first of 22R-hydroxycholesterol (22-HC) and then 20?,22R-dihydroxycholesterol (20,22-DHC), followed by C20-C22 bond cleavage. Herein we have employed EPR and ENDOR spectroscopy to characterize the intermediates in the first hydroxylation step by 77 K radiolytic one-electron cryoreduction and subsequent annealing of the ternary oxy-cytochrome P450scc-cholesterol complex. This approach is fully validated by the demonstration that the cryoreduced ternary complex of oxy-P450scc-CH is catalytically competent and hydroxylates cholesterol to form 22-HC with no detectable formation of 20-HC, just as occurs under physiological conditions. Cryoreduction of the ternary complex trapped at 77 K produces predominantly the hydroperoxy-ferriheme P450scc intermediate, along with a minor fraction of peroxo-ferriheme intermediate that converts into a new hydroperoxo-ferriheme species at 145 K. This behavior reveals that the distal pocket of the parent oxy-P450scc-cholesterol complex exhibits an efficient proton delivery network, with an ordered water molecule H-bonded to the distal oxygen of the dioxygen ligand. During annealing of the hydroperoxy-ferric P450scc intermediates at 185 K, they convert to the primary product complex in which CH has been converted to 22-HC. In this process, the hydroperoxy-ferric intermediate decays with a large solvent kinetic isotope effect, as expected when proton delivery to the terminal O leads to formation of Compound I (Cpd I). (1)H ENDOR measurements of the primary product formed in deuterated solvent show that the heme Fe(III) is coordinated to the 22R-O(1)H of 22-HC, where the (1)H is derived from substrate and exchanges to D after annealing at higher temperatures. These observations establish that Cpd I is the agent that hydroxylates CH, rather than the hydroperoxy-ferric heme.

SUBMITTER: Davydov R

PROVIDER: S-EPMC3491644 | biostudies-literature | 2012 Oct

REPOSITORIES: biostudies-literature

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Compound I is the reactive intermediate in the first monooxygenation step during conversion of cholesterol to pregnenolone by cytochrome P450scc: EPR/ENDOR/cryoreduction/annealing studies.

Davydov Roman R Gilep Andrey A AA Strushkevich Natallia V NV Usanov Sergey A SA Hoffman Brian M BM

Journal of the American Chemical Society 20121005 41

Cytochrome P450scc (CYP11A1) catalyzes conversion of cholesterol (CH) to pregnenolone, the precursor to all steroid hormones. This process proceeds via three sequential monooxygenation reactions: two stereospecific hydroxylations with formation first of 22R-hydroxycholesterol (22-HC) and then 20α,22R-dihydroxycholesterol (20,22-DHC), followed by C20-C22 bond cleavage. Herein we have employed EPR and ENDOR spectroscopy to characterize the intermediates in the first hydroxylation step by 77 K radi ...[more]

PMID: 23039857

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Project description:Cytochrome P450scc (CYP 11A1) catalyzes the conversion of cholesterol (Ch) to pregnenolone, the precursor to steroid hormones. This process proceeds via three sequential monooxygenation reactions: two hydroxylations of Ch first form 22(R)-hydroxycholesterol (HC) and then 20?,22(R)-dihydroxycholesterol (DHC); a lyase reaction then cleaves the C20-C22 bond to form pregnenolone. Recent cryoreduction/annealing studies that employed electron paramagnetic resonance (EPR)/electron nuclear double resonance (ENDOR) spectroscopy [Davydov, R., et al. (2012) J. Am. Chem. Soc. 134, 17149] showed that compound I (Cpd I) is the active intermediate in the first step, hydroxylation of Ch. Herein, we have employed EPR and ENDOR spectroscopy to characterize the intermediates in the second and third steps of the enzymatic process, as conducted by 77 K radiolytic one-electron cryoreduction and subsequent annealing of the ternary oxy-cytochrome P450scc complexes with HC and DHC. This procedure is validated by showing that the cryoreduced ternary complexes of oxy-cytochrome P450scc with HC and DHC are catalytically competent and during annealing generate DHC and pregnenolone, respectively. Cryoreduction of the oxy-P450scc-HC ternary complex trapped at 77K produces the superoxo-ferrous P450scc intermediate along with a minor fraction of ferric hydroperoxo intermediates. The superoxo-ferrous intermediate converts into a ferric-hydroperoxo species after annealing at 145 K. During subsequent annealing at 170-180 K, the ferric-hydroperoxo intermediate converts to the primary product complex with the large solvent kinetic isotope effect that indicates Cpd I is being formed, and (1)H ENDOR measurements of the primary product formed in D2O demonstrate that Cpd I is the active species. They show that the primary product contains Fe(III) coordinated to the 20-O(1)H of DHC with the (1)H derived from substrate, the signature of the Cpd I reaction. Hydroperoxo ferric intermediates are the primary species formed during cryoreduction of the oxy-P450scc-DHC ternary complex, and they decay at 185 K with a strong solvent kinetic isotope effect to form low-spin ferric P450scc. Together, these observations indicated that Cpd I also is the active intermediate in the C20,22 lyase final step. In combination with our previous results, this study thus indicates that Cpd I is the active species in each of the three sequential monooxygenation reactions by which P450scc catalytically converts Ch to pregnenolone.

Project description:Dehaloperoxidase (DHP) from Amphitrite ornata is a heme protein that can function both as a hemoglobin and as a peroxidase. This report describes the use of 77 K cryoreduction EPR/ENDOR techniques to study both functions of DHP. Cryoreduced oxyferrous [Fe(II)-O(2)] DHP exhibits two EPR signals characteristic of a peroxoferric [Fe(III)-O(2)(2-)] heme species, reflecting the presence of conformational substates in the oxyferrous precursor. (1)H ENDOR spectroscopy of the cryogenerated substates shows that H-bonding interactions between His N(?)H and heme-bound O(2) in these conformers are similar to those in the ?-chain of oxyferrous hemoglobin A (HbA) and oxyferrous myoglobin, respectively. Decay of cryogenerated peroxoferric heme DHP intermediates upon annealing at temperatures above 180 K is accompanied by the appearance of a new paramagnetic species with an axial EPR signal with g(?) = 3.75 and g(?) = 1.96, characteristic of an S = 3/2 spin state. This species is assigned to Compound I (Cpd I), in which a porphyrin ?-cation radical is ferromagnetically coupled with an S = 1 ferryl [Fe(IV)?O] ion. This species was also trapped by rapid freeze-quench of the ambient-temperature reaction mixture of ferric [Fe(III)] DHP and H(2)O(2). However, in the latter case Cpd I is reduced very rapidly by a nearby tyrosine to form Cpd ES [(Fe(IV)?O)(porphyrin)/Tyr(•)]. Addition of the substrate analogue 2,4,6-trifluorophenol (F(3)PhOH) suppresses formation of the Cpd I intermediate during annealing of cryoreduced oxyferrous DHP at 190 K but has no effect on the spectroscopic properties of the remaining cryoreduced oxyferrous DHP intermediates and kinetics of their decay. These observations indicate that substrate (i) binds to oxyferrous DHP outside of the distal pocket and (ii) can reduce Cpd I to Cpd II [Fe(IV)?O]. These assumptions are also supported by the observation that F(3)PhOH has only a small effect on the EPR properties of radiolytically cryooxidized and cryoreduced ferrous [Fe(II)] DHP. EPR spectra of cryoreduced ferrous DHP disclose the multiconformational nature of the ferrous DHP precursor. The observation and characterization of Cpds I, II, and ES in the absence and in the presence of F(3)PhOH provides definitive evidence of a mechanism involving consecutive one-electron steps and clarifies the role of all intermediates formed during turnover.

Project description:Carotenoids are indispensable molecules for life. They are present everywhere in plants, algae, bacteria whom they protect against free radicals and oxidative stress. Through the consumption of fruits and vegetables and some carotenoid-containing fish, they are introduced into the human body and, similarly, protect it. There are numerous health benefits associated with the consumption of carotenoids. Carotenoids are antioxidants but at the same time they are prone to oxidation themselves. Electron loss from the carotenoid forms a radical cation. Furthermore, proton loss from a radical cation forms a neutral radical. In this mini-review, we discuss carotenoid radicals studied in our groups by various physicochemical methods, namely the radical cations formed by electron transfer and neutral radicals formed by proton loss from the radical cations. They contain many similar hyperfine couplings due to interactions between the electron spin and numerous protons in the carotenoid. Different EPR and ENDOR methods in combination with DFT calculations have been used to distinguish the two independent carotenoid radical species. DFT predicted larger coupling constants for the neutral radical compared to the radical cation. Previously, INDO calculations miss assigned the large couplings to the radical cation. EPR and ENDOR have aided in elucidating the physisorb, electron and proton transfer processes that occur when carotenoids are adsorbed on solid artificial matrices, and predicted similar reactions in aqueous solution or in plants. After years of study of the physicochemical properties of carotenoid radicals, the different published results start to merge together for a better understanding of carotenoid radical species and their implication in biological systems. Up until 2008, the radical chemistry in artificial systems was elucidated but the correlation between quenching ability and neutral radical formation was an inspiration to look for these radical species in vivo. In addition, the EPR spin-trapping technique has been applied to study inclusion complexes of carotenoids with different delivery systems.

Dataset Information

Compound I is the reactive intermediate in the first monooxygenation step during conversion of cholesterol to pregnenolone by cytochrome P450scc: EPR/ENDOR/cryoreduction/annealing studies.

Publications

Compound I is the reactive intermediate in the first monooxygenation step during conversion of cholesterol to pregnenolone by cytochrome P450scc: EPR/ENDOR/cryoreduction/annealing studies.

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