Spin state control of the hepatic cytochrome P450 redox potential

Sligar, Stephen G.; Cinti, Dominick L.; Gibson, Gordon; Schenkman, John B.

doi:10.1016/0006-291x(79)91916-8

Cited by 124 publications

(95 citation statements)

References 25 publications

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“…With rat CPR, they are E m (FAD semiquinone/reduced ) = −0.365 V, E m (FAD oxidized/semiquinone ) = −0.290 V, E m (FMN semiquinone/reduced ) = −0.270 V and E m (FMN oxidized/semiquinone ) = −0.110 V vs NHE [23] (see also Table 1). On the other hand, the midpoint potentials of substrate (hexobarbital)-bound rat liver cytochrome P450 and microsomal cytochrome b 5 have been reported to be −0.237 V [37] and 0.02 V vs NHE [38], respectively. In the present study, the midpoint potential of the ferric/ferrous couple in the heme-rHO-1 complex was found to be −0.087 V vs NHE.…”

Section: Discussionmentioning

confidence: 99%

Electrochemical reduction of ferrous α-verdoheme in complex with heme oxygenase-1

Satō

Higashimoto

Sakamoto

et al. 2007

Journal of Inorganic Biochemistry

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The heme oxygenase (HO) reaction consists of three successive oxygenation reactions, i.e. heme to α-hydroxyheme, α-hydroxyheme to verdoheme, and verdoheme to biliverdin-iron chelate. Of these, the least understood step is the conversion of verdoheme to biliverdin-iron chelate. For the cleavage of the oxaporphyrin ring of ferrous verdoheme, involvement of a verdoheme π-neutral radical has been proposed. To probe this hypothetical mechanism in the HO reaction, we performed electrochemical reduction of ferrous verdoheme complexed with rat HO-1 under anaerobic conditions. On the basis of the electrochemical spectral changes, the midpoint potential for the oneelectron reduction of the oxaporphyrin ring of ferrous verdoheme was found to be −0.47 ± 0.01 V vs the normal hydrogen electrode (NHE). Because this potential is far lower than those of both flavins of NADPH-cytochrome P450 reductase, and of NADPH, it is concluded that the one-electron reduction of the oxaporphyrin ring of ferrous verdoheme is unlikely to occur and that the formation of the π-neutral radical cannot be the initial step in the degradation of verdoheme by HO. Rather, it appears more reasonable to consider an alternative mechanism in which binding of O 2 to the ferrous iron of verdoheme is the first step in the degradation of verdoheme.

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Section: Discussionmentioning

confidence: 99%

Electrochemical reduction of ferrous α-verdoheme in complex with heme oxygenase-1

Satō

Higashimoto

Sakamoto

et al. 2007

Journal of Inorganic Biochemistry

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“…Microsomal cytochrome b 5 (cyt b 5 ) 1 is an amphipathic electron transfer hemoprotein located in the membrane of the endoplasmic reticulum. It provides electrons for a broad range of reactions, including fatty acid desaturation, cholesterol biosynthesis, and a variety of cytochrome P450-dependent oxidation and hydroxylation reactions (1,2).…”

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confidence: 99%

“…Both ferric and oxyferrous cyt P450 receive electrons from NADPH via its redox partner, NADPH-cytochrome P450 reductase (cyt P450 reductase), whereas cyt b 5 can donate the second electron. Because of the high redox potential of Х20 mV for cyt b 5 , the first electron required to reduce cyt P450 from the ferric to the ferrous state is always transferred from NADPH via cyt P450 reductase. However, as the redox potential of cyt P450 increases from ХϪ230 mV to ϩ50 mV on transition to the oxyferrous state, the second electron can be obtained from either cyt P450 reductase or cyt b 5 (5,6).…”

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“…Depending on the substrate, isozyme of cyt P450 and the experimental conditions cyt b 5 can stimulate, inhibit, or have no effect on cyt P450 activity (2). Studies of the stoichiometry of the metabolism of benzphetamine and methoxyflurane by cyt P450 2B4 have shown that cyt b 5 increases the efficiency of catalysis by cyt P450 2B4 primarily by decreasing the formation of the side-product superoxide.…”

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confidence: 99%

“…The two domains are connected by a highly flexible peptide chain of Х15 amino acids, which is assumed to allow the heme domain enough flexibility to bind different electron partners while ensuring that the protein remains localized at the endoplasmic reticulum. Solubilization of the cyt b 5 heme domain by removal of the membrane anchor through protease cleavage of the linker region generates a protein that is capable of electron transfer to soluble electron acceptor proteins such as cytochrome c, metmyoglobin, or met-hemoglobin, but incapable of electron transfer to microsomal cyt P450 (1). This dramatic change in the properties of cyt b 5 upon elimination of its transmembrane domain indicates that it plays an essential role in the function of cyt b 5 .…”

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confidence: 99%

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The role of the length and sequence of the linker domain of cytochrome b5 in stimulating cytochrome P450 2B4 catalysis.

Clarke

Bidwai

et al. 2004

Journal of Biological Chemistry

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Cytochrome b 5 (cyt b 5 ) is a 15-kDa amphipathic protein with a cytosolic amino-terminal catalytic heme domain, which is anchored to the microsomal membrane by a hydrophobic transmembrane ␣-helix at its carboxyl terminus. These two domains are connected by an ϳ15-amino acid linker domain, Ser 90 -Asp 104 , which has been modified by site-directed mutagenesis to investigate whether the length or sequence of the linker influences the ability of cyt b 5 to bind ferric cytochrome P450 2B4 and donate an electron to oxyferrous (cyt P450 2B4), thereby stimulating catalysis. Because shortening the linker by 8 or more amino acids markedly inhibited the ability of cyt b 5 to bind cyt P450 2B4 and stimulate catalysis by this isozyme, it is postulated 7 amino acids are sufficient to allow a productive interaction. All mutant cyts b 5 except the protein lacking the entire 15-amino acid linker inserted normally into the microsomal membrane. Alternatively, lengthening the linker by 16 amino acids, reversing the sequence of the amino acids in the linker, and mutating conserved linker residues did not significantly alter the ability of cyt b 5 to interact with cyt P450 2B4. A model for the membranebound cyt b 5 -cyt P450 complex is presented.Microsomal cytochrome b 5 (cyt b 5 ) 1 is an amphipathic electron transfer hemoprotein located in the membrane of the endoplasmic reticulum. It provides electrons for a broad range of reactions, including fatty acid desaturation, cholesterol biosynthesis, and a variety of cytochrome P450-dependent oxidation and hydroxylation reactions (1, 2).Cytochrome P450 (cyt P450) is responsible for the oxidation of a large number of substrates. The overall stoichiometry of the reaction catalyzed by cyt P450 is shown in Reaction 1, where RH is the substrate (3).The cyt P450 catalytic reaction occurs via a reaction cycle that involves substrate binding, reduction of the ferric heme, O 2 binding, reduction of the oxyferrous heme (second electron transfer), substrate oxidation, and finally product dissociation (4). Both ferric and oxyferrous cyt P450 receive electrons from NADPH via its redox partner, NADPH-cytochrome P450 reductase (cyt P450 reductase), whereas cyt b 5 can donate the second electron. Because of the high redox potential of Х20 mV for cyt b 5 , the first electron required to reduce cyt P450 from the ferric to the ferrous state is always transferred from NADPH via cyt P450 reductase. However, as the redox potential of cyt P450 increases from ХϪ230 mV to ϩ50 mV on transition to the oxyferrous state, the second electron can be obtained from either cyt P450 reductase or cyt b 5 (5, 6).Depending on the substrate, isozyme of cyt P450 and the experimental conditions cyt b 5 can stimulate, inhibit, or have no effect on cyt P450 activity (2). Studies of the stoichiometry of the metabolism of benzphetamine and methoxyflurane by cyt P450 2B4 have shown that cyt b 5 increases the efficiency of catalysis by cyt P450 2B4 primarily by decreasing the formation of the side-product superoxide. However, cyt b...

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Cytochromes P450

Guengerich

2012

Metabolism of Drugs and Other Xenobiotics

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Spin state control of the hepatic cytochrome P450 redox potential

Cited by 124 publications

References 25 publications

Electrochemical reduction of ferrous α-verdoheme in complex with heme oxygenase-1

Electrochemical reduction of ferrous α-verdoheme in complex with heme oxygenase-1

The role of the length and sequence of the linker domain of cytochrome b5 in stimulating cytochrome P450 2B4 catalysis.

Cytochromes P450

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