Effect of lipid unsaturation on the binding of native and a mutant form of cytochrome b5 to membranes

The partitioning of native cytochrome b5 and a mutant form, where Trp-108 and Trp-112 were both replaced by Leu, into small unilamellar lipid vesicles was examined. The vesicles were made from phosphatidylcholines containing mono- and di-unsaturated acyl chains. As these amphipathic proteins self-associate in aqueous solution, the binding was not monitored by a simple lipid titration experiment but by an exchange assay using fluorescence quenching by brominated lipids. Each protein had a greater affinity for lipids containing mono-unsaturated chains than for vesicles containing di-unsaturated chains, and the affinities of both proteins increased in buffers of higher ionic strength. The native protein had a higher affinity than the mutant protein for all vesicles; the ratio of the affinities was relatively constant at approximately 30. This corresponds to a difference in the free energy of partitioning of 2 kcal mol(-)(1). The fluorescence quantum yields of both proteins were much lower in lipids with di-unsaturated chains whereas a similar lowering was not seen with a simple Trp compound. These data suggest that the decreased membrane hydrophobicity seen by the proteins in di-unsaturated membranes is not an inherent property of the bilayer but is induced by the insertion of the protein. Further, the similar behavior of the two proteins suggests this modulation is not sensitive to the amino acid side chains of the inserted domain.

Effect of mutation at valine 61 on the three-dimensional structure, stability, and redox potential of cytochrome b5

To elucidate the role played by Val61 of cytochrome b(5), this residue of the tryptic fragment of bovine liver cytochrome b(5) was chosen for replacement with tyrosine (Val61Tyr), histidine (Val61His), glutamic acid (Val61Glu), and lysine (Val61Lys) by means of site-directed mutagenesis. The mutants Val61Tyr, Val61Glu, Val61His, and Val61Lys exhibit electronic spectra identical to that of the wild type, suggesting that mutation at Val61 did not affect the overall protein structure significantly. The redox potentials determined by differential pulse voltammetry were -10 (wild type), -25 (Val61Glu), -33 (Val61Tyr), 12 (Val61His), and 17 mV (Val61Lys) versus NHE. The thermal stabilities and urea-mediated denaturation of wild-type cytochrome b(5) and its mutants were in the following order: wild type > Val61Glu > Val61Tyr > Val61His > Val61Lys. The kinetics of denaturation of cytochrome b(5) by urea was also analyzed. The first-order rate constants of heme transfer between cytochrome b(5) and apomyoglobin at 20 +/- 0.2 degrees C were 0.25 +/- 0.01 (wild type), 0.42 +/- 0.02 (Val61Tyr), 0.93 +/- 0.04 (Val61Glu), 2.88 +/- 0.01 (Val61His), and 3.88 +/- 0.02 h(-)(1) (Val61Lys). The crystal structure of Val61His was determined using the molecular replacement method and refined at 2.1 A resolution, showing that the imidazole side chain of His61 points away from the heme-binding pocket and extends into the solvent, the coordination distances from Fe to NE2 atoms of two axial ligands are approximately 0.6 A longer than the reported value, and the hydrogen bond network involving Val61, the heme propionates, and three water molecules no longer exists. We conclude that the conserved residue Val61 is located at one of the key positions, the "electrostatic potential" around the heme-exposed area and the hydrophobicity of the heme pocket are determinant factors modulating the redox potential of cytochrome b(5), and the hydrogen bond network around the exposed heme edge is also an important factor affecting the heme stability.

Modulation of the impaired drug metabolism in sarcoma-180-bearing mice by echitamine chloride

Echitamine chloride (EC), an indole alkaloid, extracted from the bark of Alstonia scholaris has got highly promising anticancer effect. The effect of this drug on the microsomal drug detoxifying system was studied in sarcoma-180 induced mice. When given sub-cutaneously at a dosage of 5 mg/kg body weight, it was able to alter the impaired drug detoxifying system which was observed in the Sarcoma-180 bearing mice. The levels of microsomal protein, Cyt-P450, Cyt-b5, NADH-Cyt-C-reductase, NADPH-Cyt-C-reductase, and glu-6 phosphatase were determined. The levels of these drug metabolizing enzymes were decreased in S-180 bearing mice. EC treatment corrected to near normal levels of these enzymes and microsomal hemeproteins. In order to understand the mechanism responsible for the decreased protein level and its normalization after treatment with EC, 3H-Phenylalanine incorporation study was carried out. From the results, it is observed that the synthesis of apoproteins is also altered in tumor-bearing animals. All these changes which were observed in tumor-bearing animals were corrected to near normal levels after treatment with EC.

The cytochrome b5 tail anchors and stabilizes subdomains of human DNA topoisomerase II alpha in the cytoplasm of retrovirally infected mammalian cells

DNA topoisomerase II (topo II) is the target of many anticancer drugs and is often altered in drug-resistant cell lines. In some tumor cell lines truncated isoforms of topo IIalpha are localized to the cytoplasm. To study the localization and function of individual enzyme domains, we have epitope-tagged several fragments of human topo IIalpha and expressed them by retroviral infection of rodent and human cells. We find that fusion of the topo II fragments to the hydrophobic tail of human liver cytochrome b5 anchors the fusion protein to the outer face of cytoplasmic membranes, as determined by colocalization with calnexin and selective detergent permeabilization. Moreover, whereas the minimal ATPase domain (aa 1-266) is weakly and diffusely expressed, addition of the cytb5 anchor (1-266-b5) increases its steady-state level 16-fold with no apparent toxicity. Similar results are obtained with the complete ATPase domain (aa 1-426). A C-terminal domain (aa 1030-1504) of human topo IIalpha containing an intact dimerization motif is stably expressed and accumulates in the nucleus. Fusion to the cytb5 anchor counteracts the nuclear localization signal and relocalizes the protein to cytoplasmic membranes. In conclusion, we describe a technique that stabilizes and targets retrovirally expressed proteins such that they are exposed on the cytoplasmic surface of cellular membranes. This approach may be of general use for regulating the nuclear accumulation of drugs or proteins in living cells.

Effects of human cytochrome b5 on CYP3A4 activity and stability in vivo

Cytochrome P450s (P450) form a superfamily of membrane-bound proteins that play a key role in the primary metabolism of both xenobiotics and endogenous compounds such as drugs and hormones, respectively. To be enzymically active, they require the presence of a second membrane-bound protein, NADPH P450 reductase, which transfers electrons from NADPH to the P450. Because of the diversity of P450 enzymes, much of the work on individual forms has been carried out on purified proteins, in vitro, which requires the use of complex reconstitution mixtures to allow the P450 to associate correctly with the NADPH P450 reductase. There is strong evidence from such reconstitution experiments that, when cytochrome b5 is included, the turnover of some substrates with certain P450s is increased. Here we demonstrate that allowing human P450 reductase, CYP3A4, and cytochrome b5 to associate in an in vivo-like system, by coexpressing all three proteins together in Escherichia coli for the first time, the turnover of both nifedipine and testosterone by CYP3A4 is increased in the presence of cytochrome b5. The turnover of testosterone was increased by 166% in whole cells and by 167% in preparations of bacterial membranes. The coexpression of cytochrome b5 also resulted in the stabilization of the P450 during substrate turnover in whole E. coli, with 109% of spectrally active CYP3A4 remaining in cells after 30 min in the presence of cytochrome b5 compared with 43% of the original P450 remaining in cells in the absence of cytochrome b5.

Genetic analysis of cytochrome b5 from arachidonic acid-producing fungus, Mortierella alpina 1S-4: cloning, RNA editing and expression of the gene in Escherichia coli, and purification and characterization of the gene product

Information on the amino acid sequences of the internal peptide fragments of cytochrome b5 from Mortierella hygrophila was used to prepare synthetic oligonucleotides as primers for the polymerase chain reaction. A 100-base DNA fragment was thus amplified, by using a genomic gene from Mortierella alpina 1S-4 as a template, which produced polyunsaturated fatty acids such as arachidonic acid. The amplified DNA fragment was used as the probe to clone both a 523-base cDNA fragment and a 2.1-kilobase SalI-NruI genomic fragment coding for the whole M. alpina 1S-4 cytochrome b5. On the basis of nucleotide sequences of both cytochrome b5 genomic gene and cDNA, the genomic cytochrome b5 gene was found to consist of four exons and three introns. A novel type of RNA editing, in which the cDNA included either guanine insertion or adenine-->guanine substitution at one base upstream of poly(A), was interestingly observed. The deduced amino acid sequence of M. alpina 1S-4 cytochrome b5 showed significant similarities with those of cytochrome b5s from other organisms such as rat, chicken, and yeast. The soluble form of the cytochrome b5 gene was expressed to 16% of the total soluble protein in Escherichia coli. The holo-cytochrome b5 accounted for 8% of the total cytochrome b5 in the transformants. The purified cytochrome b5 showed the oxidized and reduced absorbance spectra characteristic of fungal microsomal cytochrome b5.

Temporal and spatial gene expression of cytochrome B5 during flower and fruit development in olives

We report the characterisation of two cytochrome b5 genes and their spatial and temporal patterns of expression during development in olive, Olea europaea. A PCR-generated probe, based on a tobacco cytochrome b5 sequence, was used to isolate two full-length cDNA clones (cytochrome b5-15 and cytochrome b5-38) from a library derived from 13 WAF olive fruits. The cDNAs encoded proteins of 17.0 and 17.7 kDa, which contained all the characteristic motifs of cytochromes b5 from other organisms and exhibited 63% identity and 85% similarity with each other. The olive cytochrome b5-15 cDNA was then used as a probe for more detailed analysis. Southern blotting revealed a gene family of at least 4-6 members while northern blotting and in situ hybridisation showed a highly specific pattern of gene expression. Very low levels of cytochrome b5 mRNA were detected in tissues characterised by high rates of lipid accumulation, such as young expanding leaves, maturing seeds and ripening mesocarp. The cytochrome b5 genes were not induced at 6 degrees C and their response to ABA was relatively slow compared with fatty acid desaturase genes. In contrast, high levels of cytochrome b5 gene expression were found in young fruits at the pattern formation (globular/heart) stage of embryogenesis and in vascular and transmitting tissues of male and female reproductive organs. The data are consistent with a major role for cytochrome b5 in developmental processes related to plant reproduction in addition to being an electron donor to microsomal desaturases.

Engineering out motion: a surface disulfide bond alters the mobility of tryptophan 22 in cytochrome b5 as probed by time-resolved fluorescence and 1H NMR experiments

In the accompanying paper [Storch et al. (1999) Biochemistry 38, 5054-5064] equilibrium denaturation studies and molecular dynamics (MD) simulations were used to investigate localized dynamics on the surface of cytochrome b5 (cyt b5) that result in the formation of a cleft. In those studies, an S18C:R47C disulfide mutant was engineered to inhibit cleft mobility. Temperature- and urea-induced denaturation studies revealed significant differences in Trp 22 fluorescence between the wild-type and mutant proteins. On the basis of the results, it was proposed that wild type populates a conformational ensemble that is unavailable to the disulfide mutant and is mediated by cleft mobility. As a result, the solvent accessibility of Trp 22 is decreased in S18C:R47C, suggesting that the local environment of this residue is less mobile due to the constraining effects of the disulfide on cleft dynamics. To further probe the structural effects on the local environment of Trp 22 caused by inhibition of cleft formation, we report here the results of steady-state and time-resolved fluorescence quenching, differential phase/modulation fluorescence anisotropy, and 1H NMR studies. In Trp fluorescence experiments, the Stern-Volmer quenching constant increases in wild type versus the oxidized disulfide mutant with increasing temperature. At 50 degrees C, KSV is nearly 1.5-fold greater in wild type compared to the oxidized disulfide mutant. In the reduced disulfide mutant, KSV was the same as wild type. The bimolecular collisional quenching constant, kq, for acrylamide quenching of Trp 22 increases 2.7-fold for wild type and only 1.8-fold for S18C:R47C, upon increasing the temperature from 25 to 50 degrees C. The time-resolved anisotropy decay at 25 degrees C was fit to a double-exponential decay for both the wild type and S18C:R47C. Both proteins exhibited a minor contribution from a low-amplitude fast decay, consistent with local motion of Trp 22. This component was more prevalent in the wild type, and the fractional contribution increased significantly upon raising the temperature. The fast rotational component of the S18C:R47C mutant was less sensitive to increasing temperature. A comparison of the 1H NMR monitored temperature titration of the delta-methyl protons of Ile 76 for wild type and oxidized disulfide mutant, S18C:R47C, showed a significantly smaller downfield shift for the mutant protein, suggesting that Trp 22 in the mutant protein experiences comparatively decreased cleft dynamics in core 2 at higher temperatures. Furthermore, comparison of the delta'-methyl protons of Leu 25 in the two proteins revealed a difference in the ratio of the equilibrium heme conformers of 1.2:1 for S18C:R47C versus 1.5:1 for wild type at 40 degrees C. The difference in equilibrium heme orientations between wild type and S18C:R47C suggests that the disulfide bond affects heme binding within core 1, possibly through damped cleft fluctuations. Taken together, the NMR and fluorescence studies support the proposal that an engineered disulfide bond inhibits the formation of a dynamic cleft on the surface of cyt b5.

Engineering out motion: introduction of a de novo disulfide bond and a salt bridge designed to close a dynamic cleft on the surface of cytochrome b5

A previous molecular dynamics (MD) simulation of cytochrome b5 (cyt b5) at 25 degrees C displayed localized dynamics on the surface of the protein giving rise to the periodic formation of a cleft that provides access to the heme through a protected hydrophobic channel [Storch and Daggett (1995) Biochemistry 34, 9682]. Here we describe the production and testing of mutants designed to prevent the cleft from opening using a combination of experimental and theoretical techniques. Two mutants have been designed to close the surface cleft: S18D to introduce a salt bridge and S18C:R47C to incorporate a disulfide bond. The putative cleft forms between two separate cores of the protein: one is structural in nature and can be monitored through the fluorescence of Trp 22, and the other binds the heme prosthetic group and can be tracked via heme absorbance. An increase in motion localized to the cleft region was observed for each protein, except for the disulfide-containing variant, in MD simulations at 50 degrees C compared to simulations at 25 degrees C. For the disulfide-containing variant, the cleft remained closed. Both urea and temperature denaturation curves were nearly identical for wild-type and mutant proteins when heme absorbance was monitored. In contrast, fluorescence studies revealed oxidized S18C:R47C to be considerably more stable based on the midpoints of the denaturation transitions, Tm and U1/2. Moreover, the fluorescence changes for each protein were complete at approximately 50 degrees C and a urea concentration of approximately 3.9 M, significantly below the temperature and urea concentration (62 degrees C, 5 M urea) required to observe heme release. In addition, solvent accessibility based on acrylamide quenching of Trp 22 was lower in the S18C:R47C mutant, particularly at 50 degrees C, before heme release [presented in the accompanying paper (58)]. The results suggest that a constraining disulfide bond can be designed to inhibit dynamic cleft formation on the surface of cyt b5. Located near the heme, the native dynamics of the cleft may be functionally important for protein-protein recognition and/or complex stabilization.

Two homologous cytochrome b5s are expressed in both neurons and glial cells of the rat brain

To avoid the possibility of blood contamination and of gene rearrangement during library construction we isolated total RNA directly from cultured rat embryonic neuronal cells and glioma C6 cells to be used as template for RT-PCR. By using specific primers for both membrane-bound b5 and soluble b5, DNA bands of appropriate size were clearly amplified indicating that both neurons and glial cells expressed b5s, although soluble b5 seemed to be less expressed in these cells. Nucleotide sequence of the internal exon for soluble b5 was reinvestigated and confirmed to be 58 bp containing genetic codons for His-Ser-Ala-Leu and stop.

Isolation and characterisation of a cytochrome b5 cDNA clone from Helicoverpa armigera (Hubner): possible involvement of cytochrome b5 in cytochrome P450 CYP6B7 activity towards pyrethroids

A cDNA clone specific for cytochrome b5 was isolated from Helicoverpa armigera. This sequence corresponded to a mRNA of an estimated 544 nucleotides in length excluding the poly A tail. The mRNA contained an open reading frame of 381 nucleotides encoding a protein of 127 amino acid residues with a molecular weight of 14,564 Daltons. The encoded protein sequence showed 51% protein sequence identity with cytochrome b5 from M. domestica and 36-37% identity with mammalian and avian cytochrome b5 sequences. Northern analysis of larval RNA using this cDNA as probe, revealed that cytochrome b5 mRNA expression is tissue specific with the mRNAs being expressed in abundance in the midguts of larvae, at a lower level in fatbody but is not detectable in larval integument. During normal development this mRNA was undetectable in eggs but was present at similar levels from first to fifth instar larvae. The mRNA was expressed at very low levels in pupae and adult moths. The cytochrome b5 mRNA was found to be inducible by treatment with the monoterpene, a-pinene, and to be over-expressed in some individuals of a pyrethroid resistant population of H. armigera. The induction and over-expression patterns were identical to the cytochrome P450, CYP6B7 mRNA. The present data suggests that cytochrome b5 may be involved in CYP6B7 mediated pyrethroid resistance in H. armigera.

A cytochrome b5 is required for full activity of flavonoid 3', 5'-hydroxylase, a cytochrome P450 involved in the formation of blue flower colors

The substitution pattern of anthocyanin pigments is a main determinant of flower color. Flavonoid 3',5'-hydroxylase (F3'5'H) is a cytochrome P450 enzyme (Cyt P450) that catalyzes the 3', 5'-hydroxylation of dihydroflavonols, the precursors of purple anthocyanins. Species such as rose and carnation lack F3'5'H activity and are, therefore, unable to generate purple or blue flowers. Petunia, on the other hand, contains two loci, termed hf1 and hf2, that encode a Cyt P450 with F3'5'H activity. Here we report the identification of an additional petunia gene that is required for 3',5' substitution of anthocyanins and purple flower colors. It encodes a cytochrome b5 and is expressed exclusively in the flower. Inactivation of the gene by targeted transposon mutagenesis reduced F3'5'H enzyme activity and the accumulation of 5'-substituted anthocyanins, resulting in an altered flower color. However, no phenotypic effect on the activity of other Cyt P450s, involved in the synthesis of hormones or general phenylpropanoids, was observed. These data provide in vivo evidence for the regulation of the activity of specific Cyt P450s by a cytochrome b5.

Structure, interaction and electron transfer between cytochrome b5, its E44A and/or E56A mutants and cytochrome c

Site-directed mutagenesis has been used to produce variants of a tryptic fragment of bovine liver cytochrome b5 in which Glu44 and Glu56 are mutated to alanine. The reduction potentials measured by spectroelectrochemical titration (in the presence of 1 mM (Ru(NH3)6)3+, pH 7.0 and I=0.1 M) are 4.5, 6.0, 6.0 and 7.5 mV versus the standard hydrogen electrode (SHE) for the wild-type and E44A, E56A and E44/56A mutants of cytochrome b5, respectively. A comparative two-dimensional NMR study of cytochrome b5 and its E44/56A mutant in water solution has been achieved. Resonance assignments of side-chains have been completed successfully. The NMR results suggest that the secondary structures and global folding of the E44/56A mutant remain unchanged, but the mutation of both Glu44 and Glu56 to hydrophobic alanine may lead to the two helices containing mutated residues contracting towards the heme center. The inner mobility of the Gly42 approximately Glu44 segment in cytochrome b5 may be responsible for the difference of the binding mode between Glu44 and Glu56 with cytochrome c. The binding between cytochrome c and cytochrome b5 was studied by optical difference spectra of cytochrome c and variants of cytochrome b5. The association constants (KA) for the wild-type, E44A, E56A, and E44/56A mutants of cytochrome b5 with cytochrome c, are 4.70(+/-0. 10)x10(6) M-1, 1.88(+/-0.03)x10(6) M-1, 2.70(+/-0.13)x10(6) M-1, and 1.14(+/-0.05)x10(6) M-1, respectively. This is indicative that both Glu44 and Glu56 are involved in the complex formation between cytochrome b5 and cytochrome c. The reduction of horse heart ferricytochrome c by recombinant ferrocytochrome b5 and its mutants has been studied. The rate constant of the electron transfer reaction between ferricytochrome c and wild-type ferrocytochrome b5 (1.074(+/-0.49)x10(7) M-1 s-1) is higher than those of the mutant protein E44A (8.98(+/-0.20)x10(6) M-1 s-1), E56A (8.76(+/-0. 39)x10(6) M-1 s-1), and E44/56A (8.02(+/-0.38)x10(6) M-1 s-1) at 15 degreesC, pH 7.0, I=0.35 M. The rate constants are strongly dependent on ionic strength and temperature. These studies, by means of a series of techniques, provide conclusive results that the interaction between cytochrome b5 and cytochrome c is electrostatically guided, and, more importantly, that both Glu44 and Glu56 participate in the electron transfer reaction.

P450c17 mutations R347H and R358Q selectively disrupt 17,20-lyase activity by disrupting interactions with P450 oxidoreductase and cytochrome b5

Cytochrome P450c17 catalyzes steroid 17alpha-hydroxylase and 17,20-lyase activities and hence is a key enzyme in the production of human glucocorticoids and sex steroids. These two activities are catalyzed in a single substrate-binding site but are regulated independently in human physiology. We have recently shown that cytochrome b5 facilitates 17,20-lyase activity by allosterically promoting the interaction of P450c17 with P450 oxidoreductase (OR) and that the human P450c17 mutations, R347H and R358Q, selectively destroy 17,20-lyase activity while sparing 17alpha-hydroxylase activity. We transfected COS-1 cells with vectors for these P450c17 mutants and found that an excess of OR and b5 restored a small amount of 17,20-lyase activity, suggesting the mutations interfere with electron donation. To determine whether these mutations selectively interfere with the interaction of P450c17 and its electron-donating system, we expressed each P450cl7 mutant in yeast with or without OR, b5, or both, and measured enzyme kinetics in yeast microsomes using pregnenolone and 17alpha-hydroxypregnenolone as substrates. The apparent Michaelis-Menten (Km) values for the R347H mutant with and without coexpressed OR were 0.2 and 0.6 microM, respectively, and for the R358Q mutant with and without OR they were 0.3 and 0.4 microM, respectively; these values did not differ significantly from the wild-type values of 0.4 and 0.8 microM with and without OR, respectively. Furthermore, coincubation with 17alpha-hydroxypregnenolone showed a competitive mechanism for interference of catalysis. The similar kinetics and the competitive inhibition prove that the mutations did not affect the active site. Coexpression of the mutants with OR yielded insignificant 17,20-lyase activity, but addition of a 30:1 molar excess cytochrome b5 to these microsomes restored partial 17,20-lyase activity, with the R358Q mutant achieving twice the activity of the R347H mutant. These data indicate that both mutations selectively interfere with 17,20-lyase activity by altering the interaction of P450c17 with OR, thus proving that the lyase activity was disrupted by interfering with electron transfer. Furthermore, the data offer the first evidence that R347 is a crucial component of the site at which b5 interacts with the P450c17 x OR complex to promote electron transfer.

Charged amino acids at the carboxyl-terminal portions determine the intracellular locations of two isoforms of cytochrome b5

Outer mitochondrial membrane cytochrome b5 (OMb), which is an isoform of cytochrome b5 (cyt b5) in the endoplasmic reticulum, is a typical tail-anchored protein of the outer mitochondrial membrane. We cloned cDNA containing the complete amino acid sequence of OMb and found that the protein has no typical structural feature common to the mitochondrial targeting signal at the amino terminus. To identify the region responsible for the mitochondrial targeting of OMb, various mutated proteins were expressed in cultured mammalian cells, and the subcellular localization of the expressed proteins was analyzed. The deletion of more than 11 amino acid residues from the carboxyl-terminal end of OMb abolished the targeting of the protein to the mitochondria. When the carboxyl-terminal 10 amino acids of OMb were fused to the cyt b5 that was previously deleted in the corresponding 10 residues, the fused protein localized in the mitochondria, thereby indicating that the carboxyl-terminal 10 amino acid residues of OMb have sufficient information to transport OMb to the mitochondria. The replacement of either of the two positively charged residues within the carboxyl-terminal 10 amino acids by alanine resulted in the transport of the mutant proteins to the endoplasmic reticulum. The mutant cyt b5, in which the acidic amino acid in its carboxyl-terminal end was replaced by basic amino acid, could be transported to the mitochondria. It would thus seem that charged amino acids in the carboxyl-terminal portion of these proteins determine their locations in the cell.

A sphingolipid desaturase from higher plants. Identification of a new cytochrome b5 fusion protein

A recently cloned cDNA from sunflower codes for a fusion protein composed of an N-terminal cytochrome b5 and a domain similar to membrane-bound acyl lipid desaturases. For a functional identification, homologous cDNAs from Brassica napus and Arabidopsis thaliana were expressed in Saccharomyces cerevisiae, and sphingolipid long chain bases were analyzed. The expression of the heterologous enzyme results in significant proportions of new Delta8, 9-cis/trans-phytosphingenines that accompany the residual C18-phytosphinganine predominating in wild-type yeast cells. These results represent the first identification of a gene coding for a sphingolipid desaturase and for a stereounselective desaturase showing trans-activity from any organism. Furthermore, this fusion protein is a new member of the cytochrome b5 superfamily. The formation of the two regioisomeric phytosphingenines in the transformed yeast sheds new light on the factors controlling regioselectivity.

The influence of mutation at Glu44 and Glu56 of cytochrome b5 on the protein's stabilization and interaction between cytochrome c and cytochrome b5

To characterize the roles played by Glu44 and Glu56 of cytochrome b5 in the formation of the electrostatic complex between cytochrome c and cytochrome b5, the Glu44, Glu56, or both sites were changed to alanine by site-directed mutagenesis. The influence of these two residues on the protein stability was probed by investigating the kinetic behaviors of protein denaturation in urea or upon heating and the heme-transfer reactions between apo-myoglobin and the variants of cytochrome b5. It has been found that when the Glu44 and/or Glu56 are mutated to alanine, the protein stability increases slightly due to the fact that the hydrophilic residue is changed to a hydrophobic one, resulting in the two pairs of helices surrounding the heme taking a more compact conformation. The difference in voltammetric behavior of cytochrome c, cytochrome b5, and its three mutants, Cyt b5 E44A, E56A, and E44/56A, alone and in 1:1 protein complexes demonstrates that both Glu44 and Glu56 of cytochrome b5 take part in the electrostatic interaction with cytochrome c. The entropy changes, DeltaS degreesrc and enthalpy changes, DeltaH degrees, derived from the temperature dependence of the formal reduction potentials of each protein in different protein systems suggest that, because of the mutual interaction with cytochrome c, cytochrome b5 mutants, especially the E44A-containing mutants, in the protein complexes suffer greater conformational changes upon reduction than that of the wild type. The variation of these thermodynamic parameters indicates that the strength of mutual interactions between cytochrome c and cytochrome b5 or its mutants has the following order: Cyt c/Cyt b5 > Cyt c/Cyt b5 E56A > Cyt c/Cyt b5 E44A > Cyt c/Cyt b5 E44/56A.

Conversion of mitochondrial cytochrome b5 into a species capable of performing the efficient coupled oxidation of heme

Histidine-63, one of the heme axial ligands in outer mitochondrial membrane cytochrome b5 (OM cyt b5) has been replaced by a methionine. The H63M variant performs the efficient and regioselective coupled oxidation of heme in order to produce >90% of the alpha-isomer of verdoheme. The variant was characterized by electronic, EPR, and NMR spectroscopic studies which indicate that the ferric form is a high-spin species whose heme is coordinated by histidine-39 in the proximal site and likely by water in the distal site. The coordination of methionine to the ferric heme was ruled out on the basis of NMR spectroscopic studies. Addition of imidazole to a solution of the ferric variant results in the formation of a species axially coordinated by imidazole and histidine-63. The reduction potential of the variant was found to be +110 mV in the absence of exogenous imidazole and -92 mV in the presence of imidazole. These values compare well with the reduction potential of myoglobin (50 mV) and wild-type OM cyt b5 (-102 mV), respectively, consistent with the axial ligation described above. The ferrous variant, on the other hand, is a low-spin species coordinated by histidine-39 and methionine-63. Carbon monoxide (CO) readily displaces Met-63 from its coordination site on the ferrous heme, whereas CO cannot completely displace Met-63 from its coordination site on verdoheme. Consequently, the mechanism of inhibition for the oxidation of verdoheme to iron-biliverdin in the H63M variant appears to be similar to that observed for the heme-heme oxygenase complex in the presence of CO.

Structure-function relationships of human liver cytochromes P450 3A: aflatoxin B1 metabolism as a probe

Cytochromes P450 3A4 and 3A5, the dominant drug-metabolizing enzymes in the human liver, share >85% primary amino acid sequence identity yet exhibit different regioselectivity toward aflatoxin B1 (AFB1) biotransformation [Gillam et al., (1995) Arch. Biochem. Biophys. 317, 374-384]. P450 3A4 apparently prefers AFB1 3alpha-hydroxylation, which results in detoxification and subsequent elimination of the hepatotoxin, over AFB1 exo-8,9-oxidation. In contrast, P450 3A5 is incapable of appreciable AFB1 3alpha-hydroxylation and converts it predominantly to the exo-8,9-oxide which is genotoxic. To elucidate the structural features that govern the regioselectivity of the human liver 3A enzymes in AFB1 metabolism and bioactivation, a combination of approaches including sequence alignment, homology modeling, and site-directed mutagenesis was employed. Specifically, the switch in AFB1 regioselectivity was examined after individual substitution of the divergent amino acids in each of the six putative substrate recognition sites (SRSs) of P450 3A4 with the corresponding amino acid of P450 3A5. Of the P450 3A4 mutants examined, P107S, F108L, N206S, L210F, V376T, S478D, and L479T mutations resulted in a significant switch of P450 3A4 regioselectivity toward that of P450 3A5. The results confirmed the importance of some of these residues in substrate contact in the active site, with residue N206 (SRS-2) being critical for AFB1 detoxification via 3alpha-hydroxylation. Moreover, the P450 3A4 mutant N206S most closely mimicked P450 3A5, not only in its regioselectivity of AFB1 metabolism but also in its overall functional capacity. Furthermore, the other SRS-2 mutant, L210F, also resembled P450 3A5 in its overall AFB1 metabolism and regioselectivity. These findings reveal that a single P450 3A5 SRS domain (SRS-2) is capable of conferring the P450 3A5 phenotype on P450 3A4. In addition, some of these P450 3A4 mutations that affected AFB1 regioselectivity had little influence on testosterone 6beta-hydroxylation, thereby confirming that each substrate-P450 active site fit is indeed unique.

Role of carboxyl residues surrounding heme of human cytochrome b5 in the electrostatic interaction with NADH-cytochrome b5 reductase

To identify the cytochrome b5 residues responsible for the electrostatic interaction with NADH-cytochrome b5 reductase (b5R), we prepared and characterized the cytochrome b5 mutants in which Glu41, Glu42, Glu63, Asp70, and Glu73 were replaced by Ala, utilizing site-directed mutagenesis and the expression system for cytochrome b5 in Escherichia coli. Apparent Km values of the wild type b5R for Glu42Ala cytochrome b5 and Asp70Ala cytochrome b5 were approximately three-fold and six-fold higher than that for the wild type cytochrome b5, respectively, while the kcat values for those mutants were not remarkably affected. In contrast, Glu41Ala, Glu63Ala, and Glu73Ala cytochrome b5 showed almost the same kinetic properties as the wild type cytochrome b5. Furthermore, kinetic studies on combinations of the cytochrome b5 and b5R mutants suggested the interaction between Glu42 and Asp70 of cytochrome b5 and Lys125 and Lys41 of b5R, respectively, in the reaction.

Electrostatic interaction between NADH-cytochrome b5 reductase and cytochrome b5 studied by site-directed mutagenesis

Electrostatic interaction between NADH-cytochrome b5 reductase and cytochrome b5 was studied by site-directed mutagenesis. The target residues for mutagenesis were selected on the basis of the previously reported chemical cross-linking study of these two proteins, which implicated possible charge-pair interactions between Lys-41, Lys-125, Lys-162, and Lys-163 of the enzyme, and Glu-47, Glu-48, Glu-52, Glu-60, Asp-64 (group A), and heme propionate of cytochrome b5. Mutant reductases that lost one of the above-listed Lys residues showed higher K(m) values for cytochrome b5 and lower kcat values than those of the wild type, suggesting that all of the examined Lys residues participate in binding with cytochrome b5 as reported previously. In contrast, a removal of one of (or even all of) the group A residues from cytochrome b5 by mutagenesis caused no significant effect on the catalytic properties of cytochrome b5. Additional elimination of another set of negative residues (Glu-41, Glu-42, Asp-57, and Glu-63 (Group B)), which are also located close to heme, elevated the K(m) value by more than five folds. These results suggest that there should be other acidic residue(s) than group A in cytochrome b5 which participate in binding with NADH-cytochrome b5 reductase.

Phospholamban domain I/cytochrome b5 transmembrane sequence chimeras do not inhibit SERCA2a

A series of chimeras between the transmembrane domains of phospholamban (PLN) and cytochrome b5 were coexpressed with the Ca2+-ATPase of cardiac sarcoplasmic reticulum (SERCA2a). The chimeric molecules were not inhibitory, in line with our view that inhibitory PLN/SERCA2a interactions occur in transmembrane sequences, while cytoplasmic interactions regulate the inhibitory interactions in a four-base circuit.

Delta-9 fatty acid desaturase gene containing a carboxyl-terminal cytochrome b5 domain from the red alga Cyanidioschyzon merolae

A delta-9 fatty acid desaturase gene, homologous to animal and fungal acyl-coenzyme A (CoA) desaturases, was isolated from the red alga Cyanidioschyzon merolae using a degenerate PCR strategy. This gene, designated as CmFAD9, has no intron. The encoded delta-9 fatty acid desaturase (CmFad9p) consists of 476 amino acids and has an estimated molecular mass of 55.4 kDa. CmFad9p is a unique delta-9 fatty acid desaturase among plants, in that it is fused with the cytochrome b5 domain at its carboxyl terminus. This is characteristic of yeast acyl-CoA desaturase. Genomic Southern hybridization suggested that the C. merolae genome contains a single gene for delta-9 fatty acid desaturase of the animal and fungal type. Southern hybridization combined with pulsed-field gel electrophoresis revealed that CmFAD9 is probably located on chromosome XI of the 17 C. merolae chromosomes. A 1.6-kb product of this gene was transcribed throughout a light/dark synchronization culture. The discovery of CmFAD9 indicates the existence of a novel type of plant delta-9 fatty acid desaturase that may function in the endoplasmic reticulum, but not in the plastid.

The reduction potential of cytochrome b5 is modulated by its exposed heme edge

When the reduction potential of cytochrome b5 is measured with the aid of several different surface-modified electrodes that function on the basis of electrostatic interactions with the protein, the resultant values have been consistently more positive (40-100 mV) than the reduction potentials measured with potentiometric methods. In this paper, we report that the heme edge containing the exposed heme propionate, a heme methyl, and a heme vinyl, and which constitutes part of the surface of cytochrome b5, modulates its reduction potential. The positive shifts observed in the voltammetric measurements appear to originate from the formation of a complex between cytochrome b5 and the modified electrode surface which (a) neutralizes the charge on the heme propionate located on the exposed heme edge and (b) lowers the dielectric of the exposed heme microenvironment by excluding water from the complex interface, factors which result in the destabilization of the positive charge on the ferric heme with respect to the neutral ferrous heme. The observed positive shift, which is induced by complexation at the electrode surface, may indicate that similar shifts in the reduction potential of cytochrome b5 occur when it forms a complex with physiological partners, prior to electron transfer. The effect of the value of the dielectric constant on the reduction potential of cytochrome b5 was corroborated by preparing the V45L/V61L double mutant whose reduction potential was measured to be 50 mV more negative than the value measured for the wild type protein. The negative shift in the reduction potential of the mutant protein was explained by the increased accessibility of water to the heme binding site, as observed in its X-ray crystal structure.

Effects of tobacco smoke on the gene expression of the Cyp1a, Cyp2b, Cyp2e, and Cyp3a subfamilies in mouse liver and lung: relation to single strand breaks of DNA

Cigarette smoking is a worldwide health problem and is the greatest risk factor for lung cancer. By activating procarcinogens, hepatic and extrahepatic cytochromes P450 can participate in lung carcinogenesis. Tobacco smoke contains numerous cytochrome P450 inducers, substrates, and inhibitors. In the present study we investigated, in male NMRI mice, the effects of cigarette smoke on hepatic and pulmonary cytochrome P450 expression and their possible role in the induction of DNA lesions such as DNA single strand breaks (SSB). Hepatic and pulmonary mouse cytochrome P450 isozymes involved in carcinogenesis (Cyp1a, 2b, 2e, 3a) were differently induced by cigarette smoke. Cyp2e1 mRNA was dramatically enhanced (12.7-fold increase) while Cyp2b10 mRNA remained unchanged and Cyp1a1 was decreased or not detected. Cyp3a protein and mRNA were not detected in lung, suggesting that this isozyme is not expressed in mouse pulmonary tissue. The SSB of DNA increased in lung and liver treated mice. In contrast no modification was observed in lymphocytes that barely expressed cytochromes P450. Cimetidine and propylene glycol reduced SSB of DNA induced by smoking in liver and lung cells. The inhibition (-70%) observed in lung following treatment by propylene glycol, a CYP2E1 inhibitor, suggested that this isozyme is at least in part involved in pulmonary DNA damage induced by tobacco smoke. The high concentration of CYP2E1 function and regulation in mammals suggests that this protein could be involved in pulmonary carcinogenesis in human smokers.