Adrenodoxin supports reactions catalyzed by microsomal steroidogenic cytochrome P450s

The interaction of adrenodoxin (Adx) and NADPH cytochrome P450 reductase (CPR) with human microsomal steroidogenic cytochrome P450s was studied. It is found that Adx, mitochondrial electron transfer protein, is able to support reactions catalyzed by human microsomal P450s: full length CYP17, truncated CYP17, and truncated CYP21. CPR, but not Adx, supports activity of truncated CYP19. Truncated and the full length CYP17s show distinct preference for electron donor proteins. Truncated CYP17 has higher activity with Adx compared to CPR. The alteration in preference to electron donor does not change product profile for truncated enzymes. The electrostatic contacts play a major role in the interaction of truncated CYP17 with either CPR or Adx. Similarly electrostatic contacts are predominant in the interaction of full length CYP17 with Adx. We speculate that Adx might serve as an alternative electron donor for CYP17 at the conditions of CPR deficiency in human.     

Coexpression of redox partners increases the hydrocortisone (cortisol) production efficiency in CYP11B1 expressing fission yeast Schizosaccharomyces pombe

Cytochromes P450 play a vital role in the steroid biosynthesis pathway of the adrenal gland. An example of an essential P450 cytochrome is the steroid 11beta-hydroxylase CYP11B1, which catalyses the conversion of 11-deoxycorticol to hydrocortisone. However, despite its high biotechnological potential, this enzyme has so far been unsuccessfully employed in present-day biotechnology due to a poor expression yield and inherent protein instability. In this study, CYP11B1 was biotransformed into various strains of the yeast Schizosaccharomyces pombe, all of which also expressed the electron transfer proteins adrenodoxin and/or adrenodoxin reductase - central components of the mitochondrial P450 system - in order to maximise hydrocortisone production efficiency in our proposed model system. Site-directed mutagenesis of CYP11B1 at positions 52 and 78 was performed in order to evaluate the impact of altering the amino acids at these sites. It was found that the presence of an isoleucine at position 78 conferred the highest 11beta-hydroxylation activity of CYP11B1. Coexpression of adrenodoxin and adrenodoxin reductase appeared to further increase the 11beta-hydroxylase activity of the enzyme (3.4 fold). Adrenodoxin mutants which were found to significantly enhance enzyme efficiency in other cytochromes in previous studies were also tested in our system. It was found that, in this case, the wild type adrenodoxin was more efficient. The new fission yeast strain TH75 coexpressing the wild type Adx and AdR displays high hydrocortisone production efficiency at an average of 1mM hydrocortisone over a period of 72h, the highest value published to date for this biotransformation. Finally, our research shows that pTH2 is an ideal plasmid for the coexpression of the mitochondrial electron transfer counterparts, adrenodoxin and adrenodoxin reductase, in Schizosaccharomyces pombe, and so could serve as a convenient tool for future biotechnological applications.     

Functional expression of human mitochondrial CYP11B2 in fission yeast and identification of a new internal electron transfer protein, etp1

Mitochondrial cytochrome P450 enzymes play a crucial role in the steroid biosynthesis in human adrenals, catalyzing regio- and stereospecific hydroxylations. In search of a new model system for the study of these enzymes, we expressed the human CYP11B2 (aldosterone synthase, P450(aldo)) in fission yeast Schizosaccharomyces pombe. Analysis of the subcellular localization of the P450 enzyme by Western blot analysis, fluorescence microscopy, and electron microscopy demonstrated that the mitochondrial localization signal of the human protein is functional in S. pombe. The transformed yeasts show the inducible ability to convert in vivo considerable amounts of 11-deoxycortisol to cortisol and 11-deoxycorticosterone to corticosterone, 18-hydroxycorticosterone, and aldosterone, respectively. Although in mammalian cells, mitochondrial steroid hydroxylases depend for their activity on an electron transport chain that consists of two proteins, adrenodoxin and adrenodoxin reductase, no coexpression of these proteins is needed for efficient substrate conversion by intact fission yeast cells. Searching the fission yeast genome for adrenodoxin homologues, a gene was identified that codes for a protein with an amino terminal domain homologous to COX15 of Saccharomyces cerevisiae and a carboxy terminal ferredoxin domain. It was found that overexpression of this gene significantly enhances steroid hydroxylase activity of CYP11B2 expressing fission yeast cells. Moreover, the bacterially expressed ferredoxin domain of this protein can replace adrenodoxin in a reconstituted steroid hydroxylation assay and transfer electrons from adrenodoxin reductase to a mammalian or a bacterial cytochrome P450. Therefore, we suggest to name this protein etp1 (electron-transfer protein 1).     

Adrenodoxin--a versatile ferredoxin

Mammalian adrenodoxin (Adx) has been known for many years as an essential electron mediator in mitochondrial cytochrome P450 systems. Because of its ability to support several cytochrome P450 enzymes, it is involved not only in adrenal steroid hormone biosynthesis but also in vitamin D and bile acid metabolism. Recently, Adx is increasingly gaining attention because of its potential for pharmaceutical industry and biotechnology. With human cytochromes P450 becoming important drug targets, suitable Adx-based screening systems have to be developed to test putative new drugs. Moreover, in artificial systems, Adx has been shown to functionally interact with diverse bacterial cytochromes P450 catalyzing a variety of chemically interesting reactions. Putative biotechnological applications of such Adx-containing reconstituted systems are discussed.     

ROS production by adrenodoxin does not cause apoptosis in fission yeast

We previously showed that production of reactive oxygen species (ROS) caused by overexpression of the mitochondrial electron transfer protein adrenodoxin (Adx) induces apoptosis in mammalian cells. In the fission yeast Schizosaccharomyces pombe, ROS are also produced in cells that undergo an apoptotic-like cell death, but it is not yet clear whether they are actually causative for this phenomenon or whether they are merely produced as a by-product. Therefore, the purpose of this study was to trigger mitochondrial ROS production in fission yeast by overexpression of either wildtype Adx (Adx-WT) or of several activated Adx mutants and to investigate its consequences. It was found that strong expression of either Adx-WT or Adx-S112W did not produce any ROS, while Adx-D113Y caused a twofold and Adx^1–108 a threefold increase in ROS formation as compared to basal levels. However, no typical apoptotic markers or decreased viability could be observed in these strains. Since we previously observed that an increase in mitochondrial ROS formation of about 60% above basal levels is sufficient to strongly induce apoptosis in mammalian cells, we conclude that S. pombe is either very robust to mitochondrial ROS production or does not undergo apoptotic cell death in response to mitochondrial ROS at all.     

The adrenodoxin-like ferredoxin of Schizosaccharomyces pombe mitochondria

The single mitochondrial type I [2Fe-2S] ferredoxin of the fission yeast Schizosaccharomyces pombe is produced as the carboxy terminal part of the electron-transfer-protein 1 (etp1) and cleaved off during mitochondrial import [Biochemistry 41 (2002) 2311-2321]. The UV/Vis (UV-visible) spectrum of the purified recombinant ferredoxin domain (etp1(fd)) expressed in Escherichia coli is similar to those of bovine Adx in the oxidized as well as in the reduced state. EPR (electronic paramagnetic resonance) studies revealed a correctly incorporated iron-sulfur cluster of the axial type. The redox potential of this protein was determined to be -353 mV, which is considerably lower than that of adrenodoxin (Adx, -273 mV). Several lines of evidence indicate that the protein forms dimers under physiological and denaturating conditions. Interestingly, the fission yeast ferredoxin could be shown to be active as an electron carrier in heterologous redox systems. It is able to transfer electrons to horse heart cytochrome c and to bovine cytochromes P450(scc) (CYP11A1) and P450(11 beta) (CYP11B1), thereby receiving electrons from bovine NADPH-dependent Adx reductase. The kinetics of substrate conversion in the etp1(fd)-supported CYP11A1 and CYP11B1-dependent systems mediated was studied.     

Phosphorylation of bovine adrenodoxin by protein kinase CK2 affects the interaction with its redox partner cytochrome P450scc (CYP11A1)

Adrenodoxin (Adx), a [2Fe-2S] vertebrate-type ferredoxin, transfers electrons from the NADPH-dependent flavoprotein Adx reductase (AdR) to mitochondrial cytochrome P450 enzymes of the CYP11A and CYP11B families, which catalyze key reactions in steroid hormone biosynthesis. Adx is a known phosphoprotein, but the kinases that phosphorylate Adx have remained mostly obscure. The aim of this study was to identify previously unknown Adx phosphorylating kinases and to acquire a deeper insight into the functional consequences of such a modification. Here, we show for the first time that bovine Adx is a substrate of protein kinase CK2, whereas bovine CYP11A1, CYP11B1, and AdR are not phosphorylated by this kinase. CK2 phosphorylation of mature Adx requires the presence of both the catalytic alpha-subunit and the regulatory beta-subunit of CK2 and takes place exclusively at residue Thr-71, which is located within the redox partner interaction domain of the protein. We created two Adx mutants, Adx-T71E (imitating a phosphorylation) and Adx-T71V (which cannot be phosphorylated at this site), respectively, and investigated how these mutations affected the interaction of Adx with its redox partners. These data were supplemented with detailed spectroscopic and functional assays using the phosphorylated protein. All Adx species behaved like wild type (Adx-WT) with respect to their redox potential, iron-sulfur cluster symmetry, and overall backbone structure. Substrate conversion assays catalyzed by CYP11A1 showed an increase in product formation when Adx-T71E or CK2-phosphorylated Adx were used as electron carrier instead of Adx-WT, whereas the activity toward CYP11B1 was not altered using these Adx species. Additionally, Adx-T71E represents the only full-length Adx mutant which leads to an increase in CYP11A1 product formation. Therefore, characterizing this full-length mutant helps to improve our knowledge on the functional effects of phosphorylations on complex redox systems.     

Probing the interaction of bovine cytochrome P450scc (CYP11A1) with adrenodoxin: evaluating site-directed mutations by molecular modeling

The present study was undertaken to evaluate the role of positively charged amino acid residues proposed to reside on the proximal surface of bovine cytochrome P450 cholesterol side chain cleavage (P450scc, CYP11A1) and to determine which residues may be involved in protein-protein interactions with the electron carrier adrenodoxin (Adx). In previous studies, nine different lysine residues were identified by chemical and immunological cross-linking experiments as potentially interacting with Adx, while in the present study, two arginine residues have been identified from sequence alignments. From these 11 residues, 13 different P450scc mutants were made of which only seven were able to be expressed and characterized. Each of the seven mutants were evaluated for their ability to bind Adx, to be reduced, and for their enzymatic activity. Among these, K403Q and K405Q showed a consistent decrease in Adx binding, the ability to be reduced by Adx, and enzymatic activity, with K405Q being affected to a much greater extent. More dramatic was the complete loss of Adx binding by R426Q, while still retaining its ability to be chemically reduced and bind carbon monoxide. Independently, a homology model of P450scc was constructed and docked with the structure of Adx. Four potential sites of interaction were identified: P450scc:K403 with Adx:D76, P450scc:K405 with Adx:D72; P450scc:R426 with Adx:E73, and P450scc:K267 with Adx:E47. Thus, the biochemical and molecular modeling studies together support the hypothesis that K267, K403, K405, and R426 participate in the electrostatic interaction of P450scc with Adx.     

Reconstruction and study of a multi-enzyme system by 11 beta-hydroxylase steroids

The reconstitution of the steroid 11 beta-hydroxylase system based on the homogeneous proteins isolated from bovine adrenocortical mitochondria, cytochrome P-450 (P-450 (11 beta), 19-20.5 nmol of heme P-450 per 1 mg of protein), adrenodoxin (Adx) and adrenodoxin reductase (AR) was carried out. The reconstitution of the multienzyme system requires the presence of a non-ionic detergent due to the high hydrophobicity of P-450 (11 beta). Low concentrations of Tween 20 (below 0.015% or 115 microM) stimulate the reaction of steroid 11 beta-hydroxylation by improving the hemoprotein solubility. With a further increase in the detergent concentration, the reaction is inhibited due to the inactivation of the cytochrome and its impaired interaction with Adx. The electron transfer activity of adrenodoxin reductase and the dienzyme AR-Adx complex does not change within the Tween 20 concentration range of 0-0.4%. In solutions with the optimal concentration of Tween 20 (0.010-0.015%), the concentrations of AR and Adx providing for the half-maximum hydroxylation activity are 9 nM for AR and 280 nM for Adx. It was shown that in a reconstituted 11 beta-hydroxylase system, 75% of the reducing equivalents are involved in the formation of oxygen radicals, whereas 25%--in hydroxylation. 74% of the radical species are, in their turn, formed in the active site of the hemoprotein, while 26%--in the Fe2S2 center of adrenodoxin. The radical formation process predominates over the 11 beta-hydroxylation within a wide range of Adx/cytochrome ratios, i.e., 1.0-100. The hydroxylation substrate induces a 4-fold increase in the electron transfer rate by stimulating the enzymatic reduction of P-450 (11 beta), but only 35% of the additional reduced equivalents are consumed by the 11 beta-hydroxylation and 65%--by the oxygen radical formation.     

Kinetic and optical biosensor study of adrenodoxin mutant AdxS112W displaying an enhanced interaction towards the cholesterol side chain cleavage enzyme (CYP11A1)

In mammals, steroid hormones are synthesized from cholesterol that is metabolized by the mitochondrial CYP11A1 system leading to pregnenolone. The reduction equivalents for this reaction are provided by NADPH, via a small electron transfer chain, consisting of adrenodoxin reductase (AdR) and adrenodoxin (Adx). The reaction partners are involved in a series of transient interactions to realize the electron transfer from NADPH to CYP11A1. Here, we compared the ionic strength effect on the AdR/Adx and Adx/CYP11A1 interactions for wild-type Adx and mutant AdxS112W. Using surface plasmon resonance measurements, stopped flow kinetic investigations and analyses of the product formation, we were able to obtain new insights into the mechanism of these interactions. The replacement of serine 112 by tryptophan was demonstrated to lead to a dramatically decreased k _off rate of the Adx/CYP11A1 complex, resulting in a four-fold decreased K _d value and indicating a much higher stability of the complex involving the mutant. Stopped flow analysis at various ionic strengths and in different mixing modes revealed that the binding of reduced Adx to CYP11A1 seems to display the limiting step for electron transfer to CYP11A1 with pre-reduced AdxS112W being much more efficient than wild-type Adx. Finally, the dramatic increase in pregnenolone formation at higher ionic strength using the mutant demonstrates that the interaction of CYP11A1 with Adx is the rate-limiting step in substrate conversion and that hydrophobic interactions may considerably improve this interaction and the efficiency of product formation. The data are discussed using published structural data of the complexes.     

Adrenodoxin reductase-adrenodoxin complex structure suggests electron transfer path in steroid biosynthesis

The steroid hydroxylating system of adrenal cortex mitochondria consists of the membrane-attached NADPH-dependent adrenodoxin reductase (AR), the soluble one-electron transport protein adrenodoxin (Adx), and a membrane-integrated cytochrome P450 of the CYP11 family. In the 2.3-A resolution crystal structure of the Adx.AR complex, 580 A(2) of partly polar surface are buried. Main interaction sites are centered around Asp(79), Asp(76), Asp(72), and Asp(39) of Adx and around Arg(211), Arg(240), Arg(244), and Lys(27) of AR, respectively. In particular, the region around Asp(39) defines a new protein interaction site for Adx, similar to those found in plant and bacterial ferredoxins. Additional contacts involve the electron transfer region between the redox centers of AR and Adx and C-terminal residues of Adx. The Adx residues Asp(113) to Arg(115) adopt 3(10)-helical conformation and engage in loose intermolecular contacts within a deep cleft of AR. Complex formation is accompanied by a slight domain rearrangement in AR. The [2Fe-2S] cluster of Adx and the isoalloxazine rings of FAD of AR are 10 A apart suggesting a possible electron transfer route between these redox centers. The AR.Adx complex represents the first structure of a biologically relevant complex between a ferredoxin and its reductase.     

Complex formation in vesicle-reconstituted mitochondrial cytochrome P450 systems (CYP11A1 and CYP11B1) as evidenced by rotational diffusion experiments using EPR and ST-EPR.

Rotational diffusion measurements using EPR and saturation transfer EPR were applied to analyze complex formation between the electron-transfer components of the mitochondrial steroid-hydroxylating cytochrome P450 systems (CYP11A1 and CYP11B1) in phosphatidylcholine/phosphatidylethanolamine/cardiolipin vesicles prepared by octyl glucoside dialysis/adsorption. Octyl glucoside reconstitution of P450SCC results in large vesicles, which have an advantage over small vesicles in that vesicle tumbling does not contribute to measured rotational diffusion rates. Immobilization of spin-labeled adrenodoxin by both P450SCC and adrenodoxin reductase indicates equimolar complexation between P450SCC and adrenodoxin as well as between adrenodoxin reductase and adrenodoxin. Combination of rotational diffusion and antibody cross-linking confirmed the complexation of adrenodoxin with P450SCC and for the first time provided direct evidence of a complex between P450SCC and P45011beta in the membrane. In contrast, no evidence was found for the existence of adrenodoxin reductase-P450SCC complexes or a ternary complex of all three proteins. Thus, these experiments confirm the shuttle mechanism of electron transfer to vesicle-reconstituted P450SCC and P45011beta.     

Deletions in the loop surrounding the iron-sulfur cluster of adrenodoxin severely affect the interactions with its native redox partners adrenodoxin reductase and cytochrome P450(scc) (CYP11A1)

The redox active iron-sulfur center of bovine adrenodoxin is coordinated by four cysteine residues in positions 46, 52, 55 and 92 and is covered by a loop containing the residues Glu-47, Gly-48, Thr-49, Leu-50 and Ala-51. In plant-type [2Fe-2S] ferredoxins, the corresponding loop consists of only four amino acids. The loop is positioned at the surface of the proteins and forms a boundary separating the [2Fe-2S] cluster from solvent. In order to analyze the biological function of the five amino acids of the loop in adrenodoxin (Adx) for this electron transfer protein each residue was deleted by site-directed mutagenesis. The resulting five recombinant Adx variants show dramatic differences among each other regarding their spectroscopic characteristics and functional properties. The redox potential is affected differently depending on the position of the conducted deletion. In contrast, all mutations in the protein loop influence the binding to the redox partners adrenodoxin reductase (AdR) and cytochrome P450(scc) (CYP11A1) indicating the importance of this loop for the physiological function of this iron--sulfur protein.     

Physiological role of the N-terminal processed P4501A1 targeted to mitochondria in erythromycin metabolism and reversal of erythromycin-mediated inhibition of mitochondrial protein synthesis

Recently, we showed that the major species of beta-naphthoflavone-inducible rat liver mitochondrial P450MT2 consists of N-terminal truncated microsomal P4501A1 (+33/1A1) and that the truncated enzyme exhibits different substrate specificity as compared with intact P4501A1. The results of the present study show that P450MT2 targeted to COS cell mitochondria by transient transfection of P4501A1 cDNA is localized inside the mitochondrial inner membrane in a membrane-extrinsic orientation. Co-expression with wild type P4501A1 and adrenodoxin (Adx) cDNAs resulted in 5-7-fold higher erythromycin N-demethylation (ERND) in the mitochondrial fraction but minimal changes in the microsomal fraction of transfected cells. Erythromycin, a potent inhibitor of bacterial and mitochondrial protein synthesis, caused 8-12-fold higher accumulation of CYP1A1 mRNA, preferential accumulation of P450MT2, and 5-6-fold higher ERND activity in the mitochondrial compartment of rat C6 glioma cells. Consistent with the increased mitochondrial ERND activity, co-expression with P4501A1 and Adx in COS cells rendered complete protection against erythromycin-mediated mitochondrial translation inhibition. Mutations that specifically affect the mitochondrial targeting of P4501A1 also abolished protection against mitochondrial translation inhibition. These results for the first time suggest a physiological function for the xenobiotic inducible cytochrome P4501A1 against drug-mediated mitochondrial toxicity.     

Placental cytochrome P450scc (CYP11A1): comparison of catalytic properties between conditions of limiting and saturating adrenodoxin reductase

The mitochondrial side-chain cleavage of cholesterol, catalysed by cytochrome P450scc, is rate-limiting in the synthesis of progesterone by the human placenta. Cytochrome P450scc activity is in turn limited by the concentration of adrenodoxin reductase (AR) in placental mitochondria. In order to better understand which components of the cholesterol side-chain cleavage system are important in the regulation of placental progesterone synthesis, we have examined their effects on P450scc activity with both saturating and limiting concentrations of AR. The present study reveals that decreasing the AR concentration causes a decrease in the K(m) of cytochrome P450scc for cholesterol, facilitating saturation of the enzyme with its substrate. Decreasing AR resulted in P450scc activity becoming less sensitive to changes in P450scc concentration. The adrenodoxin (Adx) concentration in mitochondria from term placentae is near-saturating for P450scc and under these conditions, we found that decreasing AR reduces the K(m) of P450scc for adrenodoxin. Increasing either the cholesterol or P450scc concentration increased the amount of AR required for P450scc to work at half its maximum velocity. A relatively small increase in AR can support considerably higher rates of side-chain cleavage activity when there is a coordinate increase in AR and P450scc concentrations. We conclude from this study that cholesterol is near-saturating for cytochrome P450scc activity in placental mitochondria due to the P450scc displaying a low K(m) for cholesterol resulting from the low and rate-limiting concentration of AR present. This study reveals that it is unlikely that cholesterol or adrenodoxin concentrations are important regulators of placental progesterone synthesis but AR or coordinate changes in AR and P450scc concentrations are likely to be important in its regulation.     

Self-association of adrenodoxin studied by using analytical ultracentrifugation

The mitochondrial steroid hydroxylase system of vertebrates utilizes adrenodoxin (Adx), a small iron–sulfur cluster protein of about 14 kDa as an electron carrier between a reductase and cytochrome P450. Although the crystal structure of this protein has been elucidated, the solution structure of Adx was discussed contrary in the literature [I.A. Pikuleva, K. Tesh, M.R. Waterman, Y. Kim, The tertiary structure of full-length bovine adrenodoxin suggests functional dimers, Arch. Biochem. Biophys. 373 (2000) 44–55; D. Beilke, R. Weiss, F. Löhr, P. Pristovsek, F. Hannemann, R. Bernhardt, H. Rüterjans, A new electron mechanism in mitochondrial steroid hydroxylase systems based on structural changes upon the reduction of adrenodoxin, Biochemistry 41 (2002) 7969–7978]. Therefore, it was necessary to study the self-association of this protein by using analytical ultracentrifugation over a larger concentration range. As could be demonstrated in sedimentation velocity experiments, as well as sedimentation equilibrium runs with explicit consideration of thermodynamic non-ideality, the full-length protein (residues 1–128) in the oxidized state resulted in a monomer–dimer equilibrium (K_a ~ 3 × 10² M^− 1). For truncated Adx (1–108), as well as the reduced Adx, the association behavior was strongly reduced. The consequences of this behavior are discussed with respect to the physiological meaning for the Adx system.     

Induction and mitochondrial localization of cytochrome P450scc system enzymes in normal and transformed ovarian granulosa cells

After ovulation of an oocyte, granulosa cells of the ovarian follicle differentiate into luteal cells and become a major factor dedicated to the synthesis of the steroid hormone progesterone. We recently established granulosa cell lines by cotransfection of granulosa cells with SV-40 and Ha-ras oncogene. In these cells progesterone secretion can be induced by cAMP as in normal rat granulosa cells. The induction of progesterone secretion is observed only after approximately 24 h and closely follows the delayed but quantitatively dramatic induction of the mitochondrial cytochrome P450scc which catalyzes the first step in steroid hormone biosynthesis. The mitochondrial P450 system electron transport proteins, adrenodoxin and adrenodoxin reductase, are also induced but adrenodoxin shows a faster induction. Immunofluorescence studies show that the three enzymes are induced in all cells and incorporated into all mitochondria uniformly. Electron microscopic examination using immunogold technique further confirms this and reveals that adrenodoxin is predominantly located on the matrix side of the inner mitochondrial membrane. Thus, adrenodoxin, which is a small highly charged protein, shows a distribution similar to P450scc which is an integral membrane protein. The uniformity of the response of the cells provides further evidence for the homogeneity of the cell line and makes this new granulosa cell line a highly promising system for the study of the molecular mechanisms involved in changes in gene expression during the process of granulosa cell differentiation.     

Interaction of CYP11B1 (cytochrome P-45011 beta) with CYP11A1 (cytochrome P-450scc) in COS-1 cells.

The interactions of CYP11B1 (cytochrome P-45011beta), CYP11B2 (cytochrome P-450aldo) and CYP11A1 (cytochrome P-450scc) were investigated by cotransfection of their cDNA into COS-1 cells. The effect of CYP11A1 on CYP11B isozymes was examined by studying the conversion of 11-deoxycorticosterone to corticosterone, 18-hydroxycorticosterone and aldosterone. It was shown that when human or bovine CYP11B1 and CYP11A1 were cotransfected they competed for the reducing equivalents from the limiting source contained in COS-1 cells; this resulted in a decrease of the CYP11B activities without changes in the product formation patterns. The competition of human CYP11A1 with human CYP11B1 and CYP11B2 could be diminished with excess expression of bovine adrenodoxin. However, the coexpression of bovine CYP11B1 and CYP11A1 in the presence of adrenodoxin resulted in a stimulation of 11beta-hydroxylation activity of CYP11B1 and in a decrease of the 18-hydroxycorticosterone and aldosterone formation. These results suggest that the interactions of CYP11A1 with CYP11B1 and CYP11B2 do not have an identical regulatory function in human and in bovine adrenal tissue.     

Cellular surface display of dimeric Adx and whole cell P450-mediated steroid synthesis on E. coli

Bovine adrenodoxin (Adx) was expressed on the surface of Escherichia coli as a monomeric fusion protein with the translocation unit of the AIDA-I autotransporter. The fusion protein remained anchored in the outer membrane by the beta-barrel of the autotransporter. Dimeric Adx molecules were formed spontaneously on the bacterial surface with high efficiencies. Adx dimers could be activated to biological function by chemical incorporation of the [2Fe-2S] cluster. By adding purified adrenodoxin reductase and P450 CYP11A1, a whole cell biocatalyst system was obtained, which effectively synthesized pregnenolone from cholesterol. Addition of artificial membrane constituents or detergents, which was indispensable before to get functional steroidal P450 enzymes, was not necessary. The whole cell activity (0.21 nmol x h(-1) x nmol(-1) CYP11A1) was in the same range as obtained earlier for reconstitution assays. The whole cell system developed here is an easy to handle, stable tool for the expression of membrane-associated P450 enzymes without the need of microsome preparation or reconstitution of artificial membrane vesicles. Moreover, it is the first report on functional dimer formation of a protein anchored on the surface of E. coli after being transported as a monomer. This seems to be a special feature of the autotransporter translocation unit, containing a beta-barrel, motile in the outer membrane and opens a new dimension for the surface display of multimeric proteins.