The tertiary structure of full-length bovine adrenodoxin suggests functional dimers

The three-dimensional X-ray crystal structure of full-length oxidized bovine adrenodoxin (Adx) has been determined at 2.5 A resolution by molecular replacement using a structure of a truncated form as a starting model. Crystals of Adx belong to a primitive monoclinic space group P2(1) with four Adx molecules in an asymmetric unit. The unit cell dimensions are a = 59.44 A, b = 77.03 A, c = 59.68 A, and beta = 94.83 degrees. The structure has been refined to an R factor of 23.5%. Structures of the four molecules of full-length Adx (127 amino acids) in the asymmetric unit were compared with each other and also with that of the truncated Adx (4-108). The overall topology of full-length Adx remains the same as described earlier for the truncated protein. Differences that do occur are almost wholly confined to alternate side-chain conformations that reflect differing lattice contacts made by two proteins. Extensive interactions found between molecules 1 and 2 in the full-length Adx asymmetric unit may reflect the ability of Adx to form dimers in vivo and are consistent with hydrodynamic measurements which show that in solution there is an equilibrium between monomeric and dimeric forms of Adx. Dimerization of Adx could explain why the truncated form has greater affinity for the P450 redox partner than the full-length form. From these results it can be considered that the mechanism of electron transfer is not necessarily the same in different mitochondrial P450 systems.     

Light-induced reduction of bovine adrenodoxin via the covalently bound ruthenium(II) bipyridyl complex: intramolecular electron transfer and crystal structure

Bovine adrenodoxin (Adx) plays an important role in the electron-transfer process in the mitochondrial steroid hydroxylase system of the bovine adrenal cortex. Using electron paramagnetic resonance (EPR) spectroscopy, we showed that photoreduction of the [2Fe-2S] cluster of Adx via (4'-methyl-2,2'-bipyridine)bis(2,2'-bipyridine)ruthenium(II) [Ru(bpy)2(mbpy)] covalently attached to the protein surface can be used as a new approach to probe the "shuttle" hypothesis for the electron transfer by Adx. The 1.5 A resolution crystal structure of a 1:1 Ru(bpy)2(mbpy)-Adx(1-108) complex reveals the site of modification, Cys95, and allows to predict the possible intramolecular electron-transfer pathways within the complex. Photoreduction of uncoupled Adx, mutant Adx(1-108), and Ru(bpy)2(mbpy)-Adx(1-108) using safranin T as the mediating electron donor suggests that two electrons are transferred from the dye to Adx. The intramolecular photoreduction rate constant for the ruthenated Adx has been determined and is discussed according to the predicted pathways.     

Interaction of Catalytic Domains in Cytochrome P450scc–Adrenodoxin Reductase–Adrenodoxin Fusion Protein Imported into Yeast Mitochondria

We have constructed plasmids for yeast expression of the fusion protein pre-cytochrome P450scc–adrenodoxin reductase–adrenodoxin (F2) and a variant of F2 with the yeast CoxIV targeting presequence. Mitochondria isolated from transformed yeast cells contained the F2 fusion protein at about 0.5% of total protein and showed cholesterol hydroxylase activity with 22(R)-hydroxycholesterol. The activity increased 17- or 25-fold when sonicated mitochondria were supplemented with an excess of purified P450scc or a mixture of adrenodoxin (Adx) and adrenodoxin reductase (AdxRed), respectively. These data suggest that, at least in yeast mitochondria, the interactions of the catalytic domains of P450scc, Adx, and AdxRed in the common polypeptide chain are restricted.     

Adrenodoxin (Adx) and CYP11A1 (P450scc) induce apoptosis by the generation of reactive oxygen species in mitochondria

Mitochondrial cytochrome P450 systems are an indispensable component of mammalian steroid biosynthesis; they catalyze regio- and stereo-specific steroid hydroxylations and consist of three protein entities: adrenodoxin reductase (AdR), adrenodoxin (Adx), and a mitochondrial cytochrome P450 enzyme, e.g., CYP11A1 (P450 side chain cleavage, P450scc). It is known that the latter two are able to generate reactive oxygen species (ROS) in vitro . In this study, we investigated whether this ROS generation also occurs in vivo and, if so, whether it leads to the induction of apoptosis. We found that overexpression of either human or bovine Adx causes a significant loss of viability in 11 different cell lines. This loss of viability does not depend on the presence of the tumor suppressor protein p53. Transient overexpression of human Adx in HCT116 cells leads to ROS production, to a disruption of the mitochondrial transmembrane potential (DeltaPsi), to cytochrome c release from the mitochondria, and to caspase activation. In contrast, the effect of transient overexpression of human CYP11A1 on cell viability varies in different cell lines, with some being sensitive and others not. We conclude that mitochondrial cytochrome P450 systems are a source of mitochondrial ROS production and can play a role in the induction of mitochondrial apoptosis.     

Solution structure for an Encephalitozoon cuniculi adrenodoxin‐like protein in the oxidized state

Encephalitozoon cuniculi is a unicellular, obligate intracellular eukaryotic parasite in the Microsporidia family and one of the agents responsible for microsporidosis infections in humans. Like most Microsporidia, the genome of E. cuniculi is markedly reduced and the organism contains mitochondria‐like organelles called mitosomes instead of mitochondria. Here we report the solution NMR structure for a protein physically associated with mitosome‐like organelles in E. cuniculi, the 128‐residue, adrenodoxin‐like protein Ec‐Adx (UniProt ID Q8SV19) in the [2Fe‐2S] ferredoxin superfamily. Oxidized Ec‐Adx contains a mixed four‐strand β‐sheet, β2‐β1‐β4‐β3 (↓↑↑↓), loosely encircled by three α‐helices and two 3₁₀‐helices. This fold is similar to the structure observed in other adrenodoxin and adrenodoxin‐like proteins except for the absence of a fifth anti‐parallel β‐strand next to β3 and the position of α3. Cross peaks are missing or cannot be unambiguously assigned for 20 amide resonances in the ¹H‐¹⁵N HSQC spectrum of Ec‐Adx. These missing residues are clustered primarily in two regions, G48‐V61 and L94‐L98, containing the four cysteine residues predicted to ligate the paramagnetic [2Fe‐2S] cluster. Missing amide resonances in ¹H‐¹⁵N HSQC spectra are detrimental to NMR‐based solution structure calculations because ¹H‐¹H NOE restraints are absent (glass half‐empty) and this may account for the absent β‐strand (β5) and the position of α3 in oxidized Ec‐Adx. On the other hand, the missing amide resonances unambiguously identify the presence, and immediate environment, of the paramagnetic [2Fe‐2S] cluster in oxidized Ec‐Adx (glass half‐full).     

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 interaction domain of the redox protein adrenodoxin is mandatory for binding of the electron acceptor CYP11A1, but is not required for binding of the electron donor adrenodoxin reductase

Adrenodoxin (Adx) is a [2Fe-2S] ferredoxin involved in electron transfer reactions in the steroid hormone biosynthesis of mammals. In this study, we deleted the sequence coding for the complete interaction domain in the Adx cDNA. The expressed recombinant protein consists of the amino acids 1-60, followed by the residues 89-128, and represents only the core domain of Adx (Adx-cd) but still incorporates the [2Fe-2S] cluster. Adx-cd accepts electrons from its natural redox partner, adrenodoxin reductase (AdR), and forms an individual complex with this NADPH-dependent flavoprotein. In contrast, formation of a complex with the natural electron acceptor, CYP11A1, as well as electron transfer to this steroid hydroxylase is prevented. By an electrostatic and van der Waals energy minimization procedure, complexes between AdR and Adx-cd have been proposed which have binding areas different from the native complex. Electron transport remains possible, despite longer electron transfer pathways.     

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.     

Kinetic differences between two interconvertible forms of fructose-1,6-bisphosphatase from Saccharomyces cerevisiae

Newly synthesized, [³⁵S]methionine-labeled cholesterol side-chain cleavage cytochrome P-450, 11β-hydroxylase cytochrome P-450, adrenodoxin, and adrenodoxin reductase were immunoisolated from radiolabeled bovine adrenocortical cells and from rabbit reticulocyte lysate translation systems programmed with bovine adrenocortical RNA. Cholesterol side-chain cleavage cytochrome P-450 immunoisolated from a reticulocyte lysate translation system had an apparent molecular weight of 54,500 whereas this cytochrome P-450 immunoisolated from radiolabeled bovine adrenocortical cells had an apparent molecular weight of 49,000, an apparent molecular weight identical to that of the purified protein. Similarly, newly synthesized, [³⁵S]methionine-labeled 11β-hydroxylase cytochrome P-450 immunoisolated from a reticulocyte lysate translation system had an apparent molecular weight 5500 daltons larger than that immunoisolated from radiolabeled adrenocortical cells (48,000) and the authentic cytochrome (48,000). The cell-free translation products of adrenodoxin and adrenodoxin reductase were also several thousand daltons larger than the corresponding mitochondrial proteins. The apparent molecular weight of adrenodoxin immunoisolated from a reticulocyte lysate translation system was 19,000, while that of the authentic protein was 12,000. Adrenodoxin reductase immunoisolated from a lysate translation system had an apparent molecular weight of 53,400; an apparent molecular weight 2300 daltons larger than that of adrenodoxin reductase immunoisolated from radiolabeled adrenocortical cells or purified by conventional techniques. These results demonstrate that all of the components of the mitochondrial steroid hydroxylase systems of the bovine adrenal cortex are synthesized as precursor molecules of higher molecular weight. Presumably, the precursor proteins are post-translationally converted to the mature enzymes upon insertion into the mitochondrion by a process which includes the proteolytic cleavage of the precursor segments.     

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.     

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.     

Structural and thermodynamic characterization of the adrenodoxin-like domain of the electron-transfer protein Etp1 from Schizosaccharomyces pombe

The protein Etp1 of Schizosaccharomyces pombe consists of an amino-terminal COX15-like domain and a carboxy-terminal ferredoxin-like domain, Etp1^fd, which is cleaved off after mitochondrial import. The physiological function of Etp1^fd is supposed to lie in the participation in the assembly of iron-sulfur clusters and the synthesis of heme A. In addition, the protein was shown to be the first microbial ferredoxin being able to support electron transfer in mitochondrial steroid hydroxylating cytochrome P450 systems in vivo and in vitro, replacing thereby the native redox partner, adrenodoxin. Despite a sequence similarity of 39% and the fact that fission yeast is a mesophilic organism, thermodynamic studies revealed that Etp1^fd has a melting temperature more than 20 °C higher than adrenodoxin. The three-dimensional structure of Etp1^fd has been determined by crystallography. To the best of our knowledge it represents the first three-dimensional structure of a yeast ferredoxin. The structure-based sequence alignment of Etp1^fd with adrenodoxin yields a rational explanation for their observed mutual exchangeability in the cytochrome P450 system. Analysis of the electron exchange with the S. pombe redox partner Arh1 revealed differences between Etp1^fd and adrenodoxin, which might be linked to their different physiological functions in the mitochondria of mammals and yeast.     

Integration of purified adrenocortical cytochrome P-45011β into phospholipid vesicles

Cytochrome P-450 supporting steroid 11β hydroxylase activity (cyt P-450_11β) was purified from bovine adrenal cortex mitochondria using a procedure, which included an octyl-sepharose adsorption step and elution of the protein in the presence of phosphatidyl-choline. Purified cyt P-450_11β could then be included into phosphatidyl choline-phosphatidyl ethanolamine (1 : 1) spherical vesicles (20–50 nm in diameter) during their formation upon gel filtration, as demonstrated by the protein refractoriness to trypsin hydrolysis. After inclusion into the phospholipid vesicles, cyt P-450_11β remained stable and expressed full 11β hydroxylase activity in a reconstituted system including purified adrenodoxin and adrenodoxin reductase.     

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.     

The dipole moment of the electron carrier adrenodoxin is not critical for redox partner interaction and electron transfer

Dipole moments of proteins arise from helical dipoles, hydrogen bond networks and charged groups at the protein surface. High protein dipole moments were suggested to contribute to the electrostatic steering between redox partners in electron transport chains of respiration, photosynthesis and steroid biosynthesis, although so far experimental evidence for this hypothesis was missing. In order to probe this assumption, we changed the dipole moment of the electron transfer protein adrenodoxin and investigated the influence of this on protein-protein interactions and electron transfer. In bovine adrenodoxin, the [2Fe-2S] ferredoxin of the adrenal glands, a dipole moment of 803 Debye was calculated for a full-length adrenodoxin model based on the Adx(4-108) and the wild type adrenodoxin crystal structures. Large distances and asymmetric distribution of the charged residues in the molecule mainly determine the observed high value. In order to analyse the influence of the resulting inhomogeneous electric field on the biological function of this electron carrier the molecular dipole moment was systematically changed. Five recombinant adrenodoxin mutants with successively reduced dipole moment (from 600 to 200 Debye) were analysed for their redox properties, their binding affinities to the redox partner proteins and for their function during electron transfer-dependent steroid hydroxylation. None of the mutants, not even the quadruple mutant K6E/K22Q/K24Q/K98E with a dipole moment reduced by about 70% showed significant changes in the protein function as compared with the unmodified adrenodoxin demonstrating that neither the formation of the transient complex nor the biological activity of the electron transfer chain of the endocrine glands was affected. This is the first experimental evidence that the high dipole moment observed in electron transfer proteins is not involved in electrostatic steering among the proteins in the redox chain.     

Adrenodoxin: structure, stability, and electron transfer properties

Adrenodoxin is an iron-sulfur protein that belongs to the broad family of the [2Fe-2S]-type ferredoxins found in plants, animals and bacteria. Its primary function as a soluble electron carrier between the NADPH-dependent adrenodoxin reductase and several cytochromes P450 makes it an irreplaceable component of the steroid hormones biosynthesis in the adrenal mitochondria of vertebrates. This review intends to summarize current knowledge about structure, function, and biochemical behavior of this electron transferring protein. We discuss the recently solved first crystal structure of the vertebrate-type ferredoxin, the truncated adrenodoxin Adx(4-108), that offers the unique opportunity for better understanding of the structure-function relationships and stabilization of this protein, as well as of the molecular architecture of [2Fe-2S] ferredoxins in general. The aim of this review is also to discuss molecular requirements for the formation of the electron transfer complex. Essential comparison between bacterial putidaredoxin and mammalian adrenodoxin will be provided. These proteins have similar tertiary structure, but show remarkable specificity for interactions only with their own cognate cytochrome P450. The discussion will be largely centered on the protein-protein recognition and kinetics of adrenodoxin dependent reactions.     

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.     

Covalently crosslinked complexes of bovine adrenodoxin with adrenodoxin reductase and cytochrome P450scc. Mass spectrometry and Edman degradation of complexes of the steroidogenic hydroxylase system.

NADPH-dependent adrenodoxin reductase, adrenodoxin and several diverse cytochromes P450 constitute the mitochondrial steroid hydroxylase system of vertebrates. During the reaction cycle, adrenodoxin transfers electrons from the FAD of adrenodoxin reductase to the heme iron of the catalytically active cytochrome P450 (P450scc). A shuttle model for adrenodoxin or an organized cluster model of all three components has been discussed to explain electron transfer from adrenodoxin reductase to P450. Here, we characterize new covalent, zero-length crosslinks mediated by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide between bovine adrenodoxin and adrenodoxin reductase, and between adrenodoxin and P450scc, respectively, which allow to discriminate between the electron transfer models. Using Edman degradation, mass spectrometry and X-ray crystallography a crosslink between adrenodoxin reductase Lys27 and adrenodoxin Asp39 was detected, establishing a secondary polar interaction site between both molecules. No crosslink exists in the primary polar interaction site around the acidic residues Asp76 to Asp79 of adrenodoxin. However, in a covalent complex of adrenodoxin and P450scc, adrenodoxin Asp79 is involved in a crosslink to Lys403 of P450scc. No steroidogenic hydroxylase activity could be detected in an adrenodoxin -P450scc complex/adrenodoxin reductase test system. Because the acidic residues Asp76 and Asp79 belong to the binding site of adrenodoxin to adrenodoxin reductase, as well as to the P450scc, the covalent bond within the adrenodoxin-P450scc complex prevents electron transfer by a putative shuttle mechanism. Thus, chemical crosslinking provides evidence favoring the shuttle model over the cluster model for the steroid hydroxylase system.     

Modeling of electrostatic recognition processes in the mammalian mitochondrial steroid hydroxylase system

Adrenodoxin reductase (AR) and adrenodoxin (Adx) are components of the mammalian mitochondrial steroid-hydroxylating system. Crystal structures of Adx, AR and a cross-linked Adx–AR complex have recently been determined. Based on these, we have carried out a modeling and docking study to characterize the recognition between AR, Adx and cytochrome c (Cytc). To rationalize the recognition process, electrostatic potentials were calculated by solving the Poisson–Boltzmann equations. In the Adx–AR complex modeled, a negatively charged surface of Adx recognizes a positive surface of AR, as in the crystal structure of the Adx–AR complex, proving the correct parameterization for the energy calculations. After forming salt bridges between the polar primary binding sites of Adx and AR, charge compensation causes a domain movement in AR, which closes the binding cleft by 2–4 Å. Thereby, a secondary polar binding site is closed and the electron transfer pathways between the FAD of AR and the [2Fe–2S] cluster of Adx are adjusted. Next, the model structure of a complex between Adx and Cytc was derived. The lowest-energy complex between Adx and Cytc matches earlier chemical modification and cross-linking experiments, which proposed polar interactions of Lys13, Lys27, Lys72 and Lys79 of Cytc with acidic residues in Adx. Because of the short distance of 9.4 Å between the redox centers, a complex, productive in electron transfer via a different outlet pathway from the inlet route in Adx, is expected. However, a ternary complex cannot be formed between the Adx–AR complex and Cytc because of steric hindrance. Therefore, a shuttle model for the role of Adx in the electron transfer process to Cytc is preferable to a relay model. In addition, no preferable docking site could be detected for a second Adx when probing the Adx–AR complex, which is required for a quaternary organized-cluster model of all redox partners of the hydroxylase system.