Mutations of tyrosine 82 in bovine adrenodoxin that affect binding to cytochromes P45011A1 and P45011B1 but not electron transfer

Mechanistic basis of electron transfer to cytochromes p450 by natural redox partners and artificial donor constructs

Cytochromes P450 (P450s) are hemoproteins catalyzing oxidative biotransformation of a vast array of natural and xenobiotic compounds. Reducing equivalents required for dioxygen cleavage and substrate hydroxylation originate from different redox partners including diflavin reductases, flavodoxins, ferredoxins and phthalate dioxygenase reductase (PDR)-type proteins. Accordingly, circumstantial analysis of structural and physicochemical features governing donor-acceptor recognition and electron transfer poses an intriguing challenge. Thus, conformational flexibility reflected by togging between closed and open states of solvent exposed patches on the redox components was shown to be instrumental to steered electron transmission. Here, the membrane-interactive tails of the P450 enzymes and donor proteins were recognized to be crucial to proper orientation toward each other of surface sites on the redox modules steering functional coupling. Also, mobile electron shuttling may come into play. While charge-pairing mechanisms are of primary importance in attraction and complexation of the redox partners, hydrophobic and van der Waals cohesion forces play a minor role in docking events. Due to catalytic plasticity of P450 enzymes, there is considerable promise in biotechnological applications. Here, deeper insight into the mechanistic basis of the redox machinery will permit optimization of redox processes via directed evolution and DNA shuffling. Thus, creation of hybrid systems by fusion of the modified heme domain of P450s with proteinaceous electron carriers helps obviate the tedious reconstitution procedure and induces novel activities. Also, P450-based amperometric biosensors may open new vistas in pharmaceutical and clinical implementation and environmental monitoring.

Cytochromes P450 as promising catalysts for biotechnological application: chances and limitations

Cytochromes P450 (CYPs) belong to the superfamily of heme b containing monooxygenases with currently more than 21,000 members. These enzymes accept a vast range of organic molecules and catalyze diverse reactions. These extraordinary capabilities of CYP systems that are unmet by other enzymes make them attractive for biotechnology. However, the complexity of these systems due to the need of electron transfer from nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) via redox partner proteins for the initial hydroxylation step limits a broader technical implementation of CYP enzymes. There have been several reviews during the past years tackling the potential CYPs for synthetic application. The aim of this review is to give a critical overview about possibilities and chances for application of these interesting catalysts as well as to discuss drawbacks and problems related to their use. Solutions to overcome these limitations will be demonstrated, and several selected examples of successful CYP applications under industrial conditions will be reviewed.

Adrenodoxin: the archetype of vertebrate-type [2Fe-2S] cluster ferredoxins

Adrenodoxin is probably the best characterized member of the vertebrate-type [2Fe-2S]-cluster ferredoxins. It has been in the spotlight of scientific interest for many years due to its essential role in mammalian steroid hormone biosynthesis, where it acts as electron mediator between the NADPH-dependent adrenodoxin reductase and several mitochondrial cytochromes P450. In this review we will focus on the present knowledge about protein-protein recognition in the mitochondrial cytochrome P450 system and the modulation of the electron transfer between Adx and its redox partners, AdR and CYP(s). We also intend to point out the potential biotechnological applications of Adx as a versatile electron donor to different cytochromes P450, both in vitro and in vivo. Finally we will address the comparison between the mammalian cytochrome P450-associated adrenodoxin and ferredoxins involved in iron-sulfur-cluster biosynthesis. Despite their different functions, these proteins display an amazing similarity regarding their primary sequence, tertiary structure and biophysical features.

A new application of the yeast two-hybrid system in protein engineering

Cytochromes P450 are involved in the biosynthesis of steroid hormones in mitochondria of the adrenal gland. The electrons required for these reactions are provided via a redox chain consisting of adrenodoxin reductase (AdR) and adrenodoxin (Adx). A prerequisite for a fast and efficient electron transfer as well as high catalytic activity is the formation of functional complexes between the different redox partners. To improve the protein-protein interactions by directed evolution, we developed a new in vivo selection system. This high-throughput screening method is based on the yeast two-hybrid system. It enables a background-free screening for increased protein-protein interactions between stable and functional species including cofactor-containing proteins (FAD, [2Fe-2S], heme). The method was successfully applied for the directed evolution of Adx and selected variants were analyzed biochemically and biophysically. All analyzed proteins exhibit typical characteristics of [2Fe-2S]-cluster-type ferredoxins. Adx-dependent substrate conversion assays with different cytochromes demonstrated that the improved ability of the mutants to form complexes results in an enhanced catalytic efficiency of the cytochrome P450 system.

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.

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.

Molecular dynamics simulation of truncated bovine adrenodoxin

The 128 amino acid long soluble protein adrenodoxin (Adx) is a typical member of the ferredoxin protein family that are electron carrier proteins with an iron-sulfur cofactor. Adx carries electrons from adrenodoxin reductase (AdR) to cytochrome P450s. Its binding modes to these proteins were previously characterized by site-directed mutagenesis, by X-ray crystallography for the complex Adx:AdR, and by NMR. However, no clear evidence has been provided for the driving force that promotes Adx detachment from AdR upon reduction. Here, we characterized the conformational dynamics of unbound Adx in the oxidized and reduced forms using 2-20 ns long molecular dynamics simulations. The most noticeable difference between both forms is the enhanced flexibility of the loop (47-51) surrounding the iron-sulfur cluster in the reduced form. Together with several structural displacements at the binding interface, this increased flexibility may be the key factor promoting unbinding of reduced Adx from AdR. This points to an intrinsic property of reduced Adx that drives dissociation.

Modulation of aldosterone and cortisol synthesis on the molecular level

CYP11B1 and the closely related CYP11B2 are involved in the production of adrenal steroid hormones. Although in human their primary structure is 93% identical they are involved in the biosynthesis of functionally diverse products, such as glucocorticoids and mineralocorticoids, respectively. In contrast, bovine CYP11B1 combines both activities in one single enzyme. The CYP11B family belongs to class I cytochromes P450 that have been described in bacteria and mitochondria and receive their electrons from a low molecular weight iron sulphur protein which is reduced by a NADPH-dependent FAD-containing reductase. In this review, we summarise the current knowledge on the modulation of aldosterone and cortisol synthesis by transcriptional regulation, on the molecular level as consequence of mutations found in patients suffering from steroid hormone-related diseases as well as introduced by site-directed mutagenesis and as consequence of protein-protein interaction with both CYP11A1 and the natural redox partner adrenodoxin.

Bacterial (CYP101) and mitochondrial P450 systems—how comparable are they?

The bacterial CYP101 system and mitochondrial P450 systems show high similarity. Both systems contain the same protein components, a FAD containing reductase, a ferredoxin of the [2Fe2S] type, and a cytochrome P450. At a first glance they seem to be comparable but there are considerable differences among both proteins. Thus, the ferredoxin components of the two systems display significant structural homology but cannot substitute for each other in functional assays. Going into more detail, pronounced differences between the two systems that affect their biological functions are found.

Functional interaction of cytochrome P450 with its redox partners: a critical assessment and update of the topology of predicted contact regions

The problem of donor-acceptor recognition has been the most important and intriguing one in the area of P450 research. The present review outlines the topological background of electron-transfer complex formation, showing that the progress in collaborative investigations, combining physical techniques with chemical-modification and immunolocalization studies as well as site-directed mutagenesis experiments, has increasingly enabled the substantiation of hypothetical work resulting from homology modelling of P450s. Circumstantial analysis reveals the contact regions for redox proteins to cluster on the proximal face of P450s, constituting parts of the highly conserved, heme-binding core fold. However, more variable structural components located in the periphery of the hemoprotein molecules also participate in donor docking. The cross-reactivity of electron carriers, purified from pro- and eukaryotic sources, with a diversity of P450 species points at a possible evolutionary conservation of common anchoring domains. While electrostatic mechanisms appear to dominate orientation toward each other of the redox partners to generate pre-collisional encounter complexes, hydrophobic forces are likely to foster electron transfer events by through-bonding or pi-stacking interactions. Moreover, electron-tunneling pathways seem to be operative as well. The availability of new P450 crystal structures together with improved validation strategies will undoubtedly permit the production of increasingly satisfactory three-dimensional donor-acceptor models serving to better understand the molecular principles governing functional association of the redox proteins.

The human steroid hydroxylases CYP1B1 and CYP11B2

Major advances have been made during the last decade in our understanding of adrenal steroid hormone biosynthesis. Two key players in these pathways are the human mitochondrial cytochrome P450 enzymes CYP11B1 and CYP11B2, which catalyze the final steps in the biosynthesis of cortisol and aldosterone. Using data from mutations found in patients suffering from steroid hormone-related diseases, from mutagenesis studies and from the construction of three-dimensional models of these enzymes, structural information could be deduced that provide a clue to the stereo- and regiospecific steroid hydroxylation reactions carried out by these enzymes. In this review, we summarize the current knowledge on the physiological function and the biochemistry of these enzymes. Furthermore, the pharmacological and toxicological importance of these steroid hydroxylases, the means for the identification of their potential inhibitors and possible biotechnological applications are discussed.

A new electron transport mechanism in mitochondrial steroid hydroxylase systems based on structural changes upon the reduction of adrenodoxin

The adrenal ferredoxin (adrenodoxin, Adx) is an acidic 14.4-kDa [2Fe-2S] ferredoxin that belongs to the vertebrate ferredoxin family. It is involved in the electron transfer from the flavoenzyme NADPH-adrenodoxin-reductase to cytochromes P-450(scc) and P-450(11)(beta). The interaction between the redox partners during electron transport has not yet been fully established. Determining the tertiary structure of an electron-transfer protein may be very helpful in understanding the transport mechanism. In the present work, we report a structural study on the oxidized and reduced forms of bovine adrenodoxin (bAdx) in solution using high-resolution NMR spectroscopy. The protein was produced in Escherichia coli and singly or doubly labeled with (15)N or (13)C/(15)N, respectively. Approximately 70 and 75% of the (15)N, (13)C, and (1)H resonances could be assigned for the reduced and the oxidized bAdx, respectively. The secondary and tertiary structures of the reduced and oxidized states were determined using NOE distance information. (1)H(N)-T(1) relaxation times of certain residues were used to obtain additional distance constraints to the [2Fe-2S] cluster. The results suggest that the solution structure of oxidized Adx is quite similar to the X-ray structure. However, structural changes occur upon reduction of the [2Fe-2S] cluster, as indicated by NMR measurements. It could be shown that these conformational changes, especially in the C-terminal region, cause the dissociation of the Adx dimer upon reduction. A new electron transport mechanism proceeding via a modified shuttle mechanism, with both monomers and dimers acting as electron carriers, is proposed.

Regulation of the heme A biosynthetic pathway in Saccharomyces cerevisiae

Biosynthesis of heme A, a prosthetic group of cytochrome oxidase (COX), involves an initial farnesylation of heme B. The heme O product formed in this reaction is modified by hydroxylation of the methyl group at carbon C-8 of the porphyrin ring. This reaction was proposed to be catalyzed by Cox15p, ferredoxin, and ferredoxin reductase. Oxidation of the alcohol to the corresponding aldehyde yields heme A. In the present study we have assayed heme A and heme O in yeast COX mutants. The steady state concentrations of the two hemes in the different strains studied indicate that hydroxylation of heme O, catalyzed by Cox15p, is regulated either by a subunit or assembly intermediate of COX. The heme profiles of the mutants also suggest positive regulation of heme B farnesylation by the hydroxylated intermediate formed at the subsequent step or by Cox15p itself.

The interaction of bovine adrenodoxin with CYP11A1 (cytochrome P450scc) and CYP11B1 (cytochrome P45011beta ). Acceleration of reduction and substrate conversion by site-directed mutagenesis of adrenodoxin

The kinetics of protein-protein interaction and heme reduction between adrenodoxin wild type as well as eight mutants and the cytochromes P450 CYP11A1 and CYP11B1 was studied in detail. Rate constants for the formation of the reduced CYP11A1.CO and CYP11B1.CO complexes by wild type adrenodoxin, the adrenodoxin mutants Adx-(4-108), Adx-(4-114), T54S, T54A, and S112W, and the double mutants Y82F/S112W, Y82L/S112W, and Y82S/S112W (the last four mutants are Delta113-128) are presented. The rate constants observed differ by a factor of up to 10 among the respective adrenodoxin mutants for CYP11A1 but not for CYP11B1. According to their apparent rate constants for CYP11A1, the adrenodoxin mutants can be grouped into a slow (wild type, T54A, and T54S) and a fast group (all the other mutants). The adrenodoxin mutants forming the most stable complexes with CYP11A1 show the fastest rates of reduction and the highest rate constants for cholesterol to pregnenolone conversion. This strong correlation suggests that C-terminal truncation of adrenodoxin in combination with the introduction of a C-terminal tryptophan residue enables a modified protein-protein interaction rendering the system almost as effective as the bacterial putidaredoxin/CYP101 system. Such a variation of the adrenodoxin structure resulted in a mutant protein (S112W) showing a 100-fold increased efficiency in conversion of cholesterol to pregnenolone.

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.

Vertebrate-type and plant-type ferredoxins: crystal structure comparison and electron transfer pathway modelling

Crystallographic analysis of a fully functional, truncated bovine adrenodoxin, Adx(4-108), has revealed the structure of a vertebrate-type [2Fe-2S] ferredoxin at high resolution. Adrenodoxin is involved in steroid hormone biosythesis in adrenal gland mitochondria by transferring electrons from adrenodoxin reductase to different cytochromes P450. Plant-type [2Fe-2S] ferredoxins interact with photosystem I and a diverse set of reductases.A systematic structural comparison of Adx(4-108) with plant-type ferredoxins which share about 20 % sequence identity yields these results. (1) The ferredoxins of both types are partitioned into a large, strictly conserved core domain bearing the [2Fe-2S] cluster and a smaller interaction domain which is structurally different for both subfamilies. (2) In both types, residues involved in interactions with reductase are located at similar positions on the molecular surface and coupled to the [2Fe-2S] cluster via structurally equivalent hydrogen bonds. (3) The accessibility of the [2Fe-2S] cluster differs between Adx(4-108) and the plant-type ferredoxins where a solvent funnel leads from the surface to the cluster. (4) All ferredoxins are negative monopoles with a clear charge separation into two compartments, and all resulting dipoles but one point into a narrow cone located in between the interaction domain and the [2Fe-2S] cluster, possibly controlling predocking movements during interactions with redox partners. (5) Model calculations suggest that FE1 is the origin of electron transfer pathways to the surface in all analyzed [2Fe-2S] ferredoxins and that additional transfer probability for electrons tunneling from the more buried FE2 to the cysteine residue in position 92 of Adx is present in some.

cDNA cloning, overproduction and characterization of rat adrenodoxin reductase

We isolated a full-length cDNA clone for rat adrenodoxin reductase (AdR). The precursor of rat AdR was predicted to consist of 34 amino-terminal residues of extrapeptide for transport into mitochondria and the following 460 residues of the mature peptide region. The deduced amino acid sequence was 70.8 and 61.8% homologous to those of bovine and human AdRs in the extrapeptide region, respectively, and 88.5% homologous to both the sequences of bovine and human AdRs in the mature peptide region. The predicted mature form of rat AdR was directly expressed in Escherichia coli, using cDNA, and was purified with a yield of 32 mg/l of culture. The purified recombinant rat AdR showed an absorption spectrum characteristic of a flavoprotein with peaks at 270, 378 and 450 nm and shoulders at 280, 425 and 474 nm. The extinction coefficient was estimated to be 10.9 mM(-1) cm(-1) at 450 nm. The absorbance ratio at 270 nm/450 nm was 7.1. From the θ(208) value in the circular dichroism spectrum, the alpha-helix content in the rat AdR was calculated to be 30%. In NADPH-cytochrome c reductase activity reconstituted with adrenodoxin (Ad), the apparent K(m) value of rat AdR for NADPH was 0.32 microM, a value significantly lower than that of bovine AdR (1.4 microM). The rat AdR showed a higher affinity to the heterologous redox partner (bovine Ad, K(m)=9.3 nM) than to the native partner (rat Ad, K(m)=16.7 nM), whereas the affinity of bovine AdR was slightly higher to the native partner (bovine Ad, K(m)=37.1 nM) than to the heterologous partner (rat Ad, K(m)=46.8 nM). The K(m) values showed a reverse correlation to the difference of pI values between the redox partners. These results indicate that AdR binds to Ad mainly by ionic interaction.

Modulation of aldosterone biosynthesis by adrenodoxin mutants with different electron transport efficiencies

Aldosterone biosynthesis is highly regulated on different levels by hormones, potassium, lipid composition of the membrane and the molecular structure of its gene. Here, the influence of the electron transport efficiency from adrenodoxin (Adx) to CYP11B1 on the activities of bovine CYP11B1 has been investigated using a liposomal reconstitution system with truncated mutants of Adx. It could be clearly demonstrated that Adx mutants Adx 4-114 and Adx 4-108, possessing enhanced electron transfer abilities, produce increases in corticosterone and aldosterone biosynthesis. Based on the Vmax values of corticosterone and aldosterone formation, Adx 4-108 and Adx 4-114 enhance corticosterone synthesis 1.3-fold and aldosterone formation threefold and twofold, respectively. The production of 18-hydroxycorticosterone was changed only slightly in these Adx mutants. The effect of Adx 1-108 on the product patterns of bovine CYP11B1, human CYP11B1 and human CYP11B2 was confirmed in COS-1 cells by cotransfection of CYP11B- and Adx-containing expression vectors. It could be shown that Adx 1-108 enhances the formation of aldosterone by bovine CYP11B1 and by human CYP11B2, and stimulates the production of corticosterone by bovine CYP11B1 and human CYP11B1 and CYP11B2 also.

YAH1 of Saccharomyces cerevisiae: a new essential gene that codes for a protein homologous to human adrenodoxin

Here we describe the identification of a yeast gene (YAH1) with significant homology to a mammalian enzyme, adrenodoxin, encoded in open reading frame (ORF) YPL252C. Adrenodoxin is the second electron carrier that participates in a mitochondrial electron transfer chain that, in mammals, catalyses the conversion of cholesterol into pregnenolone, the first step in the synthesis of all steroid hormones. The inactivation of the yeast gene's chromosomal copy reveals that it performs an essential function. We show that the protein is targeted to the mitochondrial matrix and describe attempts to complement the yeast knockout with the human adrenodoxin gene (FDX1) and with chimerical proteins constructed with the fusion of the yeast and the human gene. The previous identification of a homolog of the first mammalian enzyme in yeast, ARH1, also shown to be essential (Manzella, L., Barros, M.H., Nobrega, F.G., 1998. Yeast 14, 839-846), strongly suggests that there is a novel electron transfer chain, unlinked to respiration, and of essential function in mitochondria.

A model for the solution structure of oxidized terpredoxin, a Fe2S2 ferredoxin from Pseudomonas

Terpredoxin (Tdx) is a 105-residue bacterial ferredoxin consisting of a single polypeptide chain and a single Fe2S2 prosthetic group. Tdx was first identified in a strain of Pseudomonas sp. capable of using alpha-terpineol as sole carbon source. The Tdx gene, previously cloned from the plasmid-encoded terp operon, that carries genes encoding for proteins involved in terpineol catabolism, has been subcloned and expressed as the holoprotein in E. coli. Physical characterization of the expressed Tdx has been performed, and a model for the solution structure of oxidized Tdx (Tdxo) has been determined. High-resolution homo- and heteronuclear NMR data have been used for structure determination in diamagnetic regions of the protein. The structure of the metal binding site (which cannot be determined directly by NMR methods due to paramagnetic broadening of resonances) was modeled using restraints obtained from a crystal structure of the homologous ferredoxin adrenodoxin (Adx) and loose restraints determined from paramagnetic broadening patterns in NMR spectra. Essentially complete 1H and 15N NMR resonance assignments have been made for the diamagnetic region of Tdxo (ca. 80% of the protein). A large five-stranded beta-sheet and a smaller two-stranded beta-sheet were identified, along with three alpha-helices. A high degree of structural homology was observed between Tdx and two other ferredoxins with sequence and functional homology to Tdx for which structures have been determined, Adx and putidaredoxin (Pdx), a homologous Pseudomonas protein. 1H/2H exchange rates for Tdx backbone NH groups were measured for both oxidation states and are rationalized in the context of the Tdx structure. In particular, an argument is made for the importance of the residue following the third ligand of the metal cluster (Arg49 in Tdx, His49 in Pdx, His56 in Adx) in modulating protein dynamics as a function of oxidation state. Some differences between Tdx and Pdx are detected by UV-visible spectroscopy, and structural differences at the C-terminal region were also observed. Tdx exhibits only 2% of the activity of Pdx in turnover assays performed using the reconstituted camphor hydroxylase system of which Pdx is the natural component.

Characterization of recombinant adrenodoxin reductase homologue (Arh1p) from yeast. Implication in in vitro cytochrome p45011beta monooxygenase system

The mammalian electron transfer chain of mitochondrial cytochrome P450 forms involved in steroidogenesis includes very specific proteins, namely adrenodoxin reductase and adrenodoxin. Adrenodoxin reductase transfers electrons from NADPH to adrenodoxin, which subsequently donates them to the cytochrome P450 forms. The Saccharomyces cerevisiae ARH1 gene product (Arh1p) presents homology to mammalian adrenodoxin reductase. We demonstrate the capacity of recombinant Arh1p, made in Escherichia coli, to substitute for its mammalian homologue in ferricyanide, cytochrome c reduction, and, more importantly, in vitro 11beta-hydroxylase assays. Electrons could be transferred from NADPH and NADH as measured in the cytochrome c reduction assay. Apparent Km values were determined to be 0.5, 0.6, and 0.1 microM for NADPH, NADH, and bovine adrenodoxin, respectively. These values differ slightly from those of mammalian adrenodoxin reductase, except for NADH, which is a very poor electron donor to the mammalian protein. Subcellular fractionation studies have localized Arh1p to the inner membrane of yeast mitochondria. The biological function of Arh1p remains unknown, and to date, no mitochondrial cytochrome P450 has been identified. ARH1 is, however, essential for yeast viability because an ARH1 gene disruption is lethal not only in aerobic growth conditions but also, surprisingly enough, during fermentation.

Structural and functional consequences of substitutions at the Pro108-Arg14 hydrogen bond in bovine adrenodoxin

Elimination of Pro108 in bovine adrenodoxin is known to result in the formation of a misfolded protein that is not able to incorporate a [2Fe-2S] cluster and rapidly degrades upon expression in E. coli. However, no experimental explanation for this phenomenon has been demonstrated so far. Using the recently obtained 3D structure of the truncated mutant Adx(4-108) we have studied the reasons of the protein stabilization by the proline residue by means of site-directed muta-genesis. Two main results have been obtained (i) the conserved hydrogen bond Pro108-Arg14, that connects different structural domains of Adx, contributes 6 kJ/mol into the protein stability and (ii) the presence of proline at position 108 provides a low conformational entropy of the unfolded state, supporting a gain in the Gibbs energy of 5.4 kJ/mol at 37 degrees C.

New aspects of electron transfer revealed by the crystal structure of a truncated bovine adrenodoxin, Adx(4-108)

BACKGROUND

Adrenodoxin (Adx) is a [2Fe-2S] ferredoxin involved in steroid hormone biosynthesis in the adrenal gland mitochondrial matrix of mammals. Adx is a small soluble protein that transfers electrons from adrenodoxin reductase (AR) to different cytochrome P450 isoforms where they are consumed in hydroxylation reactions. A crystallographic study of Adx is expected to reveal the structural basis for an important electron transfer reaction mediated by a vertebrate [2Fe-2S] ferredoxin.

RESULTS

The crystal structure of a truncated bovine adrenodoxin, Adx(4-108), was determined at 1.85 A resolution and refined to a crystallographic R value of 0.195. The structure was determined using multiple wavelength anomalous dispersion phasing techniques, making use of the iron atoms in the [2Fe-2S] cluster of the protein. The protein displays the compact (alpha + beta) fold typical for [2Fe-2S] ferredoxins. The polypeptide chain is organized into a large core domain and a smaller interaction domain which comprises 35 residues, including all those previously determined to be involved in binding to AR and cytochrome P450. A small interdomain motion is observed as a structural difference between the two independent molecules in the asymmetric unit of the crystal. Charged residues of Adx(4-108) are clustered to yield a strikingly asymmetric electric potential of the protein molecule.

CONCLUSIONS

The crystal structure of Adx(4-108) provides the first detailed description of a vertebrate [2Fe-2S] ferredoxin and serves to explain a large body of biochemical studies in terms of a three-dimensional structure. The structure suggests how a change in the redox state of the [2Fe-2S] cluster may be coupled to a domain motion of the protein. It seems likely that the clearly asymmetric charge distribution on the surface of Adx(4-108) and the resulting strong molecular dipole are involved in electrostatic steering of the interactions with AR and cytochrome P450.

Probing the interactions of putidaredoxin with redox partners in camphor P450 5-monooxygenase by mutagenesis of surface residues

The role of surface amino acid residues in the interaction of putidaredoxin (Pdx) with its redox partners in the cytochrome P450cam (CYP101) system was investigated by site-directed mutagenesis. The mutated Pdx genes were expressed in Escherichia coli, and the proteins were purified and studied in vitro. Activity of the complete reconstituted P450cam system was measured, and kinetic parameters were determined. Partial assays were also conducted to determine the effect of the mutations on interactions with each redox partner. Some mutations altered interactions of Pdx with one redox partner but not the other. Other mutations affected interactions with both redox partners, suggesting some overlap in the binding sites on Pdx for putidaredoxin reductase and CYP101. Cysteine 73 of Pdx was identified as important in the interaction of Pdx with putidaredoxin reductase, whereas aspartate 38 serves a critical role in the subunit binding and electron transfer to CYP101.

Specific aspects of electron transfer from adrenodoxin to cytochromes p450scc and p45011beta

An analysis of the electron transfer kinetics from the reduced [2Fe-2S] center of bovine adrenodoxin and its mutants to the natural electron acceptors, cytochromes P450scc and P45011beta, is the primary focus of this paper. A series of mutant proteins with distinctive structural parameters such as redox potential, microenvironment of the iron-sulfur cluster, electrostatic properties, and conformational stability was used to provide more detailed insight into the contribution of the electronic and conformational states of adrenodoxin to the driving forces of the complex formation of reduced adrenodoxin with cytochromes P450scc and P45011beta and electron transfer. The apparent rate constants of P450scc reduction were generally proportional to the adrenodoxin redox potential under conditions in which the protein-protein interactions were not affected. However, the effect of redox potential differences was shown to be masked by structural and electrostatic effects. In contrast, no correlation of the reduction rates of P45011beta with the redox potential of adrenodoxin mutants was found. Compared with the interaction with P450scc, however, the hydrophobic protein region between the iron-sulfur cluster and the acidic site on the surface of adrenodoxin seems to play an important role for precise complementarity in the tightly associated complex with P45011beta.

Molecular recognition and electron transfer in mitochondrial steroid hydroxylase systems

Mitochondrial monooxygenase systems are involved in the biosynthesis of glucocorticoids, mineralocorticoids, bile acids, and 1,25-dihydroxyvitamin D. The reactions are catalyzed by specific P450 enzymes that receive reducing equivalents via NADPH-ferredoxin oxidoreductase (adrenodoxin reductase) and ferredoxin (adrenodoxin). Although the three-dimensional structures of the individual components have not yet been solved, methods of expressing recombinant forms of these enzymes in Escherichia coli have allowed the use of site-directed mutagenesis to investigate the roles of specific amino acids in protein binding interactions, electron transfer, and catalysis. These studies have identified key charged residues in NADPH-ferredoxin oxidoreductase, ferredoxin, and P450scc, which are involved in electrostatic interactions critical for recognition, high-affinity binding, and electron transfer. The finding that the binding sites on ferredoxin for NADPH-ferredoxin oxidoreductase and P450 show significant overlap supports the proposed function for ferredoxin as a mobile electron shuttle between the reductase and P450 enzymes and is consistent with ferredoxin's role in serving multiple P450 isoforms.

Conformational stability of adrenodoxin mutant proteins

Adrenodoxin and the mutants at the positions T54, H56, D76, Y82, and C95, as well as the deletion mutants 4-114 and 4-108, were studied by high-sensitivity scanning microcalorimetry, limited proteolysis, and absorption spectroscopy. The mutants show thermal transition temperatures ranging from 46 to 56 degrees C, enthalpy changes from 250 to 370 kJ/mol, and heat capacity change delta Cp = 7.28 +/- 0.67 kJ/mol/K, except H56R. The amino acid replacement H56R produces substantial local changes in the region around positions 56 and Y82, as indicated by reduced heat capacity change (delta Cp = 4.29 +/- 0.37 kJ/mol/K) and enhanced fluorescence. Deletion mutant 4-108 is apparently more stable than the wild type, as judged by higher specific denaturation enthalpy and resistance toward proteolytic degradation. No simple correlation between conformational stability and functional properties could be found.

The influence of conserved aromatic residues on the electron transfer reactivity of 2[4Fe-4S] ferredoxins

The detailed mechanism used by [4Fe-4S] ferredoxins to exchange electrons is not known. The importance of two highly conserved aromatic residues, each located close to one cluster of 2[4Fe-4S] ferredoxins has been probed by site-directed mutagenesis of Clostridium pasteurianum ferredoxin. All generated variants are less stable than the native protein and only hydrophobic residues can replace one of the two conserved aromatic residues. With leucine substituting both aromatics, Clostridium pasteurianum ferredoxin cannot even be completely purified because of its deleterious instability. The reduction potentials of Clostridium pasteurianum ferredoxin variants do not depend on the presence of aromatic residues near the clusters. However, the ferredoxin from Entamoeba histolytica which is naturally devoid of aromatic residues displays a reduction potential nearly 60 mV less negative than that of Clostridium pasteurianum ferredoxin. The rate constants for the oxidation of the reduced ferredoxins by the inorganic complexes hexaamine-cobalt(III) chloride and sodium ethylenediaminetetra-acetatecobaltate(III) are similar. This implies that electron transfer from the clusters of these molecules is not mediated by the conserved aromatic residues. These residues rather appear to be involved in maintaining the overall stability of ferredoxins.

11 beta-hydroxylase activity in recombinant yeast mitochondria. In vivo conversion of 11-deoxycortisol to hydrocortisone

In mammals, the final 11 beta-hydroxylation step of the hydrocortisone biosynthesis pathway is performed by a mitochondrial enzyme, namely cytochrome P-450(11 beta), together with the electron carriers adrenodoxin and NADPH adrenodoxin oxidoreductase. Successful production of a functional steroid 11 beta-hydroxylase activity was obtained in recombinant yeast in vivo. This conversion was achieved by coexpression of a mitochondrially targeted adrenodoxin and a modified bovine P-450(11 beta) whose natural presequence was replaced by a yeast presequence, together with an unexpected yeast endogenous NADPH-adrenodoxin-reductase-like activity. Adrenodoxin and P-450(11 beta) behave as a mitochondrial matrix and membrane protein, respectively. Saccharomyces cerevisiae apparently produces a mitochondrial protein which is capable of transferring electrons to bovine adrenodoxin, which in turn transfers the electrons to P-450(11 beta). The endogenous adrenodoxin oxidoreductase gains electrons specifically from NADPH. The notion that a yeast microsomal NADPH P-450 oxidoreductase can transfer electrons to mammalian microsomal P-450s can be extended to mitochondria, where an NADPH adrenodoxin oxidoreductase protein transfers electrons to adrenodoxin and renders a mitochondrial mammalian P-450 functional in vivo. The physiological function of this yeast NADPH adrenodoxin oxidoreductase activity is not known.

Redox-dependent dynamics of putidaredoxin characterized by amide proton exchange

Multidimensional NMR methods were used to obtain 1H-15N correlations and 15N resonance assignments for amide and side-chain nitrogens of oxidized and reduced putidaredoxin (Pdx), the Fe2S2 ferredoxin, which acts as the physiological reductant of cytochrome P-450cam (CYP101). A model for the solution structure of oxidized Pdx has been determined recently using NMR methods (Pochapsky TC, Ye XM, Ratnaswamy G, Lyons TA, 1994, Biochemistry 33:6424-6432) and redox-dependent 1H NMR spectral features have been described (Pochapsky TC, Ratnaswamy G, Patera A, 1994, Biochemistry 33:6433-6441). 15N assignments were made with NOESY-(1H/15N) HMQC and TOCSY-(1H/15N) HSQC spectra obtained using samples of Pdx uniformly labeled with 15N. Local dynamics in both oxidation states of Pdx were then characterized by comparison of residue-specific amide proton exchange rates, which were measured by a combination of saturation transfer and H2O/D2O exchange methods at pH 6.4 and 7.4 (uncorrected for isotope effects). In general, where exchange rates for a given site exhibit significant oxidation-state dependence, the oxidized protein exchanges more rapidly than the reduced protein. The largest dependence of exchange rate upon oxidation state is found for residues near the metal center and in a region of compact structure that includes the loop-turn Val 74-Ser 82 and the C-terminal residues (Pro 102-Trp 106). The significance of these findings is discussed in light of the considerable dependence of the binding interaction between Pdx and CYP101 upon the oxidation state of Pdx.

A structure-based model for cytochrome P450cam-putidaredoxin interactions

Putidaredoxin (Pdx) is a Fe2S2 ferredoxin which acts as the physiological reductant of cytochrome P-450cam (CYP101). A model for the solution structure of oxidized Pdx has been determined using NMR methods (Pochapsky et al (1994) Biochemistry 33, 6424-6432). 1H-15N correlations and redox-dependent amide exchange rates have also been described (Lyons et al (1996) Protein Sci 5, 627-639). Data obtained from mutagenesis and kinetic measurements concerning the interactions of Pdx and CYP101 are summarized. A model for the structure of the homologous ferredoxin adrenodoxin (Adx) is also described, and data concerning Adx activity are discussed in relation to this structure. The structures of Pdx and CYP101 were used as starting points for molecular modeling and molecular dynamics simulations. Close approach between the metal centers of the two proteins and interaction between aromatic residues on the surfaces of the proteins are premised. The resulting complex exhibits three intermolecular salt bridges, five intermolecular hydrogen bonds and a 12 A distance between the metal centers. The first direct observations of interaction between Pdx and CYP101 (by two-dimensional NMR of 15N-labeled Pdx in solution with CYP101) are described. The results of the NMR experiments indicate that conformational gating of the electron transfer complex between CYP101 and Pdx may be important.

The role of threonine 54 in adrenodoxin for the properties of its iron-sulfur cluster and its electron transfer function

The amino acid in position 54 of adrenodoxin is strongly conserved among ferredoxins, consisting of a threonine or serine. Its role was studied by analyzing mutants T54S and T54A of bovine adrenodoxin. Absorption, circular dichroism, fluorescence, and electron paramagnetic resonance spectra of mutant T54S show that this substitution has no influence on the formation and stability of the ferredoxin. The redox potential of this mutant, however, was lowered by 55 mV as compared with native adrenodoxin, indicating a role for this residue in redox potential modulation. Incorporation of the iron-sulfur cluster was not impaired in the T54A mutant, although structural features of the oxidized protein were considerably changed. The decreased stability of the T54A mutant as compared with the wild type and mutant T54S indicates that a hydrogen bond donor at this position stabilizes the protein. Both mutants have been shown to be functionally active. Replacement of threonine 54 by serine or alanine, however, leads to rearrangements at the recognition sites for its redox partners. This is reflected by decreased Km and Kd values of both mutants for the cytochromes P450, whereas only T54A displayed a decreased Km value in cytochrome c reduction. Substrate conversion was accelerated (2.2- and 2.4-fold for mutants T54A and T54S, respectively) in the CYP11B1-, but not in the CYP11A1-dependent reaction.

Rabbit liver cytochrome P-450 2B5: high-level expression of the full-length protein in Escherichia coli, purification, and catalytic activity

Rabbit liver cytochrome P-450 2B5 (P-450 2B5) was expressed in Escherichia coli using the D(+)-galactose-inducible expression vector pJL-2, containing the full-length cDNA encoding P-450 2B5. Stimulation by galactose of protein synthesis in the presence of the heme precursor 5-aminolevulinic acid peaked 72 h after addition to the inducer to yield 108 nmol membrane-bound P-450 2B5 per liter of culture medium. The recombinant enzyme was purified to near homogeneity by a two-column procedure involving chromatography on DE-52 cellulose and hydroxylapatite. The hemoprotein was isolated mainly in the low-spin iron configuration and exhibited a reduced CO-difference spectrum with a Soret band at 451 nm. Second-derivative spectral analysis in the middle-UV region revealed that type I binding of 4-nitroanisole to ferric P-450 2B5 abolished absorption bands ascribable to tyrosine residues within the polypeptide chain. Pseudo-first-order rates of NADPH-driven reduction of the pigment were lower when reconstituted with NADPH-cytochrome P-450 reductase than with the mitochondrial adrenodoxin/NADPH-adrenodoxin reductase redox couple. The enzyme was catalytically active toward 4-nitroanisole and androstenedione; metabolic rates were enhanced to different extents by the presence of cytochrome b5. The recombinant hemoprotein did not catalyze bioactivation of 4-(N-methyl-N-nitrosamino)-1-(3-pyridyl)-1-butanone, a potent pulmonary carcinogen. The methods described here should facilitate further studies on the biophysical basis of the complex interactions of P-450 2B5 with its redox partners.

Mutational effects on the spectroscopic properties and biological activities of oxidized bovine adrenodoxin, and their structural implications

Of the aromatic 1H-NMR signals of oxidized bovine adrenodoxin only those of His56 showed intrinsic chemical shift changes upon replacement of Tyr82 by Ser or Leu, that must arise from a loss of a through-space ring-current effect of the tyrosine ring in these mutants. Thus, of the three His residues contained in adrenodoxin, His56 is closest to Tyr82, and hence to the highly acidic determinant region of adrenodoxin that is the interaction site for adrenodoxin reductase and P-450. The strong dependence of the fluorescence intensity of Tyr82 on the residue in position 56 supported this observation. As a consequence of this, the effects of replacement of His56 by Gln or Thr on cytochrome c reduction and cytochromes P-450(11 beta) (CYP11B1)-dependent and P-450scc (CYP11A1)-dependent substrate conversions were studied. No influence on Vmax values was observed for all reactions mediated by the mutants, implying His56 does not play a decisive role in the intramolecular or intermolecular electron transfer. In contrast, the Km values were increased, as was the Ks value for binding of CYP11A1 to the [H56T]adrenodoxin. The secondary structure deduced from further NMR data of adrenodoxin was compared with that of other ferredoxins. Tyr82 is in a region of the molecule containing no secondary-structure elements. The data for Tyr82 are in keeping with the biological activities and suggests it is in a flexible, solvent-exposed region of the molecule.

Conformational stability of bovine holo and apo adrenodoxin--a scanning calorimetric study

Holo and apo adrenodoxin were studied by differential scanning calorimetry, absorption spectroscopy, limited proteolysis, and size-exclusion chromatography. To determine the conformational stability of adrenodoxin, a method was found that prevents the irreversible destruction of the iron-sulfur center. The approach makes use of a buffer solution that contains sodium sulfide and mercaptoethanol. The thermal transition of adrenodoxin takes place at Ttrs = 46-57 degrees C, depending on the Na2S concentration with a denaturation enthalpy of delta H = 300-380 kJ/mol. From delta H versus Ttrs a heat capacity change was determined as delta Cp = 7.5 +/- 1.2 kJ/mol/K. The apo protein is less stable than the holo protein as judged by the lower denaturation enthalpy (delta H = 93 +/- 14 kJ/mol at Ttrs = 37.4 +/- 3.3 degrees C) and the higher proteolytic susceptibility. The importance of the iron-sulfur cluster for the conformational stability of adrenodoxin and some conditions for refolding of the thermally denatured protein are discussed.

Structure-function studies on mutants of adrenal ferredoxin

Mutants of the adrenal ferredoxin (adrenodoxin) have been expressed in E. coli in order to improve the understanding of its structure and function. Replacement of the ligands to the /2Fe-2S/ center, C46, C52, C55 and C92, by serine, histidine or aspartic acid lead to apoproteins not incorporating the iron-sulfur cluster, whereas C95S forms a functionally active holoprotein. C-terminal deletions up to amino acid 109 affect the conformation around the iron-sulfur cluster in adrenodoxin and the interaction with CYP11A1 and CYP11B1, but not with adrenodoxin reductase. The presence of P108 is necessary for incorporation of the /2Fe-2S/ cluster and obviously for correct folding of adrenodoxin.