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

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.

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.

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.

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.

Crystal Structure of Putidaredoxin, the [2Fe–2S] Component of the P450cam Monooxygenase System from Pseudomonas putida

Stability of the [2Fe-2S]-containing putidaredoxin (Pdx), the electron donor to cytochrome P450cam in Pseudomonas putida, was improved by mutating non-ligating cysteine residues, Cys73 and Cys85, to serine singly and in combination. The increasing order of stability is Cys73Ser/Cys85Ser>Cys73Ser>Cys85Ser>WT Pdx. Crystal structures of Cys73Ser/Cys85Ser and Cys73Ser mutants of Pdx, solved by single-wavelength anomalous dispersion phasing using the [2Fe-2S] iron atoms to 1.47 A and 1.65 A resolution, respectively, are nearly identical and very similar to those of bovine adrenodoxin (Adx) and Escherichia coli ferredoxin. However, unlike the Adx structure, no motion between the core and interaction domains of Pdx is observed. This higher conformational stability of Pdx might be due to the presence of a more extensive hydrogen bonding network at the interface between the two structural domains around the conserved His49. In particular, formation of a hydrogen bond between the side-chain of Tyr51 and the carbonyl oxygen atom of Glu77 and the presence of two well-ordered water molecules linking the interaction domain and the C-terminal peptide to the core of the molecule are unique to Pdx. The folding topology of the NMR model is similar to that of the X-ray structure of Pdx. The overall rmsd of Calpha positions between the two models is 1.59 A. The largest positional differences are observed for residues 18-21 and 33-37 in the loop regions and the C terminus. The latter two peptides display conformational heterogeneity in the crystal structures. Owing to flexibility, the aromatic ring of the C-terminal Trp106 can closely approach the side-chains of Asp38 and Thr47 (3.2-3.9 A) or move away and leave the active site solvent-exposed. Therefore, Trp106, previously shown to be important in the Pdr-to-Pdx and Pdx-to-P450cam electron transfer reactions is in a position to regulate and/or mediate electron transfer to or from the [2Fe-2S] center of Pdx.

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.

A conserved histidine in vertebrate-type ferredoxins is critical for redox-dependent dynamics

Adrenodoxin (Adx) belongs to the family of Cys(4)Fe(2)S(2) vertebrate-type ferredoxins that shuttle electrons from NAD(P)H-dependent reductases to cytochrome P450 enzymes. The vertebrate-type ferredoxins contain a conserved basic residue, usually a histidine, adjacent to the third cysteine ligand of the Cys(4)Fe(2)S(2) cluster. In bovine Adx the side chain of this residue, His 56, is involved in a hydrogen-bonding network within the domain of Adx that interacts with redox partners. It has been proposed that this network acts as a mechanical link between the metal cluster binding site and the interaction domain, transmitting redox-dependent conformational or dynamical changes from the cluster binding loop to the interaction domain. H/D exchange studies indicate that oxidized Adx (Adx(o)) is more dynamic than reduced Adx (Adx(r)) on the kilosecond time scale in many regions of the protein, including the interaction domain. Dynamical differences on picosecond to nanosecond time scales between the oxidized (Adx(o)) and reduced (Adx(r)) adrenodoxin were probed by measurement of (15)N relaxation parameters. Significant differences between (15)N R(2) rates were observed for all residues that could be measured, with those rates being faster in Adx(o) than in Adx(r). Two mutations of His 56, H56R and H56Q, were also characterized. No systematic redox-dependent differences between (15)N R(2) rates or H/D exchange rates were observed in either mutant, indicating that His 56 is required for the redox-dependent behavior observed in WT Adx. Comparison of chemical shift differences between oxidized and reduced H56Q and H56R Adx confirms that redox-dependent changes are smaller in these mutants than in the wild-type Adx.

Transient protein interactions studied by NMR spectroscopy: the case of cytochrome C and adrenodoxin

The interaction between yeast iso-1-cytochrome c (C102T) and two forms of bovine adrenodoxin, the wild type and a truncated form comprising residues 4-108, has been investigated using a combination of one- and two-dimensional heteronuclear NMR spectroscopy. Chemical shift perturbations and line broadening of amide resonances in the [(15)N,(1)H]HSQC spectrum for both (15)N-labeled cytochrome c and adrenodoxin in the presence of the unlabeled partner protein indicate the formation of a transient complex, with a K(a) of (4 +/- 1) x 10(4) M(-)(1) and a lifetime of <3 ms. The perturbed residues map over a large surface area for both proteins. For cytochrome c, the dominating effects are located around the exposed heme edge but with other areas also affected upon formation of the complex. In the case of adrenodoxin, effects are seen in both the recognition and core domains, with the largest perturbations in the recognition domain. These results indicate that the complex has a dynamic nature, with delocalized binding of cytochrome c on adrenodoxin. A comparison with other transient complexes of redox proteins places this complex between well-defined complexes such as the cytochrome c-cytochrome c peroxidase complex and entirely dynamic complexes such as the cytochrome b(5)-myoglobin complex.

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.

Comparison of functional domains in vertebrate-type ferredoxins

The vertebrate-type Cys(4)Fe(2)S(2) ferredoxins are a class of small acidic proteins that typically act as electron shuttles between NAD(P)H-dependent reductases and monoxygenases, particularly cytochromes P450. Nuclear magnetic resonance assignments and detailed analysis of nuclear Overhauser effects permit the direct comparison of the functional C-terminal domains of three vertebrate-type ferredoxins, the mammalian adrenodoxin (Adx) and the bacterial ferredoxins putidaredoxin (Pdx) and terpredoxin (Tdx). In particular, homologous hydrogen-bonding networks involving a conserved basic residue (His 49 in Pdx, His 56 in Adx, Arg 49 in Tdx) are detailed. This hydrogen bond network appears to play a role in the mechanical transmission of redox-dependent conformational and dynamic changes from the iron-sulfur binding loop to the C-terminal domain. Hydrogen/deuterium exchange measurements have been made in Adx as a function of oxidation state for comparison with previous studies of Pdx and Tdx. The results of these measurements highlight the importance of the conserved basic residue in the linkage between oxidation state and protein dynamics. Finally, a series of mutations have been made in the C-terminal domain of Pdx, including one, Y51F, that disrupts the proposed hydrogen-bonding network without perturbing steric and hydrophobic interactions in the functional domain. Although the mutant is considerably destabilized with respect to wild-type Pdx, relatively unperturbed chemical shifts for residues near the site of the mutation and NOEs between water and Phe 51 suggest that the network is reconstituted with a solvent water in place of the tyrosine hydroxyl group in this mutant.

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.

GroEL-assisted refolding of adrenodoxin during chemical cluster insertion

Chemical reconstitution of recombinant bovine adrenal mitochondrial apoadrenodoxin was carried out in the presence of the nonhomologous chaperone protein GroEL and of the cochaperone GroES, both in the presence and in the absence of ATP. The approach used here was different from the one characterizing studies on chaperone activity, as we used an adrenodoxin apoprotein, devoid of the cluster iron and sulfide, rather than a denaturant-unfolded form of the protein, and catalytic amounts of the chaperone proteins. A possible scaffolding role for two bacterial sulfur transferases, namely, rhodanese from Azotobacter vinelandii and a rhodanese-like sulfurtransferase from Escherichia coli, was also investigated in the absence of the enzyme substrates. The extent and the rate of adrenodoxin refolding following cluster insertion was measured by spectroscopy and by monitoring the activity recovery in a NADPH-cytochrome c reduction assay. These measurements were carried out on the unresolved reaction mixture and on the adrenodoxin-containing fraction obtained by HPLC fractionation of the reconstitution mixture at different reaction times. The rate and extent of cluster insertion and activity recovery were substantially improved by addition of GroEL and increased with increasing the GroEL/apoadrenodoxin ratio. GroES and ATP had no effect by themselves, and did not enhance the effect of GroEL. A. vinelandii rhodanese, the E. coli sulfurtransferase, and bovine serum albumin had no effect on the rate and yield of chemical reconstitution. The accelerated chemical reconstitution of apoadrenoxin in the presence of GroEL is therefore attributable to a scaffolding effect of this protein.

The loop region covering the iron-sulfur cluster in bovine adrenodoxin comprises a new interaction site for redox partners

The amino acid in position 49 in bovine adrenodoxin is conserved among vertebrate [2Fe-2S] ferredoxins as hydroxyl function. A corresponding residue is missing in the cluster-coordinating loop of plant-type [2Fe-2S] ferredoxins. To probe the function of Thr-49 in a vertebrate ferredoxin, replacement mutants T49A, T49S, T49L, and T49Y, and a deletion mutant, T49Delta, were generated and expressed in Escherichia coli. CD spectra of purified proteins indicate changes of the [2Fe-2S] center geometry only for mutant T49Delta, whereas NMR studies reveal no transduction of structural changes to the interaction domain. The redox potential of T49Delta (-370 mV) is lowered by approximately 100 mV compared with wild type adrenodoxin and reaches the potential range of plant-type ferredoxins (-305 to -455 mV). Substitution mutants show moderate changes in the binding affinity to the redox partners. In contrast, the binding affinity of T49Delta to adrenodoxin reductase and cytochrome P-450 11A1 (CYP11A1) is dramatically reduced. These results led to the conclusion that Thr-49 modulates the redox potential in adrenodoxin and that the cluster-binding loop around Thr-49 represents a new interaction region with the redox partners adrenodoxin reductase and CYP11A1. In addition, variations of the apparent rate constants of all mutants for CYP11A1 reduction indicate the participation of residue 49 in the electron transfer pathway between adrenodoxin and CYP11A1.

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.

GroEL-assisted and -unassisted refolding of mature and precursor adrenodoxin: the role of the precursor sequence

We have performed refolding studies on a [2Fe-2S] protein, adrenodoxin (Adx), and its precursor form, preadrenodoxin. In vitro, mature Adx is expressed as a soluble active form in Escherichia coli, but precursor Adx is expressed in inclusion bodies. Both mature and precursor Adx refolded spontaneously from their denatured forms and the recovery levels of enzyme activities were 40 and 37% for mature and precursor Adx, respectively. Furthermore, the interaction between GroEL- and Gdn-HCl-denatured mature and precursor forms was investigated. In the case of mature Adx, the activity was increased in the presence of either GroEL, GroES, or bovine serum albumin and the refolding of mature Adx is a nonspecific process. However, the GroEL-mediated reaction is specific for precursor Adx under the experimental conditions used here. A higher electron transfer activity is obtained after ATP addition to the GroEL-containing refolding mixture, and GroEL-precursor complexes were found by gel chromatography studies. Our observation suggests that the small single-domain protein Adx (mature form) folded independently of the chaperonin GroEL. The contribution of the chaperonin complexes to the folding is toward the aggregation-sensitive precursor Adx, which in vitro folded 1.3- to 1.4-fold slower than mature Adx. This demonstrates that the presequence is responsible for the formation of inclusion bodies and for the in vitro recognition motif for GroEL binding.

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.

A refined model for the solution structure of oxidized putidaredoxin

A refined model for the solution structure of oxidized putidaredoxin (Pdxo), a Cys4Fe2S2 ferredoxin, has been determined. A previous structure (Pochapsky et al. (1994) Biochemistry 33, 6424-6432; PDB entry ) was calculated using the results of homonuclear two-dimensional NMR experiments. New data has made it possible to calculate a refinement of the original Pdxo solution structure. First, essentially complete assignments for diamagnetic 15N and 13C resonances of Pdxo have been made using multidimensional NMR methods, and 15N- and 13C-resolved NOESY experiments have permitted the identification of many new NOE restraints for structural calculations. Stereospecific assignments for leucine and valine CH3 resonances were made using biosynthetically directed fractional 13C labeling, improving the precision of NOE restraints involving these residues. Backbone dihedral angle restraints have been obtained using a combination of two-dimensional J-modulated 15N,1H HSQC and 3D (HN)CO(CO)NH experiments. Second, the solution structure of a diamagnetic form of Pdx, that of the C85S variant of gallium putidaredoxin, in which a nonligand Cys is replaced by Ser, has been determined (Pochapsky et al. (1998) J. Biomol. NMR 12, 407-415), providing information concerning structural features not observable in the native ferredoxin due to paramagnetism. Third, a crystal structure of a closely related ferredoxin, bovine adrenodoxin, has been published (Müller et al. (1998) Structure 6, 269-280). This structure has been used to model the metal binding site structure in Pdx. A family of fourteen structures is presented that exhibits an rmsd of 0.51 A for backbone heavy atoms and 0.83 A for all heavy atoms. Exclusion of the modeled metal binding loop region reduces overall the rmsd to 0.30 A for backbone atoms and 0.71 A for all heavy atoms.

Structure of adrenodoxin and function in mitochondrial steroid hydroxylation

The three-dimensional structure of a truncated mutant of bovine adrenodoxin has been resolved at 1.85 A resolution by MAD. The protein consists of a large core region and a more flexible hairpin loop bearing residues which have been previously described as being involved in redox partner recognition. To study the role of distinct protein domains and amino acids of adrenodoxin in interaction with adrenodoxin reductase (AdR), CYP11A1 and CYP11B1, as well as in electron transfer, mutants of adrenodoxin have been prepared by site-directed mutagenesis and produced in Escherichia coli, and their structural and functional properties have been characterized in detail. It could be demonstrated that Tyr82 is located at the edge of the flexible interaction loop of adrenodoxin participating in interactions with AdR and P450s. His56, being close to Tyr82, forms a bridge between the core region of adrenodoxin and the interaction loop. Its role in transmitting changes of the cluster region to the interaction site has also been supported by functional studies. Pro108 of adrenodoxin, the only proline residue contained in the protein and being conserved in this position among several other vertebrate-type ferredoxins, has been demonstrated to be of importance for the correct folding of this protein.

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.

Pro108 is important for folding and stabilization of adrenal ferredoxin, but does not influence the functional properties of the protein

The truncated mutant Met-adrenodoxin-(4-107)-peptide of bovine adrenal ferredoxin was expressed as apoprotein in Escherichia coli BL21 and could be reconstituted to the holoform by chemical or enzymatic methods. The reconstituted protein had spectroscopic, functional and redox properties similar to the Met-adrenodoxin-(4-108)-peptide of adrenal ferredoxin, into which the cluster was inserted upon expression in the same Escherichia coli strain. Rate of in vitro cluster insertion into the Met-adrenodoxin-(4-107) apoprotein was much lower than for the Met-adrenodoxin-(4-108) apoprotein under identical conditions. Comparative thermodynamic studies with the Met-adrenodoxin-(4-108)-peptide indicated that removal of Pro108 resulted in an extensive decrease of the overall stability of the protein in either oxidation state. The Met-adrenodoxin-(4-107)-peptide showed a higher sensitivity to urea denaturation and had a sensibly lower denaturation temperature, 44.8 degrees C, compared with 51.7 degrees C for mutant Met-adrenodoxin-(4-108). The stability of the reduced state of both mutants is slightly lower than that of the oxidized state indicating that this protein region does not undergo major structural changes upon reduction.

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.

Reversible, non-denaturing metal substitution in bovine adrenodoxin and spinach ferredoxin and the different reactivities of [2Fe-2S]-cluster-containing proteins

The non-denaturing substitution of cluster iron by other metals was studied in spinach ferredoxin and in bovine adrenodoxin. Only some of several metal species tested (Cd2+, Zn2+, VO2+, Mn2+, Co2+, Ni2+) caused bleaching of the residual visible absorbance and of the EPR signals of the reduced ferredoxins. No formation of mixed-metal cluster was observed. The most reactive metal species were Cd2+ and Zn2+ and Cd2+ was found to react also with oxidized adrenodoxin. Metal-treated proteins were resolved into a mixture of apoprotein, metal-substituted protein and unreacted holoprotein. Their biological activity was proportional to the residual holoprotein concentration. Spinach ferredoxin and adrenodoxin were found to differ substantially with regard to their metal-substitution reactivity under oxidizing and reducing conditions, reaction time, and formation of apoprotein, which was more pronounced for spinach ferredoxin. Exchange of cluster iron with Cd2+ in adrenodoxin generated stable species containing 2 mol sulfide/mol protein and 2 or 5 mol cadmium/mol protein, respectively. The relative amount of the two substitution products depended on the experimental conditions. CD and NMR data on all the cadmium-substituted proteins suggest that iron replacement led to a significant structural rearrangement. Nevertheless, all the metal-substituted proteins could be re-converted into the native iron-containing form upon incubation with iron in the absence of reductants, of denaturing agents, and of an external source of sulfide. The different reactivity of the two proteins is discussed in terms of the cluster environment, along with the possible physiological relevance of these findings.

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.