Biochemical aspects of iron-sulfur linkage in non-heme iron protein, with special reference to “Adrenodoxin”

Cellular retinoid-binding proteins transfer retinoids to human cytochrome P450 27C1 for desaturation

Cytochrome P450 27C1 (P450 27C1) is a retinoid desaturase expressed in the skin that catalyzes the formation of 3,4-dehydroretinoids from all-trans retinoids. Within the skin, retinoids are important regulators of proliferation and differentiation. In vivo, retinoids are bound to cellular retinol-binding proteins (CRBPs) and cellular retinoic acid–binding proteins (CRABPs). Interaction with these binding proteins is a defining characteristic of physiologically relevant enzymes in retinoid metabolism. Previous studies that characterized the catalytic activity of human P450 27C1 utilized a reconstituted in vitro system with free retinoids. However, it was unknown whether P450 27C1 could directly interact with holo-retinoid-binding proteins to receive all-trans retinoid substrates. To assess this, steady-state kinetic assays were conducted with free all-trans retinoids and holo-CRBP-1, holo-CRABP-1, and holo-CRABP-2. For holo-CRBP-1 and holo-CRABP-2, the k_cat/K_m values either decreased 5-fold or were equal to the respective free retinoid values. The k_cat/K_m value for holo-CRABP-1, however, decreased ∼65-fold in comparison with reactions with free all-trans retinoic acid. These results suggest that P450 27C1 directly accepts all-trans retinol and retinaldehyde from CRBP-1 and all-trans retinoic acid from CRABP-2, but not from CRABP-1. A difference in substrate channeling between CRABP-1 and CRABP-2 was also supported by isotope dilution experiments. Analysis of retinoid transfer from holo-CRABPs to P450 27C1 suggests that the decrease in k_cat observed in steady-state kinetic assays is due to retinoid transfer becoming rate-limiting in the P450 27C1 catalytic cycle. Overall, these results illustrate that, like the CYP26 enzymes involved in retinoic acid metabolism, P450 27C1 interacts with cellular retinoid-binding proteins.

Polyamines: naturally occurring small molecule modulators of electrostatic protein-protein interactions

Modulations of protein-protein interactions are a key step in regulating protein function, especially in networks. Modulators of these interactions are supposed to be candidates for the development of novel drugs. Here, we describe the role of the small, polycationic and highly abundant natural polyamines that could efficiently bind to charged spots at protein interfaces as modulators of such protein-protein interactions. Using the mitochondrial cytochrome P45011A1 (CYP11A1) electron transfer system as a model, we have analyzed the capability of putrescine, spermidine, and spermine at physiologically relevant concentrations to affect the protein-protein interactions between adrenodoxin reductase (AdR), adrenodoxin (Adx), and CYP11A1. The actions of polyamines on the individual components, on their association/dissociation, on electron transfer, and on substrate conversion were examined. These studies revealed modulating effects of polyamines on distinct interactions and on the entire system in a complex way. Modulation via changed protein-protein interactions appeared plausible from docking experiments that suggested favourable high-affinity binding sites of polyamines (spermine>spermidine>putrescine) at the AdR-Adx interface. Our findings imply for the first time that small endogenous compounds are capable of interfering with distinct components of transient protein complexes and might control protein functions by modulating electrostatic protein-protein interactions.

Protein phosphorylation and intermolecular electron transfer: a joint experimental and computational study of a hormone biosynthesis pathway

Protein phosphorylation is a common regulator of enzyme activity. Chemical modification of a protein surface, including phosphorylation, could alter the function of biological electron-transfer reactions. However, the sensitivity of intermolecular electron-transfer kinetics to post-translational protein modifications has not been widely investigated. We have therefore combined experimental and computational studies to assess the potential role of phosphorylation in electron-transfer reactions. We investigated the steroid hydroxylating system from bovine adrenal glands, which consists of adrenodoxin (Adx), adrenodoxin reductase (AdR), and a cytochrome P450, CYP11A1. We focused on the phosphorylation of Adx at Thr-71, since this residue is located in the acidic interaction domain of Adx, and a recent study has demonstrated that this residue is phosphorylated by casein kinase 2 (CK2) in vitro.1 Optical biosensor experiments indicate that the presence of this phosphorylation slightly increases the binding affinity of oxidized Adx with CYP11A1ox but not AdRox. This tendency was confirmed by KA values extracted from Adx concentration-dependent stopped-flow experiments that characterize the interaction between AdRred and Adxox or between Adxred and CYP11A1ox. In addition, acceleration of the electron-transfer kinetics measured with stopped-flow is seen only for the phosphorylated Adx-CYP11A1 reaction. Biphasic reaction kinetics are observed only when Adx is phosphorylated at Thr-71, and the Brownian dynamics (BD) simulations suggest that this phosphorylation may enhance the formation of a secondary Adx-CYP11A1 binding complex that provides an additional electron-transfer pathway with enhanced coupling.

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.

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

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

Some properties of mitochondrial adrenodoxin associated with its nonconventional electron donor function toward rabbit liver microsomal cytochrome P450 2B4

Mitochondrial adrenodoxin (Adx) was found to cross-react with microsomal cytochrome P450 2B4 (CYP2B4) as the terminal electron acceptor. When compared with NADPH-cytochrome P450 reductase (P450R), the natural redox partner of CYP2B4, Adx was less efficient both in transferring the first electron and in coupling the system. The ferredoxin yielded an unusual reverse type I spectral change with low-spin CYP2B4, which underwent transformation to a typical type I optical perturbation upon deletion of the signal anchor sequence (Delta2-27) of the hemoprotein. Truncation of CYP2B4 slightly fostered electron transfer from Adx, but was deleterious to reduction of the engineered isozyme by P450R. Addition of manganese-substituted cytochrome b5, which failed to serve as an electron donor to CYP2B4, augmented the amount of hemoprotein existing in form of a low-spin complex with Adx and affected the ferredoxin-dependent reduction kinetics through causing a proportional rise in both Km and Vmax. Conservative replacement of Asp-76 with glutamate in the Adx molecule was associated with a drastic drop in reductive efficiency toward CYP2B4, while spectral binding of the mutant to the hemoprotein was marginally changed. The results support the concept of an evolutionary relationship between the various cytochrome P450 forms as regards the conservation of surface regions participating in contacts with heterologous donor proteins.

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.

Potent and selective aromatase inhibitor: in vitro and in vivo studies with s-triazine derivative SEF19

We found a potent aromatase inhibitor through the screening of agents for estrogen-dependent breast cancer. SEF19 (2-(imidazol-1-yl)-4,6-dimorphorino-1,3,5-triazine) decreased 50% of human placental aromatase activity in vitro at the concentration of 5.3 nM. In order to clarify the selectivity of SEF19 for enzyme inhibition, we determined the effect of SEF19 on the activities of four steroidogenic cytochrome P450 enzymes in porcine adrenal gland, P450SCC(side-chain cleavage of cholesterol), P450(11 beta) (11 beta-hydroxylase), P450(17 alpha)(17 alpha-hydroxylase/C17,20 lyase) and P450C21 (21-hydroxylase). SEF19 failed to inhibit the activities of porcine adrenal P450SCC, P450(17 alpha) and P450C21 up to the concentration of 100 microM and showed some inhibition on P450(11 beta) activity at 100 microM, while SEF19 completely nullified the aromatase activity at 1 microM. We also determined the potency of SEF19 for the suppression of aromatase activity in vivo. SEF19 suppressed dose-dependently the uterine hypertrophy of immature rats caused by administration of androstenedione (30 mg/kg, s.c.). The ED50 of SEF19 for the suppression of uterine hypertrophy was 0.8 mumol/kg. These results suggest that SEF19 may serve as a potent and selective agent for the treatment of estrogen-dependent breast cancer.

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.

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.

Cytochrome P-450scc: enzymology, and the regulation of intramitochondrial cholesterol delivery to the enzyme

The mechanism and properties of the adrenal cortex enzyme system which catalyzes the side chain cleavage of cholesterol to form pregnenolone are summarized. Cytochrome P-450scc, an integral inner mitochondrial membrane protein, interacts with its electron donor adrenodoxin via an aqueous-exposed (matrix side) site, and with its substrate cholesterol via an active site in communication with the hydrophobic phospholipid milieu. In a purified, phospholipid vesicle-reconstituted system, membrane-dissolved cholesterol interacts rapidly with and can be readily metabolized by the membrane-associated cytochrome, and thus represents a readily accessible cholesterol pool. Evidence for a rapidly metabolizable mitochondrial substrate pool (presumably that in the inner mitochondrial membrane) and the regulation by ACTH of cholesterol movement from other site(s) (presumably the outer mitochondrial membrane) into the reactive pool is reviewed; additional evidence is provided which supports the idea that the outer mitochondrial membrane/intermembrane space provides the rate-limiting block to cholesterol utilization. Possible mechanisms by which ACTH might regulate intramitochondrial cholesterol movement are discussed. ACTH has been found to regulate intramitochondrial aqueous volumes (both the matrix and the intermembrane space) in a cycloheximide-inhibitable manner, and it is proposed that these volume changes reflect an altered relationship of outer and inner membranes which may promote movement of cholesterol.

Steroidogenic electron transport in adrenal cortex mitochondria

The flavoprotein NADPH-adrenodoxin reductase and the iron sulfur protein adrenodoxin function as a short electron transport chain which donates electrons one-at-a-time to adrenal cortex mitochondrial cytochromes P-450. The soluble adrenodoxin acts as a mobile one-electron shuttle, forming a complex first with NADPH-reduced adrenodoxin reductase from which it accepts an electron, then dissociating, and finally reassociating with and donating an electron to the membrane-bound cytochrome P-450 (Fig. 9). Dissociation and reassociation with flavoprotein then allows a second cycle of electron transfers. A complex set of factors govern the sequential protein-protein interactions which comprise this adrenodoxin shuttle mechanism; among these factors, reduction of the iron sulfur center by the flavin weakens the adrenodoxin-adrenodoxin reductase interaction, thus promoting dissociation of this complex to yield free reduced adrenodoxin. Substrate (cholesterol) binding to cytochrome P-450scc both promotes the binding of the free adrenodoxin to the cytochrome, and alters the oxidation-reduction potential of the heme so as to favor reduction by adrenodoxin. The cholesterol binding site on cytochrome P-450scc appears to be in direct communication with the hydrophobic phospholipid milieu in which this substrate is dissolved. Specific effects of both phospholipid headgroups and fatty acyl side-chains regulate the interaction of cholesterol with its binding side. Cardiolipin is an extremely potent positive effector for cholesterol binding, and evidence supports the existence of a specific effector lipid binding site on cytochrome P.450scc to which this phospholipid binds.

Characterization of cytochrome P-450scc-containing liposomes

Purified cytochrome P450scc from bovine adrenocortical mitochondria was incorporated into liposomes by the cholate-dilution method utilizing either dialysis or Sephadex gel filtration. Among synthetic phospholipids tested, dioleoylglycerophosphocholine showed the best stability during the incorporation of P450scc into liposomes. A maximum amount of heme was incorporated into liposomes at a molar ratio of phospholipid to the cytochrome of approx. 200. When P450scc was incorporated into the dioleoylglycerophosphocholine liposomes by the cholate-filtration method, the P450scc-containing liposomes showed two major populations on the elution pattern of the Sepharose 4B gel filtration, and were seen at a diameter of 200-600 A and its aggregated forms. When the cytochrome was incorporated into dioleoylglycerophosphocholine liposomes or cholesterol-free adrenocortical mitochondrial liposomes, P450scc was less stable than P450scc in aqueous solution. Cholesterol or adrenodoxin markedly stabilized the liposomal P450scc. Liposomal P450scc required cholesterol for its optimum reduction with adrenodoxin, adrenodoxin reductase, and NADPH in the presence of CO. About 70% of the total heme in the dioleoylglycerophosphocholine liposomes was reduced by the enzymatic reduction in the presence of cholesterol, indicating that 70% of the total molecules are exposed to the surface of the outer monolayer. In order to see the location of the heme in membrane, the dioleoylglycerophosphocholine-liposomal P450scc was subjected to p-chloromercuriphenyl sulfonic acid treatment. This reagent destroyed the liposomal P450scc. These results suggest that the heme is located in the proximity of the p-chloromercuriphenyl sulfonic acid reacting sites which are exposed to the surface, or located on the vincinity of polar heads of the membrane.

Cytochrome P-450 from mitochondria of bovine adrenal cortex: comparison of cholesterol side-chain cleavage P-450 with steroid 11beta-hydroxylation P-450 and immunochemical cross-reactivity between adrenal mitochondrial and liver microsomal cytochromes P-450

Adrenocortical mitochondrial cytochrome P-450 specific to the cholesterol side-chain cleavage (desmolase) reaction differs from that for the 11beta-hydroxylation reaction of deoxycorticosterone. The former cytochrome appears to be more loosely bound to the inner membrane than the latter. Upon ageing at 0 degrees C or by aerobic treatment with ferrous ions, the desmolase P-450 was more stable than the 11beta-hydroxylase P-450. By utilizing artificial hydroxylating agents such as cumene hydroperoxide, H2O2, and sodium periodate, the hydroxylation reaction of deoxycorticosterone to corticosterone in the absence of NADPH was observed to a comparable extent with the reaction in the presence of adrenodoxin reductase, adrenodoxin and NADPH. However, the hydroxylation reaction of cholesterol to pregnenolone was not supported by these artificial agents. Immunochemical cross-reactivity of bovine adrenal desmolase P-450 with rabbit liver microsomal P-450LM4 was also investigated. We found a weak but significant cross-reactivity between the adrenal mitochondrial P-450 and liver microsomal P-450LM4, indicating to some extent a homology between adrenal and liver cytochromes P-450.

Studies on nitrotyrosine-82 and aminotyrosine-82 derivatives of adrenodoxin. Effects of chemical modification on the complex formation with adrenodoxin reductase

The coordination structure of the iron-sulfur center of the nitrotyrosine and the aminotyrosine derivates of bovine adrenodoxin was investigated by electron paramagnetic resonance spectroscopy. The reduced form of both modified samples exhibited signals identical with those for the native protein at g= 1.94 and g=2.01. From these results together with optical absorption and chemical analyses, it was concluded that the coordination structure of the iron-sulfur chromophore for both the derivatives was identical with the binuclear tetrahedral structure of native adrenodoxin. The configuration of the iron-binding area in nitro- and amino-adrenodoxin was studied by ovserving the circular dichroism spectra between 350 and 600 nm. The maxima for the nitro or amino derivatives were all identical with those for the native protein but different in the magnitude of their molar ellipticity. The molar ellipticities at 440 nm were 45.8 X 10(3), 14.5 X 10(3), and 9.5 X 10(6) deg cm2 per mol of iron for native adrenodoxin, nitro or amino derivative, respectively. These results suggest that the chemical modification of the tyrosine residue causes a conformational change in the iron-binding area. We have previously reported that the enzymatic activities of these reconstituted nitro and amino derivatives toware cytochrome c reduction in the presence of adrenodoxin reductase and reduced nicotinamide adenine dinucleotide phosphate were 19 and 7% of native adrenodoxin, respectively. The cytochrome c reductase activities of nitro- and aminoadrenodixin were drastically affected by the ionic strength of the assay medium, as found in native adrenodoxin. Fluorometric titration of the reductase with aminoadrenodoxin revealed that aminoadrenodoxin forms a 1:1 molar complex with the reductase. These results suggest that both the nitro and amino derivatives form a complex with the reductase. The dissociation constants of nitro- and aminoadrenodoxin for the reductase were 6.1 X 10(-7)M and 3.3 X 10(-7) M at mu = 0.04 and 1.9 X 10(-6) M and 2.0 X 10(-6) M at mu = 0.20, respectively. Comparison of these values with those of native adrenodoxin (approximately 10(-9) M at mu = 0.04 and 2.2 X 10(-7) M at mu = 0.20) suggests that an increase in the dissociation constant for the reductase is responsible for the decreased electron transferring activity of the modified adrenodoxins.

Studies on NO2-Tyr82and NH2-Tyr82 derivatives of adrenodoxin. Effects of chemical modification on electron transferring activity

Bovine apoadrenodoxin was treated with tetranitromethane to introduce a nitro group into the tyrosyl residue at position 82 of this protein. The degrees of nitration under the best conditions were estimated to be 90% and nearly 100% on the basis of amino acid analysis and the spectrophotometric method, respectively. An amino derivative was prepared by reducing the nitro group with sodium dithionite. The apoadrenodoxin derivatives could be reconstituted to have an iron-sulfur chromophore similar to the native adrenodoxin which contains a 1:1 molar ratio of labile sulfur to iron content and displays absorption peaks at 414 and 450 nm. The enzymatic acitivies of these reconstituted nitro and amino derivatives toward cytochrome c reduction in the presence of adrenodoxin reductase and NADPH were 19 and 7% of native adrenodoxin, respectively. We studied the kinetics of the direct reduction of the reconstituted amino derivative in the presence of NADPH and adrenodoxin reductase under anaerobic conditons. The initial rate of reduction for the amino derivative was 7% of the native adrenodoxin, which is in good agreement with its activity toward cytochrome c reduction. From these results, it is concluded that by modifying the tyrosyl residue at position 82 of the adrenodoxin polypeptide, the electron-transferring activity of the molecule is largely diminished.