Crystallization and X-ray examination of bovine adrenodoxin

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.

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

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

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.

Evidence for oxidation-state-dependent conformational changes in human ferredoxin from multinuclear, multidimensional NMR spectroscopy

Human ferredoxin belongs to the vertebrate ferredoxin family which includes bovine adrenodoxin. It is a small (13.8 kDa) acidic protein with a [2Fe-2S] cluster. It functions as an electron shuttle in the cholesterol side-chain cleavage reaction which is the first step of steroid hormone biosynthesis. The protein studied here was produced in Escherichia coli and doubly labeled with 13C and 15N. The diamagnetic 15N, 13C', 13C alpha, 13C beta, 1H alpha, and 1HN resonances from about 70% of the 124 amino acid residues for oxidized human ferredoxin and 80% of those for the reduced protein have been assigned primarily on the basis of results from three-dimensional, triple-resonance experiments. Secondary structure features for human ferredoxin in its oxidized and reduced states have been identified from a combination of chemical shift index and NOE data. Comparison of NMR results from the protein in its oxidized and reduced states indicates that structural changes accompany the change in the oxidation state of the [2Fe-2S] cluster. Major differences are localized at two regions: residues 29-31 and residues 109-124; the latter stretch forms the C-terminal region of the protein. The possible functional significance of these oxidation-state-dependent structural changes is discussed.

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.

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.

Purification and characterization of rat adrenodoxin

Adrenodoxin was purified from the rat adrenal gland. The A414/A280 value of the purified rat adrenodoxin was 0.90 and the oxidized spectrum showed absorption maxima at 320, 414 and 455 nm, similar to those of bovine adrenodoxin. On SDS-PAGE, the rat adrenodoxin showed a single band with a molecular mass of 11.2 kDa, while the apparent molecular mass by gel filtration through Sephadex G-75 equilibrated with 10 mM K-phosphate (pH 7.5) was 27 kDa. In the reconstituted system, Vmax of NADPH-cytochrome c reduction activity and the Km for the rat adrenodoxin were much the same as those for recombinant bovine adrenodoxin. In the case of cholesterol side-chain cleavage activity, however, these values of the rat adrenodoxin were about half of those of the bovine adrenodoxin. The CD spectrum of the rat adrenodoxin was similar to that of the bovine adrenodoxin but showed a significantly lower ellipticity value in the 195-205 nm region than that of the bovine adrenodoxin. The structural differences may possibly explain differences in the enzymic properties between rat and bovine adrenodoxins.

Genetic diseases of steroid metabolism

All major classes of biologically active steroid hormones (progestins, mineralocorticoids, glucocorticoids, and sex steroids) are synthesized from cholesterol through 11 different bioconversions. With the exception of 5 alpha-reductase, all the enzymes mediating these reactions fall into two classes, cytochromes P450 and short-chain dehydrogenases. Cytochromes P450 are heme-containing membrane-bound proteins with molecular weights of approximately 50,000 that utilize molecular oxygen and electrons from NADPH-dependent accessory proteins to hydroxylate substrates. Short-chain dehydrogenases have molecular weights of 30,000-40,000, have tyrosine and lysine residues at the active site, and remove a hydride from the substrate, transferring the electrons of the hydride to NAD+ or NADP+. In most cases, this reaction is reversible so that the dehydrogenase can also function as a reductase under appropriate conditions. Inherited disorders in enzymes required for steroid biosynthesis have varying effects. Defects that prevent cortisol from being synthesized are referred to collectively as congenital adrenal hyperplasia. Because the enzymes required for cortisol biosynthesis in the adrenal cortex are in many cases required for the synthesis of mineralocorticoids and/or sex steroids, these classes of steroids may also not be synthesized normally. Thus, cholesterol desmolase and 3 beta-hydroxysteroid dehydrogenase deficiencies affect synthesis of all classes of steroids in both the adrenals and gonads. Steroid 21-hydroxylase deficiency, the most common cause (> 90% of cases) of congenital adrenal hyperplasia, can affect both mineralocorticoid and glucocorticoid synthesis, but androgen secretion is usually abnormally high due to shunting of accumulated precursors into this pathway. Excessive secretion of androgens and mineralocorticoids occurs in 11 beta-hydroxylase deficiency (the second most frequent form of congenital adrenal hyperplasia). Mineralocorticoid excess is also seen in 17 alpha-hydroxylase deficiency, but in this disorder sex steroid synthesis is defective. All defects that affect estrogen synthesis (deficiencies of cholesterol desmolase, 3 beta-hydroxysteroid dehydrogenase, 17 alpha-hydroxylase, aromatase, and 17 beta-hydroxysteroid dehydrogenase) are very rare, suggesting that the inability to synthesize placental estrogens may adversely affect fetal survival. A number of enzymes are expressed at sites of steroid action and regulate the amount of active steroid available to steroid receptors. Steroid 5 alpha-reductase converts testosterone to the more active dihydrotestosterone. Deficiency of this activity leads to incomplete development of male genitalia; 17 beta-hydroxysteroid dehydrogenase deficiency has similar phenotypic effects.(ABSTRACT TRUNCATED AT 400 WORDS)