Evidence for the cluster model of mitochondrial steroid hydroxylase system derived from dissociation constants of the complex between adrenodoxin reductase and adrenodoxin

Mechanism of steroidogenic electron transport: role of conserved Glu429 in destabilization of CYP11A1-adrenodoxin complex

In the present work the role of conserved residue E429 of cytochrome P45011A1 has been studied. The charge neutralization of E429Q results in 3-fold decrease of K(d) as well as V(max) compared to the wild type hemoprotein indicating tighter binding and, as the result, the impaired dissociation of oxidized adrenodoxin from the complex. As cytochrome P45011A1-adrenodoxin complex formation is driven primarily by electrostatic interactions, the low activity of E429Q mutant is completely restored to that of wild type hemoprotein by increasing of ionic strength. The charge neutralization of the corresponding residue of rat cytochrome P45011B2 has the same effect: the activity is 10-fold decreased but it is restored by increasing of ionic strength without effect on the ratio of products formed. Thus, this is the first report on identification of residues involved in modulation of dissociation of redox partner from the complex with cytochrome P450s.

Self-association of adrenodoxin studied by using analytical ultracentrifugation

The mitochondrial steroid hydroxylase system of vertebrates utilizes adrenodoxin (Adx), a small iron-sulfur cluster protein of about 14 kDa as an electron carrier between a reductase and cytochrome P450. Although the crystal structure of this protein has been elucidated, the solution structure of Adx was discussed contrary in the literature [I.A. Pikuleva, K. Tesh, M.R. Waterman, Y. Kim, The tertiary structure of full-length bovine adrenodoxin suggests functional dimers, Arch. Biochem. Biophys. 373 (2000) 44-55; D. Beilke, R. Weiss, F. Löhr, P. Pristovsek, F. Hannemann, R. Bernhardt, H. Rüterjans, A new electron mechanism in mitochondrial steroid hydroxylase systems based on structural changes upon the reduction of adrenodoxin, Biochemistry 41 (2002) 7969-7978]. Therefore, it was necessary to study the self-association of this protein by using analytical ultracentrifugation over a larger concentration range. As could be demonstrated in sedimentation velocity experiments, as well as sedimentation equilibrium runs with explicit consideration of thermodynamic non-ideality, the full-length protein (residues 1-128) in the oxidized state resulted in a monomer-dimer equilibrium (K(a) approximately 3 x 10(2) M(-1)). For truncated Adx (1-108), as well as the reduced Adx, the association behavior was strongly reduced. The consequences of this behavior are discussed with respect to the physiological meaning for the Adx system.

Site-directed mutations (Asp405Ile and Glu124Ile) in cytochrome P450scc: effect on adrenodoxin binding

Cytochrome P450scc, mitochondrial adrenodoxin (Adx), and adrenodoxin reductase (AdR) are an essential components in a steroid hydroxylation system. In particular, mytochondrial cytochrome P450scc enzyme catalyses the first step in steroid hormones biosynthesis, represented by the conversion of cholesterol to pregnenolone. In order to study the effect of single mutations on the Adx binding a model of bovine cytochrome P450scc, previously optimized by molecular modeling, was utilized. It was hypothesized by molecular docking that two residues (Asp405 and Glu124) are involved in Adx binding. By site-directed mutagenesis, two mutants of cytochrome P450scc (Asp405Ile and Glu124Ile) expressed in Escherichia coli, were realized by replacing with isoleucines. The site-directed mutations effect on Adx binding was evaluated by differential spectral titration. The apparent dissociation constant values for Asp405Ile and Glu124Ile cytochrome P450scc show that the mutated residues seem to be at the interaction domain with Adx or at least close to it, as predicted by molecular modeling study. Finally, the engineered enzymes were characterized by biochemical and biophysical techniques such as circular dichroism (CD), UV/Vis spectroscopy, and electrochemical analysis.

Computational, spectroscopic, and resonant mirror biosensor analysis of the interaction of adrenodoxin with native and tryptophan-modified NADPH-adrenodoxin reductase

In steroid hydroxylation system in adrenal cortex mitochondria, NADPH-adrenodoxin reductase (AR) and adrenodoxin (Adx) form a short electron-transport chain that transfers electrons from NADPH to cytochromes P-450 through FAD in AR and [2Fe-2S] cluster in Adx. The formation of [AR/Adx] complex is essential for the electron transfer mechanism in which previous studies suggested that AR tryptophan (Trp) residue(s) might be implicated. In this study, we modified AR Trps by N-bromosuccinimide (NBS) and studied AR binding to Adx by a resonant mirror biosensor. Chemical modification of tryptophans caused inhibition of electron transport. The modified protein (AR*) retained the native secondary structure but showed a lower affinity towards Adx with respect to AR. Activity measurements and fluorescence data indicated that one Trp residue of AR may be involved in the electron transferring activity of the protein. Computational analysis of AR and [AR/Adx] complex structures suggested that Trp193 and Trp420 are the residues with the highest probability to undergo NBS-modification. In particular, the modification of Trp420 hampers the correct reorientation of AR* molecule necessary to form the native [AR/Adx] complex that is catalytically essential for electron transfer from FAD in AR to [2Fe-2S] cluster in Adx. The data support an incorrect assembly of [AR*/Adx] complex as the cause of electron transport inhibition.

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

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

Critical role of the plasma membrane for expression of mammalian mitochondrial side chain cleavage activity in yeast

Engineered yeast cells efficiently convert ergosta-5-eneol to pregnenolone and progesterone provided that endogenous pregnenolone acetylase activity is disrupted and that heterologous sterol delta7-reductase, cytochrome P450 side chain cleavage (CYP11A1) and 3beta hydroxysteroid dehydrogenase/isomerase (3beta-HSD) activities are present. CYP11A1 activity requires the expression of the mammalian NADPH-adrenodoxin reductase (Adrp) and adrenodoxin (Adxp) proteins as electron carriers. Several parameters modulate this artificial metabolic pathway: the effects of steroid products; the availability and delivery of the ergosta-5-eneol substrate to cytochrome P450; electron flux and protein localization. CYP11A1, Adxp and Adrp are usually located in contact with inner mitochondrial membranes and are directed to the outside of the mitochondria by the removal of their respective mitochondrial targeting sequences. CYP11A1 then localizes to the plasma membrane but Adrp and Adxp are detected in the endoplasmic reticulum and cytosol as expected. The electron transfer chain that involves several subcellular compartments may control side chain cleavage activity in yeast. Interestingly, Tgl1p, a potential ester hydrolase, was found to enhance steroid productivity, probably through both the availability and/or the trafficking of the CYP11A1 substrate. Thus, the observation that the highest cellular levels of free ergosta-5-eneol are found in the plasma membrane suggests that the substrate is freely available for pregnenolone synthesis.

CYP11A1 stimulates the hydroxylase activity of CYP11B1 in mitochondria of recombinant yeast in vivo and in vitro

In mammals, hydrocortisone synthesis from cholesterol is catalyzed by a set of five specialized enzymes, four of them belonging to the superfamily of cytochrome P-450 monooxygenases. A recombinant yeast expression system was recently developed for the CYP11B1 (P45011beta) enzyme, which performs the 11beta hydroxylation of steroids such as 11-deoxycortisol into hydrocortisone, one of the three mitochondrial cytochrome P-450 proteins involved in steroidogenesis in mammals. This heterologous system was used to test the potential interaction between CYP11B1 and CYP11A1 (P450scc), the mitochondrial cytochrome P-450 enzyme responsible for the side chain cleaving of cholesterol. Recombinant CYP11B1 and CYP11A1 were targeted to Saccharomyces cerevisiae mitochondria using the yeast cytochrome oxidase subunit 6 mitochondrial presequence fused to the mature form of the two proteins. In yeast, the presence of CYP11A1 appears to improve 11beta hydroxylase activity of CYP11B1 in vivo and in vitro. Fractionation experiments indicate the presence of the two proteins in the same membrane fractions, i.e. inner membrane and contact sites of mitochondria. Thus, yeast mitochondria provide interesting insights to study some molecular and cellular aspects of mammalian steroid synthesis. In particular, recombinant yeast should permit a better understanding of the mechanism permitting the synthesis of steroids (sex steroids, mineralocorticoids and glucocorticoids) with a minimal set of enzymes at physiological level, thus avoiding disease states.