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

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.

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.

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.

Chemical modification of adrenocortical cytochrome P-450scc with tetranitromethane

Selective chemical modification of adrenocortical cytochrome P-450scc, responsible for key stages of steroid biogenesis, with tetranitromethane has been carried out. Nitration of the cytochrome P-450scc tyrosine residues results in heme protein inactivation with syncatalytic loss of enzyme activity. Analysis of the cytochrome P-450scc inactivation kinetics indicates that there are several pools of tyrosine residues, differing in their accessibility to tetranitromethane. The modification of cytochrome P-450scc results in changes in the hemeprotein spectral properties and its conformation which indicates to the involvement of essential tyrosine residue(s) in the heme-protein interaction. Cholesterol and adrenodoxin (high-spin effectors) prevent the inactivation of cytochrome P-450scc with tetranitromethane, i.e., protect the essential tyrosine residue(s) from modification. Possible functions of the tyrosine residues in the cytochrome P-450scc molecule are discussed.

The role of the sugar regions of components of the cytochrome P-450-linked mixed-function oxidase (monooxygenase) system of bovine adrenocortical mitochondria

Cytochrome P-450scc from bovine adrenocortical mitochondria was found to be a glycoprotein, similar to NADPH-adrenodoxin reductase. The role of sugar moieties of components of the cytochrome P-450-linked mixed-function oxidase system was investigated. The NADPH-ferricyanide reductase activity of NADPH-adrenodoxin reductase was not affected by neuraminidase treatment, but NADPH-cytochrome c reductase activity of NADPH-adrenodoxin reductase and adrenodoxin was lost. Cytochrome P-450scc treated with neuraminidase could not be reduced by NADPH-adrenodoxin reductase and adrenodoxin, and the side-chain cleavage activity of cholesterol by the cytochrome P-450-linked mixed-function oxidase system was lost by neuraminidase treatment of cytochrome P-450scc or NADPH-adrenodoxin reductase or both. These results indicate that the sugar moieties of the components of the cytochrome P-450-linked mixed-function oxidase system are necessary for binding and attaching of the components and electron transport.

ACTH stimulation on cholesterol side chain cleavage activity of adrenocortical mitochondria. Transfer of the stimulus from plasma membrane to mitochondria

Adrenocortical mitochondrial cholesterol side chain cleavage reactions are regulated by the influence of pituitary ACTH. The mechanism of the stimulation involves adenyl cyclase, cAMP-dependent protein kinase, cholesterol esterase, and ribosomal labile protein synthesis. Through these reactions the stimulus reaches the mitochondrial side chain cleavage enzyme system. In this review article, the current implications on the stimulus transfer from the plasma membrane to the mitochondrial inner membrane are summarized. In particular the availability of cholesterol to P-450scc was discussed in terms of the distribution of cholesterol molecules in the membranes.