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. Proteins 2000;40:590–612. © 2000 Wiley‐Liss, Inc.

No abstract

Mitochondrial cytochrome P450 enzymes play a crucial role in the steroid biosynthesis in human adrenals, catalyzing regio- and stereospecific hydroxylations. In search of a new model system for the study of these enzymes, we expressed the human CYP11B2 (aldosterone synthase, P450(aldo)) in fission yeast Schizosaccharomyces pombe. Analysis of the subcellular localization of the P450 enzyme by Western blot analysis, fluorescence microscopy, and electron microscopy demonstrated that the mitochondrial localization signal of the human protein is functional in S. pombe. The transformed yeasts show the inducible ability to convert in vivo considerable amounts of 11-deoxycortisol to cortisol and 11-deoxycorticosterone to corticosterone, 18-hydroxycorticosterone, and aldosterone, respectively. Although in mammalian cells, mitochondrial steroid hydroxylases depend for their activity on an electron transport chain that consists of two proteins, adrenodoxin and adrenodoxin reductase, no coexpression of these proteins is needed for efficient substrate conversion by intact fission yeast cells. Searching the fission yeast genome for adrenodoxin homologues, a gene was identified that codes for a protein with an amino terminal domain homologous to COX15 of Saccharomyces cerevisiae and a carboxy terminal ferredoxin domain. It was found that overexpression of this gene significantly enhances steroid hydroxylase activity of CYP11B2 expressing fission yeast cells. Moreover, the bacterially expressed ferredoxin domain of this protein can replace adrenodoxin in a reconstituted steroid hydroxylation assay and transfer electrons from adrenodoxin reductase to a mammalian or a bacterial cytochrome P450. Therefore, we suggest to name this protein etp1 (electron-transfer protein 1).

The origin of steroid hormones in mammals is cholesterol that is metabolized by the mitochondrial CYP11A1 system. The cytochrome P450 is fed with reduction equivalents via a small electron transfer chain consisting of NADPH, adrenodoxin reductase, and adrenodoxin. Though the redox behavior of the individual protein components has been studied previously, the kinetics of the system in its entirety has not yet been analyzed. In this study we combine surface plasmon resonance experiments to determine the binding constants for the different pairs of redox partners with measurements of the pre-steady-state kinetics of the different reaction steps of this system and steady-state kinetics. We could correlate the individual protein-protein interactions with the effect of distinct reductionoxidation steps on the overall catalytic activity of the CYP11A1 system. For the first time, we were able to follow the reduction of each of the protein components of this system within one measurement when we mixed all oxidized protein components with NADPH. These measurements allowed the determination of the individual apparent rate constants for the reduction of all three proteins involved. In addition, variation of the ionic strength in these experiments revealed different optimum salt concentrations for the reduction of adrenodoxin reductase and adrenodoxin, respectively, and unraveled dramatically changing reduction rates of CYP11A1 by adrenodoxin.The mitochondrial steroid hydroxylase CYP11A1 1 belongs to the superfamily of cytochrome P450 enzymes. It catalyzes the rate-limiting step in the biogenesis of steroid hormones in adrenal mitochondria (1, 2). In this reaction, the side chain of the substrate cholesterol gets cleaved off to yield pregnenolone and isocaproic aldehyde at the expense of three molecules of NADPH and three molecules of oxygen (3). The reduction equivalents, provided by NADPH, are transferred to a FAD containing adrenodoxin reductase (AdR), which transfers electrons to the one electron carrier adrenodoxin (Adx), the ferredoxin of the adrenal gland (4). The [2Fe-2S] cluster-containing Adx subsequently transfers electrons, one at a time, to the heme iron of CYP11A1. These electrons are necessary for the reductive activation of oxygen and finally to hydroxylate the substrate. For a complete conversion of cholesterol to pregnenolone, six electrons have to be supplied.About 40 years ago the group of Kimura (5-8) started to characterize single components of the system followed by a multitude of investigations concerning the adrenal steroid hydroxylase system and its different components by various groups (9 -18). During the last decade much work has been done to get a better understanding of the kinetics of this system (4, 19 -23). Nevertheless, the nature of the rate-limiting step in this system is still a matter of controversial discussion and remains to be elucidated.For the first time we were able to follow the reduction of all three protein components of the CYP11A1 system within one measurement using rapid mix...

No abstract