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We have directly imaged n-alkane layers adsorbed at the liquid/graphite interface using a scanning tunneling microscope. The layers possessed a high degree of two-dimensional ordering. The adsorbate was observed to enhance the tunneling current, and the atomic structure of the images was dominated by features associated with the substrate. These systems are excellent vehicles for studies concerning the imaging mechanism of adsorbed organic layers because of their stability and simplicity.

Mammalian adrenodoxin (ferredoxin 1; Fdx1) is essential for the synthesis of various steroid hormones in adrenal glands. As a member of the [2Fe-2S] cluster-containing ferredoxin family, Fdx1 reduces mitochondrial cytochrome P450 enzymes, which then catalyze; e.g., the conversion of cholesterol to pregnenolone, aldosterone, and cortisol. The high protein sequence similarity between Fdx1 and its yeast adrenodoxin homologue (Yah1) suggested that Fdx1, like Yah1, may be involved in the biosynthesis of heme A and Fe/S clusters, two versatile and essential protein cofactors. Our study, employing RNAi technology to deplete human Fdx1, did not confirm this expectation. Instead, we identified a Fdx1-related mitochondrial protein, designated ferredoxin 2 (Fdx2) and found it to be essential for heme A and Fe/S protein biosynthesis. Unlike Fdx1, Fdx2 was unable to efficiently reduce mitochondrial cytochromes P450 and convert steroids, indicating that the two ferredoxin isoforms are highly specific for their substrates in distinct biochemical pathways. Moreover, Fdx2 deficiency had a severe impact, via impaired Fe/S protein biogenesis, on cellular iron homeostasis, leading to increased cellular iron uptake and iron accumulation in mitochondria. We conclude that mammals depend on two distinct mitochondrial ferredoxins for the specific production of either steroid hormones or heme A and Fe/S proteins.adrenodoxin | cytochrome P450 | iron | iron-sulfur cluster | IRP1

Cytochromes P450 (CYPs) belong to the superfamily of heme b containing monooxygenases with currently more than 21,000 members. These enzymes accept a vast range of organic molecules and catalyze diverse reactions. These extraordinary capabilities of CYP systems that are unmet by other enzymes make them attractive for biotechnology. However, the complexity of these systems due to the need of electron transfer from nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) via redox partner proteins for the initial hydroxylation step limits a broader technical implementation of CYP enzymes. There have been several reviews during the past years tackling the potential CYPs for synthetic application. The aim of this review is to give a critical overview about possibilities and chances for application of these interesting catalysts as well as to discuss drawbacks and problems related to their use. Solutions to overcome these limitations will be demonstrated, and several selected examples of successful CYP applications under industrial conditions will be reviewed.

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.

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.

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