Kinetic and optical biosensor study of adrenodoxin mutant AdxS112W displaying an enhanced interaction towards the cholesterol side chain cleavage enzyme (CYP11A1)

Interactions of human mitochondrial Ferredoxin 1 (Adrenodoxin) by NMR; modulation by cytochrome P450 substrate and by truncation of the C-terminal tail

Human Ferredoxin 1, also referred to as Adrenodoxin (Adx), is the sole electron carrier supporting the function of all seven mitochondrial cytochrome P450 (CYP) enzymes. Adx utilizes conserved negatively charged residues along its α-helix3 to interact with either the proximal surface of CYP enzymes or the binding surface of Adrendodoxin Reductase (AdR). However, in the oxidized state, Adx assumes a monomer-homodimer equilibrium that requires the presence of its unstructured C-terminal tail. Crystallographic structures of full-length human Adx dimers indicate that part of the binding surface necessary for its interactions with CYPs or with AdR is partially occluded by the dimer interface. In this study, protein NMR spectroscopy was used to interrogate the interactions between full-length (2-124) or truncated monomeric (2-108) human Adx and human CYP24A1 (with and without its vitamin-D substrate) as well as interactions with AdR. Here, monomeric Adx induced a similar pattern of peak broadening as that induced by addition of CYP24A1 substrate, consistent with a 1:1 Adx:CYP interaction as the functional complex. Additionally, removal of the C-terminal tail appears to enhance the interaction with AdR, despite removal of some of the AdR contacts in the tail region. This finding was also supported by an NMR competition assay. These findings suggest that the Adx dimers do not undergo meaningful interactions with either CYP or AdR, but may instead be responsible for regulating access to monomeric Adx. These conclusions are discussed in the context of a revised model of the Adx electron shuttle mechanism.

Underestimated reactions and regulation patterns of adrenal cytochromes P450

Although cytochrome P450 (CYP) systems including the adrenal ones are being investigated since many years, there are still reactions and regulation patterns that have been underestimated ever since. This review discusses neglected ones to bring them into the focus of investigators working in the field. Novel substrates and reactions described for adrenal CYPs recently point to the fact that different from what has been believed for many years, adrenal CYPs are less selective than previously thought. The conversion of steroid sulfates, intermediates of steroid biosynthesis as well as of exogenous compounds are being discussed here in more detail and consequences for further studies are drawn. Furthermore, it was shown that protein-protein interactions may have an important effect not only on the activity of adrenal CYPs, but also on the product pattern of the reactions. It was found that, as expected, the stoichiometry of CYP:redox partner plays an important role for tuning the activity. In addition, competition between different CYPs for the redox partner and for electrons and possible alterations by mutants in the efficiency of electron transfer play an important role for the activity and product pattern. Moreover, the influence of phosphorylation and small charged molecules like natural polyamines on the activity of adrenal systems has been demonstrated in-vitro indicating a possible regulation of adrenal CYP reactions by affecting redox partner recognition and binding affinity. Finally, an effect of the genetic background on the consequences of mutations in adrenal CYPs found in patients was suggested from corresponding in-vitro studies indicating that a different genetic background might be able to significantly affect the activity of a CYP mutant.

Effect of sulfonated steroids on steroidogenic cytochrome P450-dependent steroid hydroxylases

In the last decades, sulfonated steroids evolved from inactive metabolites intended for excretion to highly relevant compounds involved in many physiological processes. Investigations of the impact of sulfonated steroids on the steroid hormone biosynthesis revealed that, on the one hand, these can serve as substrate for steroidogenic cytochromes P450 and, on the other hand, these are able to influence the catalytic properties of these enzymes. In this review the relevance of sulfonated steroids for the steroid hormone biosynthesis will be discussed.

Raman and infrared spectroscopic evidence for the structural changes of the 2Fe2S cluster and its environment during the interaction of adrenodoxin and adrenodoxin reductase

Many biological functions involve the formation of protein-protein complexes. In the present study, we investigated the interaction of two proteins involved in electron transfer, adrenodoxin (Adx) and adrenodoxin reductase (AdR) by using Raman and infrared spectroscopies. Different shifts and splittings of the FeS_b/t stretching vibrational modes upon interaction of the two proteins can be reported pointing towards major structural changes in the [2Fe2S] cluster. These changes may be necessary for optimizing electron transfer. The assignment of the shifted modes to the [2Fe2S] cluster was confirmed by ⁵⁴Fe labeling of the truncated Adx (4-108) as well as the investigation of mutants close to the interaction site and in the vicinity of the [2Fe2S] cluster. Electrochemically induced FTIR difference spectra revealed that the flavin cofactor in AdR also changes due to the interaction with [2Fe2S] cluster in the Adx/AdR electron transfer complex.