Electrochemical characterization of mutant forms of rubredoxin B from Mycobacterium tuberculosis

Electron transfer in metalloproteins is a driving force for many biological processes and widely distributed in nature. Rubredoxin B (RubB) from Mycobacterium tuberculosis is a first example among [1Fe-0S] proteins that support catalytic activity of terminal sterol-monooxygenases enabling its application in metabolic engineering. To explore the tolerance of RubB to the specific amino acid changes we evaluated the effect of surface mutations on its electrochemical properties. Based on the RubB fold we also designed the mutant with a putative additional site for protein-protein interactions to further evaluate electron transfer and electrochemical properties. The investigation of redox properties of mutant variants of RubB was done using screen-printed graphite electrodes (SPEs) modified with stable dispersion of multi-walled carbon nanotubes (MWCNTs). The redox potentials (midpoint potentials, E^0Ꞌ) of mutants did not significantly differ from the wild type protein and vary in the range of -264 to -231 mV vs. Ag/AgCl electrode. However, all mutations affect electron transfer rate between the protein and electrode. Notably, the modulation of the protein-protein interactions was observed for the insertion mutant suggesting the possibility of tailoring of rubredoxin for the selected redox-partner. Overall, RubB is tolerant to the significant modifications in its structure enabling rational engineering of novel redox proteins.

A new twist of rubredoxin function in M. tuberculosis

Electron transfer mediated by metalloproteins drives many biological processes. Rubredoxins are a ubiquitous [1Fe-0S] class of electron carriers that play an important role in bacterial adaptation to changing environmental conditions. In Mycobacterium tuberculosis, oxidative and acidic stresses as well as iron starvation induce rubredoxins expression. However, their functions during M. tuberculosis infection are unknown. In the present work, we show that rubredoxin B (RubB) is able to efficiently shuttle electrons from cognate reductases, FprA and FdR to support catalytic activity of cytochrome P450s, CYP124, CYP125, and CYP142, which are important for bacterial viability and pathogenicity. We solved the crystal structure of RubB and characterized the interaction between RubB and CYPs using site-directed mutagenesis. Mutations that not only neutralize single charge but also change the specific residues on the surface of RubB did not dramatically decrease activity of studied CYPs. Together with isothermal calorimetry (ITC) experiments, the obtained results suggest that interactions are transient and not highly specific. The redox potential of RubB is -264 mV vs. Ag/AgCl and the measured extinction coefficients are 9931 M^-1cm^-1 and 8371 M^-1cm^-1 at 380 nm and 490 nm, respectively. Characteristic parameters of RubB along with the discovered function might be useful for biotechnological applications. Our findings suggest that a switch from ferredoxins to rubredoxins might be crucial for M. tuberculosis to support CYPs activity during the infection.

Estimation of the inhibiting impact of abiraterone D4A metabolite on human steroid 21-monooxygenase (CYP21A2)

Abiraterone D4A metabolite, the product of 3β-hydroxysteroid dehydrogenase activity toward abiraterone, may serve as a potential antitumor agent for the treatment of prostate cancer. The main adverse effect of abiraterone is the disruption of corticosteroid biosynthesis, and the more pharmacologically active abiraterone D4A metabolite may have the same issues. We therefore estimated the inhibiting impact of the abiraterone D4A metabolite on one of the key corticosteroidogenic enzymes - human steroid 21-monooxygenase (CYP21A2). Molecular docking of D4A into the active site of CYP21A2 has been predicted to be similar to abiraterone binding with the enzyme. Abiraterone D4A metabolite, similar to abiraterone, induces type II spectral changes of CYP21A2. The spectral dissociation constant for the abiraterone D4A metabolite-CYP21A2 complex was calculated as 3.4 ± 0.5 μM. Abiraterone D4A metabolite demonstrates competitive/mixed type CYP21A2 inhibition with an inhibitory constant of 1.8 ± 0.8 μM, as obtained by Dixon plot. These results make it possible to predict the adverse effects of the new perspective candidate compound for antitumor therapy.