Functional interactions of adrenodoxin with several human mitochondrial cytochrome P450 enzymes

The multistep oxidation of cholesterol to pregnenolone by human cytochrome P450 11A1 is highly processive

Cytochrome P450 (P450, CYP) 11A1 is the classical cholesterol side chain cleavage enzyme (P450scc) that removes six carbons of the side chain, the first and rate-limiting step in the synthesis of all mammalian steroids. The reaction is a 3-step, 6-electron oxidation that proceeds via formation of 22R-hydroxy (OH) and 20R,22R-(OH)2 cholesterol, yielding pregnenolone. We expressed human P450 11A1 in bacteria, purified the enzyme in the absence of nonionic detergents, and assayed pregnenolone formation by HPLC-mass spectrometry of the dansyl hydrazone. The reaction was inhibited by the nonionic detergent Tween 20, and several lipids did not enhance enzymatic activity. The 22R-OH and 20R,22R-(OH)2 cholesterol intermediates were bound to P450 11A1 relatively tightly, as judged by steady-state optical titrations and koff rates. The electron donor adrenodoxin had little effect on binding; the substrate cholesterol showed a ∼5-fold stimulatory effect on the binding of adrenodoxin to P450 11A1. Presteady-state single-turnover kinetic analysis was consistent with a highly processive reaction with rates of intermediate oxidation steps far exceeding dissociation rates for products and substrates. The presteady-state kinetic analysis revealed a second di-OH cholesterol product, separable by HPLC, in addition to 20R,22R-(OH)2 cholesterol, which we characterized as a rotamer that was also converted to pregnenolone at a similar rate. The first oxidation step (at C-22) is the slowest, limiting the overall rate of cleavage. d3-Cholesterol showed no kinetic deuterium isotope effect on C-22, indicating that C-H bond cleavage is not rate-limiting in the first hydroxylation step.

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.

Redox partner adrenodoxin alters cytochrome P450 11B1 ligand binding and inhibition

Human cytochrome P450 11B1 (CYP11B1) generation of the major glucocorticoid cortisol requires two electrons delivered sequentially by the iron‑sulfur protein adrenodoxin. While the expected adrenodoxin binding site is on the opposite side of the heme and 15-20 Å away, evidence is provided that adrenodoxin allosterically impacts CYP11B1 ligand binding and catalysis. The presence of adrenodoxin both decreases the dissociation constant (Kd) for substrate binding and increases the proportion of substrate that is bound at saturation. Adrenodoxin additionally decreases the Michaelis-Menten constant for the native substrate. Similar studies with several inhibitors also demonstrate the ability of adrenodoxin to modulate inhibition (IC50 values). Somewhat similar allosterism has recently been observed for the closely related CYP11B2/aldosterone synthase, but there are several marked differences in adrenodoxin effects on the two CYP11B enzymes. Comparison of the sequences and structures of these two CYP11B enzymes helps identify regions likely responsible for the functional differences. The allosteric effects of adrenodoxin on CYP11B enzymes underscore the importance of considering P450/redox partner interactions when evaluating new inhibitors.

Binding of cytochrome P450 27C1, a retinoid desaturase, to its accessory protein adrenodoxin

Of the 57 human cytochrome P450 (P450) enzymes, seven are mitochondrial: 11A1, 11B1, 11B2, 24A1, 27A1, 27B1, and 27C1. Mitochondrial P450s utilize an electron transport system with adrenodoxin (Adx) and NADPH-adrenodoxin reductase (AdR). AdR reduces Adx, which then transfers electrons to the P450. The interactions between proteins in the mitochondrial P450 system are largely driven by electrostatic interactions, though the specifics vary depending on the P450. Unlike other mitochondrial P450s, the interaction between P450 27C1, a retinoid 3,4-desaturase expressed in the skin, and Adx remains largely uncharacterized. In this work, we utilized an Alexa Fluor 488 C₅ maleimide-labeled Adx to measure binding affinities between Adx and P450 27C1 or AdR. Both proteins bound Adx tightly, with K_d values < 100 nM, and binding affinities decreased with increasing ionic strength, supporting the role of electrostatic interactions in mediating these interactions. Cross-linking mass spectrometry and computational modeling were performed to identify interactions between P450 27C1 and Adx. While the residues of Adx identified in interactions were consistent with studies of other mitochondrial P450s, the binding interface of P450 27C1 was quite large and supported multiple Adx binding positions, including ones outside of the canonical Adx binding site. Additionally, Adx did not appear to be an allosteric effector of P450 27C1 substrate binding, in contrast to some other mitochondrial P450s. Overall, we conclude that P450-Adx interactions are P450-specific.

Development of autotrophic and heterotrophic consortia via immobilized microbial beads for chemical wastewater treatment, using PTA wastewater as an approach

Studies on the symbiosis of microalgae-bacteria have been accelerating as a mean for wastewater remediation. However, there were few reports about the microalgae-bacteria consortia for chemical wastewater treatment. The aim of the present study is to develop an autotrophic and heterotrophic consortium for chemical wastewater treatment and probe whether and how bacteria could benefit from the microalgae during the treatment process, using PTA wastewater as an approach. A process-dependent strategy was applied. First of all, the results showed that the sludge beads with the sludge concentration of 30 g/L were the optimal one with the COD removal rate at 84.8% but the ceiling effect occurred (COD removal rate < 90%) even several common reinforcement methods were applied. Additionally, by adding the microalgae Chlorella vulgaris, a microalgae-activated sludge consortium was formed inside the immobilized beads, which provided better performance to shatter the ceiling effect. The COD remove rate was higher than 90%, regardless of the activated sludge was pre-culture or not. COD removal capacity could also be improved (COD removal rate > 92%) when LEDs light belt was offered as an advanced light condition. Biochemical assay and DNA analysis indicated that the microalgae could form an internal circulation of substances within the activated sludge and drove the microbial community to success and the corresponding gene functions, like metabolism and.

Tight binding of cytochrome b5 to cytochrome P450 17A1 is a critical feature of stimulation of C21 steroid lyase activity and androgen synthesis

It has been recognized for >50 years that cytochrome b₅ (b₅) stimulates some cytochrome P450 (P450)–catalyzed oxidations, but the basis of this function is still not understood well. The strongest stimulation of catalytic activity by b₅ is in the P450 17A1 lyase reaction, an essential step in androgen synthesis from 21-carbon (C21) steroids, making this an excellent model system to interrogate b₅ function. One of the issues in studying b₅–P450 interactions has been the limited solution assay methods. We constructed a fluorescently labeled variant of human b₅ that can be used in titrations. The labeled b₅ bound to WT P450 17A1 with a K_d of 2.5 nM and rapid kinetics, on the order of 1 s⁻¹. Only weak binding was observed with the clinical P450 17A1 variants E305G, R347H, and R358Q; these mutants are deficient in lyase activity, which has been hypothesized to be due to attenuated b₅ binding. K_d values were not affected by the presence of P450 17A1 substrates. A peptide containing the P450 17A1 Arg-347/Arg-358 region attenuated Alexa 488-T70C-b₅ fluorescence at higher concentrations. The addition of NADPH–P450 reductase (POR) to an Alexa 488-T70C-b₅:P450 17A1 complex resulted in a concentration-dependent partial restoration of b₅ fluorescence, indicative of a ternary P450:b₅:POR complex, which was also supported by gel filtration experiments. Overall, these results are interpreted in the context of a dynamic and tight P450 17A1:b₅ complex that also binds POR to form a catalytically competent ternary complex, and variants that disrupt this interaction have low catalytic activity.