Dynamics and Mechanism of Binding of Androstenedione to Membrane-Associated Aromatase.

Aromatase (CYP19A1) catalyzes the synthesis of estrogens from androgens and is an invaluable target of pharmacotherapy for estrogen-dependent cancers. CYP19A1 is also one of the most primordial human CYPs and, to the extent that its fundamental dynamics are conserved, is highly relevant to understanding those of the more recently evolved and promiscuous enzymes. A complementary approach employing molecular dynamics simulations and hydrogen-deuterium exchange mass spectrometry (HDX-MS) was employed to interrogate the changes in CYP19A1 dynamics coupled to binding androstenedione (ASD). Gaussian-accelerated molecular dynamics and HDX-MS agree that ASD globally suppresses CYP19A1 dynamics. Bimodal HDX patterns of the B'-C loop potentially arising from at least two conformations are present in free 19A1 only, supporting the possibility that conformational selection is operative. Random-acceleration molecular dynamics and adaptive biasing force simulations illuminate ASD's binding pathway, predicting ASD capture in the lipid headgroups and a pathway to the active site shielded from solvent. Intriguingly, the predicted access channel in 19A1 aligns well with the steroid binding sites of other human sterol-oxidizing CYPs.

Expression and Functional Characterization of Breast Cancer-Associated Cytochrome P450 4Z1 in Saccharomyces cerevisiae.

CYP4Z1 is an "orphan" cytochrome P450 (P450) enzyme that has provoked interest because of its hypothesized role in breast cancer through formation of the signaling molecule 20-hydroxyeicosatetraenoic acid (20-HETE). We expressed human CYP4Z1 in Saccharomyces cerevisiae and evaluated its catalytic capabilities toward arachidonic and lauric acids (AA and LA). Specific and sensitive mass spectrometry assays enabled discrimination of the regioselectivity of hydroxylation of these two fatty acids. CYP4Z1 generated 7-, 8-, 9-, 10-, and 11-hydroxy LA, whereas the 12-hydroxy metabolite was not detected. HET0016, the prototypic CYP4 inhibitor, only weakly inhibited laurate metabolite formation (IC50 ∼15 µM). CYP4Z1 preferentially oxidized AA to the 14(S),15(R)-epoxide with high regioselectivity and stereoselectivity, a reaction that was also insensitive to HET0016, but neither 20-HETE nor 20-carboxy-AA were detectable metabolites. Docking of LA and AA into a CYP4Z1 homology model was consistent with this preference for internal fatty acid oxidation. Thus, human CYP4Z1 has an inhibitor profile and product regioselectivity distinct from most other CYP4 enzymes, consistent with CYP4Z1's lack of a covalently linked heme. These data suggest that, if CYP4Z1 modulates breast cancer progression, it does so by a mechanism other than direct production of 20-HETE.

Membrane Fluidity Modulates Thermal Stability and Ligand Binding of Cytochrome P4503A4 in Lipid Nanodiscs.

Cytochrome P4503A4 (CYP3A4) is a peripheral membrane protein that plays a major role in enzymatic detoxification of many drugs and toxins. CYP3A4 has an integral membrane N-terminal helix and a localized patch comprised of the G' and F' helix regions that are embedded in the membrane, but the effects of membrane composition on CYP3A4 function are unknown. Here, circular dichroism and differential scanning calorimetry were used to compare the stability of CYP3A4 in lipid bilayer nanodiscs with varying ratios of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine to 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). These lipids differ in the acyl-chain length and their degree of unsaturation. The thermal denaturation of CYP3A4 in nanodiscs occurs in a temperature range distinct from that of the nanodisc denaturation so it can be monitored calorimetrically. Melting temperatures (Tm), heat capacities (ΔCp), and calorimetric enthalpies (ΔHcal) for denaturation of CYP3A4 each increased with an increasing fraction of DMPC, with a maximum at 50% DMPC, before decreasing at 75% DMPC. Addition of the inhibitor ketoconazole results in increased thermal stability, and larger ΔCp and ΔHcal values, with different sensitivities to lipid composition. Effects of lipid composition on ligand binding dynamics were also studied. Equilibrium binding affinities of both ketoconazole (KTZ) and testosterone (TST) were minimally affected by lipid composition. However, stopped-flow analyses indicate that the rates of KTZ binding reach a maximum in membranes containing 50% DMPC, whereas the rate of TST binding decreases continuously with an increasing DMPC concentration. These results indicate that CYP3A4 is highly sensitive to the acyl-chain composition of the lipids and fluidity of the membrane in which it is embedded.

Nanodisc-embedded cytochrome P450 P3A4 binds diverse ligands by distributing conformational dynamics to its flexible elements.

Cytochrome P450 3A4 (CYP3A4) metabolizes a wide range of drugs and toxins. Interactions of CYP3A4 with ligands are difficult to predict due to promiscuity and conformational flexibility. To better understand CYP3A4 conformational responses to ligands we use hydrogen deuterium exchange mass spectrometry (HDX-MS) to investigate the effect of ligands on nanodisc-embedded CYP3A4. For a subset of CYP3A4-ligand complexes, differences in the low-frequency modes derived by principal component analyses of molecular dynamics trajectories mirrored the HDX-MS results. The effects of ligands are distributed to flexible elements of CYP3A4 between stretches of secondary structure. The largest effects occur in the F- and G-helices, where most ligands increase the flexibility of the F-helix and connecting loops and decrease the flexibility of the C-term of the G-helix. Most ligands affect the E-F-G, CD and HI regions of the protein. Ligand-dependent differences are observed in the A"-A' loop, BC region, E-helix, K-ß1 region, proximal loop, and C-term loop. Correlated HDX responses were observed in the CD region and the C-term of the G-helix that were most pronounced for Type II ligands. Collectively, the HDX and molecular dynamics results suggest that CYP3A4 accommodates diverse binding partners by propagating local backbone fluctuations from the binding site onto the flexible regions of the enzyme via long-range interactions that are differentially modulated by ligands. In contrast to the paradigm wherein ligands decrease protein dynamics at their binding site, a wide range of ligands modestly increase CYP3A4 dynamics throughout the protein including effects remote from the active site.

Reversibility and Low Commitment to Forward Catalysis in the Conjugation of Lipid Alkenals by Glutathione Transferase A4-4.

High concentrations of electrophilic lipid alkenals formed during oxidative stress are implicated in cytotoxicity and disease. However, low concentrations of alkenals are required to induce antioxidative stress responses. An established clearance pathway for lipid alkenals includes conjugation to glutathione (GSH) via Michael addition, which is catalyzed mainly by glutathione transferase isoform A4 (GSTA4-4). Based on the ability of GSTs to catalyze hydrolysis or retro-Michael addition of GSH conjugates, and the antioxidant function of low concentrations of lipid alkenals, we hypothesize that GSTA4-4 contributes a homeostatic role in lipid metabolism. Enzymatic kinetic parameters for retro-Michael addition with trans-2-Nonenal (NE) reveal the chemical competence of GSTA4-4 in this putative role. The forward GSTA4-4-catalyzed Michael addition occurs with the rapid exchange of the C2 proton of NE in D2O as observed by NMR. The isotope exchange was completely dependent on the presence of GSH. The overall commitment to catalysis, or the ratio of first order kcat,f for 'forward' Michael addition to the first order kcat,ex for H/D exchange is remarkably low, approximately 3:1. This behavior is consistent with the possibility that GSTA4-4 is a regulatory enzyme that contributes to steady-state levels of lipid alkenals, rather than a strict 'one way' detoxication enzyme.