31P NMR spectroscopic studies on purified, native and cloned, expressed forms of NADPH-cytochrome P450 reductase

Variability in Loss of Multiple Enzyme Activities Due to the Human Genetic Variation P284T Located in the Flexible Hinge Region of NADPH Cytochrome P450 Oxidoreductase

Cytochromes P450 located in the endoplasmic reticulum require NADPH cytochrome P450 oxidoreductase (POR) for their catalytic activities. Mutations in POR cause multiple disorders in humans related to the biosynthesis of steroid hormones and also affect drug-metabolizing cytochrome P450 activities. Electron transfer in POR occurs from NADH to FAD to FMN, and the flexible hinge region in POR is essential for domain movements to bring the FAD and FMN close together for electron transfer. We tested the effect of variations in the hinge region of POR to check if the effects would be similar across all redox partners or there will be differences in activities. Here we are reporting the effects of a POR genetic variant P284T located in the hinge region of POR that is necessary for the domain movements and internal electron transfer between co-factors. Human wild-type and P284T mutant of POR and cytochrome P450 proteins were expressed in bacteria, purified, and reconstituted for enzyme assays. We found that for the P284T variant of POR, the cytochrome c reduction activity was reduced to 47% of the WT and MTT reduction was reduced to only 15% of the WT. No impact on ferricyanide reduction activity was observed, indicating intact direct electron transfer from FAD to ferricyanide, but a severe loss of CYP19A1 (aromatase) activity was observed (9% of WT). In the assays of drug-metabolizing cytochrome P450 enzymes, the P284T variant of POR showed 26% activity for CYP2C9, 44% activity for CYP2C19, 23% activity for CYP3A4, and 44% activity in CYP3A5 assays compared to the WT POR. These results indicate a severe effect on several cytochrome P450 activities due to the P284T variation in POR, which suggests a negative impact on both the steroid as well as drug metabolism in the individuals carrying this variation. The negative impact of P284T mutation in the hinge region of POR seems to be due to disruption of FAD to FMN electron transfer. These results further emphasize the importance of hinge region in POR for protein flexibility and electron transfer within POR as well as the interaction of POR with different redox partners.

Lipid-exchange in nanodiscs discloses membrane boundaries of cytochrome-P450 reductase

Lipids are critical for the function of membrane proteins. NADPH-cytochrome-P450-reductase, the sole electron transferase for microsomal oxygenases, possesses a conformational dynamics entwined with its topology. Here, we use peptide-nanodiscs to unveil cytochrome-P450-reductase's lipid boundaries, demonstrating a protein-driven enrichment of ethanolamine lipids (by 25%) which ameliorates by 3-fold CPR's electron-transfer ability.

FMN binding site of yeast NADPH-cytochrome P450 reductase exposed at the surface is highly specific

NADPH-cytochrome P450 reductase (CPR) transfers two reducing equivalents derived from NADPH via FAD and FMN to microsomal P450 monooxygenases in one-electron transfer steps. The crystal structure of yeast CPR (yCPR) contains a surface-exposed FMN binding site (FMN2 site) at the interface of the FMN binding and connecting domains, in addition to the single buried site that has been observed in rat CPR. This finding provides a testable hypothesis of how intramolecular (between FAD and FMN) and intermolecular (between FMN and P450) electron transfer may occur in CPR. To verify that occupancy of the FMN2 site is not an artifact of crystallization, a surface plasmon resonance (SPR) biosensor technique has been applied to probe the selectivity of this site under functional conditions. A series of kinetic and equilibrium binding experiments involving yCPR immobilized on different sensor chip surfaces was performed using FMN and FAD, as well as FMN-derived compounds, including riboflavin, dimethylalloxazine, and alloxazine, and other molecules that resemble the planar isoalloxazine ring structure. Only FMN and FAD showed stoichiometric binding responses. Binding affinity for FMN was in the submicromolar range, 30 times higher than that for FAD. Association kinetic rates for the yCPR/FMN complex were up to 60-fold higher than for the yCPR/FAD complex. Taken together, these data indicate that (i) the surface-exposed site in yCPR is highly selective toward binding flavins, (ii) binding of FMN in this site is notably favored, and finally, (iii) both the phosphate group and the isoalloxazine ring of FMN are essential for binding.

At the crossroads of steroid hormone biosynthesis: the role, substrate specificity and evolutionary development of CYP17

Cytochrome P450s play critical roles in the metabolism of various bioactive compounds. One of the crucial functions of cytochrome P450s in Chordata is in the biosynthesis of steroid hormones. Steroid 17alpha-hydroxylase/17,20-lyase (CYP17) is localized in endoplasmic reticulum membranes of steroidogenic cells. CYP17 catalyzes the 17alpha-hydroxylation reaction of delta4-C₂₁ steroids (progesterone derivatives) and delta5-C₂₁ steroids (pregnenolone derivatives) as well as the 17,20-lyase reaction producing C₁₉-steroids, a key branch point in steroid hormone biosynthesis. Depending on CYP17 activity, the steroid hormone biosynthesis pathway is directed to either the formation of mineralocorticoids and glucocorticoids or sex hormones. In the present review, the current information on CYP17 is analyzed and discussed.

Purification and characterization of a soluble form of rat liver NADPH-cytochrome P-450 reductase highly expressed in Escherichia coli

A recombinant cDNA of rat liver NADPH-cytochrome P-450 reductase (CPR), which lacks the N-terminal hydrophobic region, was amplified by PCR and cloned. The N-truncated cDNA named tCPR was ligated into a pBAce vector and expressed. The tCPR protein expressed in Escherichia coli was recovered into the soluble fraction of the cell lysate and purified to homogeneity by three sequential purification procedures; (I) anion-exchange chromatography on a DEAE-cellulose (DE-52) column, (II) affinity chromatography on 2('),5(')-ADP Sepharose 4B, and (III) chromatography on a hydroxyapatite column. The average yield was 47mg per liter of culture medium. The absorption spectrum of the purified tCPR protein was identical to that of a native full-length CPR purified from rat liver, indicating that tCPR also possesses one molecule each of FAD and FMN. The tCPR protein was able to reduce cytochrome c and was also able to assist heme degradation by a soluble form of rat heme oxygenase-1. However, it failed to support the O-deethylation of 7-ethoxycoumarin by cytochrome P-450 1A1, indicating that the presence of the N-terminal hydrophobic domain is necessary for CPR to interact with cytochrome P-450. Previously, to prepare a soluble form of CPR, full-length CPR was treated with proteinases that selectively removed the N-terminal domain. With the expression system established in this study, however, the soluble and biologically active tCPR protein can be readily prepared in large amounts. This expression system will be useful for mechanistic as well as structural studies of CPR.

31P nuclear magnetic resonance study of the flavoprotein component of the Escherichia coli sulfite reductase

SiR-FP60, the monomeric form of the Escherichia coli sulfite reductase flavoprotein component (SiR-FP), has been analysed by 31P-NMR spectroscopy. This protein was reported previously as a reliable simplified model for native SiR-FP [Zeghouf, M., Fontecave, M., Macherel, D., & Covès, J. (1998) Biochemistry 37, 6117-6123]. SiR-FP60 was examined in its native form, as a complex with NADP+ and after monoelectronic reduction either with NADPH or dithionite. In these latter cases, the stabilized FMN semiquinone radical offers a natural and internal paramagnetic probe. The paramagnetic effect of added manganese was also studied. In each case, the NMR parameters were extracted from digitalized data by a deconvolution procedure and compared with those obtained previously with cytochrome P450 reductase. Evolution of the NMR parameters and of calculated relaxation rate constants upon biochemical modifications of SiR-FP60 led us to propose that the reactive center is more compact than the one of cytochrome P450 reductase, with the redox components, FMN, FAD and NADPH, in a tighter spatial arrangement, close to the protein surface. This underlies some subtle differences between the two proteins for which a very similar overall structure is likely considering their common genetic origin and common operating cycle.

Three-dimensional structure of NADPH-cytochrome P450 reductase: prototype for FMN- and FAD-containing enzymes

Microsomal NADPH-cytochrome P450 reductase (CPR) is one of only two mammalian enzymes known to contain both FAD and FMN, the other being nitric-oxide synthase. CPR is a membrane-bound protein and catalyzes electron transfer from NADPH to all known microsomal cytochromes P450. The structure of rat liver CPR, expressed in Escherichia coli and solubilized by limited trypsinolysis, has been determined by x-ray crystallography at 2.6 A resolution. The molecule is composed of four structural domains: (from the N- to C- termini) the FMN-binding domain, the connecting domain, and the FAD- and NADPH-binding domains. The FMN-binding domain is similar to the structure of flavodoxin, whereas the two C-terminal dinucleotide-binding domains are similar to those of ferredoxin-NADP+ reductase (FNR). The connecting domain, situated between the FMN-binding and FNR-like domains, is responsible for the relative orientation of the other domains, ensuring the proper alignment of the two flavins necessary for efficient electron transfer. The two flavin isoalloxazine rings are juxtaposed, with the closest distance between them being about 4 A. The bowl-shaped surface near the FMN-binding site is likely the docking site of cytochrome c and the physiological redox partners, including cytochromes P450 and b5 and heme oxygenase.

Crystallization and preliminary x-ray studies of NADPH-cytochrome P450 reductase

NADPH-cytochrome P450 reductase (CPR; NADPH:ferrihemoprotein reductase, EC 1.6.2.4) catalyzes the transfer of electrons to all known microsomal cytochromes P450. CPR is unique in that it is one of only two mammalian enzymes known to contain both flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN), the other being the various isoforms of nitric oxide synthase. Similarities in amino acid sequence and in functional domain arrangement with other key flavoproteins, including nitric oxide synthase, make CPR an excellent prototype for studies of interactions between two flavin cofactors. We have obtained diffraction-quality crystals of rat liver CPR, expressed in Escherichia coli and solubilized by limited proteolysis with trypsin. The crystals were grown in Hepes buffer (pH 7.0), containing polyethylene glycol 4500 and NaCl. The crystals belong to the orthorhombic space group P2(1)2(1)2(1), with unit cell dimensions a = 103.3 A, b = 116.1 A, and c = 120.4 A. If we assume that there are two molecules of the 72-kDa CPR polypeptide per asymmetric unit, the calculated value of Vm is 2.54 A3/Da.

A specific interaction between NADPH-cytochrome reductase and phosphatidylserine and phosphatidylinositol

In the present study the interaction of NADPH-cytochrome reductase with phospholipids was investigated using 31P-NMR, thin-layer chromatography combined with chemical analysis, fluorescence spectroscopy and kinetic studies with purified rat liver cytochrome P450 IIB1. 31P-NMR analysis demonstrates that the composition of the phospholipids that remain associated to NADPH-cytochrome reductase upon its purification is significantly different from the phospholipid composition of the microsomal membrane. Thin-layer chromatography followed by chemical analysis of the phospholipid composition demonstrates that the isolated NADPH-cytochrome reductase was enriched in L-alpha-1,2-diacyl-sn-glycero-3-phosphoserine (acyl2GroPSer) and L-alpha-1,2-diacyl-sn-glycero-3-phosphoinositol (acyl2GroPIns) compared to the microsomal membrane. The observed preference of NADPH-cytochrome reductase for acyl2GroPSer and acyl2GroPIns appeared not to be a result of the procedure for solubilisation and/or purification of the protein. The specific interaction of NADPH-cytochrome reductase with acyl2GroPSer and acyl2GroPIns was further investigated by comparison of the effect of acyl2GroPSer and acyl2GroPIns with that of acyl2GroPCho and acyl2GroPEtn on the 2-[3-(diphenylhexatrienyl)propanoyl]-1-hexadecanoyl-sn-glycero-3- phosphocholine-(DphPamGroPCho)-dependent quenching of the tryptophan fluorescence of purified NADPH-cytochrome reductase. The results demonstrate that the addition of acyl2GroPSer or acyl2GroPIns affects the DphPamGroPCho-dependent quenching of the tryptophan fluorescence in a manner significantly different from the addition of acyl2GroPCho or acyl2GroPEtn. The relatively larger DphPamGroPCho-induced quenching of the tryptophan fluorescence of NADPH-cytochrome reductase in the presence of acyl2GroPSer and acyl2GroPIns must result from a change in the conformation of NADPH-cytochrome reductase induced by the latter two lipids. Finally, the possible consequences of this special interaction of acyl2GroPSer and acyl2GroPIns with NADPH-cytochrome reductase on the kinetic characteristics of the cytochrome P450 system were studied using cytochrome-P450-IIB1-dependent O-dealkylation of pentoxyresorufin as the model reaction. These studies demonstrate that a 1:1 mixture of acyl2GroPCho and acyl2GroPSer results in a significantly higher apparent maximum rate (V) of O-dealkylation than a 1:1 mixture of acyl2GroPCho and acyl2ProPEtn or acyl2GroPCho alone. This increase in the apparent V can be ascribed to an acyl2GroPSer-dependent improvement of the interaction of NADPH-cytochrome reductase with cytochrome P450. This improvement of the interaction of the proteins cannot, however, be exclusively ascribed to the negative charge of acyl2GroPSer, since the other negatively charged phospholipid investigated, namely acyl2GroPIns, resulted in a significant decrease in the apparent V.(ABSTRACT TRUNCATED AT 400 WORDS)