NADPH-flavodoxin reductase and flavodoxin from Escherichia coli: characteristics as a soluble microsomal P450 reductase.
In addition to their endogenous roles as an activation system for various Escherichia coli metabolic pathways, the soluble flavoproteins flavodoxin (Fld) and NADPH-flavodoxin (ferredoxin) reductase (Fpr) can serve as an electron-transfer system for microsomal cytochrome P450s. Furthermore, since Fld and Fpr are structurally similar to the functional domains (FMN binding and NADPH/FAD binding domains, respectively) of NADPH-cytochrome P450 reductases (P450 reductases), these bacterial proteins represent a potentially useful model system for eukaryotic P450 reductases. Here we delineate similarities and differences between the E. coli Fpr-Fld system and rat P450 reductase as electron donors to bovine 17alpha-hydroxylase/17,20-lyase P450 (P450c17). Importantly, recombinant Fpr, in combination with recombinant Fld, supports both the hydroxylase and lyase activities of P450c17 to the same proportional extent (hydroxylase-to-lyase ratio) as does P450 reductase. Maximum P450c17 turnover [5-6 mol of 17alpha-OH-progesterone (mol of P450c17)-1 min-1] was achieved using a large molar excess (50-100-fold over P450c17) of a 1:1 ratio of Fpr-Fld, although this rate was an order of magnitude less than the maximal P450 reductase-supported activity. Using these conditions, we have examined the effects of increasing ionic strength and the presence of cytochrome b5 (b5) on these two systems. Critical Fld-P450c17 electrostatic interactions are disrupted at moderate ionic strength (>100 mM NaCl) as evidenced by significant inhibition (>50%) of Fpr-Fld-supported P450c17 activity while much higher ionic strength (300 mM NaCl) is required to disrupt P450 reductase-P450c17 interactions to the same extent. Interestingly, cytochrome b5 was found to dramatically inhibit both P450 reductase- and Fpr-Fld-supported P450c17 progesterone 17alpha-hydroxylase activity while in contrast 17alpha-OH-pregnenolone lyase activity was stimulated by b5. Investigation of the fate of reducing equivalents from NADPH added to Fpr under aerobic conditions revealed that the majority of the protein-bound FAD of Fpr is converted to the hydroquinone form. In constrast, the FMN of Fld is reduced by Fpr to a stable blue, neutral semiquinone which serves as the predominant electron donor to P450c17 in reconstitution assays. Thus, while the Fpr-Fld system and P450 reductase are fundamentally different with respect to their electrostatic interactions with P450c17, their ability to support maximal P450c17 turnover, and the FMN redox states (one-electron-reduced for Fld and two-electron-reduced for P450 reductase) capable of transferring electrons to microsomal cytochrome P450s, these differences do not appear to influence the relative catalytic efficiency of the P450c17 hydroxylase and lyase reactions.
Jenkins
C M
CM
Department of Biochemistry, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-0146, USA.
Waterman
M R
MR
D000818:Animals; D002417:Cattle; D004579:Electron Transport; D004926:Escherichia coli / Q000201:enzymology* / Q000235:genetics; D005418:Flavodoxin / Q000737:chemistry* / Q000235:genetics; D008861:Microsomes / Q000201:enzymology*; D009247:NADH, NADPH Oxidoreductases / Q000737:chemistry* / Q000235:genetics; D009251:NADPH-Ferrihemoprotein Reductase / Q000737:chemistry*; D051381:Rats; D011994:Recombinant Proteins / Q000737:chemistry; D012995:Solubility; D013053:Spectrophotometry; D013254:Steroid 17-alpha-Hydroxylase / Q000737:chemistry / Q000235:genetics
D005418:Flavodoxin; D011994:Recombinant Proteins; D013254:Steroid 17-alpha-Hydroxylase; D009247:NADH, NADPH Oxidoreductases; D009251:NADPH-Ferrihemoprotein Reductase