Determination of the rate of reduction of oxyferrous cytochrome P450 2B4 by 5-deazariboflavin adenine dinucleotide T491V cytochrome P450 reductase.
The use of 5-deazaFAD T491V cytochrome P450 reductase has made it possible to directly measure the rate of electron transfer to microsomal oxyferrous cytochrome (cyt) P450 2B4. In this reductase the FMN moiety can be reduced to the hydroquinone, FMNH(2), while the 5-deazaFAD moiety remains oxidized [Zhang, H., et al. (2003) Biochemistry 42, 6804-6813]. The rate of electron transfer from 5-deazaFAD cyt P450 reductase to oxyferrous cyt P450 was determined by rapidly mixing the ferrous cyt P450-2-electron-reduced 5-deazaFAD T491V reductase complex with oxygen in the presence of substrate. The 5-deazaFAD T491V reductase which can only donate a single electron reduces the oxyferrous cyt P450 and oxidizes to the air-stable semiquinone, with rate constants of 8.4 and 0.37 s(-1) at 15 degrees C. Surprisingly, oxyferrous cyt P450 turns over more slowly with a rate constant of 0.09 s(-1), which is the rate of catalysis under steady-state conditions at 15 degrees C (k(cat) = 0.08 s(-1)). In contrast, the rate constant for electron transfer from ferrous cyt b(5) to oxyferrous cyt P450 is 10 s(-1) with oxyferrous cyt P450 and cyt b(5) simultaneously undergoing spectral changes. Quantitative analyses by LC-MS/MS revealed that the product, norbenzphetamine, was formed with a coupling efficiency of 52% with cyt b(5) and 32% with 5-deazaFAD T491V reductase. Collectively, these results suggest that during catalysis a relatively stable reduced oxyferrous intermediate of cyt P450 is formed in the presence of cyt P450 reductase but not cyt b(5) and that the rate-limiting step in catalysis follows introduction of the second electron.
Zhang
Haoming
H
University of Michigan and VA Medical Research Center, 2215 Fuller Road, Ann Arbor, Michigan 48105, USA.
Gruenke
Larry
L
Arscott
Dave
D
Shen
Anna
A
Kasper
Charles
C
Harris
Danni L
DL
Glavanovich
Michael
M
Johnson
Richard
R
Waskell
Lucy
L
D001189:Aryl Hydrocarbon Hydroxylases / Q000737:chemistry* / Q000378:metabolism; D001589:Benzphetamine / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism; D002384:Catalysis; D000072467:Cytochrome P450 Family 2; D015786:Cytochromes b5 / Q000737:chemistry / Q000378:metabolism; D004579:Electron Transport; D005296:Ferrous Compounds / Q000737:chemistry* / Q000378:metabolism; D005182:Flavin-Adenine Dinucleotide / Q000031:analogs & derivatives* / Q000737:chemistry* / Q000378:metabolism; D009251:NADPH-Ferrihemoprotein Reductase / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry; D013379:Substrate Specificity; D013912:Threonine / Q000235:genetics; D014633:Valine / Q000235:genetics
D005296:Ferrous Compounds; D001589:Benzphetamine; C008669:norbenzphetamine; D005182:Flavin-Adenine Dinucleotide; D013912:Threonine; C023406:1-deaza-FAD; D015786:Cytochromes b5; D001189:Aryl Hydrocarbon Hydroxylases; D000072467:Cytochrome P450 Family 2; C465241:cytochrome P-450 CYP2B4 (rabbit); D009251:NADPH-Ferrihemoprotein Reductase; D014633:Valine; D010100:Oxygen