Topological mapping of the active sites of cytochromes P4502B1 and P4502B2 by in situ rearrangement of aryl-iron complexes.
Cytochrome P4502B1 reacts with phenylhydrazine or phenyldiazene to give an iron-phenyl complex that oxidatively rearranges in situ to the two N-phenylprotoporphyrin IX regioisomers with the phenyl group on pyrrole rings A (NA) and D (ND) [Swanson, B. A., Dutton, D. R., Lunetta, J. M., Yang, C. S., & Ortiz de Montellano, P. R. (1991) J. Biol. Chem. 266, 19258-19264]. The conclusion that the active site of cytochrome P4502B1 is open above pyrrole rings A and D but not B and C is extended here by studies with larger arylhydrazines. The N-arylprotoporphyrin IX standards required for product identification were obtained by reaction of the arylhydrazines with equine myoglobin. Cytochrome P4502B1 aryl-iron complex formation followed by oxidative shift of the aryl group produces the following N-aryl-protoporphyrin IX NA:ND regioisomer ratios: phenylhydrazine (39:61), 3,5-dimethylphenylhydrazine (29:71), 4-tert-butylhydrazine (25:75), 2-naphthylhydrazine (less than 2:greater than 98), and 4-(phenyl)phenylhydrazine (87:13). Electron-withdrawing substituents (as in 3,5-dichlorophenyl) prevent the aryl group shift. The increase in the proportion of the ND regioisomer with increasing bulk of the aryl group suggests that the region over pyrrole ring A is more sterically encumbered than that over pyrrole ring D. The regiospecificity is reversed, however, with 4-(phenyl)phenylhydrazine, which primarily gives the NA regioisomer. This reversal suggests that the active site has a sloping roof that is higher over pyrrole ring A than pyrrole ring D and that provides a larger steric barrier to the shift of tall aryl moieties than the barrier over pyrrole ring A.(ABSTRACT TRUNCATED AT 250 WORDS)
Tuck
S F
SF
Department of Pharmaceutical Chemistry, School of Pharmacy, University of California, San Francisco 94143-0446.
Ortiz de Montellano
P R
PR
D000818:Animals; D001665:Binding Sites; D002851:Chromatography, High Pressure Liquid; D003577:Cytochrome P-450 Enzyme System / Q000737:chemistry* / Q000378:metabolism; D006834:Hydrazines / Q000378:metabolism*; D007097:Imines / Q000378:metabolism; D007501:Iron / Q000378:metabolism*; D008786:Metmyoglobin / Q000378:metabolism; D015394:Molecular Structure; D010659:Phenylhydrazines / Q000378:metabolism; D051381:Rats
D006834:Hydrazines; D007097:Imines; D010659:Phenylhydrazines; C030299:phenylhydrazine; D008786:Metmyoglobin; D003577:Cytochrome P-450 Enzyme System; C011981:phenyldiazene; D007501:Iron