Roles of key active-site residues in flavocytochrome P450 BM3.
The effects of mutation of key active-site residues (Arg-47, Tyr-51, Phe-42 and Phe-87) in Bacillus megaterium flavocytochrome P450 BM3 were investigated. Kinetic studies on the oxidation of laurate and arachidonate showed that the side chain of Arg-47 contributes more significantly to stabilization of the fatty acid carboxylate than does that of Tyr-51 (kinetic parameters for oxidation of laurate: R47A mutant, Km 859 microM, kcat 3960 min-1; Y51F mutant, Km 432 microM, kcat 6140 min-1; wild-type, Km 288 microM, kcat 5140 min-1). A slightly increased kcat for the Y51F-catalysed oxidation of laurate is probably due to decreased activation energy (DeltaG) resulting from a smaller DeltaG of substrate binding. The side chain of Phe-42 acts as a phenyl 'cap' over the mouth of the substrate-binding channel. With mutant F42A, Km is massively increased and kcat is decreased for oxidation of both laurate (Km 2. 08 mM, kcat 2450 min-1) and arachidonate (Km 34.9 microM, kcat 14620 min-1; compared with values of 4.7 microM and 17100 min-1 respectively for wild-type). Amino acid Phe-87 is critical for efficient catalysis. Mutants F87G and F87Y not only exhibit increased Km and decreased kcat values for fatty acid oxidation, but also undergo an irreversible conversion process from a 'fast' to a 'slow' rate of substrate turnover [for F87G (F87Y)-catalysed laurate oxidation: kcat 'fast', 760 (1620) min-1; kcat 'slow', 48.0 (44.6) min-1; kconv (rate of conversion from fast to slow form), 4.9 (23.8) min-1]. All mutants showed less than 10% uncoupling of NADPH oxidation from fatty acid oxidation. The rate of FMN-to-haem electron transfer was shown to become rate-limiting in all mutants analysed. For wild-type P450 BM3, the rate of FMN-to-haem electron transfer (8340 min-1) is twice the steady-state rate of oxidation (4100 min-1), indicating that other steps contribute to rate limitation. Active-site structures of the mutants were probed with the inhibitors 12-(imidazolyl)dodecanoic acid and 1-phenylimidazole. Mutant F87G binds 1-phenylimidazole >10-fold more tightly than does the wild-type, whereas mutant Y51F binds the haem-co-ordinating fatty acid analogue 12-(imidazolyl)dodecanoic acid >30-fold more tightly than wild-type.
Noble
M A
MA
Department of Chemistry, University of Edinburgh, The King's Buildings, West Mains Road, Edinburgh EH9 3JJ, UK.
Miles
C S
CS
Chapman
S K
SK
Lysek
D A
DA
MacKay
A C
AC
Reid
G A
GA
Hanzlik
R P
RP
Munro
A W
AW
D001426:Bacterial Proteins; D001483:Base Sequence; D001665:Binding Sites; D003577:Cytochrome P-450 Enzyme System / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D003574:Cytochrome c Group / Q000378:metabolism; D017931:DNA Primers; D004579:Electron Transport; D005227:Fatty Acids / Q000378:metabolism; D005415:Flavins / Q000378:metabolism; D006418:Heme / Q000378:metabolism; D007700:Kinetics; D006899:Mixed Function Oxygenases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D016296:Mutagenesis; D009251:NADPH-Ferrihemoprotein Reductase; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000378:metabolism
D001426:Bacterial Proteins; D003574:Cytochrome c Group; D017931:DNA Primers; D005227:Fatty Acids; D005415:Flavins; D006418:Heme; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; D010088:Oxidoreductases; D009251:NADPH-Ferrihemoprotein Reductase; C052423:flavocytochrome P450 BM3 monoxygenases