title: Identification and characterization of two functional domains in cytochrome P-450BM-3, a catalytically self-sufficient monooxygenase induced by barbiturates in Bacillus megaterium.
abstract: In a previous publication (Narhi, L. O. and Fulco, A. J. (1986) J. Biol. Chem. 261, 7160-7169) we described the characterization of a soluble 119,000-dalton P-450 cytochrome (P-450BM-3) that was induced by barbiturates in Bacillus megaterium. This single polypeptide contained 1 mol each of FAD and FMN/mol of heme and, in the presence of NADPH and O2, catalyzed the oxygenation of long-chain fatty acids without the aid of any other protein. We have now utilized limited trypsin proteolysis in the presence of substrate to cleave P-450BM-3 into two polypeptides (domains) of about 66,000 and 55,000 daltons. The 66-kDa domain contains both FAD and FMN but no heme, reduces cytochrome c in the presence of NADPH, and is derived from the C-terminal portion of P-450BM-3. The 55-kDa domain is actually a mixture of three discrete peptides (T-I, T-II, and T-III) separable by high performance liquid chromatography. All three contain heme and show a P-450 absorption peak in the presence of CO and dithionite. The major component, T-I (Mr = 55 kDa), binds fatty acid substrate and has an N-terminal amino acid sequence identical to that of intact P-450BM-3, an indication that this domain constitutes the N-terminal portion of the 119-kDa protein. T-II (54 kDa) is the same as T-I except that it is missing the first nine N-terminal amino acids and does not bind substrate. T-III (Mr = 53.5 kDa) has lost the first 15 N-terminal residues and does not bind substrate. Since trypsin digestion of P-450BM-3 carried out in the absence of substrate yields T-II and T-III but no T-I, it appears that 1 or more residues of the first nine N-terminal amino acids of this protein are intimately involved in substrate binding. Although both the heme- and flavin-containing tryptic peptides retain their original half-reactions, fatty acid monooxygenase activity cannot be reconstituted after proteolysis, and the two domains, once separated, show no affinity for each other. In most respects, the reductase domain of P-450BM-3 more closely resembles the mammalian microsomal P-450 reductases than it does any known bacterial protein.
mesh_terms: D000595:Amino Acid Sequence; D001410:Bacillus megaterium / Q000187:drug effects / Q000201:enzymology*; D001426:Bacterial Proteins; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis* / Q000302:isolation & purification; D004790:Enzyme Induction; D046911:Macromolecular Substances; D006899:Mixed Function Oxygenases / Q000096:biosynthesis* / Q000302:isolation & purification; D008970:Molecular Weight; D009251:NADPH-Ferrihemoprotein Reductase; D010446:Peptide Fragments / Q000032:analysis; D010634:Phenobarbital / Q000494:pharmacology*; D014357:Trypsin
chemical_list: D001426:Bacterial Proteins; D046911:Macromolecular Substances; D010446:Peptide Fragments; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; D009251:NADPH-Ferrihemoprotein Reductase; C052423:flavocytochrome P450 BM3 monoxygenases; D014357:Trypsin; D010634:Phenobarbital