title: Interaction of human CYP17 (P-450(17alpha), 17alpha-hydroxylase-17,20-lyase) with cytochrome b5: importance of the orientation of the hydrophobic domain of cytochrome b5.
abstract: Human CYP17 (P-450(17alpha), 17alpha-hydroxylase-17,20-lyase)-catalysed side-chain cleavage of 17alpha-hydroxyprogestogens into androgens is greatly dependent on the presence of cytochrome b5. The native form of cytochrome b5 is composed of a globular core, residues 1-98, followed by a membrane insertable C-terminal tail, residues 99-133. In the present study the abilities of five different forms of cytochrome b5 to support the side-chain cleavage activity of CYP17 were compared. The five derivatives were: the native pig cytochrome b5 (native pig), its genetically engineered rat counterpart (core-tail), the soluble core form of the latter (core), the core with the secretory signal sequence of alkaline phosphatase appended to its N-terminal (signal-core) and the latter containing the C-terminal tail of the native rat protein (signal-core-tail). When examined by Edman degradation and MS, the engineered proteins were shown to have the expected N-terminal amino acid sequences and molecular masses. The native pig was found to be acetylated at the N-terminal. The native pig and core-tail enzymes were equally efficient at enhancing the side-chain cleavage activity of human CYP17 and the signal-core-tail was 55% as efficient. The core and signal-core constructs were completely inactive in the aforementioned reaction. All the five derivatives were reduced to varying degrees by NADPH:cytochrome P-450 (NADPH-P450) reductase and the relative efficiencies of this reduction were reminiscent of the behaviour of these derivatives in supporting the side-chain cleavage reaction. In the side-chain cleavage assay, however, NADPH-P450 reductase was used in large excess so that the reduction of cytochrome b5 derivatives was not rate-limiting. The results highlight that productive interaction between cytochrome b5 and CYP17 is governed not only by the presence of a membrane insertable hydrophobic region on the cytochrome b5 but also by its defined spatial orientation at the C-terminal.
authors.affiliation: Department of Biochemistry, University of Southampton, U.K.
authors.forename: P
authors.initials: P
authors.lastname: Lee-Robichaud
authors.forename: M A
authors.initials: MA
authors.lastname: Kaderbhai
authors.forename: N
authors.initials: N
authors.lastname: Kaderbhai
authors.forename: J N
authors.initials: JN
authors.lastname: Wright
authors.forename: M
authors.initials: M
authors.lastname: Akhtar
mesh_terms: D000079:Acetaldehyde / Q000031:analogs & derivatives; D000595:Amino Acid Sequence; D000818:Animals; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D015786:Cytochromes b5 / Q000737:chemistry* / Q000378:metabolism*; D006801:Humans; D008565:Membrane Proteins / Q000737:chemistry / Q000378:metabolism; D008954:Models, Biological; D008969:Molecular Sequence Data; D015394:Molecular Structure; D010446:Peptide Fragments / Q000737:chemistry; D021382:Protein Sorting Signals / Q000378:metabolism; D051381:Rats; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017421:Sequence Analysis; D013254:Steroid 17-alpha-Hydroxylase / Q000378:metabolism*
chemical_list: D008565:Membrane Proteins; D010446:Peptide Fragments; D021382:Protein Sorting Signals; D011994:Recombinant Proteins; C049826:2-methoxyacetaldehyde; D015786:Cytochromes b5; D003577:Cytochrome P-450 Enzyme System; D013254:Steroid 17-alpha-Hydroxylase; D000079:Acetaldehyde