Site-Directed Mutagenesis of Cytochrome b₅ for Studies of Its Interaction with Cytochrome P450

M. V. Chudaev, A. A. Gilep, and S. A. Usanov^*

Institute of Bioorganic Chemistry, National Academy of Sciences of Belarus, ul. Kuprevicha 5/2, Minsk, 220141 Belarus; fax: 375 (172) 63-7274; E-mail: usanov@ns.iboch.ac.by

^* To whom correspondence should be addressed.

Received October 26, 2000; Revision received December 27, 2000

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We have shown earlier that microsomal cytochrome b₅ can form a specific complex with mitochondrial cytochrome P450 (cytochrome P450scc). The formation of the complex between these two heme proteins was proved spectrophotometrically, by affinity chromatography on immobilized cytochrome b₅, and by measuring the cholesterol side-chain cleavage activity of cytochrome P450scc in a reconstituted system in the presence of cytochrome b₅. To further study the mechanism of interaction of these heme proteins and evaluate the role of negatively charged amino acid residues Glu42, Glu48, and Asp65 of cytochrome b₅, which are located at the site responsible for interaction with electron transfer partners, we used site-directed mutagenesis to replace residues Glu42 and Glu48 with lysine and residue Asp65 with alanine. The resulting mutant forms of cytochrome b₅ were expressed in E. coli, and full-length and truncated forms (shortened from the C-terminal sequence due to cleavage of 40 amino acid residues) of these cytochrome b₅ mutants were purified. Addition of the truncated forms of cytochrome b₅ (which do not contain the hydrophobic C-terminal sequence responsible for interaction with the membrane) to the reconstituted system containing cytochrome P450scc caused practically no stimulation of catalytic activity, indicating an important role of the hydrophobic fragment of cytochrome b₅ in its interaction with cytochrome P450scc. However, full-length cytochrome b₅ and the full-length Glu48Lys and Asp65Ala mutant forms of cytochrome b₅ stimulated the cholesterol side-chain cleavage reaction catalyzed by cytochrome P450scc by 100%, suggesting that residues Glu48 and Asp65 of cytochrome b₅ are not directly involved in its interaction with cytochrome P450scc. The replacement of Glu42 for lysine, however, made the Glu42Lys mutant form of cytochrome b₅ about 40% less effective in stimulation of the cholesterol side-chain cleavage activity of cytochrome P450scc, indicating that residue Glu42 of cytochrome b₅ is involved in electrostatic interactions with cytochrome P450scc. Residues Glu42 and Glu48 of cytochrome b₅ appear to participate in electrostatic interaction with microsomal type cytochrome P450.

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KEY WORDS: cytochrome b₅, cytochrome P450, cytochrome P450scc, site-directed mutagenesis, heterologous expression, protein-protein interactions

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