A Model of the Membrane-bound Cytochrome b5-Cytochrome P450 Complex from NMR and Mutagenesis Data
Shivani Ahuja, Nicole Jahr, Sang‐Choul Im, Subramanian Vivekanandan, Nataliya Popovych, Stéphanie V. Le Clair, Rui Huang, Ronald Soong, Jiadi Xu, Kazutoshi Yamamoto, Ravi Prakash Reddy Nanga, Angela M. Bridges, Lucy A. Waskell, Ayyalusamy Ramamoorthy · Journal of Biological Chemistry · 2013
Microsomal cytochrome b 5 (cytb 5 ) is a membrane-bound protein that modulates the catalytic activity of its redox partner, cytochrome P4502B4 (cytP450). Here, we report the first structure of full-length rabbit ferric microsomal cytb 5 (16 kDa), incorporated in two different membrane mimetics (detergent micelles and lipid bicelles). Differential line broadening of the cytb 5 NMR resonances and site-directed mutagenesis data were used to characterize the cytb 5 interaction epitope recognized by ferric microsomal cytP450 (56 kDa). Subsequently, a data-driven docking algorithm, HADDOCK (high ambiguity driven biomolecular docking), was used to generate the structure of the complex between cytP4502B4 and cytb 5 using experimentally derived restraints from NMR, mutagenesis, and the double mutant cycle data obtained on the full-length proteins. Our docking and experimental results point to the formation of a dynamic electron transfer complex between the acidic convex surface of cytb 5 and the concave basic proximal surface of cytP4502B4. The majority of the binding energy for the complex is provided by interactions between residues on the C-helix and β-bulge of cytP450 and residues at the end of helix α4 of cytb 5 . The structure of the complex allows us to propose an interprotein electron transfer pathway involving the highly conserved Arg-125 on cytP450 serving as a salt bridge between the heme propionates of cytP450 and cytb 5 . We have also shown that the addition of a substrate to cytP450 likely strengthens the cytb 5 -cytP450 interaction. This study paves the way to obtaining valuable structural, functional, and dynamic information on membrane-bound complexes. Background: cytb 5 modulates catalysis performed by cytsP450, in vivo and in vitro . Results: The structure of full-length cytb 5 was solved by NMR, and the cytP450-binding site on cytb 5 was identified by mutagenesis and NMR. Conclusion: A model of the cytb 5 -cytP450 complex is presented. Addition of a substrate strengthens the cytb 5 -cytP450 interaction. Significance: The cytb 5 -cytP450 complex structure will help unravel the mechanism by which cytb 5 regulates catalysis by cytP450.