Analysis of the interactions of cytochrome b5 with flavocytochrome P450 BM3 and its domains.
ABSTRACT Interactions between a soluble form of microsomal cytochrome b(5) (b(5)) from Musca domestica (housefly) and Bacillus megaterium flavocytochrome P450 BM3 and its component reductase (CPR), heme (P450) and FAD/NADPH-binding (FAD) domains were analyzed by a combination of steady-state and stopped-flow kinetics methods, and optical spectroscopy techniques. The high affinity binding of b(5) to P450 BM3 induced a low-spin to high-spin transition in the P450 heme iron (K(d) for b(5) binding = 0.44 microM and 0.72 microM for the heme domain and intact flavocytochrome, respectively). The b(5) had modest inhibitory effects on steady-state turnover of P450 BM3 with fatty acids, and the ferrous-carbon monoxy P450 complex was substantially stabilized on binding b(5). Single turnover reduction of b(5) by BM3 using stopped-flow absorption spectroscopy (k(lim) = 116 s(-1)) was substantially faster than steady-state reduction of b(5) by P450 BM3 (or its CPR and FAD domains), indicating rate-limiting step(s) other than BM3 flavin-to-b(5) heme electron transfer in the steady-state reaction. Steady-state b(5) reduction by P450 BM3 was considerably accelerated at high ionic strength. Pre-reduction of P450 BM3 by NADPH decreased the k(lim) for b(5) reduction approximately 10-fold, and also resulted in a lag phase in steady-state b(5) reduction that was likely due to BM3 conformational perturbations sensitive to the reduction state of the flavocytochrome. Ferrous b(5) could not reduce the ferric P450 BM3 heme domain under anaerobic conditions, consistent with heme iron reduction potentials of the two proteins. However, rapid oxidation of both hemoproteins occurred on aeration of the ferrous protein mixture (and despite the much slower autoxidation rate of b(5) in isolation), consistent with electron transfer occurring from b(5) to the oxyferrous P450 BM3 in the complex. The results demonstrate that strong interactions occur between a eukaryotic b(5) and a model prokaryotic P450. Binding of b(5) perturbs BM3 heme iron spin-state equilibrium, as is seen in many physiologically relevant b(5) interactions with eukaryotic P450s. These results are consistent with the conservation of structure of P450s (particularly at the heme proximal face) between prokaryotes and eukaryotes, and may point to as yet undiscovered roles for b(5)-like proteins in the control of activities of certain prokaryotic P450s.
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ABSTRACT: In the present paper we describe studies on molecular mechanisms of protein-protein interactions between cytochrome P450 3A4 (CYP3A4) and cytochrome b 5, the latter being incorporated into the artificial recombinant protein Hmwb 5-EGFP containing full-length cytochrome b 5 (functional module) and a mutant form of the green fluorescent protein EGFP (signal module) fused into a single polypeptide chain. It is shown that cytochrome b 5 within the fusion protein Hmwb 5-EGFP can be reduced by NADPH-cytochrome P450 reductase in the presence of NADPH, the rate of reduction being dependent on solution ionic strength, indicating that the signal module does not prevent the interaction of the flavo- and hemeproteins. Interaction of cytochrome P450 3A4 and Hmwb 5-EGFP was estimated based on spin equilibrium shift of cytochrome P450 3A4 to high-spin state in the presence of Hmwb 5-EGFP, as well as based on steady-state fluorescence anisotropy of the EGFP component of the fusion protein in the presence of CYP3A4. The engineering of chimeric protein Hmwb 5-EGFP gives an independent method to determine dissociation constant for the complex of cytochrome P450 and cytochrome b 5 that is less sensitive to environmental factors compared to spectrophotometric titration used before. Reconstitution of catalytic activity of cytochrome P450 3A4 in the reaction of testosterone 6β-hydroxylation in the presence of Hmwb 5-EGFP indicates that cytochrome b 5 in the fusion protein is able to stimulate the hydroxylation reaction. Using other fusion proteins containing either cytochrome b 5 or its hydrophilic domain to reconstitute catalytic activity of cytochrome P450 3A4 showed that the hydrophobic domain of cytochrome b 5 participates not only in hemeprotein interaction, but also in electron transfer from cytochrome b 5 to cytochrome P450.Biochemistry (Moscow) 74(8):862-873. · 1.15 Impact Factor
Article: What makes a P450 tick?[Show abstract] [Hide abstract]
ABSTRACT: The cytochromes P450 (P450s) are probably nature's most versatile enzymes in terms of both their vast substrate range and the diverse types of molecular transformations performed across the P450 enzyme superfamily. The P450s exquisitely perform highly specific oxidative chemistry, utilizing a sophisticated catalytic reaction mechanism. Recent studies have provided the first definitive characterization of the transient reaction cycle intermediate (compound I) responsible for the majority of P450 oxidative reactions. This major advance comes at a time when P450 engineering has facilitated the elucidation of several mammalian P450 structures and generated P450 variants with novel substrate specificity and reactivity. This review describes recent advances in P450 research and the ramifications for biotechnological and biomedical exploitation of these enzymes.Trends in Biochemical Sciences 01/2013; · 13.08 Impact Factor
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ABSTRACT: The interactions of protein components of the xenobiotic-metabolizing cytochrome P450 system, CYP6A1, P450 reductase, and cytochrome b5 from the house fly (Musca domestica) have been characterized. CYP6A1 activity is determined by the concentration of the CYP6A1–P450 reductase complex, regardless of which protein is present in excess. Both holo- and apo-b5 stimulated CYP6A1 heptachlor epoxidase and steroid hydroxylase activities and influenced the regioselectivity of testosterone hydroxylation. The conversion of CYP6A1 to its P420 form was decreased by the addition of apo-b5. The effects of cytochrome b5 may involve allosteric modification of the P450 enzyme that modify the conformation of the active site. The overall stoichiometry of the P450 reaction was substrate-dependent. High uncoupling of CYP6A1 was observed with generation of hydrogen peroxide, in excess over the concomitant testosterone hydroxylation or heptachlor epoxidation. Inclusion of cytochrome b5 in the reconstituted system improved efficiency of oxygen consumption and electron utilization from NADPH, or coupling of the P450 reaction. Depending on the reconstitution conditions, coupling efficiency varied from 8 to 25% for heptachlor epoxidation, and from 11 to 70% for testosterone hydroxylation. Because CYP6A1 is a P450 involved in insecticide resistance, this suggests that xenobiotic metabolism by constitutively overexpressed P450s may be linked to significant oxidative stress in the cell that may carry a fitness cost.Insect Biochemistry and Molecular Biology 11/2008; · 3.23 Impact Factor