Putidaredoxin competitively inhibits cytochrome b5-cytochrome P-450cam association: a proposed molecular model for a cytochrome P-450cam electron-transfer complex

Cytochrome b 5 Binds Tightly to Several Human Cytochrome P450 Enzymes

Numerous studies have been reported in the past 50-plus years regarding the stimulatory role of cytochrome b ₅ (b ₅) in some, but not all, microsomal cytochrome P450 (P450) reactions with drugs and steroids. A missing element in most of these studies has been a sensitive and accurate measure of binding affinities of b ₅ with P450s. In the course of work with P450 17A1, we developed a fluorescent derivative of a human b ₅ site-directed mutant, Alexa 488-T70C-b ₅, that could be used in binding assays at sub-μM concentrations. Alexa 488-T70C-b ₅ bound to human P450s 1A2, 2B6, 2C8, 2C9, 2E1, 2S1, 4A11, 3A4, and 17A1, with estimated K _d values ranging from 2.5 to 61 nM. Only weak binding was detected with P450 2D6, and no fluorescence attenuation was observed with P450 2A6. All of the P450s that bound b ₅ have some reported activity stimulation except for P450 2S1. The affinity of P450 3A4 for b ₅ was decreased somewhat by the presence of a substrate or inhibitor. The fluorescence of a P450 3A4•Alexa 488-T70C-b ₅ complex was partially restored by titration with NADPH-P450 reductase (POR) (K _d,apparent 89 nM), suggesting the existence of a ternary P450 3A4-b ₅-POR complex, as observed previously with P450 17A1. Gel filtration evidence was also obtained for this ternary complex with P450 3A4. Overall, the results indicated that the affinity of b ₅ for many P450s is very high, and that ternary P450-b ₅-POR complexes are relevant in P450 3A4 reactions as opposed to a shuttle mechanism. SIGNIFICANCE STATEMENT: High-affinity binding of cytochrome b ₅ (b ₅) (K _d < 100 nM) was observed with many drug-metabolizing cytochrome P450 (P450) enzymes. There is some correlation of binding with reported stimulation, with several exceptions. Evidence is provided for a ternary P450 3A4-b ₅-NADPH-P450 reductase complex.

Tight binding of cytochrome b5 to cytochrome P450 17A1 is a critical feature of stimulation of C21 steroid lyase activity and androgen synthesis

It has been recognized for >50 years that cytochrome b₅ (b₅) stimulates some cytochrome P450 (P450)–catalyzed oxidations, but the basis of this function is still not understood well. The strongest stimulation of catalytic activity by b₅ is in the P450 17A1 lyase reaction, an essential step in androgen synthesis from 21-carbon (C21) steroids, making this an excellent model system to interrogate b₅ function. One of the issues in studying b₅–P450 interactions has been the limited solution assay methods. We constructed a fluorescently labeled variant of human b₅ that can be used in titrations. The labeled b₅ bound to WT P450 17A1 with a K_d of 2.5 nM and rapid kinetics, on the order of 1 s⁻¹. Only weak binding was observed with the clinical P450 17A1 variants E305G, R347H, and R358Q; these mutants are deficient in lyase activity, which has been hypothesized to be due to attenuated b₅ binding. K_d values were not affected by the presence of P450 17A1 substrates. A peptide containing the P450 17A1 Arg-347/Arg-358 region attenuated Alexa 488-T70C-b₅ fluorescence at higher concentrations. The addition of NADPH–P450 reductase (POR) to an Alexa 488-T70C-b₅:P450 17A1 complex resulted in a concentration-dependent partial restoration of b₅ fluorescence, indicative of a ternary P450:b₅:POR complex, which was also supported by gel filtration experiments. Overall, these results are interpreted in the context of a dynamic and tight P450 17A1:b₅ complex that also binds POR to form a catalytically competent ternary complex, and variants that disrupt this interaction have low catalytic activity.

Glutamine-451 Confers Sensitivity to Oxidative Inhibition and Heme-Thiolate Sulfenylation of Cytochrome P450 4B1

Human cytochrome P450 (P450) family 4 enzymes are involved in the metabolism of fatty acids and the bioactivation of carcinogenic arylamines and toxic natural products, e.g., 4-ipomeanol. These and other drug-metabolizing P450s are redox sensitive, showing a loss of activity resulting from preincubation with H2O2 and recovery with mild reducing agents [Albertolle, M. W., et al. (2017) J. Biol. Chem. 292, 11230-11242]. The inhibition is due to sulfenylation of the heme-thiolate ligand, as determined by chemopreoteomics and spectroscopy. This phenomenon may have implications for chemical toxicity and observed disease-drug interactions, in which the decreased metabolism of P450 substrates occurs in patients with inflammatory diseases (e.g., influenza and autoimmunity). Human P450 1A2 was determined to be redox insensitive. To determine the mechanism underlying the differential redox sensitivity, molecular dynamics (MD) simulations were employed using the crystal structure of rabbit P450 4B1 (Protein Data Bank entry 5T6Q ). In simulating either the thiolate (Cys-S-) or the sulfenic acid (Cys-SOH) at the heme ligation site, MD revealed Gln-451 in either an "open" or "closed" conformation, respectively, between the cytosol and heme-thiolate cysteine. Mutation to either an isosteric leucine (Q451L) or glutamate (Q451E) abrogated the redox sensitivity, suggesting that this "open" conformation allows for reduction of the sulfenic acid and religation of the thiolate to the heme iron. In summary, MD simulations suggest that Gln-451 in P450 4B1 adopts conformations that may stabilize and protect the heme-thiolate sulfenic acid; mutating this residue destabilizes the interaction, producing a redox insensitive enzyme.

Topography of human cytochrome b5/cytochrome b5 reductase interacting domain and redox alterations upon complex formation

Cytochrome b₅ is the main electron acceptor of cytochrome b₅ reductase. The interacting domain between both human proteins has been unidentified up to date and very little is known about its redox properties modulation upon complex formation. In this article, we characterized the protein/protein interacting interface by solution NMR and molecular docking. In addition, upon complex formation, we measured an increase of cytochrome b₅ reductase flavin autofluorescence that was dependent upon the presence of cytochrome b_5. Data analysis of these results allowed us to calculate a dissociation constant value between proteins of 0.5±0.1μM and a 1:1 stoichiometry for the complex formation. In addition, a 30mV negative shift of cytochrome b₅ reductase redox potential in presence of cytochrome b₅ was also measured. These experiments suggest that the FAD group of cytochrome b₅ reductase increase its solvent exposition upon complex formation promoting an efficient electron transfer between the proteins.

Mechanistic basis of electron transfer to cytochromes p450 by natural redox partners and artificial donor constructs

Cytochromes P450 (P450s) are hemoproteins catalyzing oxidative biotransformation of a vast array of natural and xenobiotic compounds. Reducing equivalents required for dioxygen cleavage and substrate hydroxylation originate from different redox partners including diflavin reductases, flavodoxins, ferredoxins and phthalate dioxygenase reductase (PDR)-type proteins. Accordingly, circumstantial analysis of structural and physicochemical features governing donor-acceptor recognition and electron transfer poses an intriguing challenge. Thus, conformational flexibility reflected by togging between closed and open states of solvent exposed patches on the redox components was shown to be instrumental to steered electron transmission. Here, the membrane-interactive tails of the P450 enzymes and donor proteins were recognized to be crucial to proper orientation toward each other of surface sites on the redox modules steering functional coupling. Also, mobile electron shuttling may come into play. While charge-pairing mechanisms are of primary importance in attraction and complexation of the redox partners, hydrophobic and van der Waals cohesion forces play a minor role in docking events. Due to catalytic plasticity of P450 enzymes, there is considerable promise in biotechnological applications. Here, deeper insight into the mechanistic basis of the redox machinery will permit optimization of redox processes via directed evolution and DNA shuffling. Thus, creation of hybrid systems by fusion of the modified heme domain of P450s with proteinaceous electron carriers helps obviate the tedious reconstitution procedure and induces novel activities. Also, P450-based amperometric biosensors may open new vistas in pharmaceutical and clinical implementation and environmental monitoring.

Polymorphisms of CYP51A1 from cholesterol synthesis: associations with birth weight and maternal lipid levels and impact on CYP51 protein structure

We investigated the housekeeping cytochrome P450 CYP51A1 encoding lanosterol 14α-demethylase from cholesterol synthesis that was so far not directly linked to human disorders. By direct sequencing of CYP51A1 in 188 women with spontaneous preterm delivery and 188 unrelated preterm infants (gestational age <37 weeks) we identified 22 variants where 10 are novel and rare. In infants there were two novel CYP51A1 variants where damaging effects of p.Tyr145Asp from the substrate recognition region, but not p.Asn193Asp, were predicted by PolyPhen2 and SIFT. This was confirmed by molecular modeling showing that Tyr145Asp substitution results in changed electrostatic potential of the CYP51 protein surface and lengthened distance to the heme which prevents hydrogen bonding. The CYP51 Tyr145Asp mutation is rare and thus very interesting for further structure/function relationship studies. From the 12 identified known variants rs6465348 was chosen for family based association studies due to its high minor allele frequency. Interestingly, this CYP51A1 common variant associates with small for gestational age weight in newborns (p = 0.028) and lower blood total cholesterol and low density lipoprotein cholesterol levels in mothers in 2nd trimester of pregnancy (p = 0.042 and p = 0.046 respectively). Our results indicate a new link between a cholesterol synthesis gene CYP51A1 and pregnancy pathologies.

Exhaustive computational search of ionic-charge clusters that mediate interactions between mammalian cytochrome P450 (CYP) and P450-oxidoreductase (POR) proteins

In this work, a model for the interaction between CYP2B4 and the FMN domain of rat P450-oxidoreductase is built using as template the structure of a bacterial redox complex. Amino acid residues identified in the literature as cytochrome P450 (CYP)-redox partner interfacial residues map to the interface in our model. Our model supports the view that the bacterial template represents a specific electron transfer complex and moreover provides a structural framework for explaining previous experimental data. We have used our model in an exhaustive search for complementary pairs of mammalian CYP and P450-oxidoreductase (POR) charge clusters. We quantitatively show that among the previously defined basic clusters, the 433K-434R cluster is the most dominant (32.3% of interactions) and among the acidic clusters, the 207D-208D-209D cluster is the most dominant (29%). Our analysis also reveals the previously not described basic cluster 343R-345K (16.1% of interactions) and 373K (3.2%) and the acidic clusters 113D-115E-116E (25.8%), 92E-93E (12.9%), 101D (3.2%) and 179E (3.2%). Cluster pairings among the previously defined charge clusters include the pairing of cluster 421K-422R to cluster 207D-208D-209D. Moreover, 433K-434R and 207D-208D-209D, respectively the dominant positively and negatively charged clusters, are uncorrelated. Instead our analysis suggests that the newly identified cluster 113D-115E-116E is the main partner of the 433K-434R cluster while the newly described cluster 343R-345K is correlated to the cluster 207D-208D-209D.

Electron transfer properties and catalytic competence of cytochrome b5 in the fusion protein Hmwb5-EGFP in reactions catalyzed by cytochrome P450 3A4

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 6beta-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.

Molecular evolution of a steroid hydroxylating cytochrome P450 using a versatile steroid detection system for screening

Molecular evolution is a powerful tool for improving or changing activities of enzymes for their use in biotechnological processes. Cytochromes P450 are highly interesting enzymes for biotechnological purposes because they are able to hydroxylate a broad variety of substrates with high regio- and stereoselectivity. One promising steroid hydroxylating cytochrome P450 for biotechnological applications is CYP106A2 from Bacillus megaterium ATCC 13368. It is one of a few known bacterial cytochromes P450 able to transform steroids such as progesterone and 11-deoxycortisol. CYP106A2 can be easily expressed in Escherichia coli with a high yield and can be reconstituted using the adrenal redox proteins, adrenodoxin and adrenodoxin reductase. We developed a simple screening assay for this system and performed random mutagenesis of CYP106A2, yielding variants with improved 11-deoxycortisol and progesterone hydroxylation activity. After two generations of directed evolution, we were able to improve the k (cat)/K (m) of the 11-deoxycortisol hydroxylation by a factor of more than four. At the same time progesterone conversion was improved about 1.4-fold. Mapping the mutations identified in catalytically improved CYP106A2 variants into the structure of a CYP106A2 model suggests that these mutations influence the mobility of the F/G loop, and the interaction with the redox partner adrenodoxin. The results show the evolution of a soluble steroid hydroxylase as a potential new catalyst for the production of steroidogenic compounds.

Solution NMR structure of putidaredoxin-cytochrome P450cam complex via a combined residual dipolar coupling-spin labeling approach suggests a role for Trp106 of putidaredoxin in complex formation

The 58-kDa complex formed between the [2Fe-2S] ferredoxin, putidaredoxin (Pdx), and cytochrome P450cam (CYP101) from the bacterium Pseudomonas putida has been investigated by high-resolution solution NMR spectroscopy. Pdx serves as both the physiological reductant and effector for CYP101 in the enzymatic reaction involving conversion of substrate camphor to 5-exo-hydroxycamphor. In order to obtain an experimental structure for the oxidized Pdx-CYP101 complex, a combined approach using orientational data on the two proteins derived from residual dipolar couplings and distance restraints from site-specific spin labeling of Pdx has been applied. Spectral changes for residues in and near the paramagnetic metal cluster region of Pdx in complex with CYP101 have also been mapped for the first time using (15)N and (13)C NMR spectroscopy, leading to direct identification of the residues strongly affected by CYP101 binding. The new NMR structure of the Pdx-CYP101 complex agrees well with results from previous mutagenesis and biophysical studies involving residues at the binding interface such as formation of a salt bridge between Asp38 of Pdx and Arg112 of CYP101, while at the same time identifying key features different from those of earlier modeling studies. Analysis of the binding interface of the complex reveals that the side chain of Trp106, the C-terminal residue of Pdx and critical for binding to CYP101, is located across from the heme-binding loop of CYP101 and forms non-polar contacts with several residues in the vicinity of the heme group on CYP101, pointing to a potentially important role in complex formation.

Phospholipid membranes affect tertiary structure of the soluble cytochrome b5 heme-binding domain

The influence of charged phospholipid membranes on the conformational state of the water-soluble fragment of cytochrome b5 has been investigated by a variety of techniques at neutral pH. The results of this work provide the first evidence that aqueous solutions with high phospholipid/protein molar ratios (pH 7.2) induce the cytochrome to undergo a structural transition from the native conformation to an intermediate state with molten-globule like properties that occur in the presence of an artificial membrane surface and that leads to binding of the protein to the membrane. At other phospholipid/protein ratios, equilibrium was observed between cytochrome free in solution and cytochrome bound to the surface of vesicles. Inhibition of protein binding to the vesicles with increasing ionic strength indicated for the most part an electrostatic contribution to the stability of cytochrome b5-vesicle interactions at pH 7.2. The possible physiological role of membrane-induced conformational change in the structure of cytochrome b5 upon the interaction with its redox partners is discussed.

Analysis of the interactions of cytochrome b5 with flavocytochrome P450 BM3 and its domains

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.

Engineering, expression, purification, and physicochemical characterization of a chimeric protein, full-length cytochrome b(5)--green fluorescence protein (HMWb5--EGFP)

In this article we report on construction of expression vector, heterologous expression in Escherichia coli, isolation, purification, and physicochemical characterization of an artificial chimeric protein HMWb(5)-EGFP consisting of full-length cytochrome b(5) (HMWb(5)) and green fluorescence protein (EGFP) from Aequorea. Optimization of expression conditions yielded an expression level up to 1500 nmol of chimeric protein per liter of culture. Recombinant chimeric protein HMWb(5)-EGFP was purified from cell membranes by using metal-affinity chromatography. It possesses physicochemical, spectral, and fluorescence properties of cytochrome b(5) and EGFP indicating independent character of protein folding in frames of the chimera. It is shown that there is a fluorescent resonance energy transfer in HMWb(5)-EGFP between the fluorophore of EGFP and heme of cytochrome b(5), and the distance between chromophores in the chimeric protein is approximately 67.3 A. The chimeric protein was shown to exist as a monomer in aqueous solution in the presence of detergents. The data indicate that the HMWb(5)-EGFP designed in the present work is a very promising model for modern biosensors and an instrument to study protein-protein interactions.

FTIR studies of the redox partner interaction in cytochrome P450: The Pdx–P450cam couple

Recently we have developed a new approach to study protein-protein interactions using Fourier transform infrared spectroscopy in combination with titration experiments and principal component analysis (FTIR-TPCA). In the present paper we review the FTIR-TPCA results obtained for the interaction between cytochrome P450 and the redox partner protein in two P450 systems, the Pseudomonas putida P450cam (CYP101) with putidaredoxin (P450cam-Pdx), and the Bacillus megaterium P450BM-3 (CYP102) heme domain with the FMN domain (P450BMP-FMND). Both P450 systems reveal similarities in the structural changes that occur upon redox partner complex formation. These involve an increase in beta-sheets and alpha-helix content, a decrease in the population of random coil/3(10)-helix structure, a redistribution of turn structures within the interacting proteins and changes in the protonation states or hydrogen-bonding of amino acid carboxylic side chains. We discuss in detail the P450cam-Pdx interaction in comparison with literature data and conclusions drawn from experiments obtained by other spectroscopic techniques. The results are also interpreted in the context of a 3D structural model of the Pdx-P450cam complex.

Putidaredoxin-to-cytochrome P450cam electron transfer: differences between the two reductive steps required for catalysis

Cytochrome P450cam (P450cam) is the terminal monooxygenase in a three-component camphor-hydroxylating system from Pseudomonas putida. The reaction cycle requires two distinct electron transfer (ET) processes from the [2Fe-2S] containing putidaredoxin (Pdx) to P450cam. Even though the mechanism of interaction and ET between the two proteins has been under investigation for over 30 years, the second reductive step and the effector role of Pdx are not fully understood. We utilized mutagenesis, kinetic, and computer modeling approaches to better understand differences between the two Pdx-to-P450cam ET events. Our results indicate that interacting residues and the ET pathways in the complexes formed between reduced Pdx (Pdx(r)) and the ferric and ferrous dioxygen-bound forms of P450cam (oxy-P450cam) are different. Pdx Asp38 and Trp106 were found to be key players in both reductive steps. Compared to the wild-type Pdx, the D38A, W106A, and delta106 mutants exhibited considerably higher Kd values for ferric P450cam and retained ca. 20% of the first electron transferring ability. In contrast, the binding affinity of the mutants for oxy-P450cam was not substantially altered while the second ET rates were <1%. On the basis of the kinetic and modeling data we conclude that (i) P450cam-Pdx interaction is highly specific in part because it is guided/controlled by the redox state of both partners; (ii) there are alternative ET routes from Pdx(r) to ferric P450cam and a unique pathway to oxy-P450cam involving Asp38; (iii) Pdx Trp106 is a key structural element that couples the second ET event to product formation possibly via its "push" effect on the heme-binding loop.

Expression of outer mitochondrial membrane cytochrome b 5 in Escherichia coli. Purification of the recombinant protein and studies of its interaction with electron-transfer partners

In the present work, we report expression in Escherichia coli, purification, and characterization of recombinant full-length cytochrome b(5) from outer mitochondrial membrane. Optimization of expression conditions for cytochrome b(5) from outer mitochondrial membrane allowed reaching expression level up to 10(4) nmol of the hemeprotein per liter of culture. Recombinant cytochrome b(5) from outer mitochondrial membrane was purified from cell lysate by using metal-affinity chromatography. It has physicochemical, spectral, and immunochemical properties similar to those of cytochrome b(5) from rat liver outer mitochondrial membrane. Immobilized recombinant mitochondrial cytochrome b(5) was used as affinity ligand to study its interaction with electron transfer proteins. By using this approach, it is shown that in interaction of NADPH:cytochrome P450 reductase with both forms of cytochrome b(5) an important role is played by hydrophobic interactions between proteins, although the contribution of these interactions in complex formation with NADPH:cytochrome P450 reductase is different for isoforms of cytochrome b(5).

Comparison of the complexes formed by cytochrome P450cam with cytochrome b5 and putidaredoxin, two effectors of camphor hydroxylase activity

Structural perturbations in cytochrome P450cam (CYP101) induced by the soluble fragment of cytochrome b5, a nonphysiological effector of CYP101, were investigated by NMR spectroscopy and compared with the perturbations induced by the physiological reductant and effector putidaredoxin (Pdx). Chemical shifts of perdeuterated [U-15N]CYP101 backbone amide (NH) resonances were monitored as a function of cytochrome b5 concentration by 1H-15N TROSY-HSQC experiments. The association of cytochrome b5 with the reduced CYP101-camphor-carbon monoxide complex (CYP-S-CO) perturbs many of the same resonances that Pdx does, including regions of the CYP101 molecule implicated in substrate access and orientation. The perturbations are smaller in magnitude than those observed with Pdx(r) due to a lower binding affinity (a Kd of 13 +/- 3 mM, for the reduced cytochrome b5-CYP-S-CO complex compared to a Kd of 26 +/- 12 microM for the Pdx-CYP-S-CO complex). The results are in accord with our previous suggestion that the observed perturbations are related to effector activity and support the proposal that the primary role of the effector is to populate the active conformation of CYP101 to prevent uncoupling [Pochapsky, S. S., et al. (2003) Biochemistry 42, 5649-5656]. A titratable perturbation is observed at the 1H resonance of the 8-CH3 group of CYP101-bound camphor upon addition of cytochrome b5, a phenomenon also associated with the formation of the CYP101 x Pdx complex, albeit with larger perturbations [Wei, J. Y., et al. (2005) J. Am. Chem. Soc. 127, 6974-6976]. The effector activity of the particular rat cytochrome b5 construct used for NMR studies was confirmed by monitoring the enzymatic turnover that yielded 5-exo-hydroxycamphor using gas chromatography and mass spectrometry. Finally, the common features of the perturbations observed in the NMR spectra of the two complexes are discussed, and their relevance to effector activity is considered.

Identification of the interactions between cytochrome P450 2E1 and cytochrome b5 by mass spectrometry and site-directed mutagenesis

The reaction cycles of cytochrome P450s (P450) require input of two electrons. Electrostatic interactions are considered important driving forces in the association of P450s with their redox partners, which in turn facilitates the transfer of the two electrons. In this study, the cross-linking reagent, 1-ethyl-3-[3-dimethylaminopropyl]carbodiimide hydrochloride (EDC), was used to covalently link cytochrome P450 2E1 (CYP2E1) with cytochrome b(5) (b(5)) through the formation of specific amide bonds between complementary charged residue pairs. Cross-linked peptides in the resulting protein complex were distinguished from non-cross-linked peptides using an (18)O-labeling method on the basis that cross-linked peptides incorporate twice as many (18)O atoms as non-cross-linked peptides during proteolysis conducted in (18)O-water. Subsequent tandem mass spectrometric (MS/MS) analysis of the selected cross-linked peptide candidates led to the identification of two intermolecular cross-links, Lys(428)(CYP2E1)-Asp(53)(b(5)) and Lys(434)(CYP2E1)-Glu(56)(b(5)), which provides the first direct experimental evidence for the interacting orientations of a microsomal P450 and its redox partner. The biological importance of the two ion pairs for the CYP2E1-b(5) interaction, and the stimulatory effect of b(5), was confirmed by site-directed mutagenesis. Based on the characterized cross-links, a CYP2E1-b(5) complex model was constructed, leading to improved insights into the protein interaction. The described method is potentially useful for mapping the interactions of various P450 isoforms and their redox partners, because the method is relatively rapid and sensitive, and is capable of suggesting not only protein interacting regions, but also interacting orientations.

Structural aspects of ligand binding to and electron transfer in bacterial and fungal P450s

Cytochrome P450 enzymes are heme-containing monooxygenases that are named after an absorption band at 450 nm when complexed with carbon monoxide. They catalyze a wide variety of reactions and are unique in their ability to hydroxylate nonactivated hydrocarbons. P450 enzymes are involved in numerous biological processes, which include the biosynthesis of lipids, steroids, antibiotics, and the degradation of xenobiotics. In line with the variety of reactions catalyzed, the size of their substrates varies significantly. Some P450s have open active sites (e.g., BM3), and some have shielded active sites that open only transiently (e.g., P450cam), whereas others bind the substrate only when attached to carrier proteins (e.g., Oxy proteins). Structural aspects of both organic and gaseous ligand binding and electron transfer are described.

The cationic charges on Arg347, Arg358 and Arg449 of human cytochrome P450c17 (CYP17) are essential for the enzyme's cytochrome b5-dependent acyl-carbon cleavage activities

CYP17 (17alpha-hydroxylase-17,20-lyase; also P450c17 or P450(17alpha)) catalyses the17alpha-hydroxylation of progestogens and the subsequent acyl-carbon cleavage of the 17alpha-hydroxylated products (lyase activity) in the biosynthesis of androgens. The enzyme also catalyses another type of acyl-carbon cleavage (direct cleavage activity) in which the 17alpha-hydroxylation reaction is by-passed. Human CYP17 is heavily dependent on the presence of the membrane form of cytochrome b(5) for both its lyase and direct cleavage activities. In the present study it was found that substitution of human CYP17 amino acids, Arg(347), Arg(358) and Arg(449), with non-cationic residues, yielded variants that were impaired in the two acyl-carbon bond cleavage activities, quantitatively to the same extent and these were reduced to between 3 and 4% of the wild-type protein. When the arginines were replaced by lysines, the sensitivity to cytochrome b(5) was restored and the acyl-carbon cleavage activities were recovered. All of the human mutant CYP17 proteins displayed wild-type hydroxylase activity, in the absence of cytochrome b(5). The results suggest that the bifurcated cationic charges at Arg(347), Arg(358) and Arg(449) make important contributions to the formation of catalytically competent CYP17.cytochrome b(5) complex. The results support our original proposal that the main role of cytochrome b(5) is to promote protein conformational changes which allow the iron-peroxo anion to form a tetrahedral adduct that fragments to produce the acyl-carbon cleavage products.

NMR study on the structural changes of cytochrome P450cam upon the complex formation with putidaredoxin. Functional significance of the putidaredoxin-induced structural changes

We investigated putidaredoxin-induced structural changes in carbonmonoxy P450cam by using NMR spectroscopy. The resonance from the beta-proton of the axial cysteine was upfield shifted by 0.12 ppm upon the putidaredoxin binding, indicating that the axial cysteine approaches to the heme-iron by about 0.1 A. The approach of the axial cysteine to the heme-iron would enhance the electronic donation from the axial thiolate to the heme-iron, resulting in the enhanced heterolysis of the dioxygen bond. In addition to the structural perturbation on the axial ligand, the structural changes in the substrate and ligand binding site were observed. The resonances from the 5-exo- and 9-methyl-protons of d-camphor, which were newly identified in this study, were upfield shifted by 1.28 and 0.20 ppm, respectively, implying that d-camphor moves to the heme-iron by 0.15-0.7 A. Based on the radical rebound mechanism, the approach of d-camphor to the heme-iron could promote the oxygen transfer reaction. On the other hand, the downfield shift of the resonance from the gamma-methyl group of Thr-252 reflects the movement of the side chain away from the heme-iron by approximately 0.25 A. Because Thr-252 regulates the heterolysis of the dioxygen bond, the positional rearrangement of Thr-252 might assist the scission of the dioxygen bond. We, therefore, conclude that putidaredoxin induces the specific heme environmental changes of P450cam, which would facilitate the oxygen activation and the oxygen transfer reaction.

Functional interaction of cytochrome P450 with its redox partners: a critical assessment and update of the topology of predicted contact regions

The problem of donor-acceptor recognition has been the most important and intriguing one in the area of P450 research. The present review outlines the topological background of electron-transfer complex formation, showing that the progress in collaborative investigations, combining physical techniques with chemical-modification and immunolocalization studies as well as site-directed mutagenesis experiments, has increasingly enabled the substantiation of hypothetical work resulting from homology modelling of P450s. Circumstantial analysis reveals the contact regions for redox proteins to cluster on the proximal face of P450s, constituting parts of the highly conserved, heme-binding core fold. However, more variable structural components located in the periphery of the hemoprotein molecules also participate in donor docking. The cross-reactivity of electron carriers, purified from pro- and eukaryotic sources, with a diversity of P450 species points at a possible evolutionary conservation of common anchoring domains. While electrostatic mechanisms appear to dominate orientation toward each other of the redox partners to generate pre-collisional encounter complexes, hydrophobic forces are likely to foster electron transfer events by through-bonding or pi-stacking interactions. Moreover, electron-tunneling pathways seem to be operative as well. The availability of new P450 crystal structures together with improved validation strategies will undoubtedly permit the production of increasingly satisfactory three-dimensional donor-acceptor models serving to better understand the molecular principles governing functional association of the redox proteins.

The many roles of cytochrome b5

Four distinct suggestions have been made to explain the mechanism of the cytochrome b(5)-imposed positive modifier action of the cytochrome P450 monooxygenase reaction. The first mechanism involves a direct input of an electron into the monooxygenase cycle. This is the second of the two electrons necessary for activation of molecular oxygen, and appears to be a rate-limiting step in the monooxygenase reaction. P450 monooxygenases all appear to be uncoupled to varying extents, releasing superoxide and hydrogen peroxide instead of oxidized substrate. A second mechanism suggests that cytochrome b(5) acts as a positive modifier of the monooxygenase by decreasing the extent of uncoupling of the monooxygenase reaction. The implication is that a slow input of the second electron allows uncoupling of a superoxide anion instead of formation of two-electron reduced oxygen. Faster input of the second electron via cytochrome b(5) would result in formation of more of the activated oxygen that reacts with substrate to form product. A third suggestion involves formation of a two-hemoprotein complex between cytochrome b(5) and cytochrome P450 that allows acceptance of two electrons from NADPH-cytochrome P450 reductase. Uncomplexed cytochrome P450 accepts an electron from the reductase, dissociates from it, binds oxygen, and re-associates with the reductase to accept another electron. Complexation with cytochrome b(5) enhances the rate of formation of the active oxygen by obviating the need for two interactions with reductase. The fourth mechanism has cytochrome b(5) serving as an effector without a reduction-oxidation role in the monooxygenation reaction. This effector function may be to enhance the breakdown of the oxygenated hemoprotein to products or to facilitate flow of electrons through the system.

X-ray crystallography, CD and kinetic studies revealed the essence of the abnormal behaviors of the cytochrome b5 Phe35-->Tyr mutant

Conserved phenylalanine 35 is one of the hydrophobic patch residues on the surface of cytochrome b5 (cyt b5). This patch is partially exposed on the surface of cyt b5 while its buried face is in direct van der Waals' contact with heme b. Residues Phe35 and Phe/Tyr74 also form an aromatic channel with His39, which is one of the axial ligands of heme b. By site-directed mutagenesis we have produced three mutants of cyt b5: Phe35-->Tyr, Phe35-->Leu, and Phe35-->His. We found that of these three mutants, the Phe35-->Tyr mutant displays abnormal properties. The redox potential of the Phe35-->Tyr mutant is 66 mV more negative than that of the wild-type cyt b5 and the oxidized Phe35-->Tyr mutant is more stable towards thermal and chemical denaturation than wild-type cyt b5. In this study we studied the most interesting mutant, Phe35-->Tyr, by X-ray crystallography, thermal denaturation, CD and kinetic studies of heme dissociation to explore the origin of its unusual behaviors. Analysis of crystal structure of the Phe35-->Tyr mutant shows that the overall structure of the mutant is basically the same as that of the wild-type protein. However, the introduction of a hydroxyl group in the heme pocket, and the increased van der Waals' and electrostatic interactions between the side chain of Tyr35 and the heme probably result in enhancement of stability of the Phe35-->Tyr mutant. The kinetic difference of the heme trapped by the heme pocket also supports this conclusion. The detailed conformational changes of the proteins in response to heat have been studied by CD for the first time, revealing the existence of the folding intermediate.

Enhanced electron transfer and lauric acid hydroxylation by site-directed mutagenesis of CYP119

CYP119, a cytochrome P450 from a thermophilic organism for which a crystal structure is available, is shown here to hydroxylate lauric acid in a reaction supported by putidaredoxin and putidaredoxin reductase. This fatty acid hydroxylation activity is increased 15-fold by T214V and D77R mutations. The T214V mutation increases the rate by facilitating substrate binding and enhancing the associated spin state change, whereas the D77R mutation improves binding of the heterologous redox partner putidaredoxin to CYP119 and the rate of electron transfer from it to the heme group. A sequence alignment with P450(cam) can, therefore, be used to identify a part of the binding site for putidaredoxin on an unrelated P450 enzyme. This information can be used to engineer by mutagenesis an improved complementarity of the protein-protein interface that results in improved electron transfer from putidaredoxin to the P450 enzyme. As a result, the catalytic activity of the thermo- and barostable CYP119 has been incorporated into a catalytic system that hydroxylates fatty acids.

Cloning, sequencing, and characterization of CYP1A1 cDNA from leaping mullet (Liza Saliens) liver and implications for the potential functions of its conserved amino acids

A 2,037 bp CYP1A1 cDNA (GenBank AF072899) was cloned through screening of a lambdaZipLox cDNA library constructed from the liver of a leaping mullet (Liza saliens) fish captured from Izmir Bay on the Aegean coast of Turkey using rainbow trout CYP1A1 cDNA as a probe. This clone has a 130 bp 5'-flanking region, a 1,563 bp open reading frame (ORF) encoding a 521-amino acid protein (58,972 Da), and a 344 bp 3'-untranslated region without a poly (A) tail. Alignment of the deduced amino acids of CYP1A1 cDNAs showed 58% and 69-96% identities with human and 12 other fish species, respectively. Southern blot analysis suggested that this CYP1A1 cDNA was from a single-copy gene. Based on the comparison with CYP1A1 genes reported for fish and mammals, the leaping mullet CYP1A1 gene is probably split into 7 exons. The intron insertion sites were predicted. Alignment of the CYP1A1 cDNA encoded amino acids from 13 fish and 7 mammalian species disclosed differences in highly conserved amino acids between aquatic and land vertebrates. The possible associated secondary structure; conserved motifs and substrate-binding sites were discussed. The phylogenetic relationships of CYP1A1s among 13 fish species were analyzed by a distance method.

Complex formation of cytochrome P450cam with Putidaredoxin. Evidence for protein-specific interactions involving the proximal thiolate ligand

We have performed resonance Raman studies on ferrous NO- and CO-adducts of cytochrome P450(cam) and investigated the effects of diprotein complex formation with reduced putidaredoxin. We have found that the Fe-NO stretching mode of NO-P450(cam) can be resolved into two peaks at 551 and 561 cm(-1), and the binding of putidaredoxin increases the intensity of the high frequency component. Because the Fe-NO mode has been shown to be more sensitive to the nature of the heme proximal ligand than to the distal pocket environment, such a perturbation upon putidaredoxin binding is suggestive of changes in conformation or electronic structure that affect the proximal iron-cysteine bond. In accordance with this idea, the isotope shifts for the Fe-XO stretching and Fe-X-O bending modes (X = N or C) are insensitive to the presence or absence of putidaredoxin, indicating that the geometry of the Fe-X-O unit is not significantly altered by the complex formation. On the other hand, complex formation does induce a perturbation of the low frequency heme vibrational modes, suggesting that alterations of the heme electronic structure and/or geometry take place when putidaredoxin binds. We also find that cytochrome b(5) minimally affects the heme active site of the enzyme, although both putidaredoxin and cytochrome b(5) bind to the same or similar site on P450(cam). These observations suggest that there is a key specific interaction between P450(cam) and putidaredoxin, and that this interaction increases the population of a protein conformation that exhibits structural and/or electronic distortions of the heme group associated with the proximal side of the heme pocket and the S --> Fe electron donation. These electronic and structural changes are potentially correlated with H-bonding to the proximal cysteine.

Revelation of ternary complexes between redox partners in cytochrome P450-containing monooxygenase systems by the optical biosensor method

Formation of binary and ternary complexes in the water-soluble cytochrome P450cam (P450cam)-containing as well as in the membrane P4502B4(2B4)- and the mixed P450scc-containing monooxygenase systems was investigated in real time by the 'resonant mirror' optical biosensor method. It was shown that the inter-protein electron transfer occurs not only during complex formation but also upon random collision--as was the case with the d-Fp/d-b5 pair (2B4 system). Binary complexes may be either facilitative to electron transfer (electron-transfer complexes) or prohibitive to it (non-productive complexes). Although the binary PdR/Pd and P450cam/Pd complex formation (within the P450cam-system) as well as the binary AdR/Ad and P450scc/Ad complex formation (within the P450scc-system) does occur, the lifetimes of these complexes formed are several orders of magnitude higher than the time required for realization of a complete hydroxylation cycle. At the same time, the lifetimes of the ternary PdR/Pd/P450cam and AdR/Ad/P450scc complexes are sufficient to permit the realization of a complete hydroxylation cycle in either of these systems. For the membrane P450 2B4 system, the formation of both the binary (Fp/2B4 and 2B4/b5) and ternary (Fp/2B4/b5) complexes was registered. The lifetimes of the binary Fp/2B4 and the ternary Fp/2B4/b5 complexes are sufficient for realization of a complete hydroxylation cycle in each of them.

Molecular recognition in the p450cam monooxygenase system: direct monitoring of protein-protein interactions by using optical biosensor

A real-time optical biosensor study on the interactions between putidaredoxin reductase (PdR), putidaredoxin (Pd), and cytochrome P450cam (P450cam) within the P450cam system was conducted. The binary Pd/P450cam and Pd/PdR complexes were revealed and kinetically characterized. The dominant role of electrostatic interactions in formation of productive electron transfer complexes was demonstrated. It was found that Pd/P450cam complex formation and decay obeys biphasic kinetics in contrast to the monophasic one for complexes formed by other redox partners within the system. Evidence for PdR/P450cam complex formation was obtained. It was found that, in contrast to Pd, which binds only to its redox partners, PdR and P450cam were able to form PdR/PdR and P450cam/P450cam complexes. A ternary PdR/Pd/P450cam complex was also registered. Its lifetime was sufficient to permit up to 60 turnovers to occur. The binding of Pd to P450cam and to PdR within the ternary complex occurred at distinct sites, with Pd serving as a bridge between the two proteins.

Charge-Dependent Sidedness of Cytochrome P450 Forms Studied by Quartz Crystal Microbalance and Atomic Force Microscopy

Quartz crystal microbalance (QCM) resonance measurements were used to examine the surface charge characteristics of cytochrome P450 forms and the influence of charge on the docking of redox partners like cytochrome b5. The distal surface of cytochrome P450 (CYP)101 (pI = 4.5), relative to the heme, is fairly anionic, as is the proximal surface. The latter, however, also has two cationic clusters. A considerably greater extent of CYP101 binding was seen to the cationic, polyethylene-surfaced resonators. CYP2B4 (pI = 8.5) preferentially bound to the polyanionic, polystyrene sulfonate-surfaced resonators. Cytochrome b5 is an acidic protein that had a preferential binding to the poly(ethyleneimine (PEI)-surfaced resonators. When binding to CYP2B4-surfaced films, cytochrome b5 preferentially bound to those cytochrome P450 molecules that were adsorbed to cationic (PEI) films. It is suggested that adsorption of CYP2B4 to an anionic poly(styrenesulfonate) (PSS) surface is with cationic clusters that include the cytochrome b5 docking domain. This diminishes the extent of docking of the cytochrome b5. In contrast, when CYP2B4 is adsorbed to a cationic film the proximal surface with the cytochrome b5-docking site is available for cytochrome b5 binding. A film of the polycation PEI was adsorbed to the silver QCM surface. It formed polymer islands when viewed with atomic force microscopy. Polyanionic PSS was adsorbed intermittently with the PEI. By the third and fourth layer of polyions the polymer islands were essentially merged and protein adsorption as a fourth or fifth layer formed a nearly continuous film. CYP101 was seen to adsorb as globules with a molecular diameter of about 10 nm. CYP2B4 adsorbed to the polyionic films had a slightly elliptical globular shape, also with a molecular diameter of about 10 nm.

Proximal cysteine residue is essential for the enzymatic activities of cytochrome P450cam

To investigate the functional and structural roles of the proximal thiolate ligand in cytochrome P450cam, we prepared the C357H mutant of the enzyme in which the axial cysteine residue (Cys357) was replaced with a histidine residue. We obtained the unstable C357H mutant by developing a new preparation procedure involving in vitro folding of P450cam from the inclusion bodies. The C357H mutant in the ferrous-CO form exhibited the Soret peak at 420 nm and the Fe-CO stretching line at 498 cm-1, indicating a neutral histidine residue as the axial ligand. However, another internal ligand is coordinated to the heme iron as the sixth ligand in the ferric and ferrous forms of the C357H mutant, suggesting the collapse of the substrate-binding site. The C357H mutant showed no catalytic activity for camphor hydroxylation and the reduced heterolytic/homolytic ratio of the O-O bond scission in the reaction with cumene hydroperoxide. The present observations indicate that the thiolate coordination in P450cam is important for the construction of the heme pocket and the heterolysis of the O-O bond.

Protein engineering of cytochromes P-450

The cytochromes P-450 are an immensely important superfamily of heme-containing enzymes. They catalyze the monooxygenation of an enormous range of substrates. In bacteria, cytochromes P-450 are known to catalyze the hydroxylation of environmentally significant substrates such as camphor, phenolic compounds and many herbicides. In eukaryotes, these enzymes perform key roles in the synthesis and interconversion of steroids, while in mammals hepatic cytochromes P-450 are vital for the detoxification of many drugs. As such, the cytochromes P-450 are of considerable interest in medicine and biotechnology and are obvious targets for protein engineering. The purpose of this article is to illustrate the ways in which protein engineering has been used to investigate and modify the properties of cytochromes P-450. Illustrative examples include: the manipulation of substrate selectivity and regiospecificity, the alteration of membrane binding properties, and probing the route of electron transfer.

Interactions between redox partners in various cytochrome P450 systems: functional and structural aspects

The various types of redox partner interactions employed in cytochrome P450 systems are described. The similarities and differences between the redox components in the major categories of P450 systems present in bacteria, mitochondria and microsomes are discussed in the light of the accumulated evidence from X-ray crystallographic and NMR spectroscopic determinations. Molecular modeling of the interactions between the redox components in various P450 mono-oxygenase systems is proposed on the basis of structural and mutagenesis information, together with experimental findings based on chemical modification of key residues likely to be associated with complementary binding sites on certain typical P450 isoforms and their respective redox partners.

The effect of mutation at valine-45 on the stability and redox potentials of trypsin-cleaved cytochrome b5

In an attempt to elucidate the determinants of redox potential and protein stability in cytochrome b5, three mutants at a highly conserved residue Val45, which is a member of heme hydrophobic pocket residues have been characterized. The V45Y mutant was designed to introduce a bulkier residue and a hydroxyl group to the heme pocket. The mutants V45H and V45E were constructed to test the effect of positive and negative charge on the stability and redox potential of proteins. The influence of these mutants on the protein stability towards thermal, urea, acid, ethanol and on the redox potential were studied. It is concluded that the decrease of hydrophobic free energy and the larger volume of the tyrosine make the phenylhydroxyl group of tyrosine still sitting inside the hydrophobic pocket, while the side chain of the mutant V45E and V45H shift away from the heme pocket. The redox potentials of mutants V45Y, V45H, V45E and wild-type of cytochrome b5 are -35 mV, 8 mV, -26 mV and -3 mV, respectively. The bigger change of the V45Y on redox potential is due to the close contact between the hydroxyl group and the heme, while the changes of the V45E and V45H result from the alteration of charge density and distribution around the heme. Different relative stability of these mutants towards heat have been observed with the order: WT > V45Y-V45H > V45E being both in the oxidized and reduced state. The relative stability induced by addition of urea decreases in the order: WT > V45Y > V45H > V45E. These results suggest that the difference in the hydrophobic free energy is a major factor contributing to the stability of the Val45 mutants. Also the loose of the helix III in the mutant V45E makes it more unstable. These results indicate that residue Val45 plays an important role in the stability and redox potential of the protein.

Synthetic peptide mimics of a predicted topographical interaction surface: the cytochrome P450 2B1 recognition domain for NADPH-cytochrome P450 reductase

In order to identify the cytochrome P450-binding domain for NADPH-cytochrome P450 reductase, synthetic peptide mimics of predicted surface regions of rat cytochrome P450 2B 1 were constructed and evaluated for inhibition of the P450-reductase interaction. A peptide corresponding to residues 116-134, which includes the C helix, completely inhibited reductase-mediated benzphetamine demethylation by purified P450 2B1. Replacement of Arg-125 by Glu yielded a noninhibitory peptide, suggesting that this residue significantly contributes to the reductase-P450 interaction. Additional P450 peptides were prepared which correspond to combinations of regions distant in primary sequence, but predicted to be spatially proximate. A peptide derived from segments of the C and L helices was a more potent inhibitor than peptides derived from either segment alone. This topographically designed peptide not only inhibited P450 2B1 in its purified form, but also when membrane-bound in rat liver microsomes. The peptide also inhibited microsomal aryl hydrocarbon hydroxylase, aniline hydroxylase, and erythromycin demethylase activities derived from other P450s. These results indicate that the C and L helices contribute to a reductase-binding site common to multiple P450s, and present a peptide mimic for this region that is useful for inhibition of P450-mediated microsomal activities.

Mutations of glutamate-84 at the putative potassium-binding site affect camphor binding and oxidation by cytochrome p450cam

Cytochrome P450cam (CYP101) from Pseudomonas putida is unusual among P450 enzymes in that it exhibits co-operative binding between the substrate camphor and a potassium ion. This behaviour has been investigated by mutagenesis of Glu84, a surface residue which forms part of the cation-binding site. Substitutions that neutralize or reverse the charge of this side chain are shown to disrupt the co-operativity of potassium and camphor binding by P450cam, and also to influence the catalytic activity. In particular, replacement of Glu84 by positively charged residues such as lysine results in increased high-spin haem fractions and camphor turnover activities in the absence of potassium, along with decreased camphor dissociation constants. However, in the presence of potassium the camphor dissociation constants of these mutants are significantly increased compared with the wild-type, although the camphor turnover activities remain marginally higher. In contrast, substitution by aspartate results in tighter binding of both potassium and camphor, but has little effect on the enzymatic activity. In all cases the reaction remains essentially 100% coupled and gives 5-exo-hydroxycamphor as the only product. These results suggest that an anionic side chain at the 84 position is crucial for the co-operativity of camphor and cation binding, and that the physiological role for potassium binding by cytochrome P450cam is to promote camphor binding even at the expense of turnover rate, thus allowing the organism to utilize low environmental concentrations of this substrate for growth.

Methanol-induced unfolding and refolding of cytochrome b5 and its P40V mutant monitored by UV-visible, CD, and fluorescence spectra

In order to illustrate the structural importance of proline-40 of cytochrome b5 (Cyt b5), the P40V mutant gene was constructed. Unfolding and refolding of Cyt b5 induced by methanol was investigated by means of the UV-visible spectrum, circular dichroism, and the fluorescence spectrum. Methanol denaturation of Cyt b5 is a cooperative process, that is, the heme group dissociates from the heme pocket accompanied by unfolding of the polypeptide chain both in the secondary and tertiary structures. Substitution of proline by valine reduces the stability of the mutant under methanol denaturation. The unfolding process is almost reversible by dilution. During refolding, the denatured polypeptide must be folded to a more ordered structure prior to the heme capture. Pro40 plays an important role in modulating the protein's stability. The role of tyrosine in the unfolding and refolding of Cyt b5 is evaluated for the first time. A mechanism of methanol denaturation is also proposed.

Surface-modified mutants of cytochrome P450cam: enzymatic properties and electrochemistry

We report the electrochemistry of genetic variants of the haem monooxygenase cytochrome P450cam. A surface cysteine-free mutant (abbreviated as SCF) was prepared in which the five surface cysteine residues Cys-58, Cys-85, Cys-136, Cys-148 and Cys-334 were changed to alanines. Four single surface cysteine mutants with an additional mutation, R72C, R112C, K344C or R364C, were also prepared. The haem spin-state equilibria, NADH turnover rates and camphor-hydroxylation properties, as well as the electrochemistry of these mutants are reported. The coupling of a redox-active label, N-ferrocenylmaleimide, to the single surface cysteine mutant SCF-K344C, and the electrochemistry of this modified mutant are also described.