Mutagenesis and molecular dynamics suggest structural and functional roles for residues in the N-terminal portion of the cytochrome P450 2B1 I helix

Crystal Structure of CYP2B6 in Complex with an Efavirenz Analog

The over two dozen CYP2B structures of human, rabbit, and woodrat enzymes solved in the last decade have significantly enhanced our understanding of the structure-function relationships of drug metabolizing enzymes. More recently, an important role has emerged for halogen-π interactions in the CYP2B6 active site in substrate selectivity, explaining in part the preference for halogenated ligands as substrates. The mechanism by which such ligands interact with CYP2B enzymes involves conserved phenylalanine side chains, in particular F108, F115, or F297, in the active site, which form π bonds with halogens. To illustrate such halogen-π interactions using drugs that are major substrates of CYP2B6, we present here a crystal structure of CYP2B6 in complex with an analog of the widely used anti-HIV drug efavirenz, which contains a methyl group in place of the carbonyl oxygen. The chlorine of the efavirenz analog forms a π bond with the aromatic ring of F108, whereas the putative metabolism site on the distal end of the molecule is oriented towards the heme iron. The crystal structure showcases how CYP2B6 accommodates this important drug analog of considerable size in the active site by movement of various side chains without substantially increasing the active site volume. Furthermore, the CYP2B6-efavirenz analog complex provides a useful platform to investigate computationally as well as biophysically the effect of genetic polymorphisms on binding of the widely studied efavirenz.

The role of cytochrome P450 2B6 and 2B4 substrate access channel residues predicted based on crystal structures of the amlodipine complexes

Recent X-ray crystal structures of human cytochrome P450 2B6 and rabbit cytochrome P450 2B4 in complex with amlodipine showed two bound ligand molecules, one in the active site and one in the substrate access channel. Based on the X-ray crystal structures, we investigated the interactions of P450 2B4 and 2B6 with amlodipine using absorbance spectroscopy, and determined the steady-state kinetics of 7-ethoxy-4-(trifluoromethyl)coumarin and 7-benzyloxyresorufin oxidation by some access channel mutants to evaluate the functional role of these residues in substrate turnover. The results of absorbance titrations are consistent with a simple mechanism with two parallel binding events that result in the formation of the enzyme complex with two molecules of amlodipine. Using this model we were able to resolve two separate ligand-binding events, which are characterized by two distinct KD values in each enzyme. The access channel mutants R73K in P450 2B6 and R73K, V216W, L219W, and F220W in P450 2B4 showed a significant decrease in kcat/KM with the both substrates. Overall, the results suggest that P450 2B4 and 2B6 form an enzyme complex with two molecules of amlodipine in solution, and R73, V216, L219 and F220 in P450 2B4 may play an important role in substrate metabolism.

Conformational adaptation of human cytochrome P450 2B6 and rabbit cytochrome P450 2B4 revealed upon binding multiple amlodipine molecules

Structures of human cytochrome P450 2B6 and rabbit cytochrome P450 2B4 in complex with two molecules of the calcium channel blocker amlodipine have been determined by X-ray crystallography. The presence of two drug molecules suggests clear substrate access channels in each P450. According to a previously established nomenclature, amlodipine molecules were trapped in access pathway 2f in P450 2B6 and in pathway 2a or 2f in P450 2B4. These pathways overlap for part of the length and then diverge as they extend toward the protein surface. A previously described solvent channel was also found in each enzyme. The results indicate that key residues located on the surface and at the entrance of the substrate access channels in each of these P450s may play a crucial role in guiding substrate entry. In addition, the region of P450 2B6 and 2B4 involving helices B', F, F', and G' and part of helix G is substantially more open in the amlodipine complexes than in the corresponding 4-(4-chlorophenyl)imidazole complexes. The increased active site volume observed results from the major retraction of helices F, F', and B' and the β4 sheet region located close to the binding cavity to accommodate amlodipine. These structures demonstrate novel insight into distinct conformational states not observed with previous P450 2B structures and provide clear evidence of the substrate access channels in two drug-metabolizing P450s. In addition, the structures exhibit the versatility that can be exploited via in silico studies with other P450 2B6 ligands as large as raloxifene and itraconazole.

Computational prediction of metabolism: sites, products, SAR, P450 enzyme dynamics, and mechanisms

Metabolism of xenobiotics remains a central challenge for the discovery and development of drugs, cosmetics, nutritional supplements, and agrochemicals. Metabolic transformations are frequently related to the incidence of toxic effects that may result from the emergence of reactive species, the systemic accumulation of metabolites, or by induction of metabolic pathways. Experimental investigation of the metabolism of small organic molecules is particularly resource demanding; hence, computational methods are of considerable interest to complement experimental approaches. This review provides a broad overview of structure- and ligand-based computational methods for the prediction of xenobiotic metabolism. Current computational approaches to address xenobiotic metabolism are discussed from three major perspectives: (i) prediction of sites of metabolism (SOMs), (ii) elucidation of potential metabolites and their chemical structures, and (iii) prediction of direct and indirect effects of xenobiotics on metabolizing enzymes, where the focus is on the cytochrome P450 (CYP) superfamily of enzymes, the cardinal xenobiotics metabolizing enzymes. For each of these domains, a variety of approaches and their applications are systematically reviewed, including expert systems, data mining approaches, quantitative structure–activity relationships (QSARs), and machine learning-based methods, pharmacophore-based algorithms, shape-focused techniques, molecular interaction fields (MIFs), reactivity-focused techniques, protein–ligand docking, molecular dynamics (MD) simulations, and combinations of methods. Predictive metabolism is a developing area, and there is still enormous potential for improvement. However, it is clear that the combination of rapidly increasing amounts of available ligand- and structure-related experimental data (in particular, quantitative data) with novel and diverse simulation and modeling approaches is accelerating the development of effective tools for prediction of in vivo metabolism, which is reflected by the diverse and comprehensive data sources and methods for metabolism prediction reviewed here. This review attempts to survey the range and scope of computational methods applied to metabolism prediction and also to compare and contrast their applicability and performance.

Investigation by site-directed mutagenesis of the role of cytochrome P450 2B4 non-active-site residues in protein-ligand interactions based on crystal structures of the ligand-bound enzyme

Residues located outside the active site of cytochromes P450 2B have exhibited importance in ligand binding, structural stability and drug metabolism. However, contributions of non-active-site residues to the plasticity of these enzymes are not known. Thus, a systematic investigation was undertaken of unique residue-residue interactions found in crystal structures of P450 2B4 in complex with 4-(4-chlorophenyl)imidazole (4-CPI), a closed conformation, or in complex with bifonazole, an expanded conformation. Nineteen mutants distributed over 11 sites were constructed, expressed in Escherichia coli and purified. Most mutants showed significantly decreased expression, especially in the case of interactions found in the 4-CPI structure. Six mutants (H172A, H172F, H172Q, L437A, E474D and E474Q) were chosen for detailed functional analysis. Among these, the K(s) of H172F for bifonazole was ∼ 20 times higher than for wild-type 2B4, and the K(s) of L437A for 4-CPI was ∼ 50 times higher than for wild-type, leading to significantly altered inhibitor selectivity. Enzyme function was tested with the substrates 7-ethoxy-4-(trifluoromethyl)coumarin, 7-methoxy-4-(trifluoromethyl)coumarin and 7-benzyloxyresorufin (7-BR). H172F was inactive with all three substrates, and L437A did not turn over 7-BR. Furthermore, H172A, H172Q, E474D and E474Q showed large changes in k(cat)/K(M) for each of the three substrates, in some cases up to 50-fold. Concurrent molecular dynamics simulations yielded distances between some of the residues in these putative interaction pairs that are not consistent with contact. The results indicate that small changes in the protein scaffold lead to large differences in solution behavior and enzyme function.

Molecular dynamics analysis reveals structural insights into mechanism of nicotine N-demethylation catalyzed by tobacco cytochrome P450 mono-oxygenase

CYP82E4, a cytochrome P450 monooxygenase, has nicotine N-demethylase (NND) activity, which mediates the bioconversion of nicotine into nornicotine in senescing tobacco leaves. Nornicotine is a precursor of the carcinogen, tobacco-specific nitrosamine. CYP82E3 is an ortholog of CYP82E4 with 95% sequence identity, but it lacks NND activity. A recent site-directed mutagenesis study revealed that a single amino acid substitution, i.e., cysteine to tryptophan at the 330 position in the middle of protein, restores the NND activity of CYP82E3 entirely. However, the same amino acid change caused the loss of the NND activity of CYP82E4. To determine the mechanism of the functional turnover of the two molecules, four 3D structures, i.e., the two molecules and their corresponding cys–trp mutants were modeled. The resulting structures exhibited that the mutation site is far from the active site, which suggests that no direct interaction occurs between the two sites. Simulation studies in different biological scenarios revealed that the mutation introduces a conformation drift with the largest change at the F-G loop. The dynamics trajectories analysis using principal component analysis and covariance analysis suggests that the single amino acid change causes the opening and closing of the transfer channels of the substrates, products, and water by altering the motion of the F-G and B-C loops. The motion of helix I is also correlated with the motion of both the F-G loop and the B-C loop and; the single amino acid mutation resulted in the curvature of helix I. These results suggest that the single amino acid mutation outside the active site region may have indirectly mediated the flexibility of the F-G and B-C loops through helix I, causing a functional turnover of the P450 monooxygenase.

A single amino acid residue, Ala 105, confers 16alpha-hydroxylase activity to human cytochrome P450 17alpha-hydroxylase/17,20 lyase

In adrenal steroidogenesis, CYP17 catalyses the 17alpha-hydroxylation of pregnenolone and progesterone and the subsequent 17,20-lyase reaction, yielding adrenal androgens. The enzyme exhibits distinctly different selectivities towards these substrates in various species. CYP17 has also been shown to exhibit 16alpha-hydroxylase activity towards progesterone in some species, with only human and chimp CYP17 catalysing the biosynthesis of substantial amounts of 16-OHprogesterone. The 16alpha-hydroxylase activity was investigated by introducing an Ala105Leu substitution into human CYP17. The converse mutation, Leu105Ala was introduced into the baboon, goat and pig enzymes. Wt human CYP17 converted approximately 30% progesterone to 16-OHprogesterone while the Ala105Leu mutant converted negligible amounts to 16-OHprogesterone ( approximately 9%), comparable to wt CYP17 of the other three species when expressed in COS-1 cells. The ratio of 17-hydroxylated products to 16-OHprogesterone of human CYP17 was 2.7 and that of the mutant human construct 10.5. Similar ratios were observed for human and goat CYP17 with the corresponding Ala or Leu residues. Although the Leu105Ala mutation of both baboon and pig CYP17 exhibited the same trend regarding the ratios, the rate of progesterone conversion was reduced. Coexpression with cytochrome b(5) significantly decreased the ratio of 17-hydroxylated products to 16-OHprogesterone in the Leu105 constructs, while effects were negligible with Ala at this position. Homology models show that Ala105 faces towards the active pocket in the predicted B'-C domain of CYP17. The smaller residue allows more flexibility of movement in the active pocket than Leu, presenting both the C16 and C17 of progesterone to the iron-oxy complex.

Structural insight into the altered substrate specificity of human cytochrome P450 2A6 mutants

Human P450 2A6 displays a small active site that is well adapted for the oxidation of small planar substrates. Mutagenesis of CYP2A6 resulted in an increased catalytic efficiency for indole biotransformation to pigments and conferred a capacity to oxidize substituted indoles (Wu, Z.-L., Podust, L.M., Guengerich, F.P. J. Biol. Chem. 49 (2005) 41090-41100.). Here, we describe the structural basis that underlies the altered metabolic profile of three mutant enzymes, P450 2A6 N297Q, L240C/N297Q and N297Q/I300V. The Asn297 substitution abolishes a potential hydrogen bonding interaction with substrates in the active site, and replaces a structural water molecule between the helix B'-C region and helix I while maintaining structural hydrogen bonding interactions. The structures of the P450 2A6 N297Q/L240C and N297Q/I300V mutants provide clues as to how the protein can adapt to fit the larger substituted indoles in the active site, and enable a comparison with other P450 family 2 enzymes for which the residue at the equivalent position was seen to function in isozyme specificity, structural integrity and protein flexibility.

Re-engineering cytochrome P450 2B11dH for enhanced metabolism of several substrates including the anti-cancer prodrugs cyclophosphamide and ifosfamide

Based on recent directed evolution of P450 2B1, six P450 2B11 mutants at three positions were created in an N-terminal modified construct termed P450 2B11dH and characterized for enzyme catalysis using five substrates. Mutant I209A demonstrated a 3.2-fold enhanced k(cat)/K(m) for 7-ethoxy-4-trifluoromethylcourmarin O-deethylation, largely due to a dramatic decrease in K(m) (0.72 microM vs. 18 microM). I209A also demonstrated enhanced selectivity for testosterone 16beta-hydroxylation over 16alpha-hydroxylation. In contrast, V183L showed a 4-fold increased k(cat) for 7-benzyloxyresorufin debenzylation and a 4.7-fold increased k(cat)/K(m) for testosterone 16alpha-hydroxylation. V183L also displayed a 1.7-fold higher k(cat)/K(m) than P450 2B11dH with the anti-cancer prodrugs cyclophosphamide and ifosfamide, resulting from a approximately 4-fold decrease in K(m). Introduction of the V183L mutation into full-length P450 2B11 did not enhance the k(cat)/K(m). Overall, the re-engineered P450 2B11dH enzymes exhibited enhanced catalytic efficiency with several substrates including the anti-cancer prodrugs.

Evidence for the functional role of residues in the B'-C loop of baboon cytochrome P450 side-chain cleavage (CYP11A1) obtained by site-directed mutagenesis, kinetic analysis and homology modelling

To gain further insight into the structure/function relationship of cytochrome P450 side-chain cleavage (CYP11A1), this enzyme was investigated in the Cape baboon (Papio ursinus). Four constructs were cloned and characterised in non-steroidogenic mammalian COS-1 cells. Wild type recombinant baboon CYP11A1 cDNA yielded a K(m) value of 1.6 microM for 25-hydroxycholesterol. The single amino acid substitutions, I98Q and I98K resulted in a 1.7- and 2.8-fold increases in K(m) values, respectively. Conversely, the introduction of the mutation, K103A, resulted in a 1.8-fold decrease in K(m). A homology model of CYP11A1, based on the crystal structures of CYP102 and CYP2C5, revealed that residues 98 and 103 lie within the B'-C loop and contribute to the spatial orientation and structural integrity of this domain. Based on these results we propose a topological model of the CYP11A1 active pocket, which is supported by substrate docking analysis and kinetic studies.

Investigation of the role of cytochrome P450 2B4 active site residues in substrate metabolism based on crystal structures of the ligand-bound enzyme

Based on the X-ray crystal structures of 4-(4-chlorophenyl)imidazole (4-CPI)- and bifonazole (BIF)-bound P450 2B4, eight active site mutants at six positions were created in an N-terminal modified construct termed 2B4dH and characterized for enzyme inhibition and catalysis. I363A showed a >4-fold decrease in differential inhibition by BIF and 4-CPI (IC(50,BIF)/IC(50,4-CPI)). F296A, T302A, I363A, V367A, and V477A showed a 2-fold decreased k(cat) for 7-ethoxy-4-trifluoromethylcoumarin O-deethylation, whereas V367A and V477F showed an altered K(m). T302A, V367L, and V477A showed >4-fold decrease in total testosterone hydroxylation, whereas I363A, V367A, and V477F showed altered stereo- and regioselectivity. Interestingly, I363A showed a 150-fold enhanced k(cat)/K(m) with testosterone, and yielded a new metabolite. Furthermore, testosterone docking into three-dimensional models of selected mutants based on the 4-CPI-bound structure suggested a re-positioning of residues 363 and 477 to yield products. In conclusion, our results suggest that the 4-CPI-bound 2B4dH/H226Y crystal structure is an appropriate model for predicting enzyme catalysis.

Structure-function analysis of cytochromes P450 2B

In the last 4 years, breakthroughs were made in the field of P450 2B (CYP2B) structure-function through determination of one ligand-free and two inhibitor-bound X-ray crystal structures of CYP2B4, which revealed many of the structural features required for binding ligands of different size and shape. Large conformational changes of several plastic regions of CYP2B4 can dramatically reshape the active site of the enzyme to fit the size and shape of the bound ligand without perturbing the overall P450 fold. Solution biophysical studies using isothermal titration calorimetry (ITC) have revealed the large difference in the thermodynamic parameters of CYP2B4 in binding inhibitors of different ring chemistry and side chains. Other studies have revealed that the effects of site-specific mutations on steady-state kinetic parameters and mechanism-based inactivation are often substrate dependent. These findings agree with the structural data that the enzymes adopt different conformations to bind various ligands. Thus, the substrate specificity of an individual enzyme is determined not only by active site residues but also non-active site residues that modulate conformational changes that are important for substrate access and rearrangement of the active site to accommodate the bound substrate.

Functional interaction of nitrogenous organic bases with cytochrome P450: A critical assessment and update of substrate features and predicted key active-site elements steering the access, binding, and orientation of amines

The widespread use of nitrogenous organic bases as environmental chemicals, food additives, and clinically important drugs necessitates precise knowledge about the molecular principles governing biotransformation of this category of substrates. In this regard, analysis of the topological background of complex formation between amines and P450s, acting as major catalysts in C- and N-oxidative attack, is of paramount importance. Thus, progress in collaborative investigations, combining physico-chemical techniques with chemical-modification as well as genetic engineering experiments, enables substantiation of hypothetical work resulting from the design of pharmacophores or homology modelling of P450s. Based on a general, CYP2D6-related construct, the majority of prospective amine-docking residues was found to cluster near the distal heme face in the six known SRSs, made up by the highly variant helices B', F and G as well as the N-terminal portion of helix C and certain beta-structures. Most of the contact sites examined show a frequency of conservation < 20%, hinting at the requirement of some degree of conformational versatility, while a limited number of amino acids exhibiting a higher level of conservation reside close to the heme core. Some key determinants may have a dual role in amine binding and/or maintenance of protein integrity. Importantly, a series of non-SRS elements are likely to be operative via long-range effects. While hydrophobic mechanisms appear to dominate orientation of the nitrogenous compounds toward the iron-oxene species, polar residues seem to foster binding events through H-bonding or salt-bridge formation. Careful uncovering of structure-function relationships in amine-enzyme association together with recently developed unsupervised machine learning approaches will be helpful in both tailoring of novel amine-type drugs and early elimination of potentially toxic or mutagenic candidates. Also, chimeragenesis might serve in the construction of more efficient P450s for activation of amine drugs and/or bioremediation.

Use of directed evolution of mammalian cytochromes P450 for investigating the molecular basis of enzyme function and generating novel biocatalysts

Directed evolution has been successfully applied to the design of industrial biocatalysts for enhanced catalytic efficiency and stability, and for examining the molecular basis of enzyme function. Xenobiotic-metabolizing mammalian cytochromes P450 with their catalytic versatility and broad substrate specificity offer the possibility of widespread applications in industrial synthesis, medicine, and bioremediation. However, the requirement for NADPH-cytochrome P450 reductase, often cytochrome b5, and an expensive cofactor, NADPH, complicates the design of mammalian P450 enzymes as biocatalysts. Recently, Guengerich and colleagues have successfully performed directed evolution of P450s 1A2 and 2A6 initially by using colony-based colorimetric and genotoxicity screening assays, respectively, followed by in vitro fluorescence-based activity screening assays. More recently, our laboratory has developed a fluorescence-based in vitro activity screening assay system for enhanced catalytic activity of P450s 2B1 and 3A4. The studies indicate an important role of amino acid residues outside of the active site, which would be difficult to target by other methods. The approach can now be expanded to design these as well as new P450s using more targeted substrates of environmental, industrial, and medical importance.

Do mammalian cytochrome P450s show multiple ligand access pathways and ligand channelling?

Understanding substrate binding and product release in cytochrome P450 (CYP) enzymes is important for explaining their key role in drug metabolism, toxicity, xenobiotic degradation and biosynthesis. Here, molecular simulations of substrate and product exit from the buried active site of a mammalian P450, the microsomal CYP2C5, identified a dominant exit channel, termed pathway (pw) 2c. Previous simulations with soluble bacterial P450s showed a different dominant egress channel, pw2a. Combining these, we propose two mechanisms in CYP2C5: (i) a one-way route by which lipophilic substrates access the enzyme from the membrane by pw2a and hydroxylated products egress along pw2c; and (ii) a two-way route for access and egress, along pw2c, for soluble compounds. The proposed differences in substrate access and product egress routes between membrane-bound mammalian P450s and soluble bacterial P450s highlight the adaptability of the P450 fold to the requirements of differing cellular locations and substrate specificity profiles.

Directed Evolution of Mammalian Cytochrome P450 2B1

Cytochrome P450 2B1 has been subjected to directed evolution to investigate the role of amino acid residues outside of the active site and to engineer novel, more active P450 catalysts. A high throughput screening system was developed to measure H(2)O(2)-supported oxidation of the marker fluorogenic substrate 7-ethoxy-4-trifluoromethylcoumarin (7-EFC). Random mutagenesis by error-prone polymerase chain reaction and activity screening were optimized using the L209A mutant of P450 2B1 in an N-terminally modified construct with a C-terminal His tag (P450 2B1dH). Two rounds of mutagenesis and screening and one subcloning step yielded the P450 2B1 quadruple mutant V183L/F202L/L209A/S334P, which demonstrated a 6-fold higher k(cat) than L209A. Further random or site-directed mutagenesis did not improve the activity. When assayed in an NADPH-supported reconstituted system, V183L/L209A demonstrated lower 7-EFC oxidation than L209A. Therefore, F202L/L209A/S334P was generated, which showed a 2.5-fold higher k(cat)/K(m) for NADPH-dependent 7-EFC oxidation than L209A. F202L/L209A/S334P also showed enhanced catalytic efficiency with 7-benzyloxyresorufin, benzphetamine, and testosterone, and a 10-fold increase in stereoselectivity for testosterone 16alpha-versus 16beta-hydroxylation compared with 2B1dH. Enhanced catalytic efficiency of F202L/L209A/S334P was also retained in the full-length P450 2B1 background with 7-EFC and testosterone as substrates. Finally, the individual mutants were tested for metabolism of the anti-cancer prodrugs cyclophosphamide and ifosfamide. Several of the mutants showed increased metabolism via the therapeutically beneficial 4-hydroxylation pathway, with L209A/S334P showing 2.8-fold enhancement of k(cat)/K(m) with cyclophosphamide and V183L/L209A showing 3.5-fold enhancement with ifosfamide. Directed evolution can thus be used to enhance P450 2B1 catalytic efficiency across a panel of substrates and to identify functionally important residues distant from the active site.

POSSIBLE PATHWAY(S) OF TESTOSTERONE EGRESS FROM THE ACTIVE SITE OF CYTOCHROME P450 2B1: A STEERED MOLECULAR DYNAMICS SIMULATION

To probe the possible substrate exit channel(s) in cytochrome P450 (P450) 2B1 and to clarify the role of residues previously identified by site-directed mutagenesis, a homology model was constructed based on the X-ray crystal structure of a P450 2B4-inhibitor complex. Testosterone was docked into the active site of P450 2B1 and was then pulled out through three putative channels using steered molecular dynamics simulations. The results indicated that of the three channels, the "solvent channel," lined by helices E, F, and I and the beta3 hairpin, required the largest rupture force and backbone motion, which rendered it unlikely as an exit route. The relatively small rupture forces and backbone motions for the other two channels suggested them as possible candidates for testosterone passage. The opening of channel 1, located between helices G and I and the B'-C loop, is characterized by rotation of the aromatic ring of Phe297 together with a bending of the B'-C loop. The opening of channel 2, penetrating through the B'-C loop/B' helix, is achieved by an expansion of this region and a small displacement of the backbone. Interestingly, during the egress of testosterone along channel 1, Phe297 and Phe108 appear to act as two clamps to stabilize testosterone binding and prevent it from leaving the active site. Phe115 acts as a gatekeeper for channel 2. These results are in agreement with previous site-directed mutagenesis experiments.

Functional role of residues in the helix B' region of cytochrome P450 2B1

Comparison of several recently determined X-ray crystal structures of mammalian cytochrome P450 family 2 enzymes suggests considerable movement of helix B' when ligands bind. To investigate the functional role of helix B' in P450 2B1, residues 100-109 were substituted with alanine and phenylalanine. Kinetic properties were examined with the typical 2B substrates 7-benzyloxyresorufin, 7-ethoxy-4-trifluoromethylcoumarin, benzphetamine, and testosterone. Several mutants showed 2- to 3-fold changes in k(cat) values and significant differences in catalytic efficiencies among the substrates examined, consistent with structural information suggesting that the helix B' region can adopt multiple conformations with different contact residues depending on the substrate. Homology modeling of P450 2B1 was performed based on an inhibitor-bound P450 2B4 structure, and the docking analyses were consistent with experimental results. The findings suggest that residues in the helix B' region affect regio- and stereoselective oxidation in P450 family 2 enzymes as well as substrate entry.