(Received for publication, August 28, 1996, and in revised form, October 28, 1996)
From the Departments of Biochemistry and
§ Molecular Genetics, University of Texas Southwestern
Medical Center, Dallas, Texas 75235 and the Departments of
¶ Medicine and
Biochemistry, Vanderbilt University Medical
School, Nashville, Tennessee 37232
Cytochrome P450 BM-3 catalyzes the high turnover regio- and stereoselective metabolism of arachidonic and eicosapentaenoic acids. To map structural determinants of productive active site fatty acid binding, we mutated two amino acid residues, arginine 47 and phenylalanine 87, which flank the surface and heme ends of the enzyme's substrate access channel, respectively.
Replacement of arginine 47 with glutamic acid resulted in a catalytically inactive mutant. Replacement of arginine 47 with alanine yielded a protein with reduced substrate binding affinity and arachidonate sp3 carbon hydroxylation activity (72% of control wild type). On the other hand, arachidonic and eicosapentaenoic acid epoxidation was significantly enhanced (154 and 137%, of control wild type, respectively). As with wild type, the alanine 47 mutant generated (18R)-hydroxyeicosatetraenoic, (14S,15R)-epoxyeicosatrienoic, and (17S,18R)-epoxyeicosatetraenoic acids nearly enantiomerically pure.
Replacement of phenylalanine 87 with valine converted cytochrome P450 BM-3 into a regio- and stereoselective arachidonic acid epoxygenase ((14S,15R)epoxyeicosatrienoic acid, 99% of total products). Conversely, metabolism of eicosapentaenoic acid by the valine 87 mutant yielded a mixture of (14S,15R)- and (17S,18R)-epoxyeicosatetraenoic acids (26 and 69% of total, 94 and 96% optical purity, respectively). Finally, replacement of phenylalanine 87 with tyrosine yielded an inactive protein.
We propose that: (a) fatty acid oxidation by P450 BM-3 is incompatible with the presence of residues with negatively charged side chains at the surface opening of the substrate access channel or a polar aromatic side chain in the vicinity of the heme iron; (b) the high turnover regio- and stereoselective metabolism of arachidonic and eicosapentaenoic acids involves charge-dependent anchoring of the fatty acids at the mouth of the access channel by arginine 47, as well as steric gating of the heme-bound oxidant by phenylalanine 87; and (c) substrate binding coordinates, as opposed to oxygen chemistries, are the determining factors responsible for reaction rates, product chemistry, and, thus, catalytic outcome.
Recombinant DNA techniques have greatly facilitated the molecular
characterization of highly homologous P4501
isoforms by providing access to sufficient quantities of recombinant proteins and have expedite the task of purifying and characterizing, as
single molecular entities, otherwise nearly unresolvable protein isoforms. Nevertheless, a three-dimensional structural description of a
single mammalian, membrane-bound P450 is lacking, despite the
availability of several highly purified eukaryotic P450 isoforms. Atomic structures are currently available for four water-soluble hemoproteins of bacterial origin (1, 2, 3, 4). Among these, the hemoprotein
domain of P450 BM-3, a fatty acid hydroxylase isolated from
Bacillus megaterium, has been crystallized and its three-dimensional structure determined at 2-Å resolution (2). Of all
the structurally characterized P450s, the hemoprotein domain of P450
BM-3 shows the highest degree of functional and sequence homology to
the mammalian enzymes and, in particular, to members of the CYP 4A gene
subfamily of microsomal fatty acid hydroxylases (5, 6, 7). Native P450
BM-3 is a single polypeptide (119 kDa) containing a cytochrome P450
domain fused to a cytochrome P450 reductase domain with a 1:1:1
stoichiometric ratio between heme, FAD, and FMN (5, 6, 7). The recombinant
P450 BM-3 holoenzyme has been shown to catalyze the
NADPH-dependent hydroxylation of medium and long chain
saturated fatty acids with optimal lengths of 14-16 carbons (8). While
the regiochemistry of saturated fatty acid hydroxylation by P450 BM-3
is more or less carbon chain length-dependent,
i.e. as chain length increases, regioselectivity shifts from
the -1 to the
-2 or
-3 fatty acid carbon (7, 8, 9), the enzyme
appears unable to catalyze fatty acid
-oxidation, a reaction common
to several mammalian P450 4A isoforms (6). Purified forms of P450 BM-3
were shown to catalyze the hydroxylation and the epoxidation of
monounsaturated fatty acids such as palmitoleic acid (10). Furthermore,
studies with palmitoleic acid indicated that the partition ratio
between
-2 hydroxylation and olefin epoxidation was
pH-dependent and subject to differential inhibition by
antibodies raised against purified P450 BM-3 (10, 11). Since, in those
studies, a single protein was shown to be responsible for fatty acid
sp3 carbon hydroxylation and olefin epoxidation,
it was then concluded that P450 BM-3 existed in two or more
pH-dependent substrate binding conformations and that
antigen-antibody interactions could alter the equilibrium between these
conformations (10, 11).
Microsomal P450 catalyzes the NADPH-dependent oxidation of AA to regioisomeric monohydroxylated and epoxidized metabolites (12, 13, 14, 15, 16). Depending on the P450 isoform composition of the enzyme preparation under study, arachidonic acid metabolism by P450 can proceed by one or more of the following pathways or reaction types: 1) allylic oxidation, 2) sp3 carbon hydroxylation at C16 through C20, and 3) olefin epoxidation. While the physiological significance of P450-dependent allylic oxidation remains obscure, the products of the latter two reactions display a variety of potent biological activities and have been implicated in processes such as hormonal signaling (12, 13, 14, 15, 16), the control of cell ion permeability and vascular tone (12, 13, 14, 15, 16), as well as the pathophysiology of experimental hypertension (12, 13, 14, 15, 16). The demonstration of in vivo chiral EET formation by human, rat, and rabbit organs established P450 as a member of the endogenous AA metabolic cascade and suggested new and important roles for the hemoprotein in the biosynthesis of lipid derived mediators of cell and organ function (9, 13). As with most eicosanoids, the regio- and stereochemical features of the P450-derived metabolites of AA define biological activity and/or potency and thus, functional significance (9). Studies using purified and/or recombinant mammalian P450 isoforms confirmed that P450 controlled, in an isoform-specific fashion, the regio- and stereoselectivity of oxygen insertion into the AA molecular template (9, 11, 13).
To initiate the delineation of protein and substrate determinants involved in productive substrate binding and metabolism, we recently characterized the metabolism of AA and several AA analogs by P450 BM-3 (17). These studies demonstrated that AA was metabolized by P450 BM-3 at the fastest rates ever reported for an NADPH-dependent P450 monooxygenase (3.2 µmol of product/nmol of P450/min at 30 °C), and that AA metabolism by P450 BM-3 regio- and stereoselectively generated (18R)-OH-AA and (14S,15R)-EET in a 5:1 molar ratio and with 96 and 99% optical purities, respectively (17). Similarly, EPA was metabolized by P450 BM-3 to (17S,18R)-EETA (97% optical purity) as the sole reaction product (17). Based on these results and similar experiments performed with the AA analogs EPA and eicosatrienoic acid, we developed a working model of the AA-bound active site of P450 BM-3 from which one concludes that during AA metabolism: 1) a single oxidant species accounts for fatty acid sp3 carbon hydroxylation and olefin epoxidation; 2) that catalytic outcome is critically dependent on active site spatial coordinates responsible for productive substrate binding and orientation between the heme-bound active oxygen and the acceptor carbon bond(s); and 3) the active site spatial coordinates that control AA binding restricts the degrees of freedom of substrate C-C bond rotation while, at the same time, allowing the molecule a moderate degree of lateral mobility (17). The above mentioned high catalytic rate, as well as the regio- and stereoselectivity displayed by P450 BM-3 during AA metabolism are indicative of a high degree of structural evolutionary specialization and, in particular, of those spatial coordinates that define protein substrate binding and orientation. We have utilized site-specific mutagenesis to change the properties of two amino acid residues flanking either the surface end or the heme end of the P450 BM-3 substrate access channel (2). Here we report that single amino acid replacements in the substrate access channel can either markedly alter the enzyme's regioselectivity or result in inactive proteins. Furthermore, a valine for phenylalanine replacement at position 87 coverts P450 BM-3 into an active and stereoselective (14S,15R)-AA epoxygenase.
The
original plasmid containing the cDNA coding for P450 BM-3 was a
gift of Dr. Armand Fulco (Department of Biochemistry, UCLA, Los
Angeles, CA). The cDNA was subcloned into the pIBI expression vector and expressed in DH5 Escherichia coli exactly as
described (5). Site-specific mutations were introduced into the P450 domain using the M13 two-primer method of Kunkel et al.
(18). Mutants were screened with the radiolabeled mutagenic oligomers and the incorporated changes confirmed by DNA sequencing. Mutated cDNAs were subcloned into the pIBI vector and expressed as
above.
The recombinant P450 BM-3 proteins were purified as described in Ref.
5. Briefly, P450 BM-3 was collected from cell lysates by precipitation
with ammonium sulfate (60% saturation). After centrifugation and
dialysis versus 50 mM MOPS buffer (pH 7.4) containing 50 mM KCl, the sample was applied to a
DEAE-cellulose column equilibrated with dialysis buffer and the
proteins were eluted using a linear salt gradient from 50 to 250 mM KCl. Fractions with a 416 nm to 280 nm absorbance ratio
greater than 0.9 were pooled, concentrated, applied to a Sephacryl 200 HR column and eluted with 50 mM KCl and 50 mM
MOPS buffer (pH 7.4). Fractions with 416 nm to 280 nm absorbance ratios
greater than 1.2 were pooled and utilized as such. P450 concentrations
were determined from the difference absorbance spectrum of the
CO-complex of the ferrous form of the enzyme using an extinction
coefficient of 91 mM1 cm
1 for
the wavelength pair of 450 versus 490 nm (19). The substrate binding spectra used for the determination of Ks,
the spectral binding constant, were obtained after the addition of increasing concentration of substrate at 25 °C. Difference spectra were recorded with a Hewlett Packard diode array spectrophotometer equipped with a Peltier temperature control device. Absorbance data for
substrate binding were analyzed with the program Microsoft Excel©.
Oxygen and NADPH consumption
were measured with a Clark-type oxygen electrode (YSI Instruments,
Yellow Springs, OH) and a Hewlett Packard spectrophotometer,
respectively, at room temperature and using 20-50 nM
solutions of the enzyme in 50 mM MOPS buffer (pH 7.4)
containing 50-100 µM fatty acid. After a 5-min
preincubation, reactions were started by the addition of NADPH. NADPH
utilization was measured at 340 nm and concentrations calculated using
an extinction coefficient of 6.22 mM1
cm
1. For product quantification and structural
characterization, incubations were performed at 30 °C under
atmospheric air and with vigorous mixing. Reaction mixtures in 50 mM Tris-Cl buffer (pH 7.4) containing 10 mM
MgCl2, 150 mM KCl, 8 µM sodium
isocitrate, isocitrate dehydrogenase (1.0 IU/ml),
dilauroylphosphatidylcholine (0.05 µg/ml), and P450 BM-3 (2-10
nM, final concentration) were incubated 2.5 min prior to
the addition of the sodium salt of either AA or EPA (25 mM
each in 0.05 mM Tris-Cl buffer (pH 8.0) to final
concentrations of 50-100 µM, each. After 1 min,
reactions were started by the addition of NADPH (1 mM,
final concentration). At different time points, aliquots of the
incubates were withdrawn and the organic soluble products extracted
thrice with equal volumes of ethyl ether containing HOAc (0.05%, v/v).
After solvent evaporation under an stream of N2, the
products were resolved by RP-HPLC on a 5-µm Dynamax Microsorb
C18 column (4.6 × 250 mm, Rainin Instruments Co.,
Woburn, MA) using a linear solvent gradient from 49.95%
CH3CN, 49.95% H2O, 0.1% HOAc to 99.9%
CH3CN, 0.1% HOAc over 40 min at 1 ml/min (20). Products
were quantified by on-line liquid scintillation using a Radiomatic
Flo-one
-Detector (Radiomatic Instruments, Tampa, FL).
The identification of 18-OH-AA, 14,15-EET, and of 17,18-EETA was done using published methodology (20, 21, 22) and confirmed with synthetic standards. For the characterization of 14,15-EETA, the organic soluble material extracted from incubates containing [1-14C]EPA (100 µM final concentration, 0.1 µCi/µmol), 5 nM P450 BM-3, and 1 mM NADPH was resolved by RP-HPLC as described above. The radioactive fraction eluting from the HPLC column with a retention time of 19.7 min was collected batchwise and submitted to catalytic hydrogenation under PtO2. For structural characterization, the NICI/GC/MS and chromatographic properties of the metabolite were compared to those of authentic 14,15-epoxyeicosanoic acid. PFB esters were prepared by reaction with pentafluorobenzyl bromide in triethylamine as described (21, 22). RP-HPLC was done on a 5-µm Dynamax Microsorb C18 column (4.6 × 250 mm, Rainin Instruments Co.) using a linear solvent gradient from 49.95% CH3CN, 49.95% H2O, 0.1% HOAc to 99.9% CH3CN, 0.1% HOAc over 40 min at 1 ml/min. Normal phase HPLC was performed on a 5-µm Dynamax Microsorb Silica column (4.6 × 250 mm, Rainin Instruments Co.) using either a solvent mixture of 99.65% n-hexane, 0.25% isopropanol, and 0.1% HOAc at 2 ml ml/min (for the eicosanoic acid) or 99.95% n-hexane, 0.05% isopropanol at 2 ml/min (for the PFB ester). The RP-HPLC retention times for the hydrogenated EPA metabolite and for synthetic 14,15-epoxyeicosanoic acid and for the corresponding PFB esters were 33.1 and 46 min, respectively, The normal phase HPLC retention time for the hydrogenated EPA metabolite and the corresponding synthetic standards were 21 and 12 min, respectively, for the free acid and PFB ester.
Stereochemical Analysis of Methyl-14,15-epoxyeicosatetraenoateSamples of enzymatically
derived [1-14C]14,15-epoxyeicosatetraenoic acid (25 µg,
1 µCi/µmol) and of synthetic (14S,15R)-EET and of (14R,15S)-EET were catalytically
hydrogenated over PtO2 and esterified using excess
pentafluorobenzyl bromine as described (21). The resulting PFB- esters
were purified by RP-HPLC and, after solvent evaporation, the optical
antipodes of the purified PFB-14,15-epoxyeicosanoate were resolved by
HPLC on a Chiralcel OD column (4.6 × 250 mm, J.T. Baker) with an
isocratic mixture of 0.11% isopropanol, 99.89%% n-hexane
at 1 ml/min with UV monitoring at 210 nm (Fig. 5). The retention times
for the PFB esters of synthetic (14R,15S)- and
(14S,15R)-epoxyeicosanoic acid were 70.6 and 78.9 min, respectively (Fig. 6).
Gas Chromatography-Mass Spectral Analysis
Samples were dissolved in dodecane and analyzed by NICI/GC/MS on a Nermag R1010C quadrupole instrument interfaced to a Varian Vista Gas chromatograph utilizing helium and CH4 as carrier and reagent gases, respectively. Splitless injections were made onto a 30 m SPB-1 fused silica capillary column (0.32 mm (inner diameter), 0.25 µm coating thickness, Supelco Inc., Bellefonte, PA). After 1 min at 100 °C, the oven temperature was raised to 300 °C at 10 °C/min.
The three-dimensional structure of P450 BM-3 revealed a heme
prosthetic group positioned at the end of a long and narrow channel (8-10 Å wide) lined with mostly hydrophobic amino acid residues (2).
It was proposed that during catalysis, this channel, "the substrate
access channel," provided access to the heme active site and could
participate in determining substrate selectivity and the regio- and
stereoselectivity of oxygen insertion (2). An arginine residue
(Arg47) flanks the end of the substrate access channel
facing the surface of the molecule (2). At the opposite end, the heme
end of the channel, a phenylalanine residue (Phe87) is
located in close proximity to the distal face of the heme prosthetic
group (2). Based on the unique spatial geometries of these residues, it
was proposed that, on the one hand, Arg47 controls entry to
the substrate access channel and, therefore, fatty acid selectivity
and, on the other hand, the aromatic side chain of Phe87
limits and/or controls substrate C-H acceptor bond access to the
enzyme's heme iron and thus, the regioselectivity of oxygen attack
(2). In the absence of atomic coordinates for substrate-bound P450
BM-3, we utilized a molecular modeling approach to visualize AA binding
to P450 BM-3 (17). For these studies, the fatty acid carboxylate was
placed within charge coupling distance of the Arg47
guanidinium group and the remainder of the molecule was built into the
volume of the access channel defined by Arg47 at the
surface and Phe87 above the heme (2, 17). To optimize
occupancy, Phe87 was moved slightly back, thus allowing the
-end of AA to occupy an available active site cavity (2, 17). In
this active site induced conformation, the fatty acid methyl end,
i.e. the
carbon, bends upward and away from the heme
iron and the AA C18 pro-R hydrogen is positioned in close proximity to
the enzyme's heme iron (17) (Fig. 1). Moreover, an
additional small displacement of Phe87 places the AA
14,15-olefin in optimal proximity to the P450 BM-3 heme iron (17).
Based on this model, we suggested that AA-bound P450 BM-3 oscillates
between two alternate conformers characterized by the relative position
of Phe87 with respect to the heme iron. These alternate
conformations allow for a small degree of substrate lateral
displacement, responsible for oxygen attack at C18 and the 14,15-olefin
of AA (2, 17). To further define the roles of Arg47 and
Phe87, as the outer and inner boundaries of the substrate
access channel (2) (Fig. 1), we replaced them by site-specific
mutagenesis. Mutated cDNAs were expressed, the corresponding
recombinant proteins purified and their enzymatic properties studied as
described under "Materials and Methods." A summary of the amino
acid replacements introduced into the protein are shown in Table
I. The positively charged side chain of
Arg47 (Fig. 1) was either eliminated by replacement with
alanine or replaced with a negatively charged glutamic acid (Table I).
The aromatic ring of Phe87 (Fig. 1) was either removed by
replacement with valine or, alternatively, its polarity was increased
by substitution with tyrosine (Table I).
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The
poorly defined nature of Arg47 in the two
crystallographically independent P450 BM-3 molecules analyzed indicated
high structural flexibility and suggested a role for this residue in
the monooxygenation of fatty acids of different chain lengths (2). The
protein crystal structure showed that the flexible, positively charged, side chain of Arg47 protruded into the lumen of the
substrate access channel (2). The catalytic significance of these
Arg47-associated structural features, i.e. side
chain length and charge, were addressed by replacing the amino acid
with either glutamic acid or alanine (Fig. 1) (Table I). Replacement of
Arg47 by a negatively charged glutamic acid residue
resulted in a mutant enzyme (Table I) spectroscopically identical to
the wild type, however, unable to metabolize AA or EPA (data not
shown). To reduced the potential for charge repulsion between the
ionizable fatty acid carboxylate and the side chain of
Glu47 in the mutant enzyme, we prepared the methyl ester
derivatives of AA and EPA and incubated them with the mutant protein.
Even at high P450 concentrations (0.5 µM), the R47E
mutant was unable to metabolize the methyl esters of AA and EPA (data
not shown). Furthermore, no significant NADPH utilization or
NADPH-dependent oxygen reduction was observed with the R47E
mutant P450 BM-3 incubated in the absence or presence of AA. Thus,
catalytic turnover appears to be incompatible with the presence of a
negative charge at the surface opening of the substrate access channel.
Finally, the addition of AA or EPA (from 1 to 50 µM) to a
solution of this mutant P450 BM-3 failed to induce a typical spectral
manifestation of binding to the enzyme's active site.
The replacement of Arg47 (Fig. 1) by alanine (Table I)
generated a mutant protein lacking a flexible and positively charged protruding side chain (2) and with potentially enhanced active site
accessibility. Incubation of the R47A mutant P450 BM-3 with [1-14C]AA and NADPH (2.5 nM, 100 µM, and 1 mM final concentrations, respectively) resulted in the formation of two radioactive metabolites with HPLC retention times identical to those of authentic 18-OH-AA and
14,15-EET (Fig. 2). While the products formed are the
same as those previously reported for wild type P450 BM-3 (17), the Arg
for Ala replacement resulted in a reduced overall rate of AA oxidation
(82% of wild type rates, Table II) (17). Furthermore, the amino acid replacement affected AA sp3
carbon hydroxylation and olefin epoxidation in opposite manners, i.e. an increased AA 14,15-olefin epoxygenase activity was
accompanied by a concomitant reduction in AA -2 carbon hydroxylation
(154 and 72% of wild type rates, respectively) (Fig. 2, Table II). An
enhanced epoxygenase activity was also observed when the R47A mutant
was incubated with EPA. As shown in Fig. 2 and Table
III, the mutant enzyme metabolized EPA to 17,18-EETA
with nearly absolute regioselectivity and at reaction rates higher than
those reported for wild type P450 BM-3 (137% of wild type rates; Fig.
2, Table III) (17). Neither wild type P450 BM-3 nor the R47A mutant
metabolized methyl-AA or methyl-EPA (data not shown). As with wild type
enzyme, during AA metabolism by the R47A mutant, NADPH oxidation is
coupled to oxygen reduction and substrate oxidation, as revealed by a 1:1 stoichiometry between NADPH and oxygen utilization (Table IV). However, in contrast with wild type P450 BM-3, the
R47A mutant enzyme diverts a small but significant portion of NADPH
supplied electrons to H2O2 formation (Table
IV). These result indicate that electron flow and oxygen chemistries
are only marginally disturbed by the Arg47 for Ala
replacement (Tables III and IV).
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To probe for changes in substrate binding affinity and/or spatial
orientation resulting from the Arg47 for Ala substitution,
we determined the spectral dissociation constants for AA and EPA as
well as the chirality of the AA and EPA metabolites generated by the
R47A mutant. Compared to wild type P450 BM-3, and as estimated by the
values of the corresponding spectral dissociation constants shown in
Table V, the mutated enzyme showed a substantially
decreased binding affinity for AA and EPA. These results, in
conjunction with the above mentioned changes in reaction rates, suggest
that charge interactions between the Arg47 side chain and
the fatty acid carboxylates may stabilize substrate binding in the
spatial conformation that favors oxygenation at the substrate C18 C-H
bond, i.e. AA sp3 -2 carbon
hydroxylation and EPA 17,18-epoxidation. Similarly, the increased AA
and EPA epoxygenase activity observed with the R47A mutant may reflect
a decreased binding rigidity and greater freedom of substrate lateral
displacement brought about by the removal of
Arg47-associated charge interactions with the fatty acid
carboxylate (17) (Fig. 1). Chiral phase HPLC analysis of the 18-OH-AA,
14,15-EET, and 17,18-EETA isolated from incubates containing either AA
or EPA, R47A P450 BM-3, and NADPH showed that the enantiofacial
selectivity of the mutant enzyme was not only identical to that
reported for wild type P450 BM-3 (Table VI) (17) but
also that the high degree of catalytic asymmetry was preserved (Table
VI). It was therefore concluded that removal of the positively charged
and protruding Arg47 side chain does not change and/or
modify the active site spatial coordinates responsible for active site
substrate orientation with regard to the heme-iron oxygenating locus,
i.e. nearly enantiomerically pure (18R)-OH-AA,
(14S,15R)-EET and
(17S,18R)-EETA are the metabolites generated by
mutant and wild type P450s (17) (Table VI).
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Based on the above results, we concluded that: (a) while a positively charged residue at position 47 is not essential for catalysis, catalytic turnover is incompatible with the presence of a negatively charged glutamic acid residue at this position; and (b) the change of Arg47 to alanine appears to enhance the occupancy of a second binding orientation, which facilitates 14,15-epoxidation over AA hydroxylation at carbon 18 (Table II). This second substrate binding orientation in the R47A mutant most likely allows the AA molecule to penetrate deeper into the access channel, permitting the 14,15-olefin to approach the heme iron at a higher frequency. In summary, charge interactions between the Arg47 side chain and the fatty acid carboxylic acid may provide an anchoring point, which leads to increased substrate binding rigidity and, thus, higher turnover and regiochemical selectivity (Fig. 1).
Substrate Heme-Iron Access: Phe87 ReplacementsA
distinctive feature of AA regioselective metabolism, and particularly
of oxidations occurring at the fatty acid ,
-1,
-2, or
-3
carbon atoms, is the efficient, active site-dependent insulation of reactive olefins and bis-allylic methylene
carbons from the heme-bound reactive oxygen. As mentioned, in P450 BM-3 the aromatic ring of Phe87 extends into the heme pocket and
is positioned above and nearly perpendicular to the porphyrin plane
(Fig. 1) (2). It is readily apparent from the protein crystal structure
(2), as well as from the active site model of AA-bound P450 BM-3 (17),
that this residue could restrict substrate access to the enzyme's
heme-iron and thus control the regioselectivity of oxygenation (17).
Using the atomic coordinates for P450 BM-3, we constructed a model of the solvent-accessible surface at the enzyme's substrate access channel (2, 17). As shown in Fig. 3A, the
aromatic side chain of Phe87 occupies a substantial portion
of the access channel volume proximal to the heme face, markedly
reducing substrate heme accessibility and thus increasing binding
rigidity (17). Furthermore, the volume of the substrate binding region
of P450 BM-3 available to solvent approximates that of a regular cone
with its base facing the surface opening of the access channel and its
vertex the heme end (Fig. 3A). A similarly constructed model
of the F87V mutant substrate access channel is shown in Fig.
3B. Replacement of Phe87 by Val eliminates the
steric effects of the Phe aromatic side chain in the vicinity of the
heme and increases substrate heme accessibility. Importantly, while the
replacement of Phe87 by Val results in a significant
expansion of the volume available for substrate binding near the heme
prosthetic group, it has little or no effect at the opposite end, the
surface opening end of the access channel. Thus, compared to wild type
P450 BM-3, the volume of the substrate binding region in F87V is more
or less cylindrical instead of conical (Fig. 3, A and
B). To study the functional role of Phe87, we
replaced it with tyrosine or valine. With the Tyr for Phe87
replacement (Table I), we sought to increase the polarity of the phenyl
ring and thus add a polarity component to its steric effects. On the
other hand, in the valine mutant (F87V) (Table I), we sought to remove
from the heme environment steric effects attributable to the aromatic
ring.
The addition of a hydroxyl group to the Phe87 aromatic side chain, i.e. replacing this residue for tyrosine, generated a mutant (F87Y) protein catalytically inactive toward AA, EPA, and their corresponding methyl esters. Importantly, the Soret region absorbance spectra of the F87Y mutant were identical to those of the wild type P450 BM-3 (data not shown). Assuming that the tyrosine replacement does not introduce significant protein structural alterations, the metabolism of polyunsaturated fatty acids by P450 BM-3 appears to be incompatible with a heme environment containing the oxygen binding site in close proximity to a polar aromatic amino acid side chain. Significant restrictions in substrate heme accessibility are likely to be responsible for the aforementioned lack of catalytic turnover. In support of the latter, the addition of AA or EPA to a solution of F87Y P450 BM-3 failed to elicit typical spectral manifestations of substrate binding and heme iron spin change. While unable to metabolize AA, the F87Y mutant did support significant NADPH-dependent oxygen reduction (Table IV). Furthermore, the absence of measurable substrate oxidation and of H2O2 formation, as well as the observed 1.9-2.1 stoichiometric ratio between NADPH and oxygen utilization (Table IV), indicates that the F87Y mutant catalyzes a four-electron reduction of molecular oxygen to H2O. Control experiments demonstrated that: (a) the F87Y mutant protein lacked significant catalase-like activity, and (b) NADPH and oxygen utilization was partially fatty acid-independent (data not shown). The four-electron reduction of oxygen to water is characteristic of cytochrome oxidase(s), the structurally complex and multi-functional hemoproteins that serve as the terminal oxidases of the respiratory chain. Because of its importance for hemoprotein biochemistry and function, this unusual activity of the F87Y mutant is currently under investigation.
The nearly absolute degree of enantiofacial selectivity with which wild
type P450 BM-3 catalyzed AA sp3 hydroxylation
and 14,15-olefin epoxidation (17) indicated that (a) AA
binding to the active site of P450 BM-3 severely limits the molecule's
freedom of C-C bond rotation, (b) the active site binding
coordinates allow for a limited degree of substrate displacement along
the longitudinal axis of the access channel resulting in more than one
catalytically productive active site-AA complex, and (c)
Phe87 provides a distinct steric barrier to active
site-substrate configurations favoring oxygen delivery to the AA or EPA
14,15-olefin (17) (Figs. 2 and 3A). Incubation of
[1-14C]AA with the F87V mutant P450 BM-3 and NADPH,
followed by product extraction and HPLC analysis, revealed this single
amino acid substitution caused important alterations in the
regioselectivity of AA oxidation by P450 BM-3. Thus, as reported (17)
and shown in Fig. 4, AA metabolism by wild type P450
BM-3 generated a 5:1 mixture of 18-OH-AA and 14,15-EET. Significantly,
removal of the protruding aromatic side chain associated with
Phe87 completely eliminated the catalysis of AA
sp3 carbon hydroxylation and converted the mutant protein
into a regio- and stereoselective AA
(14S,15R)-epoxygenase (100% of total products,
99% optical purity) (Fig. 4, Tables VII and
VIII). Importantly, the enantiofacial selectivity of the
mutant F87V epoxygenase is identical to that of wild type P450 BM-3
(Table VIII) ,suggesting that: (a) the amino acid
replacement did not substantially change the spatial orientation of the
acceptor bond(s), and (b) the wild type and mutant P450s
utilize similar substrate binding coordinates. In agreement with this,
the AA binding constants for wild type and the F87V mutant enzymes were
also similar (Table V).
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Compared to wild type, the F87V mutant enzyme metabolized AA at a significantly reduced overall rate (approximately 36% of wild type rates, Table VII). This reduced catalytic rate could be accounted for by the loss of AA sp3 carbon hydroxylation activity since, under identical conditions, the mutant enzyme catalyzed 14,15-EET formation at a rate twice that of wild type P450 BM-3 (Table VII). It is of interest that, in addition to the above effects on reaction rates, the replacement of Phe87 by Val resulted in the generation of small amounts of H2O2 due to non-productive cycles of O2 reduction and increased NADPH oxidase activity (Table IV). These results indicate that the aromatic side chain of Phe87 may facilitate tight coupling between electron transfer, oxygen reduction and activation, and acceptor C-H bond insertion. Assuming that the Phe87 for Val replacement does not introduce major protein structural alterations and/or changes in the kinetics of product release, the documented catalytic dissociation between P450 BM-3 supported hydroxylation and epoxidation reactions is in agreement with the proposal that these reactions are mediated by distinct P-450 BM-3 active site-substrate conformations (Fig. 3, A and B) (2, 17). Studies of the metabolism of AA and of several AA analogs by P450 BM-3 indicated that steric factors, as opposed to the chemistry of the reactive oxygen intermediate, may play the predominant role in determining the nature of the reaction products (17). Furthermore, our current understanding of the biochemistry of oxygen activation by P450 enzymes supports the concept that sp3 carbon hydroxylation and olefin epoxidation are mediated by a common oxidant. Inasmuch as olefin epoxidation is thermodynamically preferred over sp3 carbon hydroxylation, the decreased rates of AA metabolism observed with the F87V mutant enzyme may reflect changes in electron transfer kinetics brought about by the removal of the Phe87 aromatic side chain from the environment of the protein redox active center. Alternatively, charge interactions between the side chain of Arg47 and the substrate carboxylic acid, the proposed substrate anchoring site, may limit heme-iron access to the AA and EPA 14,15-olefins by providing a significant energy barrier to the freedom of substrate longitudinal displacement.
Incubation of [1-14C]EPA with the F87V mutant protein resulted in the NADPH-dependent formation of two radioactive products with HPLC retention times identical to those of authentic 17,18-EETA and its hydration product, 17,18-DHETA (17) (Fig. 5). A third metabolite, eluting with an HPLC retention of 19.7 min was identified as 14,15-EETA based on the following criteria: (a) catalytic hydrogenation over PtO2 generated a product with normal and RP-HPLC chromatographic properties identical to those of authentic 14,15-epoxyeicosanoic acid (21 and 33.1 min for normal and RP-HPLC, respectively), (b) catalytic hydrogenation followed by derivatization to the corresponding PFB ester yielded a product with normal and RP-HPLC retention times identical to those of the synthetic PFB ester of 14,15-epoxyeicosanoic acid (12 and 46 min for normal and RP-HPLC, respectively), and (c) catalytic hydrogenation, followed by derivatization to the corresponding PFB ester and NICI/GC/MS analysis showed the presence of major negative fragment ions at m/z 325 (base peak; M-PFB), 309 (25% abundance; loss of O and PFB) and 307 (21% abundance; loss of PFB and H2O). This mass spectrum was identical to that generated, under similar conditions by the PFB ester of synthetic 14,15-epoxyeicosanoic acid (data not shown). For chiral characterization of enzymatically derived 14,15-EETA, the metabolite was reduced by catalytic hydrogenation, converted to the corresponding PFB ester, and compared by chiral phase HPLC with racemic and/or enantiomerically pure synthetic standards. As shown in Fig. 6, Chiralcel OD chromatography separated the optical antipodes of synthetic 14,15-epoxyeicosanoate-PFB with base-line resolution. Co-injection of a mixture of racemic standards with enzymatically produced, 114C-labeled metabolite showed coelution with (14S,15R)-epoxyeicosanoate-PFB (Fig. 6).
The reported regio- and stereoselective epoxidation of EPA to
(17S,18R)-EETA by P450 BM-3 was attributed to
efficient trapping of the heme-bound active oxygen by the fatty acid
reactive 17,18-olefin electron cloud (17). As illustrated by the
HPLC chromatograms in Fig. 5, the F87V mutant P450 BM-3 catalyzed the
epoxidation of EPA to 14,15-EETA and 17,18-EETA. Additionally, small
and variable amounts of the 17,18-EETA hydration product 17,18-DHETA
were recovered (28 and 72% of total products for 14,15- and 17,18-EETA + 17,18-DHETA, respectively) (Fig. 5, Table IX). As with
AA, the removal of the Phe87 aromatic ring from the heme
environment was associated with decreased rates of EPA oxidation (Table
IX) and concomitant increases in substrate-dependent, NADPH
oxidase (Table IV). Furthermore, as shown in Tables V and VIII, the
Phe87 replacement by Val did not significantly change the
affinity of the protein for EPA, nor did it affect the enantiofacial
selectivity of oxygenation (Table VIII). Thus, while the removal of the
aromatic ring in Phe87 allows further displacement of the
EPA molecule along the longitudinal axis of the access channel (Fig.
3B), the overall EPA binding coordinates do not appear to be
significantly altered by this replacement (Table VIII). It is of
interest that regardless of the fatty acid, the si, re faces
of the 14,15- and 17,18-olefin are always selected for epoxidation
(Fig. 7), indicating that regioselectivity results
predominantly from freedom of longitudinal displacement within the
confines of the substrate access channel and not from oxygen
chemistries and/or unique binding configurations.
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The presence in P450 BM-3 of an hydrophobic substrate binding site uniquely adapted to AA binding and metabolism not only indicates high evolutionary specialization, it also raises interesting questions with regards to the mechanism and/or driving force(s) responsible for the release of the oxygenated products. As reported (17), the products of AA oxidation are efficiently recycled through the enzyme system and undergo extensive secondary oxidations, even in the presence of high AA concentrations (17). Thus, (14S,15R)-EET and (18R)-OH-AA compete effectively with AA for binding to the enzyme active site. Hence, it is likely that during catalytic turnover, reversible changes in active site configuration may participate in facilitating oxygenated product release.
All the changes in catalytic outcome and/or reaction rates reported here are the consequence of single amino acid replacements in the vicinity of the heme prosthetic group. Furthermore, these substitution-dependent changes appear to take place in the absence of major protein and/or active site structural modifications, i.e. changes in enzyme regioselectivity occurred without alterations in stereoselectivity. The pioneering studies of Negishi and collaborators and others with microsomal P450 isoforms (23, 24), in conjunction with the results reported here, show that substrate selectivity and catalytic outcome can be critically dependent on the chemical nature of unique residues located in finite, more or less predictable, areas of the protein active site. Thus, instead of the chemical properties of the oxidant species, the chemistry of the reaction products may be controlled by active site binding coordinates and the resulting spatial orientation of acceptor bonds with respect to the heme-bound reactive oxygen. These as well as other published studies are beginning to provide a molecular description of active site determinants for regio- and stereoselective metabolism in different P450 isoforms (24). These studies also contribute to the delineation of the molecular basis responsible for the catalytic versatility of these functionally diverse but, otherwise, structurally homologous proteins.
Finally, while it is apparent that charge interactions associated with the side chain of Arg47 serve as a fatty acid anchoring point and thus may regulate substrate displacement along the longitudinal axis of the access channel, the Phe87 aromatic side chain controls heme access at the distal end of the access channel. Consequently, we propose that the high turnover regio- and stereoselective metabolism of fatty acids such as AA and EPA is controlled by substrate anchoring at the surface opening of the access channel and by steric gating of the heme-bound reactive oxygen. Furthermore, the experimental data also shows that substrate-active site binding coordinates, as opposed to oxygen chemistries, are the predominant factors responsible for reaction rates and the chemistry of the reaction products.