ARCHIVES OF BIOCHEMISTRY AND BIOPHYSICS
`Vol. 228, No. 2, February 1, pp. 493-502, 1984
`
`Regioselectivity in the Cytochromes P-450: Control by Protein
`Constraints and by Chemical Reactivities 1
`RONALD E. WHITE,* MARY-BETH McCARTHY,* KAREN D. EGEBERG,
`AND STEPHEN G. SLIGAR
`*Department of Pharmacology, University ofConnect'icut Health Center, Farmington, Connect'icut 06032, and
`Department of Biochemistry, University of Illinois, Urbana, fllinois 61801
`Received July 18, 1983, and in revised form September 16, 1983
`
`Three alicyclic compounds (o-camphor, adamantanone, adamantane) were found to
`be hydroxylated by the cytochrome P-450 isoenzymes P-450cam and P-450r.M2 • With P-
`450cam as the catalyst only one product was formed from each of the substrates: 5-exo(cid:173)
`hydroxycamphor, 5-hydroxyadamantanone, and 1-adamantanol. With P-450r.M2 as the
`catalyst, two or more isomeric products were formed from each substrate: 3-endo-, 5-
`exo-, and 5-endo-hydroxycamphor; 4-anti- and 5-hydroxyadamantanone; and 1- and 2-
`adamantanol. The products from P-450cam hydroxylations were found to be isosteric
`with one another, suggesting that each of them was attacked at a topologically congruent
`position within a rigid enzyme-substrate complex. The distribution of products from
`P-450r.M2 hydroxylations, on the other hand, were similar to the distributions expected
`during solution-phase hydroxylations. Thus, it would appear that the complex which
`P-450r.M2 forms with its substrate allows considerable movement of the substrate mol(cid:173)
`ecule, such that most of the hydrogens in the substrate are exposed to the enzymatic
`hydrogen abstractor. Under these conditions, the distribution of products more nearly
`reflects the rank order of chemical reactivities of the various hydroxylatable positions,
`with only a moderate protein-based steric constraint being expressed. These suggestions
`were also evident in the tightness of binding of the substrates to the two enzymes and
`in the magnitude of coupling between the substrate binding and the spin-state equilibria.
`Thus, the product from P-450cam-catalyzed hydroxylation may be predicted by a con(cid:173)
`sideration of the relation of the topology of the prospective substrate to that of o(cid:173)
`camphor. The products from P-450r.M2-catalyzed hydroxylations, on the other hand,
`may be approximately predicted from the chemical reactivities of the various ab(cid:173)
`stractable hydrogens in the prospective substrate.
`
`The cytochromes P-45<>2 comprise a
`family of ubiquitous enzymes whose func-
`
`1 This work was supported by United States Public
`Health Service Research Grants GM 28737 (R.E.W.)
`and GM 31756 (S.G.S.).
`2 Abbreviations used: P-450cam, soluble, camphor(cid:173)
`hydroxylating cytochrome P-450 isolated from Pseu(cid:173)
`domonas putida; P-450..M2 , rabbit liver microsomal
`cytochrome P-450 inducible by in vivo pretreatment
`with phenobarbital; P-450i.M4 , rabbit liver microsomal
`cytochrome P-450 inducible by in vivo pretreatment
`with ~-naphthoflavone; P-450, generic term encom(cid:173)
`passing all forms of cytochrome P-450; GC-MS, gas
`chromatography-mass spectrometry.
`
`tion is to introduce a hydroxyl group into
`an organic compound. The reasons for such
`functionalization of the organic molecule
`are as diverse as the biological locations
`of the enzymes themselves. For instance,
`P-450cam hydroxylates D-camphor as the
`first step in the catabolism of this terpene
`when Pseudmnonas putida is grown on
`camphor as the sole carbon source (9). On
`the other hand, the mammalian adrenal
`cytochromes P-450 are involved in several
`hydroxylations of the steroid nucleus dur(cid:173)
`ing steroidogenesis (9). Still another ex(cid:173)
`ample is the hydroxylation of various li(cid:173)
`pophilic xenobiotics by mammalian liver
`
`493
`
`0003-9861/84 $3.00
`Copyright © 1984 by Academic Presa, Inc.
`All rights of reproduction in any form reserved.
`
`AstraZeneca Exhibit 2170
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`
`Page 1 of 10
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`

`
`494
`
`WHITE ET AL.
`
`microsomal cytochromes for the purpose
`of providing a suitable nucleophilic func(cid:173)
`tional group for subsequent hydrophilic
`derivatization by conjugating enzymes
`such as UDP-glucuronyl transferase.
`As we would expect, the substrate spec(cid:173)
`ificities of the various P-450 enzymes vary
`widely. However, since the basic chemical
`transformation is the same among them
`all, we can inquire as to the relative roles
`of the substrate-binding site on the protein
`and the intrinsic chemistries of the sub(cid:173)
`strate carbon skeleton and of the P-450
`reactive oxygen intermediate in determin(cid:173)
`ing which molecules and, just as impor(cid:173)
`tantly, which positions on those molecules
`are hydroxylated. In this paper, we have
`examined the hydroxylation of three sub(cid:173)
`strates by two P-450 isozymes. The sub(cid:173)
`strates (camphor, adamantanone, and
`adamantane) were chosen such that both
`enzymes exhibited hydroxylase activity
`toward them. We report two near extremes
`of substrate specificity and regioselectivity.
`The selection of substrates and hydrox(cid:173)
`ylation sites by P-450cam is strongly con(cid:173)
`trolled by the substrate-binding site, while
`P-450r.M2 shows little selection among sub(cid:173)
`strates and a regioselectivity suggestive of
`chemical rather than protein control.
`
`MATERIALS AND METHODS
`
`Chemical,s. Three of the hydroxycamphors, the 5-
`exo- and 5-endo epimers, and 6-endo were a gift from
`Dr. I. C. Gunsalus. 3-exo-Hydroxycamphor was pre(cid:173)
`pared by zinc dust reduction of 3-ketocamphor (cam(cid:173)
`phoroquinone) as described by Kreiser (11). 5-Keto(cid:173)
`camphor and adamantane-2,4-dione were prepared
`by Jones oxidation of 5-exo-hydroxycamphor and
`of 4-hydroxyadamantanone. Authentic samples of
`4-syn-, 4-anti-, and 5-hydroxyadamantanones were the
`generous gift of Dr. James Henkel, Department of
`Medicinal Chemistry, University of Connecticut. Di(cid:173)
`lauroyl glyceryl-3-phosphorylcholine, isocitric acid,
`and NADP+ were purchased from Sigma Chemical
`Company. All other chemicals were purchased from
`Aldrich Chemical Company.
`Enzymes. Previously described procedures were
`followed for the preparation of electrophoretically
`homogeneous P-450 .. m, putidaredoxin, and putida(cid:173)
`redoxin reductase (10); P-4~ and P-450i,M4 (1); and
`NADPH-cytochrome P-450 oxidoreductase (3). Cat(cid:173)
`alase and isocitric dehydrogenase were purchased
`from Sigma.
`
`Enzyme-substrate titrations. With P-4~, a so(cid:173)
`lution of the protein (1 nmol) and dilauroyl glyceryl-
`3-phosphorylcholine (50 µ.g) in 1.0 ml of potassium
`phosphate buffer (0.1 M, pH 7.4) at 25°C was titrated
`with small aliquots of a methanolic solution of the
`substrate. Optical spectra in the region 350 to 500 nm
`were collected with a Varian-Cary 219 spectropho(cid:173)
`tometer interfaced to an Apple II Plus microcomputer.
`Difference spectra were calculated by subtracting the
`original spectrum from the subsequent substrate(cid:173)
`perturbed spectra, using 405 nm as an isosbestic point.
`Plots of the reciprocal of the magnitude of the dif(cid:173)
`ference spectrum versus the reciprocal of the substrate
`concentration were linear, with correlation coefficients
`from 0.991 to 0.998. From such plots apparent dis(cid:173)
`sociation constants for the enzyme-substrate com(cid:173)
`plexes and maximal mole fractions of high-spin heme
`were calculated. In the case of tight binding sub(cid:173)
`strates, corrections for substrate depletion effects
`were applied by computer iteration. Substrate titra(cid:173)
`tions of P-450cam were performed as previously de(cid:173)
`scribed (6).
`Enzymatic hydroxylation reactions. Hydroxylation
`reactions involving P-4~ or P-450Llu utilized an
`NADPH-generating system consisting of isocitric de(cid:173)
`hydrogenase (0.4 units), NADP+ (100 µ.M), isocitric
`acid (5 mM), and magnesium chloride (10 mM). Cat(cid:173)
`alase (6000 units) was included to prevent the ac(cid:173)
`cumulation of hydrogen peroxide. Reaction mixtures
`contained P-450u,i2 or P-450u,i, (1.0 nmol), NADPH(cid:173)
`cytochrome P-450 oxidoreductase (3.0 nmol), dilauroyl
`glyceryl-3-phosphorylcholine (50 µ.g, added as a son(cid:173)
`icated suspension in water), potassium phosphate (100
`µmol, pH 7.4), one of the substrates (added as a con(cid:173)
`centrated solution in methanol), catalase, and the
`NADPH-generating system in a total volume of 1.0
`ml. Reactions were initiated by the addition of isocitric
`dehydrogenase and were allowed to proceed for 3 to
`6 hat 25°C. Accumulation of product in such reactions
`was linear with time for at least 3 h. The generating
`system prevents the accumulation of NADP+, which
`inhibits reductase, and the catalase removes H 20 2 ,
`which can destroy P-450. In these circumstances, rates
`will be constant until one of the substrates becomes
`depleted or until product inhibition becomes impor(cid:173)
`tant. These effects do not occur for hours with the
`rates observed here. The observed regioselectivity was
`invariant with the time period of reaction. Thus,
`product yields were maximized by allowing the re(cid:173)
`action to run to completion. Rate measurements were,
`of course, made within linear reaction times. Control
`reactions were also run in which the cytochrome P-
`450 reductase was omitted. No background levels of
`the reported products were present in these controls.
`Substrate concentrations were: n-camphor, 5 mM;
`adamantanone, 8 mM; and adamantane, 0.3 mM.
`Hydroxylation reaction mixtures involving P-450cam
`contained P-450cam (0.5 nmol), putidaredoxin (2.2
`
`Page 2 of 10
`
`

`
`REGIOSELECTIVITY IN THE CYTOCHROMES P-450
`
`495
`
`nmol), putidaredoxin reductase (0.5 nmol), and po(cid:173)
`tassium phosphate (100 µmo!, pH 7.0) in a total volume
`of 1.0 ml. Reactions were initiated by the addition of
`NADH (200 nmol) and were conducted at 20°C. Sub(cid:173)
`strate concentrations were all 1 mM with P-450 .....
`At the end of the reaction periods an appropriate
`amount of a suitable internal standard was added
`and the products were extracted with 1 ml of chlo(cid:173)
`roform. The standards used were p-chlorobenzyl al(cid:173)
`cohol with camphor and adamantanone, and 1-phen(cid:173)
`ylethanol with adamantane. The chloroform solutions
`were concentrated by evaporation under nitrogen and
`subjected to gas chromatographic analysis on both a
`polar and a nonpolar column, using flame ionization
`detection. Column A (polar) was 10% Carbowax 20
`Mon Supelcoport (80/HJO). Column B (nonpolar) was
`3% OV-17 on Supelcoport (80/100). Both were 1/8 in.
`by 6 ft stainless-steel columns. Gas chromatographic
`peaks were quantitated with a Hewlett-Packard
`3390A Integrator. Gas chromatography-mass spec(cid:173)
`trometry was performed on a Hewlett-Packard 5992
`instrument, using 3-ft versions of Columns A or B.
`Oxidation of secondary alcohols to ketones for
`structural identification purposes was accomplished
`by use of the Jones reagent (0.1 M sodium dichromate
`in 2.5 M sulfuric acid). The alcohol sample in chlo(cid:173)
`roform (0.1 ml) was shaken with Jones reagent (0.2
`ml) for a few minutes at room temperature until the
`orange color had changed to green. The sample was
`centrifuged and the aqueous layer was removed. The
`chloroform layer was washed once with water and
`then subjected to gas chromatography.
`
`RESULTS
`
`Hydroxylation of I>-camphor. Ten iso(cid:173)
`meric hydroxycamphors could in principle
`result from enzymatic hydroxylation of D(cid:173)
`camphor. These are 3-exo-, 3-endo-, 4-, 5-
`exo-, 5-endo, 6-exo, 6-endo-, 8-, 9-, and 10-
`hydroxycamphor. Exposure of D-camphor
`to the reconstituted P-450cam enzyme sys(cid:173)
`tem leads to the accumulation of a single
`product which has been rigorously shown
`to be 5-exo-hydroxycamphor (5). Under the
`conditions described here, this product is
`produced as a catalytic rate of 60 mol/mol
`P-450cam1min. Greater rates may be ob(cid:173)
`tained by manipulation of putidaredoxin
`concentrations and extrapolation of mea(cid:173)
`sured values to infinite redoxin concentra(cid:173)
`tion. However, moderate concentrations of
`the redoxin were used here in the interests
`of conservation of enzyme preparations.
`When a reconstituted P-450i,M2 enzyme
`system is substituted for the P-450cam sys-
`
`tern, three products accumulate which GC(cid:173)
`MS demonstrates to be hydroxycamphors
`(designated A, B, and C; see Fig. 1). Gas
`chromatography on both polar and non(cid:173)
`polar columns (Table I) as well as their
`mass spectra (Table II) demonstrated
`hydroxycamphors B and C to be 5-exo(cid:173)
`and 5-endo-hydroxycamphor, respectively.
`The evidence strongly indicates hydroxy(cid:173)
`camphor A to be 3-endo-hydroxycam(cid:173)
`phor. The mass spectrum of A showed a
`substantial M+ -73 peak (m/z 95) due
`to loss of hydroxyketene followed by
`methyl, indicating the hydroxyl group to
`be in the 3 position, adjacent to the car(cid:173)
`bonyl. An appreciable M+ -18 peak (m/z
`150) due to loss of water would indicate a
`3-endo-hydroxyl stereochemistry since a 3-
`exo-hydroxyl has no readily abstractable
`hydrogen atoms nearby. Furthermore, ox(cid:173)
`idation of the mixture of A, B, and C by
`the Jones reagent gave two ketocamphors
`D and E, which were shown by gas chro(cid:173)
`matography (Table I) and GC-MS (Table
`II) to be 3-keto- and 5-ketocamphor, re(cid:173)
`spectively, with the aid of the authentic
`compounds. Also, the mass spectrum of
`authentic 3-exo-hydroxycamphor was dis(cid:173)
`tinct from that of C. However, in the ab(cid:173)
`sence of an authentic sample of 3-endo(cid:173)
`hydroxycamphor one cannot be absolutely
`certain of our assignment. As seen in Table
`
`oiJS
`
`OH
`
`A
`
`B
`
`JI/
`
`F
`
`OH
`
`o~
`
`C
`
`OH
`
`!{Jo
`
`OH
`
`G
`
`J
`FIG. 1. Hydroxylation products with P-450r.M2 •
`
`Page 3 of 10
`
`

`
`496
`
`WHITE ET AL.
`
`TABLE I
`
`GAS CHROMATOGRAPHIC DATA
`
`Compound
`
`3-exo-Hydroxycamphor
`5-exo-Hydroxycamphor
`5-endo-Hydroxycamphor
`6-endo-Hydroxycamphor
`Hydroxycamphor A
`Hydroxycamphor B
`Hydroxycamphor C
`3-Ketocamphor
`5-Ketocamphor
`Ketocamphor D
`Ketocamphor E
`4-syn-Hydroxyadamantanone
`4-anti-Hydroxyadamantanone
`5-Hydroxyadamantanone
`Hydroxyadamantanone F
`Hydroxyadamantanone G
`Adamantane-2,4-dione
`Adamantanedione H
`1-Adamantanol
`2-Adamantanol
`Adamantanol I
`Adamantanol J
`
`Retention times, min
`(at °C)
`
`Car bow ax
`20M
`
`2.90 (180)
`5.64 (180)
`7.28 (180)
`
`2.90 (180)
`5.68 (180)
`7.28 (180)
`2.15 (180)
`1.96 (180)
`2.15 (180)
`1.93 (180)
`10.30 (200)
`9.32 (200)
`6.18 (200)
`9.24 (200)
`6.14 (200)
`4.99 (200)
`4.98 (200)
`6.44 (140)
`9.69 (140)
`6.44 (140)
`9.68 (140)
`
`OV-17
`
`1.70 (150)
`2.77 (150)
`3.18 (150)
`3.5 (150)
`1.79 (150)
`2.75 (150)
`3.17 (150)
`2.18 (150)
`1.90 (150)
`2.18 (150)
`1.82 (150)
`7.84 (150)
`6.98 (150)
`5.94 (150)
`8.09 (150)
`5.72 (150)
`7.39 (150)
`7.41 (150)
`2.12 (130)
`2.75 (130)
`2.11 (130)
`2.74 (130)
`
`III, 5-endo-hydroxycamphor was by far the
`predominant product, with the 3-endo- and
`5-exo-isomers being about equally formed.
`Small amounts of two or three compounds
`not present in no-enzyme controls were
`detected. Their total amount was only
`about 7% of the total, but because of their
`low yields they could not be identified. None
`of the product peaks had a gas chromato(cid:173)
`graphic retention time similar to that of
`6-endo-hydroxycamphor (Table I). The
`overall catalytic rate of camphor hydrox(cid:173)
`ylation by P-450i.M2 was 1.3 mol/mol P-
`450i.M2/ min.
`A third P-450 isozyme, P-450LM4 , was al(cid:173)
`lowed to oxidize D-camphor as well. In this
`case, the rate was exceedingly low, only
`0.006 mol/mol P-450i.M4/min. The only
`product detected was 5-exo-hydroxycam(cid:173)
`phor, although, in view of the amount of
`product, small amounts (10-20%) of other
`isomeric hydroxycamphors could have es(cid:173)
`caped notice.
`
`Hydroxylation of adamantanone. Be(cid:173)
`cause of the symmetry of the adamantan(cid:173)
`one skeleton (C2v symmetry), only five iso(cid:173)
`meric hydroxyadamantanones are possible,
`neglecting enantiomers. These are 1-, 4-
`syn-, 4-anti-, 5-, and 6-hydroxyadaman(cid:173)
`tanone. The 4-syn- and 4-anti-hydroxy(cid:173)
`adamantanones may exhibit enantiomer(cid:173)
`ism, but the present analytical methods
`would not distinguish enantiomers. Ada(cid:173)
`mantanone was hydroxylated by P-450cam
`to a single product, hydroxyadamantanone
`G, at a rate of 52 mol/mol P-450cam1min.
`P-450LM2 , on the other hand, produced two
`hydroxyadamantanones F and G, in a 3:2
`ratio (Table III). The rate was 1.9 mol/mol
`P-450i.M2/min. Fortunately, authentic sam(cid:173)
`ples were available to us for three of the
`five possible hydroxyadamantanones. Gas
`chromatographic retention times (Table I)
`and mass spectra (Table II) indicated that
`F and G were 4-anti- and 5-hydroxyada(cid:173)
`mantanone, respectively. Deconvolution of
`the mass spectra in order to rule out the
`possibility that F and G were other isomers
`such as the 1- or 6-isomers was not feasible
`due to the complexity of fragmentation
`pathways inherent in the symmetric tri(cid:173)
`cyclic skeleton. However, Jones oxidation
`of the mixture of F and G produced an
`adamantanedione H and left G unchanged.
`H was identified as adamantane-2,4-dione
`by gas chromatography (Table I). This
`proved that G is a tertiary alcohol while
`F is a 4-hydroxyadamantanone. Since the
`gas chromatographic behavior and mass
`spectra of 4-syn- and 4-anti-hydroxyada(cid:173)
`mantanone were similar (cf. Tables I and
`II), we cannot be absolutely sure of our
`assignment that F is 4-anti-hydroxyada(cid:173)
`mantanone as opposed to the syn stereo(cid:173)
`isomer. However, the assignment that G
`is the 5-hydroxy isomer is certain.
`Hydroxylation of adamantane. The ada(cid:173)
`mantane skeleton has an even higher
`symmetry than does adamantanone (Td
`symmetry). As a consequence, only two
`isomeric adamantanols (1- and 2-adaman(cid:173)
`tanol) are possible, neither of which may
`exhibit enantiomerism. As before, P-450cam
`produced a single product, I, identified as
`1-adamantanol. The gas chromatographic
`technique used would have detected the
`
`Page 4 of 10
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`

`
`REGIOSELECTIVITY IN THE CYTOCHROMES P-450
`
`497
`
`TABLE II
`
`MASS SPECTROMETRIC DATA
`
`Compound
`
`Prominent Ions, m/z (%Abundance)
`
`3-exo-Hydroxycamphor
`
`168 (26% ), 125 (36% ), 95 (20% ), 84 (75% ), 83 (91 % ), 70 (52% ), 41 (100%)
`
`5-exo-Hydroxycamphor
`
`5-endo-Hydroxycamphor
`
`Hydroxycamphor A
`Hydroxycamphor B
`
`Hydroxycamphor C
`
`3-Ketocamphor
`5-Ketocamphor
`
`Ketocamphor D
`Ketocamphor E
`4-syn-Hydroxyadamantanone
`4-anti-Hydroxyadamantanone
`5-Hydroxyadamantanone
`Hydroxyadamantanone F
`Hydroxyadamantanone G
`
`168 (32% ), 153 (13% ), 125 ( 41 % ), 124 (25% ), 123 (23% ), 111 (100% ), 109
`(25% ), 107 (27%)
`168(23%),153(69%),125(29%),124(42%),123(41%),111(88%),109
`(75% ), 108 (100% ), 107 (98% ), 93 (67%)
`168 (29% ), 150 (14% ), 135 (58% ), 125 (46% ), 95 (31 % ), 84 (93% ), 83 (100%)
`168 (35%), 153 (4%), 125(50%),124 (27%), 123(21%),111(100%),109
`(15% ), 107 (27%)
`168 (13% ), 153 (36% ), 125 (8% ), 124 (10% ), 123 (10% ), 111 (31 % ), 109 (37% ),
`108 (100%), 107 (54%), 93 (59%)
`166 (12% ), 138 (19% ), 123 (19% ), 110 (11 % ), 95 (100% ), 83 (51 % )
`166 (87% ), 151 (11 % ), 138 (11 % ), 123 (71 % ), 109 (81 % ), 107 (25% ), 95 ( 41 % ),
`69 (100%)
`166 (50%), 138(32%),123(49%),110 (26%), 95 (100%), 83 (76%)
`166 (99% ), 151 (21 % ), 138 (20% ), 123 (80% ), 109 (96% ), 95 (27% ), 69 (100%)
`166 (20%), 148(18%),138(31%),120(12%),109 (10%), 96 (40%), 79 (100%)
`166(24%),148 (14%), 138 (46%), 120(13%),109 (18%), 96 (73%), 79 (100%)
`166 (26%), 148(10%),108 (22%), 95 (100%), 79 (14%)
`166 (21%),148 (14% ), 138 (47% ), 120 (15% ), 109 (19% ), 96 (70% ), 79 (100%)
`166 (20% ), 148 (9% ), 108 (19% ), 95 (100%)
`
`Note. Electron-impact mass spectra were recorded on various authentic standards and on cytochrome P-
`450 reaction products using a Hewlett-Packard 5992 gas chromatograph-mass spectrometer. The ionization
`energy was 14 eV.
`
`presence of less than 1 % of 2-adamantanol
`because of the good separation (see Table
`I). However, none was detectable. The cat(cid:173)
`alytic rate was 43 mol/mol P-450cam1min.
`Also, as before, P-450LM2 gave more than
`one product, specifically I and J, which were
`identified by gas chromatography (Table
`I) as 1- and 2-adamantanol. Jones oxidation
`of the mixture of I and J gave 2-adaman(cid:173)
`tanone and unchanged I. The catalytic rate
`was 1.6 mol/mol P-450LM2/min.
`Substrate-binding phenomena. Dissocia(cid:173)
`tion constants for the various enzyme(cid:173)
`substrate complexes were measured by
`monitoring the substrate-induced spectral
`change as the enzyme was titrated with
`substrate (14). The spectral change arises
`from a perturbation of the ligand field ex(cid:173)
`perienced by the heme iron, resulting in a
`change in the equilibrium constant be(cid:173)
`tween the high-spin and low-spin states
`(15). The maximal spectral change (AA.max)
`at saturating substrate concentration was
`extrapolated from iterative double-recip-
`
`rocal plots of substrate concentration ver(cid:173)
`sus absorbance change. The maximum
`change in the mole fraction of high-spin
`heme was calculated by dividing AA.max by
`the differential extinction coefficient (Af)
`and the total heme concentration. The total
`mole fraction of high-spin heme (X~~J is
`given by the sum of this change and the
`initial high-spin mole fraction (Xfils). Thus,
`the new, substrate-perturbed equilibrium
`constant between high- and low-spin heme
`can be calculated from x~~x·
`X~~x = [AAmaxf (Ad[heme])] + Xfils
`[high-spin]
`x~~
`=----
`[low-spin]
`1 - x~~x
`
`K
`eq
`
`=
`
`P-450cam bound its ketonic substrates
`tightly with dissociation constants in the
`low micromolar region, while the hydro(cid:173)
`carbon adamantane bound less tightly.
`Camphor, the natural substrate, is well
`known to cause nearly a complete shift of
`
`Page 5 of 10
`
`

`
`498
`
`WHITE ET AL.
`
`TABLE III
`
`HYDROXYLATION OF ALICYCLIC SUBSTRATES BY VARIOUS CYTOCHROMES P-450
`
`Enzyme
`
`Substrate
`
`P-450cam
`
`P-450.am
`
`P-450 .. m
`
`P-450i.M2
`
`n-Camphor
`
`Adamantanone
`
`Adamantane
`
`n-Camphor
`
`Kd
`
`2.9
`
`3.5
`
`50
`
`44
`
`x~
`
`K ..
`
`Rate
`
`0.94
`
`0.98
`
`0.99
`
`0.37
`
`16
`
`49
`
`99
`
`60
`
`52
`
`43
`
`0.59
`
`1.3
`
`P-451\Mz
`
`Adamantanone
`
`65
`
`0.25
`
`0.33
`
`P-45~
`
`Adamantane
`
`2.0
`
`0.54
`
`1.2
`
`1.9
`
`1.6
`
`Regioselectivity
`(%)
`
`5-exo (100)
`
`5- (100)
`
`1- (100)
`
`3-endo (16)
`5-exo (14)
`5-endo (63)
`other (7)
`
`4-anti (57)
`5- (43)
`
`1- (91)
`2- (9)
`
`n-Camphor
`
`0.006
`
`5-exo (100)
`
`Note. Hydroxylation reactions were conducted as described under Materials and Methods. Rates are expressed
`as total mole product per mole enzyme per minute, while regioselectivity is expressed as the positions of
`hydroxylation of that substrate, with the corresponding percentage of the total product in parentheses.
`Dissociation constants (K4) and mole fractions of high-spin heme were determined from titrations of the
`ferric cytochromes with the respective substrates as described under Materials and Methods. Dissociation
`constants are expressed in micromolar values.
`•Dash indicates that this quantity was not measured.
`
`resting P-450cam to the high-spin form (14).
`We were therefore surprised to find that
`adamantanone pushed the enzyme to a
`higher mole fraction of high-spin form
`than did camphor (Table Ill). Adamantane
`pushed the equilibrium even more toward
`high-spin. The binding of the ketonic sub(cid:173)
`strates to P-45~M2 was about 10-20 times
`weaker than to P-450cam• but adamantane
`actually bound much more tightly to P-
`than
`to P-450cam· Adamantane
`450LM2
`bound some 30 times tighter than did ada(cid:173)
`mantanone to P-450LM2, in agreement with
`the general phenomenon of more polar
`substrates displaying higher Kd values
`with this enzyme. In general, P-45~M2 does
`not exhibit spin-state changes as large as
`does P-450cam· With P-45~M2 , camphor
`produced a larger spin shift than did ada(cid:173)
`mantanone, in contrast to the observation
`with P-450cam· However, the biggest change
`with P-450LM2, as with P-450cam• was ob(cid:173)
`served with adamantane.
`
`DISCUSSION
`
`In a purely chemical, nonenzymatic, so(cid:173)
`lution-phase hydroxylation reaction, every
`molecule dissolved in the medium will be
`attacked by the hydroxylating reagent
`during the frequent bimolecular collisions.
`If the molecule contains C-H bonds in var(cid:173)
`ious positions, then the potential exists for
`several isomeric hydroxylated products to
`result. The factors which determine the
`percentage composition of this product
`mixture include (a) the ground-state elec(cid:173)
`tron density of the various C-H bonds, (b)
`the efficiency of resonance dispersal of the
`unpaired spin of each potential carbon(cid:173)
`centered radical, (c) the ability of each
`carbon center to adopt a planar geometry
`after hydrogen abstraction, (d) the degree
`of steric hindrance of approach of the hy(cid:173)
`drogen abstractor to the several C-H
`bonds, (e) the steric requirements of the
`hydrogen abstractor, and (f) the electro-
`
`Page 6 of 10
`
`

`
`REGIOSELECTIVITY IN THE CYTOCHROMES P-450
`
`499
`
`philic reactivity of the hydrogen ab(cid:173)
`stractor.
`A hydroxylase enzyme must also act ac(cid:173)
`cording to these factors; control of which
`molecules will be hydroxylated in which
`positions can only be exerted at four levels.
`First, at a level prior to (a) through (f),
`the topography of the active site may deny
`access to the substrate, so that collisions
`between the hydrogen abstractor and the
`substrate never occur. In addition, the en(cid:173)
`zyme may allow the substrate to bind, but
`only in a fashion which restricts collisions
`to selected loci on the substr~te (i.e., a spe(cid:173)
`cial case of (e) above). With a flexible mol(cid:173)
`ecule, the enzyme might bind only a special
`conformation which removes some of the
`steric hindrance referred to in (d). Of
`course, this is not possible with rigid sub(cid:173)
`strates like camphor or adamantane. Fi(cid:173)
`nally, the enzyme may modulate the elec(cid:173)
`trophilicity of the hydrogen abstractor so
`that some of the substrate positions are
`no longer reactive enough to be successfully
`attacked (i.e., a special case of (f) above).
`However, an enzyme may do little about
`factors (a), (b), and (c). It is, of course,
`conceivable that some substrate-binding
`energy might be used to deform the sub(cid:173)
`strate and thereby modulate reactivity of
`the various C-H bonds, but such effects
`must necessarily be small, except in large
`substrate molecules with one or more polar
`functional groups. For instance, steroid
`hydroxylases might owe some of their high
`catalytic specificity to deformation effects.
`Nonetheless, on a given molecule, a poorly
`reactive position cannot be efficiently hy(cid:173)
`droxylated by any enzyme without making
`the hydrogen abstractor hyperreactive.
`Conversely, if it is necessary to avoid hy(cid:173)
`droxylating very reactive positions on a
`substrate, the enzyme must set in place
`difficult stereochemical barriers to "fence
`off" the undesired regions. In particular,
`it is not possible to use effects such as hy(cid:173)
`drogen-bonding, ion-pairing, and general
`acid-base catalysis to specifically activate
`a particular C-H bond, as is possible with
`other reactions by enzymes such as pro(cid:173)
`teases. In short, the P-450 hydroxylases
`must rely primarily on steric effects of the
`protein envelope at the active site to pro-
`
`duce high regiospecificity in their hydrox(cid:173)
`ylation reactions.
`In the following discussion, we make the
`assumption that P-450i.M2 and P-450cam
`utilize the same reactive oxygen inter(cid:173)
`mediate, although the species produced by
`the two isozymes may show small differ(cid:173)
`ences in their reactivities. We will present
`arguments that the hydroxylation products
`of P-450LM2 are approximately predicted
`by simple chemical considerations of the
`substrate molecules (i.e., factors (a)-(f)
`above). Ullrich earlier made a similar sug(cid:173)
`gestion with respect to hydroxylations
`carried out by liver microsomal suspen(cid:173)
`sions (4). On the other hand, the products
`of P-450cam are not predicted by chemical
`principles, but can be rationalized on ste(cid:173)
`reochemical grounds by a 'J)1"i<Yri knowledge
`of the regiospecificity of any one of its sub(cid:173)
`strates. We make note of the general phe(cid:173)
`nomenon observed in the present data that
`P-450cam always makes a single isomer,
`while P-450i.M2 characteristically makes
`two or more isomers. Figure 1 shows the
`seven alcohols produced by P-450i.M2 from
`the three substrates examined here. The
`three alcohols produced by P-450cam are
`depicted in Fig. 2.
`One potential alicyclic substrate for P-
`450cam, norcamphor, has the same bicyclic
`skeleton as camphor but lacks the three
`bulky methyl groups; it is not attacked (12).
`Apparently, the steric effects associated
`with the methyl groups are essential for
`a productive association of norcamphor. 5-
`Bromocamphor was previously shown to
`be hydroxylated at the 5-position and sub(cid:173)
`sequently to undergo loss of HBr to yield
`only 5-ketocamphor as the observed prod(cid:173)
`uct (6). As shown under Results, the three
`substrates tested here each yield a single
`product as well. It is obvious that both
`camphor and 5-bromocamphor are at(cid:173)
`tacked by the enzyme in isosteric locations
`(i.e., the 5-position). However, as seen in
`
`FIG. 2. Views of P-450cam hydroxylation products
`perpendicular to the cyclohexanone ring.
`
`Page 7 of 10
`
`

`
`500
`
`WHITE ET AL.
`
`Fig. 2, adamantanone is also attacked at
`a position isosteric with the 5-position of
`camphor. This position is directly opposite
`the carbonyl group in a cyclohexanone ring.
`It will be noticed that in each case the
`cyclohexanone ring is capped by a bulky
`bridge containing three or four carbons.
`Thus, it would appear that the binding
`pocket of P-450cam holds both camphor de(cid:173)
`rivatives and adamantanone rigidly in the
`same steric configuration, essentially al(cid:173)
`lowing the hydrogen abstractor access to
`only one position on the molecule. The hy(cid:173)
`drocarbon adamantane, of course, contains
`no carbonyl group and so we cannot in(cid:173)
`dicate the position of hydroxylation with
`respect to the cyclohexanone ring. How(cid:173)
`ever, inspection of Fig. 2 makes it clear
`that adamantane is hydroxylated with ex(cid:173)
`actly the same topology as with adaman(cid:173)
`tanone and therefore the same as with
`camphor. With camphor, adamantanone,
`and adamantane, the position of hydrox(cid:173)
`ylation corresponds to one of the most re(cid:173)
`active positions, but with 5-bromocamphor
`the 3- and 6-positions are probably similar
`in reactivity to the 5-position. Nonetheless,
`attack is limited to the 5-carbon. Thus, the
`position of attack by P-450cam is selected
`by the imposition of a strong steric effect
`on the normal reactivity profile of the sub(cid:173)
`strate molecule. In fact, the product from
`hydroxylation by P-450cam is in all cases
`one of the products from P-450r.M2.
`P-450i,M2 appears to define its regiose(cid:173)
`lectivity somewhat differently. Steric ef(cid:173)
`fects due to the binding pocket are still
`present, but they do not overwhelm the
`chemical reactivity profile of the substrate
`molecule. We find that P-450r.M2 is much
`less selective about the molecules which
`can be hydroxylated. For instance, while
`P-450cam will not attack norcamphor, P-
`450i,M2 will readily hydroxylate norbor(cid:173)
`nane, which, like norcamphor, lacks the
`three methyl groups (7). We also find that
`P-450i,M2 is not regiospecific, giving instead
`a mixture of isomers in all cases. As shown
`below, the distribution of isomers in the
`product mixture can be largely rationalized
`by applying the reactivity factors outlined
`above.
`
`P-450i,M2 hydroxylates camphor only at
`the secondary positions on the molecule.
`This may be rationalized by reference to
`factors (a) through (f) above. Primary po(cid:173)
`sitions (methyl groups) are expected to be
`much less reactive toward hydrogen ab(cid:173)
`straction, and
`the
`tertiary positions
`(bridgeheads) cannot achieve planarity
`following hydrogen abstraction, making
`them much less prone to hydroxylation. Of
`those left, the 3-position is a to a deacti(cid:173)
`vating carbonyl, while the 6-position is
`sterically blocked by the 10-methyl. Thus,
`the 5-carbon is the most reactive. Fur(cid:173)
`thermore, examination of molecular mod(cid:173)
`els shows that the 5-exo-position is m