Hydroperoxysterols as a probe for the mechanism of cytochrome P-450scc-mediated hydroxylation. Homolytic versus heterolytic oxygen-oxygen bond scission

Review of Ligand Specificity Factors for CYP1A Subfamily Enzymes from Molecular Modeling Studies Reported to-Date

The cytochrome P450 (CYP) family 1A enzymes, CYP1A1 and CYP1A2, are two of the most important enzymes implicated in the metabolism of endogenous and exogenous compounds through oxidation. These enzymes are also known to metabolize environmental procarcinogens into carcinogenic species, leading to the advent of several types of cancer. The development of selective inhibitors for these P450 enzymes, mitigating procarcinogenic oxidative effects, has been the focus of many studies in recent years. CYP1A1 is mainly found in extrahepatic tissues while CYP1A2 is the major CYP enzyme in human liver. Many molecules have been found to be metabolized by both of these enzymes, with varying rates and/or positions of oxidation. A complete understanding of the factors that govern the specificity and potency for the two CYP 1A enzymes is critical to the development of effective inhibitors. Computational molecular modeling tools have been used by several research groups to decipher the specificity and potency factors of the CYP1A1 and CYP1A2 substrates. In this review, we perform a thorough analysis of the computational studies that are ligand-based and protein-ligand complex-based to catalog the various factors that govern the specificity/potency toward these two enzymes.

Cholesterol hydroperoxides as substrates for cholesterol-metabolizing cytochrome P450 enzymes and alternative sources of 25-hydroxycholesterol and other oxysterols

The interaction of the primary autoxidation products of cholesterol, namely 25- and 20ξ-hydroperoxides, with the four principal cholesterol-metabolizing cytochrome P450 enzymes is reported. Addition of cholesterol 25-hydroperoxide to the enzymes CYP27A1 and CYP11A1 induced well-defined spectral changes while generating 25-hydroxycholesterol as the major product. The 20ξ-hydroperoxides induced spectral shifts in CYP27A1 and CYP11A1 but glycol metabolites were detected only with CYP11A1. CYP7A1 and CYP46A1 failed to give metabolites with any of the hydroperoxides. A P450 hydroperoxide-shunt reaction is proposed, where the hydroperoxides serve as both donor for reduced oxygen and substrate. CYP27A1 was shown to mediate the reduction of cholesterol 25-hydroperoxide to 25-hydroxycholesterol, a role of potential significance for cholesterol-rich tissues with high oxidative stress. CYP27A1 may participate in the removal of harmful autoxidation products in these tissues, while providing a complementary source of 25-hydroxycholesterol, a modulator of immune cell function and mediator of viral cell entry.

Structural basis for pregnenolone biosynthesis by the mitochondrial monooxygenase system

In humans, the precursor to all steroid hormones, pregnenolone, is synthesized from cholesterol by an enzyme complex comprising adrenodoxin reductase (AdR), adrenodoxin (Adx), and a cytochrome P450 (P450scc or CYP11A1). This complex not only plays a key role in steroidogenesis, but also has long been a model to study electron transfer, multistep catalysis, and C-C bond cleavage performed by monooxygenases. Detailed mechanistic understanding of these processes has been hindered by a lack of structural information. Here we present the crystal structure of the complex of human Adx and CYP11A1--the first of a complex between a eukaryotic CYP and its redox partner. The structures with substrate and a series of reaction intermediates allow us to define the mechanism underlying sequential hydroxylations of the cholesterol and suggest the mechanism of C-C bond cleavage. In the complex the [2Fe-2S] cluster of Adx is positioned 17.4 Å away from the heme iron of CYP11A1. This structure suggests that after an initial protein-protein association driven by electrostatic forces, the complex adopts an optimized geometry between the redox centers. Conservation of the interaction interface suggests that this mechanism is common for all mitochondrial P450s.

Metabolism of 1alpha,25-dihydroxyvitamin D2 by human CYP24A1

The metabolism of 1alpha,25-dihydroxyvitamin D2 (1alpha,25(OH)2D2) by human CYP24A1 was examined using the recombinant enzyme expressed in Escherichia coli cells. HPLC analysis revealed that human CYP24A1 produces at least 10 metabolites, while rat CYP24A1 produces only three metabolites, indicating a remarkable species-based difference in the CYP24A1-dependent metabolism of 1alpha,25(OH)2D2 between humans and rats. LC-MS analysis and periodate treatment of the metabolites strongly suggest that human CYP24A1 converts 1alpha,25(OH)2D2 to 1alpha,24,25,26(OH)4D2, 1alpha,24,25,28(OH)4D2, and 24-oxo-25,26,27-trinor-1alpha(OH)D2 via 1alpha,24,25(OH)3D2. These results indicate that human CYP24A1 catalyzes the C24-C25 bond cleavage of 1alpha,24,25(OH)2D2, which is quite effective in the inactivation of the active form of vitamin D2. The combination of hydroxylation at multiple sites and C-C bond cleavage could form a large number of metabolites. Our findings appear to be useful to predict the metabolism of vitamin D2 and its analogs in the human body.

Comparisons of hydroperoxide isomerase and monooxygenase activities of cytochrome P450 for conversions of allylic hydroperoxides and alcohols to epoxyalcohols and diols: probing substrate reorientation in the active site

Ferric cytochrome P450 (P450) catalyzes intramolecular rearrangements of hydroperoxides to diols by heterolytic scission of the O-O bond and insertion of the terminal oxygen atom into the resulting alcohol. The goals of this work were to further characterize the regio- and stereochemistry of P450 isomerase activity using allylic hydroperoxides and to compare these reactions with NADPH-supported monooxygenations of the corresponding alcohols. Microsomes from phenobarbital-treated rats or purified P450 2B1 catalyzed the conversions of several peroxyquinols, including 2-tert-butyl-4-hydroperoxy-4-methylcyclohexa-2,5-dien-1-one (BMPOOH) and its 2,6-dimethyl derivative (TMPOOH), to diols and to alpha,beta-epoxyquinols by predominant (>93%) cis addition of oxygen to the least hindered double bond. Monooxygenation of the 4-hydroxy analogues (quinols) yielded identical cis-epoxyquinols, and both isomerization and monooxygenation reactions exhibited similar enantioselectivities (32-74% enantiomer excess). Regioselectivities were similar for BMPOOH and BMPOH (epoxyquinol:diol ratios of 5.8-7.6), but quite different for TMPOOH and TMPOH (ratios of 0.6 and 6.4, respectively). Bulky peroxyquinols and quinols derived from the A-rings of 17beta-estradiol and estrone were utilized to further compare these reactions. Both estradiol derivatives underwent approximately equal amounts of 6beta-hydroxylation and 1,2-epoxidation. The estrone derivatives also underwent 6beta-hydroxylation, but only estrone quinol yielded a second product consistent with hydroxylation at position 16. The results support several conclusions. (i) Allylic hydroperoxides may be isomerized to alpha,beta-epoxyalcohols by a heterolytic O-O cleavage mechanism with high stereoselectivity. (ii) Hydroperoxide isomerization is an efficient process relative to monooxygenation. (iii) Isomerase substrates remain in proximity to the P450 oxoferryl intermediate and are rapidly captured by the oxidant. Monooxygenase substrates, on the other hand, may bind to ferric P450 in multiple orientations and undergo more extensive substrate reorientation prior to oxidative attack.

Reflections on sterol sidechain cleavage process catalyzed by cytochrome P450(scc)

The essay examines the evidence upon which the presently accepted version of the mechanism of the cytochrome P450(scc)-catalyzed-cleavage of the sidechain of cholesterol is based. This analysis indicates that the generally held view of the process (two consecutive hydroxylations, followed by cleavage of the resulting glycol) most likely does not describe the true mechanism. The available evidence can not be used to support this traditional notion. Two alternative hypotheses are proposed.

Role of THR501 Residue in Substrate Binding and Catalytic Activity of Cytochrome P4501A1

A putative binding region for cumene hydroperoxide in the active site of cytochrome P4501A1 was identified using photoaffinity labeling. Thr501 was determined as the most likely site of modification by azidocumene used as the photoaffinity label (T. Cvrk and H. W. Strobel, (1998) Arch. Biochem. Biophys. 349, 95-104). To evaluate further the role of this amino acid residue a site-directed mutagenesis approach was employed. P4501A1 wild type and two mutants, P4501A1Glu501 and P4501A1Phe501, were expressed in and purified from Escherichia coli and used for kinetic analysis to confirm the role of Thr501 residue in cumene hydroperoxide binding. The mutation resulted in a two- to fourfold decrease in the rate of heme degradation in the presence of 0.5 mM cumene hydroperoxide. The mutations do not prevent or significantly alter binding of the tested substrates; however, binding of 2-phenyl-2-propanol (product generated from cumene hydroperoxide) to P4501A1Glu501 and P4501A1Phe501 exhibited four- and eightfold decreases, respectively, suggesting that the mutations strongly affected the affinity of cumene hydroperoxide for the P4501A1 active site. The kinetic analysis of cumene hydroperoxide-supported reactions showed that both mutants exhibit increased Km and decreased VMax values for all tested substrates. Furthermore, the mutations affected product distribution in testosterone hydroxylation. On the basis of P4501A1Glu501 and P4501A1Phe501 characterization, it can be concluded that Thr501 plays an important role in cumene hydroperoxide/P4501A1 interaction.

Detection of P450c17-independent pathways for dehydroepiandrosterone (DHEA) biosynthesis in brain glial tumor cells

Dehydroepiandrosterone (D) is biosynthesized in the brain by a pathway different from that existing in the adrenal cortex. C6 rat glioma tumor cells in culture biosynthesize both pregnenolone (P) and D. They possess the mRNA, protein, and side-chain cleavage activity of P450scc. On the other hand, P450c17 was not detected. Adding FeSO4 to C6 cells increased the synthesis of both P and D. Even in the presence of aminoglutethimide, an inhibitor of P450scc, FeSO4 increased the synthesis of both steroids, indicating that the Fe2+-sensitive process does not involve P450scc. Likewise, the FeSO4-induced formation of D was not blocked by the P450c17 inhibitor, SU-10603. These results suggest that the FeSO4-induced synthesis of D as well as of P in C6 cells may be due to the fragmentation of in situ-formed tertiary hydroperoxides. It is likely, however, that the effect of the Fe2+ is not limited to this one reaction. When exogenous P was added to C6 microsomes, along with FeSO4, the amount of D formed was greater than control values, indicating that Fe2+ facilitated the conversion of P to D. Unlike the constituents that are converted by Fe2+ to P, the precursor of D in C6 cells is not soluble in a 1:1 mixture of ether and ethylacetate. Treatment of C6 cells with KI, NaBH4, or HIO4 resulted in an increase in D synthesis. From this it seems clear that a precursor of the D produced in C6 cells is a steroid where both C-17 and C-20 are oxygenated.

Photoaffinity labeling of cytochrome P4501A1 with azidocumene: identification of cumene hydroperoxide binding region

Cumene hydroperoxide can support cytochrome P450-catalyzed reactions in the absence of molecular oxygen, NADPH, and cytochrome P450-NADPH oxidoreductase. Its binding at the cytochrome P450 active site is governed by the structure of the cumene hydroperoxide binding region. In order to define the region of cytochrome P4501A1 at which cumene hydroperoxide binds, we prepared an analog of cumene hydroperoxide for use as a photoaffinity label. p-Azido-isopro-pylbenzene (azidocumene) and its tritiated derivative were photolyzed in water solution by uv light with a half-life of 29 s. The 7-ethoxycoumarin deethylatation catalyzed by P450 using the cumene hydroperoxide-supported system was strongly inhibited by the presence of the label. Covalent binding to the protein after photoactivation was blocked by 50% in the presence of cumene hydroperoxide. HPLC analysis after trypsin digestion of the labeled protein showed that [3H]-azidocumene was attached covalently to the peptide VDMTPAYGLTLK corresponding to residues 492-503 in the 1A1 sequence. The radioactivity level of this fraction was reduced by 50% when the labeling was carried out in the presence of cumene hydroperoxide. To confirm the identified region the labeled protein was cleaved by cyanogen bromide. HPLC separation of the CNBr digest showed two peaks with a high level of radioactivity. The SDS/Tricine PAGE analysis of the radioactive fraction with an elution time of 43 min revealed a 2.4-kDa peptide carrying a high level of covalently bound radioactivity. The N-terminal sequence identified the labeled peptide to be a fragment generated by CNBr corresponding to residues 494-512. The N-terminal sequence of the labeled peptide with elution time of 27 min, TLKH, matches amino acid residues 501-504 in the P4501A1 sequence. We can conclude that in the overlapping region of all three identified peptides, T501-L502-K503, is the site where azidocumene covalently binds to P4501A1. The sequence alignment of cytochrome P4501A1 with cytochrome P450102 predicts that this region might correspond to beta-sheet structure localized on the distal side of the heme ring near the I helix and the oxygen binding pocket. To our knowledge, this is the first report to localize the cumene hydroperoxide binding region in the cytochrome P450 active site.

Dynamics of carbon monoxide binding with cytochromes P-450

The dynamics of CO rebinding with cytochromes P-450cam, P-450scc, and P-450LM2 after laser flash photolysis have been investigated from 293 to 77 K, and the distribution functions of the rate parameters P(k) and of the activation enthalpy P(H) were determined using the maximum entropy method. In a fluid solvent, geminate rebinding is nonexponential, presumably because of a spectral shift induced by protein relaxation on the same time scale. Substrate binding increases the yield of the bimolecular process and decreases the bimolecular rate by 1 or 2 orders of magnitude. The amplitude of these effects seems to correlate with substrate specificity. In a rigid environment at low temperature, cytochromes P-450 exhibit a bimodal distribution of activation enthalpy; P(H) consists of two distinct bands which are in a thermal equilibrium even at 77 K. The results lead to a scheme in which a common structural perturbation splits the conformational substates of cytochromes P-450 into pairs of "doublet" substates with different dynamic properties. The hierarchy of conformational substates of cytochromes P-450 thus contrasts with that of oxygen-binding hemoproteins such as myoglobin.

Conformational analysis of mitochondrial and microsomal cytochrome P-450 by resonance Raman spectroscopy

Mitochondrial and microsomal cytochromes P-450SCC and P-450LM2 in the ferric substrate-free and substrate-bound states were studied by resonance Raman spectroscopy. In the spectra of cytochrome P-450SCC two conformational states (A and B) were detected, each of them constituting an equilibrium between a six-coordinated low-spin and a high-spin form. Both the conformational and the spin equilibria are pH- and temperature-dependent, which is in line with previously published results [Lange, R., Larroque, C., & Anzenbacher, P. (1992) Eur. J. Biochem. 207, 69-73)]. On the basis of well-resolved resonance Raman spectra, measured at different pH and temperatures, these equilibria were analyzed quantitatively. Both low-spin configurations of A and B exhibit different band patterns in the spin state marker band region, indicating differences in the active-site structures. While in the high-spin configuration of state A the heme iron remains weakly bound by a sixth ligand, the high-spin form of state B is five-coordinated. Binding of cholesterol to cytochrome P-450SCC causes a significant population of the high-spin forms, particularly of state A (62%). On the other hand, binding of 22R-hydroxycholesterol to the substrate-free enzyme leaves the overall spin equilibrium largely unchanged, i.e., six-coordinated low spin (76% A and 24% B). In both substrate-bound complexes, interactions between the substrate and the heme lead to small but distinct differences in the resonance Raman spectra of the low-spin form of state A. In contrast to cytochrome P-450SCC, the resonance Raman spectra of microsomal cytochrome P-450LM2 provide no indications for multiple conformers at 22 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Detection of radicals produced by reaction of hydroperoxides with rat liver microsomal fractions

EPR spin trapping using the spin traps 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and 3,5-dibromo-4-nitrosobenzene sulphonic acid (DBNBS) has been employed to examine the generation of radicals produced on reaction of a number of primary, secondary and lipid hydroperoxides with rat liver microsomal fractions in both the presence and absence of reducing equivalents. Two major mechanisms of radical generation have been elucidated. In the absence of NADPH or NADH, oxidative degradation of the hydroperoxide occurs to give initially a peroxyl radical which in the majority of cases can be detected as a spin adduct to DMPO; these radicals can undergo further reactions which result in the generation of alkoxyl and carbon-centered radicals. In the presence of NADPH (and to a lesser extent NADH) alkoxyl radicals are generated directly via reductive cleavage of the hydroperoxide. These alkoxyl radicals undergo further fragmentation and rearrangement reactions to give carbon-centered species which can be identified by trapping with DBNBS. The type of transformation that occurs is highly dependent on the structure of the alkoxyl radical with species arising from beta-scission, 1,2-hydrogen shifts and ring closure reactions being identified; these processes are in accord with previous chemical studies and are characteristic of alkoxyl radicals present in free solution. Studies using specific enzyme inhibitors and metal-ion chelators suggest that most of the radical generation occurs via a catalytic process involving haem proteins and in particular cytochrome P-450. An unusual species (an acyl radical) is observed with lipid hydroperoxides; this is believed to arise via a cage reaction after beta-scission of an initial alkoxyl radical.

The high-spin/low-spin equilibrium in cytochrome P-450--a new method for determination of the high-spin content

A new method for determination of the population of the high-spin state (high-spin content) in ferric cytochrome P-450 is presented. Based on curve fitting the electronic absorption spectra with a linear combination of gaussian bands analytical functions for the pure high-spin and pure low-spin states were constructed. These functions were used to fit the high-spin/low-spin mixed spectra. A good fit of the absorption spectra of six different cytochrome P-450 proteins in the presence and absence of substrates was found, indicating a similar pi-electron structure of the porphyrin and a similar chemical nature of the nearest coordination sphere of the iron in all cytochrome P-450 proteins.

The involvement of free radicals in the mechanisms of monooxygenases

The occurrence of free radicals in the mechanisms of monooxygenases reflects the chemistry of dioxygen and the inertness of typical substrates. Thus, oxidation of such substrates requires attack by reduced dioxygen-derived free radicals. Consequently, a molecule of NAD(P)H must be invested for each substrate molecule oxidized. Furthermore, since free radicals are difficult to control, deviations from the intended reaction course are frequent. These considerations are illustrated by examination of the generation and fate of enzyme- and substrate-derived free radicals at various stages in the catalytic cycles of two monooxygenases important in xenobiotic biotransformation, dopamine beta-hydroxylase and cytochrome P-450.

On the nature of the cytochrome P450scc "ultimate oxidant": characterization of a productive radical intermediate

The electron paramagnetic (EPR) properties of a transient species detected during a cytochrome P450 (P450)-mediated peroxidative reaction have been compared with those of peroxidases Compound I and model metalloporphyrins. The reaction which was studied with cholesterol-specific P450scc and (20R)-20-hydroperoxycholesterol, occurred without enzyme denaturation. The resulting transient species, which reached its maximum after 50 s reaction, was characterized by a one-line EPR spectrum, g = 2.0035, delta 1/2 = 1.08 mT. The decay of this radical was concomitant with the production of (20R)-20,21-dihydroxycholesterol. The reaction (almost 100% yield) conserved the stereo-specificity of the natural pathway. We suggest this intermediate is a candidate for the in vivo ultimate oxidant in the P450-mediated hydroxylation process. The comparison of the observed EPR spectrum with those of peroxidase Compound I and related synthetic models allows us to propose a FeIV porphyrin II cation radical structure for the intermediate.

tert-butyl hydroperoxide-dependent microsomal release of iron and lipid peroxidation. I. Evidence for the reductive release of nonheme, nonferritin iron

Rat liver microsomes were found to contain a small pool of nonheme, nonferritin iron. In the presence of ADP, low concentrations of tert-butyl hydroperoxide promoted the reductive release of nonheme, nonferritin iron, as evidenced by mobilization of bathophenanthroline-chelatable Fe2+. Iron release was inhibited by SKF 525-A and metyrapone, which are known to interfere with cytochrome P450-catalyzed reactions. Iron release was also inhibited by high concentrations of t-BOOH, which caused rapid and extensive destruction of cytochrome P450. These observations suggested that iron release was catalyzed by cytochrome P450. Treatment of rats with phenobarbital (PB) caused simultaneous increase of cytochrome P450 and decrease of nonheme, nonferritin iron. The effects of PB were minimized by simultaneous administration of hematin, an inhibitor of heme synthesis, indicating that the nonheme iron was utilized for the synthesis of the heme iron of inducible cytochrome P450 isozymes. Consistently, microsomes from the liver of PB-treated rats were found to release low amounts of Fe2+, unless rats had also been treated with hematin to prevent utilization of nonheme, nonferritin iron for the synthesis of heme iron.

Interaction of a spin-labelled cholesterol derivative with the cytochrome P-450scc active site

The cholesterol analogue 25-doxyl-27-nor-cholesterol (CNO), was found to be a substrate for cytochrome P-450scc. Upon incubation with the cytochrome P-450scc electron transfer system, CNO is transformed to pregnenolone (Km = 33 microM, Vmax = 0.32 min-1). The pregnenolone formation from endogenous cholesterol is strongly inhibited by CNO (50% at 5 microM). It binds tightly to cytochrome P-450scc as evidenced by a reversed type I spectral absorbance change (Kd = 5.9 microM) which is paralleled by a greater hyperfine splitting of the room-temperature CNO ESR spectrum due to an enhanced probe immobilization (Kd = 1.9 microM). This finding is in accord with a rotational correlation time of about 10(-7) s, which is close to the tumbling rate of the protein. At 110 K the CNO-bound cytochrome P-450scc displays the ESR g-values gx = 2.404/2.456, gy = 2.245 and gz = 1.916; these are different from those of cholesterol-liganded cytochrome P-450scc and may thus serve as a marker for cytochrome P-450scc. Our data indicate that the stereospecificity of the cytochrome P-450scc side-chain-cleaving activity is not dependent on the nature of the cholesterol side-chain termination (C25 to C27). The substrate binding site is however rather sensitive to a modification of the side chain. The doxyl ring confers a stronger affinity of the substrate to the enzyme. Upon binding it becomes embedded in the protein matrix, and we estimate that its final position is 0.6-1.0 nm from the heme moiety.

Cholesterol autoxidation 1981-1986

Literature published between 1980 and 1986 dealing broadly with the topic of cholesterol autoxidation is reviewed. The review builds on the detailed 1981 monographic treatment of the topic by the author and covers new items of chemistry, analysis, and metabolism.

Free radical mechanisms in enzyme reactions

Free radicals are formed in prosthetic groups or amino acid residues of certain enzymes. These free radicals are closely related to the activation process in enzyme catalysis, but their formation does not always result in the formation of substrate free radicals as a product of the enzyme reaction. The role of free radicals in enzyme catalysis is discussed.

The mechanism of cumene hydroperoxide-dependent lipid peroxidation: the function of cytochrome P-450

The addition of limiting amounts of cumene hydroperoxide to rat liver microsomes resulted in the rapid uptake of molecular oxygen, the formation of thiobarbituric acid reactive products, and the loss of hydroperoxide. The stoichiometry of lipid peroxidation and the yields of 2-phenyl-2-propanol (a major product of the reaction) and acetophenone (a minor product) observed with liver microsomes prepared from untreated rats is greater than that seen with liver microsomes from ciprofibrate-treated rats which, in turn, is greater than that observed with liver microsomes from phenobarbital-treated rats. The Km's and Vmax's of oxygen uptake varied with the type of rat liver microsomes used. Cytochrome P-450 substrates and inhibitors decreased the extents and initial rates of oxygen uptake and thiobarbituric acid reactive product formation. A mechanism is proposed involving the cytochrome P-450-catalyzed homolytic cleavage of the cumene hydroperoxide O-O bond to give the cumyloxyl radical. It is proposed that this oxygen-centered radical abstracts a hydrogen atom from an unsaturated fatty acid associated with a lipid (initiating lipid peroxidation) to give 2-phenyl-2-propanol or that the radical undergoes beta-scission to produce acetophenone and a methyl radical.

Cytochrome P-450scc a review of the specificity and properties of the cholesterol binding site

Cytochrome P-450scc is unusual among members of this class of enzymes in showing a high degree of substrate specificity. Features of the cholesterol structure which are particularly important for binding include the 3 beta-hydroxyl, the delta 5-ring configuration, and the side-chain organization in the 20-22 region. Regarding the ring system, binding appears to require planarity and limited size at the 4-5-6 carbons (the A-B ring juncture). In the region of the 3 beta-hydroxyl, a "cleft" in the binding site extends about 4 A beyond the hydroxyl and can accommodate two additional ether-linked carbons. Evidence indicates that an enzyme residue hydrogen-bonds to the oxygen of the 3 beta hydroxyl, providing much of the energy for the initial enzyme-substrate interaction. The cytochrome shows less specificity for the side-chain structure, except in the region of carbons 20-22 where hydroxylation/side-chain cleavage takes place. The binding cleft for the side-chain is limited to approximately the length of the isocaproic group but can accommodate structural variations beyond the 22-position. Evidence indicates that the region near the 20-22 bond is more limited in size, and that an amino acid residue near the heme iron binds strongly and stereospecifically to the 22R-hydroxyl of the cleavage intermediates, 22R-hydroxycholesterol and 20 alpha, 22R-dihydroxycholesterol. The 22R-hydrogen of cholesterol is very close to the heme iron (approximately 3 A), while the 22S-hydrogen is slightly further (about 4 A). The size and bonding properties of the steroid binding/active site suggest a mechanism which accounts for the stereospecificity and sequence of reactions catalyzed by cytochrome P-450scc.