Role of the 8-double bond of lanosterol in the enzyme-substrate interaction of cytochrome P-45014DM (lanosterol 14α-demethylase)

Processive kinetics in the three-step lanosterol 14α-demethylation reaction catalyzed by human cytochrome P450 51A1

Cytochrome P450 (P450, CYP) family 51 enzymes catalyze the 14α-demethylation of sterols, leading to critical products used for membranes and the production of steroids, as well as signaling molecules. In mammals, P450 51 catalyzes the 3-step, 6-electron oxidation of lanosterol to form (4β,5α)-4,4-dimethyl-cholestra-8,14,24-trien-3-ol (FF-MAS). P450 51A1 can also use 24,25-dihydrolanosterol (a natural substrate in the Kandutsch-Russell cholesterol pathway). 24,25-Dihydrolanosterol and the corresponding P450 51A1 reaction intermediates, the 14α-alcohol and -aldehyde derivatives of dihydrolanosterol, were synthesized to study the kinetic processivity of the overall 14α-demethylation reaction of human P450 51A1. A combination of steady-state kinetic parameters, steady-state binding constants, dissociation rates of P450-sterol complexes, and kinetic modeling of the time course of oxidation of a P450-dihydrolanosterol complex showed that the overall reaction is highly processive, with koff rates of P450 51A1-dihydrolanosterol and the 14α-alcohol and 14α-aldehyde complexes being 1 to 2 orders of magnitude less than the forward rates of competing oxidations. epi-Dihydrolanosterol (the 3α-hydroxy analog) was as efficient as the common 3β-hydroxy isomer in the binding and formation of dihydro FF-MAS. The common lanosterol contaminant dihydroagnosterol was found to be a substrate of human P450 51A1, with roughly one-half the activity of dihydrolanosterol. Steady-state experiments with 14α-methyl deuterated dihydrolanosterol showed no kinetic isotope effect, indicating that C-14α C-H bond breaking is not rate-limiting in any of the individual steps. The high processivity of this reaction generates higher efficiency and also renders the reaction less sensitive to inhibitors.

Synthesis of obtusifoliol and analogues as CYP51 substrates

Sterol 14α-demethylases (CYP51s) are a ubiquitous superfamily of cytochrome P450 enzymes that play an essential role in sterol biosynthesis. As fungal CYP51s are the target of azole-based antifungal agents, which are facing the problem of increasing resistance, the substrate specificity of this enzyme subclass has recently garnered significant attention. Herein we report the first chemical synthesis of the final endogenous substrate of this enzyme class, obtusifoliol, in 1.3% yield across ten steps from a commercially available lanosterol mixture. Intermediates along this pathway provide a basis for further derivatisation of the sterol skeleton and future investigation into CYP51 inhibition to overcome pathogens' azole resistance.

Three-dimensional model of lanosterol 14 alpha-demethylase from Cryptococcus neoformans: active-site characterization and insights into azole binding

Cryptococcus neoformans is one of the most important causes of life-threatening fungal infections in immunocompromised patients. Lanosterol 14 alpha-demethylase (CYP51) is the target of azole antifungal agents. This study describes, for the first time, the 3-dimensional model of CYP51 from Cryptococcus neoformans (CnCYP51). The model was further refined by energy minimization and molecular-dynamics simulations. The active site of CnCYP51 was well characterized by multiple-copy simultaneous-search calculations, and four functional regions important for rational drug design were identified. The mode of binding of the natural substrate and azole antifungal agents with CnCYP51 was identified by flexible molecular docking. A G484S substitution mechanism for azole resistance in CnCYP51, which might be important for the conformation of the heme environment, is suggested.

Homology Modeling of Lanosterol 14α-Demethylase ofCandida albicansandAspergillus fumigatusand Insights into the Enzyme-Substrate Interactions

The crystal structure of 14alpha-sterol demethylase from Mycobacterium tuberculosis (MT_14DM) provides a good template for modeling the three dimensional structure of lanosterol 14alpha-demethylase, which is the target of azole antifungal agents. Homologous 3D models of lanosterol 14alpha-demethylase from Candida albicans (CA_14DM) and Aspergillus fumigatus (AF_14DM) were built on the basis of the crystal coordinates of MT_14DM in complex with 4-phenylimidazole and fluconazole. The reliability of the two models was assessed by Ramachandran plots, Profile-3D analysis, and by analyzing the consistency of the two models with the experimental data on the P450(14DM). The overall structures of the resulting CA_14DM model and AF_14DM model are similar to those of the template structures. The two models remain the core structure characteristic for cytochrome P450s and most of the insertions and deletions expose the molecular surface. The structurally and functionally important residues such as the heme binding residues, the residues lining the substrate access channel, and residues in active site were identified from the model. To explore the binding mode of the substrate with the two models, 24(28)-methylene-24,25-dihydrolanosterol was docked into the active site of the two models and hydrophobic interaction and hydrogen-bonding were found to play an important role in substrate recognition and orientation. These results provided a basis for experiments to probe structure-function relationships in the P450(14DM). Although CA_14DM and AF_14DM shared similar core structural character, the active site of the two models were quite different, thus allowing the rational design of specific inhibitors to the target enzyme and the discovery of novel antifungal agents with broad spectrum.

Role of the non-respiratory pathways in the utilization of molecular oxygen by Saccharomyces cerevisiae

Saccharomyces cerevisiae is a facultative anaerobe devoid of mitochondrial alternative oxidase. In this yeast, the structure and biogenesis of the respiratory chain, on the one hand, and the functional interactions of oxidative phosphorylation with the cellular energetic metabolism, on the other, are well documented. However, to our knowledge, the molecular aspects and the physiological roles of the non-respiratory pathways that utilize molecular oxygen have not yet been reviewed. In this paper, we review the various non-respiratory pathways in a global context of utilization of molecular oxygen in S. cerevisiae. The roles of these pathways are examined as a function of environmental conditions, using either physiological, biochemical or molecular data. Special attention is paid to the characterization of the so-called 'cyanide-resistant respiration' that is induced by respiratory deficiency, catabolic repression and oxygen limitation during growth. Finally, several aspects of oxygen sensing are discussed.

Oxygen consumption by anaerobic Saccharomyces cerevisiae under enological conditions: effect on fermentation kinetics

The anaerobic growth of the yeast Saccharomyces cerevisiae normally requires the addition of molecular oxygen, which is used to synthesize sterols and unsaturated fatty acids (UFAs). A single oxygen pulse can stimulate enological fermentation, but the biochemical pathways involved in this phenomenon remain to be elucidated. We showed that the addition of oxygen (0.3 to 1.5 mg/g [dry mass] of yeast) to a lipid-depleted medium mainly resulted in the synthesis of the sterols and UFAs required for cell growth. However, the addition of oxygen during the stationary phase in a medium containing excess ergosterol and oleic acid increased the specific fermentation rate, increased cell viability, and shortened the fermentation period. Neither the respiratory chain nor de novo protein synthesis was required for these medium- and long-term effects. As de novo lipid synthesis may be involved in ethanol tolerance, we studied the effect of oxygen addition on sterol and UFA auxotrophs (erg1 and ole1 mutants, respectively). Both mutants exhibited normal anaerobic fermentation kinetics. However, only the ole1 mutant strain responded to the oxygen pulse during the stationary phase, suggesting that de novo sterol synthesis is required for the oxygen-induced increase of the specific fermentation rate. In conclusion, the sterol pathway appears to contribute significantly to the oxygen consumption capacities of cells under anaerobic conditions. Nevertheless, we demonstrated the existence of alternative oxygen consumption pathways that are neither linked to the respiratory chain nor linked to heme, sterol, or UFA synthesis. These pathways dissipate the oxygen added during the stationary phase, without affecting the fermentation kinetics.

Molecular diversity of sterol 14alpha-demethylase substrates in plants, fungi and humans

Metabolism of lanosterol (LAN), 24-methylene-24,25-dihydrolanosterol (24-methyleneDHL), dihydrolanosterol (DHL) and obtusifoliol (OBT) by purified human, plant (Sorghum bicolor) and fungal (Candida albicans) sterol 14alpha-demethylase (CYP51; P450(14DM)) reconstituted with NADPH cytochrome P450 reductases was studied in order to elucidate the substrate specificity and sterol stereo- and regio-structural requirements for optimal CYP51 activity. Both human and C. albicans CYP51 could catalyse 14alpha-demethylation of each substrate with varying levels of activity, but having slightly higher activity for their respective endogenous substrates in vivo, dihydrolanosterol for human CYP51 (Vmax = 0.5 nmol/min/nmol CYP51) and 24-methylene-24,25-dihydrolanosterol for C. albicans CYP51 (Vmax = 0.3 nmol/min/nmol CYP51). In contrast, S. bicolor CYP51 showed strict substrate specificity and selectivity towards its own endogenous substrate, obtusifoliol (Vmax = 5.5 nmol/min/nmol CYP51) and was inactive towards 14alpha-demethylation of lanosterol, 24-methylene-24,25-dihydrolanosterol and dihydrolanosterol. These findings confirm that the presence of the 4beta-methyl group in the sterol molecule renders the plant CYP51 incapable of 14alpha-demethylation thus revealing the strict active site conservation of plant CYP51 during evolution.

Substrate-based inhibitors of lanosterol 14 alpha-methyl demethylase: I. Assessment of inhibitor structure-activity relationship and cholesterol biosynthesis inhibition properties

A series of 15-, 32-, and 15,32-substituted lanost-8-en-3 beta-ols is described which function as inhibitors of cholesterol biosynthesis. These agents inhibit lanosterol 14 alpha-methyl demethylase activity as well as suppress HMG-CoA reduction activity in cultured cells. Several of these agents are extremely potent as both demethylase inhibitors and reductase suppressors, while others are more selective in their activities. Selected regio double bond isomers show preference for demethylase inhibition with the following order: delta 8 > delta 7 > delta 6 = unsaturated sterols. Comparisons also show that 4,4-dimethyl sterols are always more potent demethylase inhibitors and reductase suppressors than their 4,4-bisnomethyl counterparts. However, evaluation of an extensive oxylanosterol series leads us to conclude that demethylase inhibition and reductase suppression are not parallel in the same molecule. In addition, the oxylanosterols, but not the oxycholesterols, are able to disrupt coordinate regulation of HMG-CoA reductase from the LDL receptor. Thus, oxylanosterol treatment at levels which suppress reductase activity enhances LDL receptor activity. These results demonstrate that compounds can be made which (1) are selective reductase suppressors enabling dissection of the dual inhibitor nature of these compounds and (2) maximize reductase suppression and LDL receptor induction without demethylase inhibition which could lead to novel agents for serum cholesterol lowering.

Modeling cytochrome P450 14 alpha demethylase (Candida albicans) from P450cam

The tertiary structure of cytochrome P450 14 alpha demethylase--Candida albicans (P450 CA) is modeled on the basis of sequence alignment with two closely related proteins and the crystallographic structure of Pseudomonas putida P450cam. The secondary structure prediction system used combines the information from several algorithms and trains the data to offer an optimized prediction of the known P450cam. The trained algorithm was then used to predict the secondary structure of the other P450 sequences. The prediction of the surface coil regions was aided by an alignment between P450 CA and the homologous sequences P450 14 alpha demethylase--Saccharomyces cerevisiae (66 SD) and P450 14 alpha demethylase--Candida tropicalis (72 SD). The prediction and alignment information was combined to establish an alignment between P450 CA and P450cam, and to assign full secondary structure to the target protein. This secondary structure was folded from the template of P450cam and the predicted structure was relaxed by molecular dynamics. Model checking highlighted minor adjustments in the alignment, correctly orienting hydrophobic and hydrophilic side chains. The model offers explanations for several known experimental results and suggests further investigations that may prove fruitful in understanding the structure and mechanisms of the P450 family (Porter, T.D. and Coon, M.J. Minireview cytochrome P450. J. Biol. Chem. 1991, 266, 13469-13472. Waterman, M.R. Cytochrome P450 cellular distribution and structural considerations. Current Opinion in Structural Biology 1992, 2, 384-387. Aoyama, Y., Yoshida, Y., Sonohdo, Y. and Sato, Y. Structural analysis of the interaction between the side-chain of substrates and the active site of lanosterol 14 alpha demethylase (P450 14DM) of yeast. Biochim. Biophys. Acta 1992, 1122, 251-255.).

Cytochrome P450 lanosterol 14α-demethylase (CYP51): insights from molecular genetic analysis of the ERG11 gene in Saccharomyces cerevisiae

Eukaryotes characteristically express a cytochrome P450-catalyzed sterol 14α-methyl demethylase as an essential step in the production of membrane sterols. Lanosterol 14α-demethylase of Saccharomyces cerevisiae is the best characterized representative of these enzymes among fungi and provides a model system for the molecular genetic analysis of the reaction. The gene for this P450 and the gene for the S. cerevisiae NADPH-cytochrome P450 reductase have been examined by mutational inactivation and for their regulation of expression. Our results have contributed to a better understanding of sterol biosynthesis in relation to mechanisms of resistance to fungicidal demethylase inhibitors, and promote the rationale for using S. cerevisiae in the further characterization of structure function relationships among sterol 14α-demethylases.

Inhibition by a novel azole antifungal agent with a geranyl group on lanosterol 14 alpha-demethylase of yeast

AFK-108 (1-[2-(2,4-dichlorophenyl)-2-((2E)-3,7-dimethylocta-2,6- dienyloxy)ethyl]-1H-imidazole) is a new imidazole derivative characterized by a geranyl substituent showing strong antifungal activity. Azole antifungal agents are known to be potent inhibitors of lanosterol 14 alpha-demethylase (P450(14)DM) of fungi. The role of the geranyl group of AFK-108 on interaction of AFK-108 with the target was studied by using Saccharomyces cerevisiae P450(14)DM as the model enzyme. AFK-108 and some of its derivatives bound to oxidized P450(14)DM with one-to-one stoichiometry and inhibited the demethylase activity. AFK-108 derivatives having the longer farnesyl or the shorter prenyl group showed lower affinity than AFK-108 for the enzyme. AFK-108 caused 100% inhibition at the equivalent concentration to P450(14)DM in the reaction mixture (0.07 microM), while the farnesyl derivative inhibited the activity by 60% at the same concentration. AFK-108 interfered with the binding of CO to the ferrous P450(14)DM. However, the interfering effect of the prenyl derivative was lower than that of AFK-108. Another AFK-108 derivative having the saturated 3,7-dimethyloctyl group was also a weaker inhibitor than AFK-108. These experiments suggest that the geranyl group of AFK-108 interacts with the substrate binding site of P450(14)DM that recognises the side chain of the substrate. AFK-108 is the first example of an azole derivative interacting with the side chain recognising region of the substrate binding site of P450(14)DM.

Structural analysis of the interaction between the side-chain of substrates and the active site of lanosterol 14 alpha-demethylase (P-450(14)DM) of yeast

The role of the side-chain of lanosterol in the enzyme-substrate interaction of yeast P-450(14)DM (lanosterol 14 alpha-demethylase) was analyzed with lanosterol derivatives having functional groups on the side-chain. Purified P-450(14)DM from Saccharomyces cerevisiae catalyzed 14 alpha-demethylation of 26-hydroxylanosterol and 25-hydroxy-24,25-dihydrolanosterol with a lower activity than lanosterol and 24,25-dihydrolanosterol. This enzyme demethylated the (Z)-24-ethylidene-24,25-dihydrolanosterol with a low rate, but did not metabolize the E-isomer. The apparent Km of 26-hydroxylanosterol was 10.8 microM, which was higher than that of lanosterol, but lower than that of 24,25-dihydrolanosterol. On the other hand, competition experiments suggested that the affinity of 25-hydroxy-24,25-dihydrolanosterol and (Z)-24-ethylidene-24,25-dihydrolanosterol for P-450(14)DM was significantly lower than that of 24,25-dihydrolanosterol. Integration of the present results with the preceding ones (Aoyama, Y., Yoshida, Y., Sonoda, Y. and Sato, Y. (1991) Biochim. Biophys. Acta, 1081, 262-266 and Aoyama, Y. and Yoshida, Y. (1991) Biochem. Biophys. Res. Commun., 178, 1064-1071) suggests that yeast P-450(14)DM recognizes two parts of the side-chain, the structure around C-24 and the terminal fork consisting of C-25, C-26 and C-27.

The 4 beta-methyl group of substrate does not affect the activity of lanosterol 14 alpha-demethylase (P-450(14)DM) of yeast: difference between the substrate recognition by yeast and plant sterol 14 alpha-demethylases

Interaction of obtusifoliol and 24,28-dihydroobtusifoliol with yeast lanosterol 14 alpha-demethylase (P-450(14)DM) was studied to elucidate the role of the 4 beta-methyl group of substrate. P-450(14)DM of Saccharomyces cerevisiae catalyzed 14 alpha-demethylation of obtusifoliol. Apparent Vmax of obtusifoliol demethylation (15.4 nmol/min/nmol P-450) was similar to that of 24-methylene-24,25-dihydrolanosterol demethylation and was a little higher than those of lanosterol and 24,25-dihydrolanosterol demethylations. Apparent Km for obtusifoliol (12.0 microM) was higher than those for lanosterol and 24-methylene-24,25-dihydrolanosterol but was lower than that for 24,25-dihydrolanosterol. 24,28-Dihydroobtusifoliol was a very poor substrate for yeast P-450(14)DM. These facts suggest that the 4 beta-methyl group of sterol slightly affects the activity of yeast P-450(14)DM, while hydrogenation of a double bond in the sterol side-chain considerably impairs the activity. This finding is a contrast to the fact that the plant P-450(14)DM could not catalyze demethylation of sterols having 4 beta-methyl group, but favorably interacts with sterols having saturated side chain.

Different substrate specificities of lanosterol 14a-demethylase (P-45014DM) of Saccharomyces cerevisiae and rat liver for 24-methylene-24,25-dihydrolanosterol and 24,25-dihydrolanosterol

The purified lanosterol 14a-demethylase (P-45014DM) of S. cerevisiae catalyzed the 14a-demethylation of 24-methylene-24,25-dihydrolanosterol (24-methylenelanost-8-en-3 beta-ol, 24-methylene-DHL), the natural substrate of the demethylase of filamentous fungi, as well as its natural substrate, lanosterol. Lanosterol 14a-demethylase of rat liver microsomes also catalyzed the 14a-demethylation of 24-methylene-DHL, but the activity was considerably lower than that for lanosterol. The activity of the rat liver enzyme for 24-methylene-DHL was also lower than that for 24,25-dihydrolanosterol (DHL), while the activity of yeast P-45014DM for 24-methylene-DHL was considerably higher than that for DHL. Since 24-substituted sterols are not found in mammals and DHL is not an intermediate of ergosterol biosynthesis by yeast, above-mentioned different substrate specificities between the yeast and the mammalian 14a-demethylases may reflect certain evolutional alteration in their active sites in relation to the difference in their sterol biosynthetic pathways.

Role of the side chain of lanosterol in substrate recognition and catalytic activity of lanosterol 14 alpha-demethylase (cytochrome P-450 (14DM)) of yeast

The 14 alpha-demethylation of 24,25-dihydrolanosterol (DHL) derivatives having trimmed side chains, 27-nor-DHL, 26,27-dinor-DHL, 25,26,27-trinor-DHL, 24,25,26,27-tetranor-DHL, 23,24,25,26,27-pentanor-DHL and 22,23,24,25,26,27-hexanor-DHL, was studied with the reconstituted lanosterol 14 alpha-demethylase system consisting of cytochrome P-450(14DM) and NADPH-cytochrome P-450 reductase both purified from yeast microsomes. The demethylase catalyzed the 14 alpha-demethylation of the derivatives having the side chains longer than tetranor but the activities for the trinor- and tetranor-derivatives were lower. Kinetic analysis indicated that affinity of the trinor-derivative for the demethylase was considerably higher than that of DHL. The affinities of the 27-nor- and dinor-derivatives were increased by this order and were the intermediates of DHL and the trinor derivative. On the other hand, Vmax values of the demethylase for the DHL derivatives were decreased depending on their side-chain lengths, and the substrate-dependent reduction rate of cytochrome P-450(14DM) was also decreased in the same manner. Based on these observations, it was concluded that interaction of the side chain of lanosterol especially C-25, 26 and 27 with the substrate site of lanosterol 14 alpha-demethylase was necessary for enhancing the catalytic activity of the enzyme. However, this interaction was considered not to be essential for substrate binding.

Induction and substrate specificity of the lanosterol 14 alpha-demethylase from Saccharomyces cerevisiae Y222

The potential inducibility of the lanosterol 14 alpha-demethylase (P-45014DM) from Saccharomyces cerevisiae Y222 by xenobiotics was investigated. This enzyme and NADPH-cytochrome P-450 reductase were unaffected by a number of compounds known to induce mammalian and some yeast cytochrome P-450 monooxygenases. Furthermore, dibutyryl cyclic AMP did not affect P-45014DM or P-450 reductase levels, while growth at 37 degrees C resulted in a slight decrease. P-45014DM was found to be specific for lanosterol and did not metabolize a number of P-450 substrates including benzo[a]pyrene.

Stereoselective interaction of an azole antifungal agent with its target, lanosterol 14 alpha-demethylase (cytochrome P-45014DM): a model study with stereoisomers of triadimenol and purified cytochrome P-45014DM from yeast

The effect of the four triadimenol stereoisomers on the purified yeast lanosterol 14 alpha-demethylase (cytochrome P-45014DM), the primary target of azole antifungal agents, was studied. (1S,2R)-Triadimenol was the most potent demethylase inhibitor and bound quantitatively to the enzyme below 0.05 microM. This isomer also interfered with the chemical reduction of cytochrome P-45014DM and the binding of CO to the cytochrome. The other isomers showed a lower inhibitory effect on the enzyme, and the order of activity was (1R,2R) greater than (1R,2S) greater than or equal to (1S,2S). Based on these findings and the reported preferred conformations for the triadimenol stereoisomers (Anderson, N.H. et al., Pestic. Sci. 15:310-316, 1984), it is predicted that orientation of the hydrophobic tert-butyl and p-chlorophenyl groups relative to the azole nitrogen is important to fit the antifungal agent in the active site of the demethylase.

The 3-hydroxy group of lanosterol is essential for orienting the substrate site of cytochrome P-450(14DM) (lanosterol 14 alpha- demethylase)

Interaction of lanosterol, 3-epilanosterol, 3-oxolanosta-8,24-diene, 3-methylenelanost-8-ene and lanosterol acetate with cytochrome P-450(14DM) were studied. The cytochrome mediated the 14alpha-demethylation of 3-epilanosterol with nearly the same activity as lanosterol but could not mediate the 14alpha-demethylation of the 3-methylene derivative and the 3-acetate. The cytochrome catalyzed the 14alpha-demethylation of the 3-oxo derivative with low rate. Based on these and some additional observations the hydrogen bond formation between the 3-hydroxy group of lanosterol and the specific amino acid residue in the substrate site is assumed to be essential for orienting the substrate in the substrate site of the cytochrome.