Controlling the regiospecificity and coupling of cytochrome P450cam: T185F mutant increases coupling and abolishes 3-hydroxynorcamphor product.

Cytochrome P450cam (P450CIA1) catalyzes the hydroxylation of camphor and several substrate analogues such as norcamphor and 1-methyl-norcamphor. Hydroxylation was found experimentally at the 3, 5, and 6 positions of norcamphor, but only at the 5 and 6 positions of 1-methyl-norcamphor. In the catalytic cycle, the hydroxylation of substrate is coupled to the consumption of NADH. For camphor, the degree of coupling is 100%, but for both norcamphor and 1-methyl-norcamphor, the efficiency is dramatically lowered to 12% and 50%, respectively. Based on an examination of the active site of P450cam, it appeared that mutating position 185 might dramatically alter the product specificity and coupling of hydroxylation of norcamphor by P450cam. Analysis of molecular dynamics trajectories of norcamphor bound to the T185F mutant of cytochrome P450cam predicted that hydroxylation at the 3 position should be abolished and that the coupling should be dramatically increased. This mutant was constructed and the product profile and coupling experimentally determined. The coupling was doubled, and hydroxylation at the 3 position was essentially abolished. Both of these results are in agreement with the prediction.     

Substrate mobility in a deeply buried active site: analysis of norcamphor bound to cytochrome P-450cam as determined by a 201-psec molecular dynamics simulation.

While cytochrome P-450cam catalyzes the hydroxylation of camphor to 5-exo-hydroxycamphor with 100% stereospecificity, norcamphor is hydroxylated by this enzyme yielding 45% 5-exo-, 47% 6-exo-, and 8% 3-exo-hydroxynorcamphor (Atkins, W.M., Sligar, S.G., J. Am. Chem. Soc. 109:3754-3760, 1987). The present study describes a 201-psec molecular dynamics (MD) stimulation of norcamphorbound cytochrome P-450cam to elucidate the relationship between substrate conformational mobility and formation of alternative products. First, these data suggest that the product specificity is, at least partially, due to the mobility of the substrate within the active site. Second, the high mobility of norcamphor in the active site leads to an average increase in separation between the heme iron and the substrate of about 1.0 A; this increase in separation may be the cause of the uncoupling of electron transfer when norcamphor is the substrate. Third, the active site water located in the norcamphorbound crystal structure possesses mobility that correlates well with the spin-state equilibrium of this enzyme-substrate complex.     

The structural basis for substrate-induced changes in redox potential and spin equilibrium in cytochrome P-450CAM

The crystal structures of cytochrome P-450CAM complexed with the alternative substrates norcamphor and adamantanone have been refined at 2.0-A resolution and compared with the native, camphor-bound form of the enzyme. Norcamphor lacks the 8-, 9-, and 10-methyl groups of camphor. Thus, specific interactions between these groups and phenylalanine 87 and valines 247 and 295 are missing in the norcamphor complex. As a result, norcamphor binds about 0.9 A further from the oxygen-binding site than does camphor, which allows sufficient room for a water molecule or hydroxide ion to remain coordinated with the heme iron atom. The larger adamantanone occupies a position closer to that of camphor and, as in the camphor-bound enzyme, the heme iron remains pentacoordinate with no solvent molecule coordinated as a sixth ligand. A comparison of crystallographic temperature factors indicates that norcamphor is more "loosely" bound than are either camphor or adamantanone, as might be expected from the relative sizes of the different substrates. The looser fit of norcamphor in the active-site pocket results in a less specific pattern of hydroxylation. The presence of an aqua ligand is the likely structural basis for the norcamphor-P-450CAM complex having both a lower redox potential and higher percentage of low-spin heme than do either the camphor-P-450CAM or adamantanone-P-450CAM complexes.     

Autooxidation and hydroxylation reactions of oxygenated cytochrome P-450cam.

Oxy-ferrous substrate-bound cytochrome P-450cam (mrsO2) autooxidizes in the absence of its specific effector protein, putidaredoxin, without hydroxylating the substrate, camphor. The autooxidation is first order with an activation energy of 17 kcal mol-1 at 25 degrees, pH 7.0. Substrate removal and low pH accelerate the reaction. The product, 5-exo-OH camphor, and a nonhydroxylated pseudosubstrate, norcamphor, stabilize the complex in a manner similar to camphor. Increased oxidation rate of mrsO2 and substrate hydroxylation are induced by putidaredoxin, rebredoxin, cytochrome b5, and the apoproteins of the latter two. Dihydrolipoic acid and other dithiols also replace putidaredoxin as effector molecules, but 1000-fold higher concentrations are required. Effector molecules do not increase the autooxidation rate of mrsO2 unless camphor, norcamphor, or another pseudosubstrate is present. Kinetic evidence is presented showing that an active complex between mrsO2 and effector is a required intermediate in mixed function oxidation.     

Spring-loading the active site of cytochrome P450cam

A hydrogen bond network has been identified that adjusts protein-substrate contacts in cytochrome P450(cam) (CYP101A1). Replacing the native substrate camphor with adamantanone or norcamphor causes perturbations in NMR-detected NH correlations assigned to the network, which includes portions of a β sheet and an adjacent helix that is remote from the active site. A mutation in this helix reduces enzyme efficiency and perturbs the extent of substrate-induced spin state changes at the haem iron that accompany substrate binding. In turn, the magnitude of the spin state changes induced by alternate substrate binding parallel the NMR-detected perturbations observed near the haem in the enzyme active site.     

Deuterium isotope effects in norcamphor metabolism by cytochrome P-450cam: kinetic evidence for the two-electron reduction of a high-valent iron-oxo intermediate

The kinetics of NADH consumption, oxygen uptake, and hydrogen peroxide production have been studied for norcamphor metabolism by cytochrome P-450cam. The kinetic deuterium isotope effects on these processes, with specifically deuteriated norcamphor, are 0.77, 1.22, and 1.16, respectively. Steady-state UV-visible spectroscopy indicates that transfer of the second electron to the dioxy ferrous P-450 is the rate-limiting step, as it is when camphor is the substrate. The inverse deuterium isotope effect for NADH consumption is consistent with an isotope-dependent branching between monooxygenase and oxidase activity, where these reactivities differ in their NADH:oxygen stoichiometries. However, no isotope-dependent redistribution of steady-state intermediates was detected by isotopic difference UV-visible spectroscopy in the presence of norcamphor. The kinetic isotope effects and steady-state spectral results suggest that the high-valent iron-oxo hydroxylating intermediate [FeO]3+ is reduced by NADH and the physiological electron-transfer proteins to afford water.     

Theoretical study of the product specificity in the hydroxylation of camphor, norcamphor, 5,5-difluorocamphor, and pericyclocamphanone by cytochrome P-450cam

The hydroxylations of d-camphor, norcamphor, pericyclocamphanone, and 5,5-difluorocamphor by cytochrome P-450cam have been examined using theoretical methods to identify and characterize properties which determine product specificity. Experimental results indicate that each molecule is hydroxylated with quite different regio-specificity when metabolized by P-450cam. This result is surprising in view of their overall structural similiarity. Herein we report the results of calculations on d-camphor and three of its analogues which suggest that all of these molecules should, when metabolized by P-450cam, form hydroxylation products and predict the product distribution for each. Our conclusions are based on two fundamental criteria which are consistent with a generally accepted radical mechanism in determining product specificity in these molecules: 1) relative heats of formation of the radicals formed by abstracting a hydrogen, and 2) orientation of the substrate molecule with respect to the putative active oxygen species bound to iron. Our results explain the experimental observations for camphor and 5,5-difluorocamphor but disagree with original published results for norcamphor and pericyclocamphanone. In light of our results, new experiments have been performed for norcamphor and the original data reexamined for pericyclocamphanone. Our predictions have recently been experimentally confirmed for norcamphor, and unpublished data (Dr. S. Sligar) suggest that the same is true for pericyclocamphanone.     

Structural alterations of the heme environment of cytochrome P450cam and the Y96F mutant as deduced by resonance Raman spectroscopy

Resonance Raman spectroscopy at 2.5cm(-1) resolution was used to probe differences in wild-type and Y96F mutant P450cam (CYP101), both with and without bound camphor or styrene substrates. In the substrate-free state, the spin state equilibrium is shifted from 6-coordinate low spin (6CLS) toward more 5-coordinate high spin (5CHS) when tyrosine-96 in the substrate pocket is replaced by phenylalanine. About 25% of substrate-free Y96F mutant is 5CHS as opposed to 8% for substrate-free wild-type P450cam. Spin equilibrium constants calculated from Raman intensities indicate that the driving force for electron transfer from putidaredoxin, the natural redox partner of P450cam, is significantly smaller on styrene binding than for camphor binding. Spectral differences suggest that there is a tilt in camphor toward the pyrrole III ring on Y96F mutation. This finding is consistent with the altered product distribution found for camphor hydroxylation by the Y96F mutant relative to the single enantiomer produced by the wild-type enzyme.     

The roles of active site hydrogen bonding in cytochrome P-450cam as revealed by site-directed mutagenesis

The role of the active site hydrogen bond of cytochrome P-450cam has been studied utilizing a combination of site-directed mutagenesis and substrate analogues with altered hydrogen bonding capabilities. Cytochrome P-450cam normally catalyzes the regiospecific hydroxylation of the monoterpene camphor. The x-ray crystal structure of this soluble bacterial cytochrome P-450 (Poulos, T. L., Finzel, B. C., Gunsalus, I. C., Wagner, G. C., and Kraut, J. (1985) J. Biol. Chem. 260, 16122-16128) indicates a specific hydrogen bond between tyrosine 96 and the carbonyl moiety of the camphor substrate. The site-directed mutant in which tyrosine 96 has been changed to a phenylalanine and the substrate analogues thiocamphor and camphane have been used to probe this interaction in several aspects of catalysis. At room temperature, both the mutant enzyme with camphor and the wild type enzyme with thiocamphor bound result in 59 and 65% high-spin ferric enzyme as compared to the 95% high spin population obtained with native enzyme and camphor as substrate. The equilibrium dissociation constant is moderately increased, from 1.6 microM for the wild type protein to 3.0 and 3.3 microM for wild type-thiocamphor and mutant-camphor complexes, respectively. Camphane bound to cytochrome P-450cam exhibits a larger decrease in high spin fraction (45%) and a correspondingly larger KD (46 microM), suggesting that the carbonyl moiety of camphor plays an important steric role in addition to its interaction as a hydrogen bond acceptor. The absolute regioselectivity of the mutant enzyme, and of the wild type enzyme with thiocamphor, is lost resulting in production of several hydroxylated products in addition to the 5-exo-hydroxy isomer. Based on rates of NADH oxidation, comparison of the substrate specificity for these systems (kcat/KD) indicates a 5- and 7-fold decrease in specificity for the mutant enzyme and thiocamphor-wild type complex, respectively. The replacement of the cytochrome P-450cam active site tyrosine with phenylalanine does not affect the branching ratio of monooxygenase versus oxidase chemistry or peroxygenase activity (Atkins, W.M., and Sligar, S.G. (1987) J. Am. Chem. Soc. 109, 3754-3760).     

Differential behavior of the sub-sites of cytochrome 450 active site in binding of substrates, and products (implications for coupling/uncoupling)

The cytochrome P450 catalyzes hydroxylation of many substrates in the presence of O(2) and specific electron transport system. The ternary complex S-Fe(+)O(2) with substrate and O(2) bound to their respective sites on the reduced enzyme is an important intermediate in the formation of the hydroxylating species. Then the active site may be considered as having two sub-sites geared for entirely different types of functionally relevant interactions. The two sites are the substrate binding site, the specific protein residues (Site I), and the L(6) position of the iron (Site II) to which O(2) binds upon reduction. In the ferric enzyme, when substrate binds to Site I, the low spin six-coordinated P450 is converted to the readily reducible high spin five coordinated state. Certain amines and OH compounds, such as products of P450-catalyzed reactions, can bind to Site II resulting in six coordinated inhibited complexes. Then the substrate and product interactions with the two sub-sites can regulate the functional state of the enzyme during catalysis. Product interactions have received very little attention. CYP101 is the only P450 in which X-ray and spectroscopic data on all three structures, the substrate-free, camphor-bound and the 5-exo-OHcamphor-bound are available. The substrate-free CYP101 is low spin and six-coordinated with a water molecule ligated at the L(6) position of the iron. The substrate camphor binds to Site I, and releases the L(6) water despite its inability to bind to this site, indicating that Site I binding can inhibit Site II ligation. The product 5-exo-OHcamphor in addition to binding to Site I, binds to Site II through its -OH group forming Fe-O bond, resulting in the low spin six-coordinated complex. New temperature-jump relaxation kinetic data indicating that Site II ligation inhibits Site I binding are presented. It appears that the Site I and Site II function as interacting sub-sites. The inhibitory allosteric interactions between the two sub-sites are also reflected in the data on binding of the substrate camphor (S) in the presence of the product 5-exo-OH camphor (P) to CYP101 (E). The data are in accordance with the two-site model involving the ternary complex ESP. The affinity of the substrate to the product-bound enzyme as well as the affinity of the product to the substrate-bound enzyme decreased with increase in product concentration, which is consistent with mixed inhibition indicative of inhibitory allosteric interactions between the two sub-sites. Implications of these observations for coupling/uncoupling mechanisms are discussed in the light of the published findings consistent with the two-site behavior of the P450 active site. In addition, kinetic data indicating that the transient high spin intermediate may have to be taken into account for understanding how some P450s have been able to express appreciable hydroxylation activities in the absence of substrate-induced low to high spin transition, observable by the traditional static spectroscopy, are presented.     

Role of protein and substrate dynamics in catalysis by Pseudomonas putida cytochrome P450cam

The role of protein structural flexibility and substrate dynamics in catalysis by cytochrome P450 enzymes is an area of current interest. We have addressed these in cytochrome P450(cam) (P450(cam)) and its Y96A mutant with camphor and its related compounds using fluorescence spectroscopy. Previously [Prasad et al. (2000) FEBS Lett. 477, 157-160], we provided experimental support to dynamic fluctuations in P450(cam), and substrate access into the active site region via the channel next to the flexible F-G helix-loop-helix segment. In the investigation described here, we show that the dynamic fluctuations in the enzyme are substrate dependent as reflected by tryptophan fluorescence quenching experiments. The orientation of tryptophan relative to heme (kappa(2)) for W42 obtained from time-resolved tryptophan fluorescence measurements show variation with type of substrate bound to P450(cam) suggesting regions distant from heme-binding site are affected by physicochemical and steric characteristics/protein-substrate interactions of P450(cam) active site. We monitored substrate dynamics in the active site region of P450(cam) by time-resolved substrate anisotropy measurements. The anisotropy decay of substrates bound to P450(cam) indicate that mobility of substrates is modulated by physicochemical and steric characteristics/protein-substrate interactions of local active site structure, and provides an understanding of factors controlling observed hydroxylated products for substrate bound P450(cam) complexes. The present study shows that P450(cam) local and peripheral structural flexibility and heterogeneity along with substrate mobility play an important role in regulating substrate binding orientation during catalysis and accommodating diverse range of substrates within P450(cam) heme pocket.     

Cytochrome P450cam catalyzed epoxidation of dehydrocamphor

In addition to the normal 5-$\text{exo}$-hydroxylation of camphor, bacterial cytochrome P450 is shown to carry out the facile epoxidation of dehydrocamphor to give $\text{exo}$-5,6-epoxycamphor. A detailed kinetic study of the reaction demonstrates that epoxidation and hydroxylation reactions occur with nearly identical rates both in the reconstituted system containing flavoprotein dehydrogenase, iron-sulfur protein, and NADH as well as in the single turnover reaction beginning with ferrous, oxygenated cytochrome P450. Dehydrocamphor is not a suicide substrate for the enzyme since competent enzyme remains after several thousand reaction cycles per P450 molecule.     

The influence of substrate on the spectral properties of oxyferrous wild-type and T252A cytochrome P450-CAM

To probe whether the nature of the substrate can directly influence the spectral properties of oxyferrous cytochrome P450-CAM, the complex has been investigated in the absence and in the presence of the natural substrate (1R)-camphor (camphor) and of several camphor analogs. The oxyferrous complex of T252A P450-CAM, a mutant lacking the hydroxyl group that forms a hydrogen bond to the heme iron-coordinated dioxygen, has also been studied to gauge the influence of this hydrogen bond. UV-visible absorption and magnetic circular dichroism (MCD) spectra of these oxyferrous adducts prepared and stabilized at -40 degrees C in 60% (v/v) ethylene glycol are generally similar, exhibiting absorption bands at approximately 355, approximately 420, approximately 554, and approximately 585 nm (shoulder) and a characteristic MCD trough at approximately 585 nm. The MCD spectrum of camphor-bound oxyferrous P450-CAM is similar to that of the substrate-free oxyferrous enzyme, but the spectrum of the oxyferrous enzyme differs detectably in the presence of substrate analogs. The spectra of the oxyferrous T252A mutant and wild-type enzyme are overall similar except for Soret band position blue shifts by 2-6 nm for the mutant. 5-Methylenylcamphor (epoxidation substrate) appears to have an anomalous binding mode for the mutant compared with that for the wild-type enzyme. The present results indicate that the structures of the camphor analogs can sensitively influence the physical (spectroscopic) properties of the P450 dioxygen complex and could also affect its reactivity. The ability of substrate to modulate the reactivity of P450 intermediates could be a relevant factor in explaining the remarkable diversity of reactions catalyzed by the enzyme.     

Substrate modulates compound I formation in peroxide shunt pathway of Pseudomonas putida cytochrome P450(cam)

The active oxygenating intermediate, a ferryl-oxo-(II) porphyrin cation radical (compound I), in substrate-bound cytochrome P450(cam) (P450(cam)) has eluded detection and kinetic analysis for several decades. Upon rapid mixing of peroxides-H(2)O(2) and m-CPBA with substrate-bound forms of P450(cam), we observed an intermediate with spectral features characteristic of compound I. Unlike in H(2)O(2), kinetic investigation on the reaction of m-CPBA with various substrate (camphor, adamantone, and norcamphor)-bound P450(cam) and its Y96A mutant shows a preferential binding of the aromatic end group of m-CPBA to the active-site of the enzyme and modulation of compound I formation by the local environment of heme active-site. The results presented in this paper describe the importance of heme environment in modulating formation of compound I, and form the first kinetic analysis of this intermediate in the peroxide shunt pathway of substrate-bound P450(cam).     

A functional proline switch in cytochrome P450cam

The two-protein complex between putidaredoxin (Pdx) and cytochrome P450(cam) (CYP101) is the catalytically competent species for camphor hydroxylation by CYP101. We detected a conformational change in CYP101 upon binding of Pdx that reorients bound camphor appropriately for hydroxylation. Experimental evidence shows that binding of Pdx converts a single X-proline amide bond in CYP101 from trans or distorted trans to cis. Mutation of proline 89 to isoleucine yields a mixture of both bound camphor orientations, that seen in Pdx-free and that seen in Pdx-bound CYP101. A mutation in CYP101 that destabilizes the cis conformer of the Ile 88-Pro 89 amide bond results in weaker binding of Pdx. This work provides direct experimental evidence for involvement of X-proline isomerization in enzyme function.     

Effect of alcohols on binding of camphor to cytochrome P450cam: spectroscopic and stopped flow transient kinetic studies

Addition of alcohols to cytochrome P450cam (CYP101) was shown to release the substrate camphor from the heme pocket of the enzyme. The release of the substrate was found to be caused both due to increased solubility of the substrate in solution in presence of alcohol and due to change in the tertiary structure of the active site of the enzyme. The far-UV CD and near-UV CD spectra reveal that addition of alcohols to cytochrome P450cam cause a small change in the secondary structural elements but a significant change in the tertiary structural organization of this enzyme. The CD spectra at the heme region at various concentrations of alcohols indicate a substantial change in the tertiary structural organization around the heme moiety too. The equilibrium constant associated with the binding of camphor to Cyt P450cam is strongly dependent on the concentration of alcohols and the corresponding free energy associated with the binding is found to scale linearly with the concentration of alcohols. Kinetic experiments on binding of camphor to Cyt P450cam show that both k(on) and k(off) rate constants are strongly affected by addition of alcohols suggesting that alcohol expel camphor out of the heme cavity of Cyt P450cam by affecting tertiary structure of Cyt P450cam as well as by modifying the solubility properties of camphor in aqueous medium.     

Structural evidence for a functionally relevant second camphor binding site in P450cam: model for substrate entry into a P450 active site

P450cam has long served as a prototype for the cytochrome P450 (CYP) gene family. But, little is known about how substrate enters its active site pocket, and how access is achieved in a way that minimizes exposure of the reactive heme. We hypothesize that P450cam may first bind substrate transiently near the mobile F-G helix that covers the active site pocket. Such a two-step binding process is kinetically required if P450cam rarely populates an open conformation-as suggested by previous literature and the inability to obtain a crystal structure of P450cam in an open conformation. Such a mechanism would minimize exposure of the heme by allowing P450cam to stay in a closed conformation as long as possible, since only brief flexing into an open conformation would be required to allow substrate entry. To test this model, we have attempted to dock a second camphor molecule into the crystal structure of camphor-bound P450cam. The docking identified only one potential entry site pocket, a well-defined cavity on the F-helix side of the F-G flap, 16 A from the heme iron. Location of this entry site pocket is consistent with our NMR T1 relaxation-based measurements of distances for a camphor that binds in fast exchange (active site camphor is known to bind in slow exchange). Presence of a second camphor binding site is also confirmed with [(1)H-(13)C] HSQC titrations of (13)CH3-threonine labeled P450cam. To confirm that camphor can bind outside of the active site pocket, (13)CH3-S-pyridine was bound to the heme iron to physically block the active site, and to serve as an NMR chemical shift probe. Titration of this P450cam-pyridine complex confirms that camphor can bind to a site outside the active site pocket, with an estimated Kd of 43 microM. The two-site binding model that is proposed based on these data is analogous to that recently proposed for CYP3A4, and is consistent with recent crystal structures of P450cam bound to tethered-substrates, which force a partially opened conformation.     

Substrate analogue induced changes of the CO-stretching mode in the cytochrome P450cam-carbon monoxide complex.

The CO-stretching mode of the carbon monoxide ligand in reduced cytochrome P450cam, in the absence or presence of camphor and in the presence of nine different camphor analogues, was measured at room temperature using Fourier transform infrared spectroscopy. Substrate-free cytochrome P450cam--CO reveals a broad, slightly structured band resulting from an overlap of several stretching mode signals. The multitude of the signals indicates that cytochrome P450 exists in a dynamic equilibrium of several conformational substates. Binding of camphor or camphor analogues strongly influences this equilibrium. For substrate analogues which are not able to form a hydrogen bond to the hydroxyl group of tyrosine 96, the CO-stretching band is rather broad and asymmetric. In contrast, substrate analogues with one quinone group which form a hydrogen bond to the Tyr96 OH induce a shift and a sharpening of the CO-stretching mode band. For substrate analogues with two hetero groups, the infrared spectrum is slightly asymmetric or a minor band appears. Sterical hindrance, substrate mobility, and protein flexibility finally determine the position and width of the CO-stretching mode signals.     

Regiochemistry of Camphor Analog Oxidation by Pseudomonas putida

Pseudomonas putida cooxidized norcamphor and pericyclocamphanone to hydroxylated and lactonized products during growth on camphor. Norcamphor was hydroxylated at the 5 position, similar to the corresponding process in camphor, but pericyclocamphanone was oxidized at the 6 position. We conclude that the regiochemistry of the hydroxylation may be substrate controlled.     

A recombinant Escherichia coli whole cell biocatalyst harboring a cytochrome P450cam monooxygenase system coupled with enzymatic cofactor regeneration

A cytochrome P450cam monooxygenase (P450cam) system from the soil bacterium Pseudomonas putida requires electron transfer among three different proteins and a cofactor, nicotinamide adenine dinucleotide (NADH), for oxygenation of its natural substrate, camphor. Herein, we report a facile way to significantly enhance the catalytic efficiency of the P450cam system by the coupling of its native electron transfer system with enzymatic NADH regeneration catalyzed by glycerol dehydrogenase (GLD) in Escherichia coli whole cell biocatalysts. Recombinant E. coli harboring the P450cam system, but lacking GLD, exhibited little activity for camphor hydroxylation. In contrast, coexpression of GLD with the proteinaceous electron transfer components of P450cam resulted in about tenfold improvement in the substrate conversion, implying that the whole cell biocatalyst utilized molecular oxygen, endogenous NADH, and glycerol in the cell for catalysis. The addition of glycerol to the reaction media further promoted camphor hydroxylation, suggesting that exogenous glycerol is also available for GLD in the host cell and actively participates in the catalytic cycle. These results clearly show the utility of GLD towards functional reconstruction of the native P450cam system. The present approach may also be useful for E. coli whole cell biocatalysts with the other NADH-dependent oxygenases and oxidoreductases.