Directed evolution of cholesterol oxidase with improved thermostability using error-prone PCR

Cholesterol oxidase is industrially important as it is frequently used as a biosensor in food and agriculture industries and measurement of cholesterol. Although, most natural enzymes show low thermostability, which limits their application. Here, we obtained an improved variant of Chromobacterium sp. DS1 cholesterol oxidase (ChOS) with enhanced thermostability by random mutant library applying two forms of error-prone PCR (serial dilution and single step). Wild-type ChOS indicated an optimal temperature and pH of 70 ºC and pH 7.5, respectively. The best mutant ChOS-M acquired three amino acid substitutions (S112T, I240V and A500S) and enhanced thermostability (at 50 °C for 5 h) by 30%. The optimum temperature and pH in the mutant were not changed. In comparison to wild type, circular dichroism disclosed no significant secondary structural alterations in mutants. These findings show that error-prone PCR is an effective method for enhancing enzyme characteristics and offers a platform for the practical use of ChOS as a thermal-resistance enzyme in industrial fields and clinical diagnosis.

Heterologous Expression and Function of Cholesterol Oxidase: A Review

Cholesterol was first found in gallstones as an animal sterol; hence it is called cholesterol. Cholesterol oxidase is the chief enzyme in the process of cholesterol degradation. Its role is obtained by the coenzyme FAD, which catalyzes the isomerization and oxidation of cholesterol to produce cholesteric 4-ene-3-ketone and hydrogen peroxide at the same time. Recently, a great advance has been made in the discovery of the structure and function of cholesterol oxidase, and it has proven added value in clinical discovery, medical care, food and biopesticides development and other conditions. By recombinant DNA technology, we can insert the gene in the heterologous host. Heterologous expression (HE) is a successful methodology to produce enzymes for function studies and manufacturing applications, where Escherichia coli has been extensively used as a heterologous host because of its economical cultivation, rapid growth, and efficiency in offering exogenous genes. Heterologous expression of cholesterol oxidase has been considered for several microbial sources, such as Rhodococcus equi, Brevibacterium sp., Rhodococcus sp., Streptomyces coelicolor, Burkholderia cepacia ST-200, Chromobacterium, and Streptomyces spp. All related publications of numerous researchers and scholars were searched in ScienceDirect, Scopus, PubMed, and Google Scholar. In this article, the present situation and promotion of heterologous expression of cholesterol oxidase, the role of protease, and the perspective of its possible applications were reviewed.

Hyperexpression and Analysis ofchoBEncoding Cholesterol Oxidase ofBrevibacterium sterolicuminEscherichia coliandStreptomyces lividans

We examined the expression of choB, encoding cholesterol oxidase of Brevibacterium sterolicum ATCC 21387, in Escherichia coli JM105 and Streptomyces lividans TK23 using various deletion DNA fragments within the 5'-flanking region. The enzyme activity could be detected intracellularly in E. coli only when the 5'-flanking region was reduced to less than 256-bp and choB was transcribed by the lac promoter. A large amount of the enzyme were produced as inactive inclusion bodies when ChoB protein was fused with the NH2-terminal portion of LacZ protein. In contrast, choB with more than 256-bp of the 5'-flanking region was efficiently expressed in S. lividans TK23, and about 85 times as much of the active enzyme (170 U/ml) was secreted into the culture filtrate as with B. sterolicum in flask culture. These results suggest that the promoter of choB exist within 256-bp of the 5'-flanking region and can be efficiently recognized by the RNA polymerase of S. lividans. The characteristics of the enzyme purified from the culture filtrate of the S. lividans transformant and that of B. sterolicum were identical although the NH2-terminal amino acid sequence of the enzyme from the S. lividans transformant was 6 amino acids shorter than that from B. sterolicum.

Cholesterol to cholestenone oxidation by ChoG, the main extracellular cholesterol oxidase of Rhodococcus ruber strain Chol-4

The choG ORF of Rhodococcus ruber strain Chol-4 (referred from now as Chol-4) encodes a putative extracellular cholesterol oxidase. In the Chol-4 genome this ORF is located in a gene cluster that includes kstD3 and hsd4B, showing the same genomic context as that found in other Rhodococcus species. The putative ChoG protein is grouped into the class II of cholesterol oxidases, close to the Rhodococcus sp. CECT3014 ChoG homolog. The Chol-4 choG was cloned and expressed in a CECT3014 ΔchoG host strain in order to assess its ability to convert cholesterol into cholestenone. The RT-PCR analysis showed that choG gene was constitutively expressed in all the conditions assayed, but a higher induction could be inferred when cells were growing in the presence of cholesterol. A Chol-4 ΔchoG mutant strain was still able to grow in minimal medium supplemented with cholesterol, although at a slower rate. A comparative study of the removal of both cholesterol and cholestenone from the culture medium of either the wild type Chol-4 or its choG deletion mutant revealed a major role of ChoG in the extracellular production of cholestenone from cholesterol and, therefore, this enzyme may be related with the maintenance of a convenient supply of cholestenone for the succeeding steps of the catabolic pathway.

Cholesterol oxidase is indispensable in the pathogenesis of Mycobacterium tuberculosis

Despite considerable research effort, the molecular mechanisms of Mycobacterium tuberculosis (Mtb) virulence remain unclear. Cholesterol oxidase (ChoD), an extracellular enzyme capable of converting cholesterol to its 3-keto-4-ene derivative, cholestenone, has been proposed to play a role in the virulence of Mtb. Here, we verified the hypothesis that ChoD is capable of modifying the bactericidal and pro-inflammatory activity of human macrophages. We also sought to determine the contribution of complement receptor 3 (CR3)- and Toll-like receptor 2 (TLR2)-mediated signaling pathways in the development of macrophage responses to Mtb. We found that intracellular replication of an Mtb mutant lacking a functional choD gene (ΔchoD) was less efficient in macrophages than that of the wild-type strain. Blocking CR3 and TLR2 with monoclonal antibodies enhanced survival of ΔchoD inside macrophages. We also showed that, in contrast to wild-type Mtb, the ΔchoD strain induced nitric oxide production in macrophages, an action that depended on the TLR2, but not the CR3, signaling pathway. Both wild-type and mutant strains inhibited the production of reactive oxygen species (ROS), but the ΔchoD strain did so to a significantly lesser extent. Blocking TLR2-mediated signaling abolished the inhibitory effect of wild-type Mtb on ROS production by macrophages. Wild-type Mtb, but not the ΔchoD strain, decreased phorbol myristate acetate-induced phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2), which are involved in both TLR2- and CR3-mediated signaling pathways. Our finding also revealed that the production of interleukin 10 by macrophages was significantly lower in ΔchoD-infected macrophages than in wild-type Mtb-infected macrophages. However, tumor necrosis factor-α production by macrophages was the same after infection with mutant or wild-type strains. In summary, we demonstrate here that ChoD is required for Mtb interference with the TLR2-mediated signaling pathway and subsequent intracellular growth and survival of the pathogen in human macrophages.

ChoD and HsdD can be dispensable for cholesterol degradation in mycobacteria

Cholesterol degradation is achieved through a complex metabolic pathway that starts with the oxidation of the 17-alkyl side chain and the steroid ring system. In bacteria, the oxidation of the 3β-hydroxyl group and isomerization of the resulting cholest-5-en-3-one to cholest-4-en-3-one is catalyzed by hydroxysteroid dehydrogenase (HsdD) or cholesterol oxidase (ChoD). Genes encoding both enzymes were annotated in both fast and slow growing mycobacteria, however the enzymatic activity was confirmed for HsdD, exclusively. Here, we used homologous recombination to engineer multiple mutants, and directly show that both ChoD and HsdD are dispensable for cholesterol degradation in fast-growing Mycobacterium smegmatis mc(2)155 and slow-growing Mycobacterium tuberculosis H37Rv strains. The mutants deffective in the synthesis of ChoD, HsdD or both enzymes were able to grow in minimal media supplemented with cholesterol as a sole source of carbon and energy. Multiple mutants, defective in synthesis of ChoD, HsdD and ketosteroid dehydrogenase (KstD), showed attenuated growth in minimal medium supplemented with cholesterol and accumulated cholesterol degradation intermediates: androstendion (AD) and 9-hydroxy androstendion (9OHAD).

Cholesterol oxidase ChoD is not a critical enzyme accounting for oxidation of sterols to 3-keto-4-ene steroids in fast-growing Mycobacterium sp. VKM Ac-1815D

Fast-growing strain of Mycobacterium sp. VKM Ac-1815D is capable of effective oxidizing of sterols (phytosterol, cholesterol, ergosterol) to androstenedione and other valuable 3-oxo-steroids. To elucidate the role of cholesterol oxidase in sterol catabolism by the strain, the choD gene has been cloned and sequenced. The deduced gene product (M(r) 63.5kDa) showed homologies over its entire length to a large number of proteins belonging to the InterPro-family EPR006076, which includes various FAD dependent oxidoreductases. The expression of choD in Escherichia coli was shown to result in the synthesis of membrane associated cholesterol oxidase. In addition to cholesterol, the enzyme oxidized β-sitosterol, dehydroepiandrosterone, ergosterol, pregnenolone, and lithocholic acid. Knock-out of choD in Mycobacterium sp. VKM Ac-1815D strain was obtained by the gene replacement technique. The mutant strain transformed sitosterol forming exclusively 3-keto-4-ene steroids with androstenedione as a major product, thus evidencing that choD knock out did not abrogate sterol A-ring oxidation. The results indicated that ChoD is not a critical enzyme responsible for modification of 3β-hydroxy-5-ene- to 3-keto-4-ene steroids in Mycobacterium sp. VKM Ac-1815D. Article from a special issue on steroids and microorganisms.

Extracellular cholesterol oxidase from Rhodococcus sp.: isolation and molecular characterization

BACKGROUND

Cholesterol oxidase (CHO) has various clinical and industrial applications. Recently, microbial CHO have received a great attention for their wide usage in medicine. Here, taxonomic characterizations of isolated strain from soil, optimization of the conditions for CHO production and biochemical characterizations of produced CHO enzyme were described. Finally, CHO gene was cloned into a cloning vector.

METHODS

Various samples were collected and cultivated in a screening medium consisting of cholesterol. For isolation of CHO-producing bacteria, well-grown colonies were inoculated into an optimized medium. Different biochemical and microbiological tests were performed on isolated bacteria to identify their properties. For phylogenic analysis, a partial sequence of l6s rRNA was amplified by PCR using universally conserved primers. A modified method was applied for determination of CHO activity. Then, extracellular CHO activity was assessed under different temperature, pH and cholesterol concentration conditions. Finally, CHO gene was amplified by PCR and cloned into STV28.

RESULTS

According to the morphological, cultural and biochemical tests, the isolated bacterium was identified as Rhodococcus sp. strain 501 and deposited in GenBank with accession number FN298676. Results showed that optimum temperature and pH for CHO activity were 35 degrees C and 7.5, respectively. Alignment of nucleotide sequence of CHO gene showed 99% homology with other bacterial CHO genes.

CONCLUSION

Rhodococcus sp. strain 501 produced significant levels of extracellular CHO in an optimized medium for a short period. CHO gene was cloned into cloning vector that can be a valuable tool for better identification and further studies on gene expression.

A hydrogen-bonding network is important for oxidation and isomerization in the reaction catalyzed by cholesterol oxidase

Cholesterol oxidase is a flavoenzyme that catalyzes the oxidation and isomerization of 3beta-hydroxysteroids. Structural and mutagenesis studies have shown that Asn485 plays a key role in substrate oxidation. The side chain makes an NH...pi interaction with the reduced form of the flavin cofactor. A N485D mutant was constructed to further test the role of the amide group in catalysis. The mutation resulted in a 1800-fold drop in the overall k(cat). Atomic resolution structures were determined for both the N485L and N485D mutants. The structure of the N485D mutant enzyme (at 1.0 A resolution) reveals significant perturbations in the active site. As predicted, Asp485 is oriented away from the flavin moiety, such that any stabilizing interaction with the reduced flavin is abolished. Met122 and Glu361 form unusual hydrogen bonds to the functional group of Asp485 and are displaced from the positions they occupy in the wild-type active site. The overall effect is to disrupt the stabilization of the reduced FAD cofactor during catalysis. Furthermore, a narrow transient channel that is shown to form when the wild-type Asn485 forms the NH...pi interaction with FAD and that has been proposed to function as an access route of molecular oxygen, is not observed in either of the mutant structures, suggesting that the dynamics of the active site are altered.

Purification and characterization of Chromobacterium sp. DS-1 cholesterol oxidase with thermal, organic solvent, and detergent tolerance

A new screening method for 6beta-hydroperoxycholest-4-en-3-one (HCEO)-forming cholesterol oxidase was devised in this study. As the result of the screening, a novel cholesterol oxidase producer (strain DS-1) was isolated and identified as Chromobacterium sp. Extracellular cholesterol oxidase of strain DS-1 was purified from the culture supernatant. The molecular mass of the purified enzyme was 58 kDa. This enzyme showed a visible adsorption spectrum having peaks at 355 and 450 nm, like a typical flavoprotein. The enzyme oxidized cholesterol to HCEO, with the consumption of 2 mol of O2 and the formation of 1 mol of H2O2 for every 1 mol of cholesterol oxidized. The enzyme oxidized 3beta-hydroxysteroids such as cholesterol, beta-cholestanol, and pregnenolone at high rates. The Km value for cholesterol was 26 microM. The enzyme was stable at pH 3 to 11 and most active at pH 7.0-7.5, showing optimal activity at pH 7.0 and 65 degrees C. The enzyme retained about 80% of its activity after incubation for 30 min at 85 degrees C. The thermal stability of the enzyme was the highest among the cholesterol oxidases tested. Moreover, the enzyme was more stable in the presence of various organic solvents and detergents than commercially available cholesterol oxidases.

Cholesterol oxidase is required for virulence ofMycobacterium tuberculosis

Recent reports have indicated that cholesterol plays a crucial role during the uptake of mycobacteria by macrophages. However, the significance of cholesterol modification enzymes encoded by Mycobacterium tuberculosis for bacterial pathogenicity remains unknown. Here, the authors explored whether the well-known cholesterol modification enzyme, cholesterol oxidase (ChoD), is important for virulence of the tubercle bacillus. Homologous recombination was used to replace the choD gene from the M. tuberculosis genome with a nonfunctional copy. The resultant mutant (delta choD) was attenuated in peritoneal macrophages. No attenuation in macrophages was observed when the same strain was complemented with an intact choD gene controlled by a heat shock promoter (delta choDP(hsp)choD). The mice infection experiments confirm the significance of ChoD in the pathogenesis of M. tuberculosis.

Heterologous expression of cholesterol oxidase in Bifidobacterium longum under the control of 16S rRNA gene promoter of bifidobacteria

We have constructed a constitutive high-level-expression vector for the genus Bifidobacterium and used it to express cholesterol oxidase from Streptomyces coelicola. The promoter region of the 16S rRNA gene was amplified by inverse PCR and used for the construction of pBES16PR. The optimal ribosome-binding site (RBS) for Bifidobacterium was incorporated in pBES16PR. In order to test the efficacy of this expression vector, we constructed pBES16PR-CHOL with the structural gene for cholesterol oxidase under the control of the 16S rRNA promoter, and used it to transform Bifidobacterium longum. The gene was successfully expressed and high level of cholesterol oxidase activity was obtained in B. longum. This is the first report of an expression vector for the genus Bifidobacterium using a 16S rRNA gene promoter and successful expression of cholesterol oxidase.

Immunization by intrabronchial administration to 1-week-old foals of an unmarked double gene disruption strain of Rhodococcus equi strain 103+

Rhodococcus equi causes fatal granulomatous pneumonia in foals and immunocompromised animals and humans. However, there is no effective vaccine against this infection. In this study, the chromosomal genes isocitrate lyase (icl) and cholesterol oxidase (choE) were chosen as targets for mutation and assessment of the double mutant as an intrabronchial vaccine in 1-week-old foals. Using a modification of a suicide plasmid previously developed in this laboratory, we developed a choE-icl unmarked deletion mutant of R. equi strain 103+. Five 1-week-old foals were infected intrabronchially with the mutant and challenged intrabronchially with the parent, virulent, strain 2 weeks later. Three of the foals were protected against pneumonia caused by the virulent strain, but the other two foals developed pneumonia caused by the mutant strain during the post-challenge period. Since infection of 3-week-old foals by an icl mutant in an earlier study had shown complete attenuation of the strain, we conclude that a proportion of foals in the 1st week or so of life are predisposed to developing R. equi pneumonia because of an inability to mount an effective immune response. This has been suspected previously but this is the first time that this has been demonstrated experimentally.

Cholesterol Oxidases Act as Signaling Proteins for the Biosynthesis of the Polyene Macrolide Pimaricin

The gene cluster responsible for pimaricin biosynthesis in Streptomyces natalensis contains a cholesterol oxidase-encoding gene (pimE) surrounded by genes involved in pimaricin production. Gene-inactivation and -complementation experiments revealed that pimE encodes a functional cholesterol oxidase and, surprisingly, that it is also involved in pimaricin biosynthesis. This extracellular enzyme was purified from S. natalensis culture broths to homogeneity, and it was shown to restore pimaricin production when added to the mutant culture broths. Other cholesterol oxidases also triggered pimaricin production, suggesting that these enzymes could act as signaling proteins for polyene biosynthesis. This finding constitutes the description of a cholesterol oxidase gene with an involvement in antibiotic biosynthesis, and it broadens the scope of the biological functions for this type of oxidase.

Coenzyme precursor-assisted expression of a cholesterol oxidase from Brevibacterium sp. in Escherichia coli

The gene (choB(b)), encoding cholesterol oxidase from Brevibacterium sp. CCTCC M201008, was cloned and sequenced by PCR (GenBank accession number: DQ345780). The gene consists of 1653 base pairs and encodes a protein of 551 amino acids. ChoB(b) exhibited a homology of 98% with cholesterol oxidase gene from Brevibacterium sterolicum ATCC 21387. The cholesterol oxidase gene, cloned in the vector pET-28a, was over-expressed in Escherichia coli BL21-CodonPlus (DE3)-RP grown at 23 degrees C in Luria-Bertani medium containing 50 microM riboflavin, the precursor of the FAD coenzyme of the enzyme. A maximum activity of 3.7 U/mg was obtained from cell free extract of E. coli BL21-CodonPlus (DE3)-RP harboring the pET-28a-choB(b).

Cholesterol oxidase (ChoE) is not important in the virulence of Rhodococcus equi

To analyze further the role in virulence of the prominent cholesterol oxidase (ChoE) of Rhodococcus equi, an allelic exchange choE mutant from strain 103+ was constructed and assessed for virulence in macrophages, in mice, and in foals. There was no difference between the mutant and parent strain in cytotoxic activity for macrophages or in intra-macrophage multiplication. No evidence of attenuation was obtained in macrophages and in mice, but there was slight attenuation apparent in four intra-bronchially infected foals compared to infection of four foals with the virulent parent strain, based on a delayed rise in temperature of the choE-mutant infected foals. However, bacterial colony counts in the lung 2 weeks after infection were not significantly different, although there was a slight but non-significant (P=0.12) difference in lung:body weight ratio of the choE mutant versus virulent parent infected foals (mean 2.67+/-0.25% compared to 4.58+/-0.96%). We conclude that the cholesterol oxidase is not important for the virulence of R. equi.

Structural and kinetic analyses of the H121A mutant of cholesterol oxidase

Cholesterol oxidase is a monomeric flavoenzyme that catalyses the oxidation of cholesterol to cholest-5-en-3-one followed by isomerization to cholest-4-en-3-one. The enzyme from Brevibacterium sterolicum contains the FAD cofactor covalently bound to His121. It was previously demonstrated that the H121A substitution results in a approximately 100 mV decrease in the midpoint redox potential and a approximately 40-fold decrease in turnover number compared to wild-type enzyme [Motteran, Pilone, Molla, Ghisla and Pollegioni (2001) Journal of Biological Chemistry 276, 18024-18030]. A detailed kinetic analysis of the H121A mutant enzyme shows that the decrease in turnover number is largely due to a corresponding decrease in the rate constant of flavin reduction, whilst the re-oxidation reaction is only marginally altered and the isomerization reaction is not affected by the substitution and precedes product dissociation. The X-ray structure of the mutant protein, determined to 1.7 A resolution (1 A identical with 0.1 nm), reveals only minor changes in the overall fold of the protein, namely: two loops have slight movements and a tryptophan residue changes conformation by a rotation of 180 degrees about chi1 compared to the native enzyme. Comparison of the isoalloxazine ring moiety of the FAD cofactor between the structures of the native and mutant proteins shows a change from a non-planar to a planar geometry (resulting in a more tetrahedral-like geometry for N5). This change is proposed to be a major factor contributing to the observed alteration in redox potential. Since a similar distortion of the flavin has not been observed in other covalent flavoproteins, it is proposed to represent a specific mode to facilitate flavin reduction in covalent cholesterol oxidase.

Cholesterol oxidases: a study of nature's approach to protein design

Cholesterol oxidases are important as clinical reagents, potential larvicides, and tools in cell biology, and they are implicated in bacterial pathogenesis. Here we review chemical aspects of their function. We describe our current structural and mechanistic understanding of the type I and II cholesterol oxidases, our identification of an NH-pi hydrogen bond motif for stabilization of reduced flavins, our structural hypothesis of how O(2) gains access to the flavin, and our present understanding of type I cholesterol oxidase-lipid bilayer interactions.

Rapid identification of Rhodococcus equi by a PCR assay targeting the choE gene

The actinomycete Rhodococcus equi is an important pathogen of horses and an emerging opportunistic pathogen of humans. Identification of R. equi by classical bacteriological techniques is sometimes difficult, and misclassification of an isolate is not uncommon. We report here on a specific PCR assay for the rapid and reliable identification of R. equi. It is based on the amplification of a fragment of the choE gene encoding cholesterol oxidase. The choE-based PCR was assessed by using a panel of strains comprising 132 isolates from different sources and of different geographical origins, all initially identified biochemically as R. equi, and 30 isolates of representative non-R. equi actinomycete species, including cholesterol oxidase producers. The expected 959-bp amplicon was observed only with R. equi isolates, as confirmed by sequencing of a variable region of the 16S RNA gene from a random sample of 20 PCR-positive isolates. All R. equi isolates gave a positive choE-based PCR result, which correlated with a high degree of conservation of the choE gene. Three of the 132 strains originally identified as R. equi were negative for the choE gene, and subsequent analysis of their 16S RNA gene sequences confirmed that they belonged to other bacterial species (Dietzia maris, Mycobacterium peregrinum, and Staphylococcus epidermidis). All non-R. equi isolates were negative by the choE-based PCR. ATCC 21387, the only known isolate of Brevibacterium sterolicum, gave a 959-bp amplicon whose DNA sequence was virtually identical to that of R. equi choE. Comparison of the 16S RNA genes indicated that ATCC 21387 should be considered an R. equi isolate.

Purification and properties of a new Brevibacterium sterolicum cholesterol oxidase produced by E. coli MM294/pnH10

A gene encoding a cholesterol oxidase from Brevibacterium sterolicum nov. sp. ATCC21387 was isolated by an expression cloning method and highly expressed by a recombinant strain Escherichia coli MM294/pnH10. The purified cholesterol oxidase was a typical flavoprotein with a molecular mass of 46.5 kDa, absorption peaks at 280, 360, and 450 nm. Optimum pH and temperature were found at pH 6.5 and 55 degrees C, respectively. The enzyme acted on 3beta-hydroxysteroids such as cholesterol, pregnenolone, and beta-sitosterol at high rates, but on dehydro-epi-androsterone to a lesser degree. The molecular and catalytic properties were different from those of cholesterol oxidase I, which was initially discovered in B. sterolicum nov. sp. ATCC21387. The new enzyme, designated cholesterol oxidase II, was distinguished by its high affinity toward cholesterol (K(m)=30 microM).

Construction of a catalytically inactive cholesterol oxidase mutant: investigation of the interplay between active site-residues glutamate 361 and histidine 447

Cholesterol oxidase catalyzes the oxidation of cholesterol to cholest-5-en-3-one and its subsequent isomerization into cholest-4-en-3-one. Two active-site residues, His447 and Glu361, are important for catalyzing the oxidation and isomerization reactions, respectively. Double-mutants were constructed to test the interplay between these residues in catalysis. We observed that the k(cat) of oxidation for the H447Q/E361Q mutant was 3-fold less than that for H447Q and that the k(cat) of oxidation for the H447E/E361Q mutant was 10-fold slower than that for H447E. Because both doubles-mutants do not have a carboxylate at position 361, they do not catalyze isomerization of the reaction intermediate cholest-5-en-3-one to cholest-4-en-3-one. These results suggest that Glu361 can compensate for the loss of histidine at position 447 by acting as a general base catalyst for oxidation of cholesterol. Importantly, the construction of the double-mutant H447E/E361Q yields an enzyme that is 31,000-fold slower than wild type in k(cat) for oxidation. The H447E/E361Q mutant is folded like native enzyme and still associates with model membranes. Thus, this mutant may be used to study the effects of membrane binding in the absence of catalytic activity. It is demonstrated that in assays with caveolae membrane fractions, the wild-type enzyme uncouples platelet-derived growth factor receptor beta (PDGFRbeta) autophosphorylation from tyrosine phosphorylation of neighboring proteins, and the H447E/E361Q mutant does not. Thus maintenance of membrane structure by cholesterol is important for PDGFRbeta-mediated signaling. The cholesterol oxidase mutant probe described will be generally useful for investigating the role of membrane structure in signal transduction pathways in addition to the PDGFRbeta-dependent pathway tested.

Alteration of substrate specificity of cholesterol oxidase from Streptomyces sp. by site-directed mutagenesis

Despite the structural similarities between cholesterol oxidase from Streptomyces and that from Brevibacterium, both enzymes exhibit different characteristics, such as catalytic activity, optimum pH and temperature. In attempts to define the molecular basis of differences in catalytic activity or stability, substitutions at six amino acid residues were introduced into cholesterol oxidase using site-directed mutagenesis of its gene. The amino acid substitutions chosen were based on structural comparisons of cholesterol oxidases from Streptomyces and BREVIBACTERIUM: Seven mutant enzymes were constructed with the following amino acid substitutions: L117P, L119A, L119F, V145Q, Q286R, P357N and S379T. All the mutant enzymes exhibited activity with the exception of that with the L117P mutation. The resulting V145Q mutant enzyme has low activities for all substrates examined and the S379T mutant enzyme showed markedly altered substrate specificity compared with the wild-type enzyme. To evaluate the role of V145 and S379 residues in the reaction, mutants with two additional substitutions in V145 and four in S379 were constructed. The mutant enzymes created by the replacement of V145 by Asp and Glu had much lower catalytic efficiency for cholesterol and pregnenolone as substrates than the wild-type enzyme. From previous studies and this study, the V145 residue seems to be important for the stability and substrate binding of the cholesterol oxidase. In contrast, the catalytic efficiencies (k(cat)/K(m)) of the S379T mutant enzyme for cholesterol and pregnenolone were 1.8- and 6.0-fold higher, respectively, than those of the wild-type enzyme. The enhanced catalytic efficiency of the S379T mutant enzyme for pregnenolone was due to a slightly high k(cat) value and a low K(m) value. These findings will provide several ideas for the design of more powerful enzymes that can be applied to clinical determination of serum cholesterol levels and as sterol probes.

The presence of a hydrogen bond between asparagine 485 and the pi system of FAD modulates the redox potential in the reaction catalyzed by cholesterol oxidase

Cholesterol oxidase catalyzes the oxidation and isomerization of cholesterol to cholest-4-en-3-one. An asparagine residue (Asn485) at the active site is believed to play an important role in catalysis. To test the precise role of Asn485, we mutated it to a leucine and carried out kinetic and crystallographic studies. Steady-state kinetic analysis revealed a 1300-fold decrease in the oxidation k(cat)/K(m) for the mutant enzyme whereas the k(cat)/K(m) for isomerization is only 60-fold slower. The primary kinetic isotope effect in the mutant-catalyzed reaction indicates that 3alpha-H transfer remains the rate-determining step. Measurement of the reduction potentials for the wild-type and N485L enzymes reveals a 76 mV decrease in the reduction potential of the FAD for the mutant enzyme relative to wild type. The crystal structure of the mutant, determined to 1.5 A resolution, reveals a repositioning of the side chain of Met122 near Leu485 to form a hydrophobic pocket. Furthermore, the movement of Met122 facilitates the binding of an additional water molecule, possibly mimicking the position of the equatorial hydroxyl group of the steroid substrate. The wild-type enzyme shows a novel N-H...pi interaction between the side chain of Asn485 and the pyrimidine ring of the cofactor. The loss of this interaction in the N485L mutant destabilizes the reduced flavin and accounts for the decreased reduction potential and rate of oxidation. Thus, the observed structural rearrangement of residues at the active site, as well as the kinetic data and thermodynamic data for the mutant, suggests that Asn485 is important for creating an electrostatic potential around the FAD cofactor enhancing the oxidation reaction.

Cloning, sequence analysis and expression of a gene encoding an organic solvent- and detergent-tolerant cholesterol oxidase of Burkholderia cepacia strain ST-200

Burkholderia cepacia strain ST-200 produces an extracellular cholesterol oxidase which is stable and highly active in the presence of organic solvents. This cholesterol oxidase produces 6beta-hydroperoxycholest-4-en-3-one from cholesterol, with the consumption of two moles of O2 and the formation of one mole of H2O2. The structural gene encoding the cholesterol oxidase was cloned and sequenced. The primary translation product was predicted to be 582 amino acid residues. The mature product is composed of 539 amino acid residues and is preceded by a signal sequence of 43 residues. The cloned gene was expressed as an active product in Escherichia coli and the product was localized in the periplasmic space. The cholesterol oxidase produced from E. coli was purified to homogeneity from the periplasmic fraction. The purified enzyme was highly stable in the presence of various organic solvents or detergents, as compared with the commercially available cholesterol oxidases tested.

Identification and mutagenesis by allelic exchange of choE, encoding a cholesterol oxidase from the intracellular pathogen Rhodococcus equi

The virulence mechanisms of the facultative intracellular parasite Rhodococcus equi remain largely unknown. Among the candidate virulence factors of this pathogenic actinomycete is a secreted cholesterol oxidase, a putative membrane-damaging toxin. We identified and characterized the gene encoding this enzyme, the choE monocistron. Its protein product, ChoE, is homologous to other secreted cholesterol oxidases identified in Brevibacterium sterolicum and Streptomyces spp. ChoE also exhibits significant similarities to putative cholesterol oxidases encoded by Mycobacterium tuberculosis and Mycobacterium leprae. Genetic tools for use with R. equi are poorly developed. Here we describe the first targeted mutagenesis system available for this bacterium. It is based on a suicide plasmid, a selectable marker (the aacC4 apramycin resistance gene from Salmonella), and homologous recombination. The choE allele was disrupted by insertion of the aacC4 gene, cloned in pUC19 and introduced by electroporation in R. equi. choE recombinants were isolated at frequencies between 10(-2) and 10(-3). Twelve percent of the recombinants were double-crossover choE mutants. The choE mutation was associated with loss of cooperative (CAMP-like) hemolysis with sphingomyelinase-producing bacteria (Listeria ivanovii). Functional complementation was achieved by expression of choE from pVK173-T, a pAL5000 derivative conferring hygromycin resistance. Our data demonstrate that ChoE is an important cytolytic factor for R. equi. The highly efficient targeted mutagenesis procedure that we used to generate choE isogenic mutants will be a valuable tool for the molecular analysis of R. equi virulence.

Expression and chloroplast targeting of cholesterol oxidase in transgenic tobacco plants

Cholesterol oxidase represents a novel type of insecticidal protein with potent activity against the cotton boll weevil (Anthonomus grandis grandis Boheman). We transformed tobacco (Nicotiana tabacum) plants with the cholesterol oxidase choM gene and expressed cytosolic and chloroplast-targeted versions of the ChoM protein. Transgenic leaf tissues expressing cholesterol oxidase exerted insecticidal activity against boll weevil larvae. Our results indicate that cholesterol oxidase can metabolize phytosterols in vivo when produced cytosolically or when targeted to chloroplasts. The transgenic plants exhibiting cytosolic expression accumulated low levels of saturated sterols known as stanols, and displayed severe developmental aberrations. In contrast, the transgenic plants expressing chloroplast-targeted cholesterol oxidase maintained a greater accumulation of stanols, and appeared phenotypically and developmentally normal. These results are discussed within the context of plant sterol distribution and metabolism.

Use of the parallax-quench method to determine the position of the active-site loop of cholesterol oxidase in lipid bilayers

To elucidate the cholesterol oxidase-membrane bilayer interaction, a cysteine was introduced into the active site lid at position-81 using the Brevibacterium enzyme. To eliminate the possibility of labeling native cysteine, the single cysteine in the wild-type enzyme was mutated to a serine without any change in activity. The loop-cysteine mutant was then labeled with acrylodan, an environment-sensitive fluorescence probe. The fluorescence increased and blue-shifted upon binding to lipid vesicles, consistent with a change into a more hydrophobic, i.e., lipid, environment. This acrylodan-labeled cholesterol oxidase was used to explore the pH, ionic strength, and headgroup dependence of binding. Between pH 6 and 10, there was no significant change in binding affinity. Incorporation of anionic lipids (phosphatidylserine) into the vesicles did not increase the binding affinity nor did altering the ionic strength. These experiments suggested that the interactions are primarily driven by hydrophobic effects not ionic effects. Using vesicles doped with either 5-doxyl phosphatidylcholine, 10-doxyl phosphatidylcholine, or phosphatidyl-tempocholine, quenching of acrylodan fluorescence was observed upon binding. Using the parallax method of London [Chattopadhyay, A., and London, E. (1987) Biochemistry 26, 39-45], the acrylodan ring is calculated to be 8.1 +/- 2.5 A from the center of the lipid bilayer. Modeling the acrylodan-cysteine residue as an extended chain suggests that the backbone of the loop does not penetrate into the lipid bilayer but interacts with the headgroups, i.e., the choline. These results demonstrate that cholesterol oxidase interacts directly with the lipid bilayer and sits on the surface of the membrane.

Alteration of substrate affinity of Streptomyces cholesterol oxidase for application to the rate assay of cholesterol in serum

The Streptomyces cholesterol oxidase (ChoA) can not be adopted for the rate assay, because the Km value of the enzyme for cholesterol is very small. The choA gene was subjected to random mutagenesis in vivo, and a mutant ChoA (designated E-ChoA) that showed altered substrate affinity was obtained by screening. The Km value of E-ChoA was approximately ten times larger than that of the wild type. Unexpectedly, the thermal stability was also improved. The amino acid substitutions of E-ChoA were identified to be the valine to glutamate at position 145, which has been previously identified as one of the thermostable mutations, and the glycine to serine at position 405. The mutational effects on the structure of E-ChoA are discussed on the basis of a three-dimensional model. E-ChoA has been successfully applied to the rate assay of cholesterol in serum.

Crystal structure determination of cholesterol oxidase from Streptomyces and structural characterization of key active site mutants

Cholesterol oxidase is a monomeric flavoenzyme which catalyzes the oxidation and isomerization of cholesterol to cholest-4-en-3-one. The enzyme interacts with lipid bilayers in order to bind its steroid substrate. The X-ray structure of the enzyme from Brevibacterium sterolicum revealed two loops, comprising residues 78-87 and residues 433-436, which act as a lid over the active site and facilitate binding of the substrate [Vrielink et al. (1991) J. Mol. Biol. 219, 533-554; Li et al. (1993) Biochemistry 32, 11507-11515]. It was postulated that these loops must open, forming a hydrophobic channel between the membrane and the active site of the protein and thus sequestering the cholesterol substrate from the aqueous environment. Here we describe the three-dimensional structure of the homologous enzyme from Streptomyces refined to 1.5 A resolution. Structural comparisons to the enzyme from B. sterolicum reveal significant conformational differences in these loop regions; in particular, a region of the loop comprising residues 78-87 adopts a small amphipathic helical turn with hydrophobic residues directed toward the active site cavity and hydrophilic residues directed toward the external surface of the molecule. It seems reasonable that this increased rigidity reduces the entropy loss that occurs upon binding substrate. Consequently, the Streptomyces enzyme is a more efficient catalyst. In addition, we have determined the structures of three active site mutants which have significantly reduced activity for either the oxidation (His447Asn and His447Gln) or the isomerization (Glu361Gln). Our structural and kinetic data indicate that His447 and Glu361 act as general base catalysts in association with conserved water H2O541 and Asn485. The His447, Glu361, H2O541, and Asn485 hydrogen bond network is conserved among other oxidoreductases. This catalytic tetrad appears to be a structural motif that occurs in flavoenzymes that catalyze the oxidation of unactivated alcohols.

Evaluation of the role of His447 in the reaction catalyzed by cholesterol oxidase

Cholesterol oxidase catalyzes the oxidation and isomerization of cholesterol to cholest-4-en-3-one via cholest-5-en-3-one. It has been proposed that His447 acts as the general base catalyst for oxidation, and that the resulting imidazolium ion formed acts as an electrophile for isomerization. In this work, we undertook an assessment of the proposed dual roles of His447 in the oxidation and isomerization reactions. To test its role, we constructed five mutants, H447Q, H447N, H447E, H447D, and H447K, that introduce hydrogen bond donors and acceptors and carboxylate bases at this position, and a sixth mutant, E361Q, to test the interplay between His447 and Glu361. These mutants were characterized using steady-state kinetics and deuterium substrate and solvent isotope effects. For those mutants that catalyze either oxidation of cholesterol or isomerization of cholest-5-en-3-one, the Km's vary no more than 3-fold relative to wild type. H447K is inactive in both oxidation (> 100,000-fold reduced) and isomerization assays (> 10,000-fold reduced). H447E and H447D do not catalyze oxidation (> 100,000-fold reduced), but do catalyze isomerization, 10(4) times slower than wild type. The k(cat) for H447Q is 120-fold lower than wild type for oxidation, and the same as wild type for isomerization. The k(cat) for H447N is 4400-fold lower than wild type for oxidation, and is 30-fold lower than wild type for isomerization. E361Q does not catalyze isomerization (> 10,000-fold reduced), and the k(cat) for oxidation is 30-fold lower than wild type. The substrate deuterium kinetic isotope effects for the wild-type and mutant-catalyzed oxidation reactions suggest that mutation of His447 to an amide results in a change of the rate-determining step from hydride transfer to hydroxyl deprotonation. The deuterium solvent and substrate kinetic isotope effects for isomerization indicate that an amide at position 447 is an effective electrophile to catalyze formation of a dienolic intermediate. Moreover, consideration of kinetic and structural results together suggests that a hydrogen bonding network involving His447, Glu361 and Asn485, Wat541, and substrate serves to position the substrate and coordinate general base and electrophilic catalysis. That is, in addition to its previously demonstrated role as base for deprotonation of carbon-4 during isomerization, Glu361 has a structural role and may act as a general base during oxidation. The His447, Asn485, Glu361, and Wat541 residues are conserved in other GMC oxidoreductases. Observation of this catalytic tetrad in flavoproteins of unknown function may be diagnostic for an ability to oxidize unactivated alcohols.

Separation of the two reactions, oxidation and isomerization, catalyzed by Streptomyces cholesterol oxidase

Site-directed mutagenesis was used to identify key amino acid residues of the cholesterol oxidase from Streptomyces sp., which catalyzes the oxidation of cholesterol and the isomerization of 5-cholesten-3-one. Eight mutant enzymes were constructed and the following amino acid substitutions were identified: N318A, N318H, E356A, E356D, H441A, H441N, N480A and N480Q. Mutants N318A and N318H retained both oxidation and isomerization activities. The mutant E356D retained oxidation but not isomerization activity. On the other hand, mutants N480A and N480Q showed no oxidation activity but retained their isomerization activities. The two catalytic reactions, oxidation and isomerization, in cholesterol oxidase were thus successfully separated. When the H441A or H441N mutation was introduced, both the oxidase and isomerase activities were completely lost. The H441, E356 and N480 residues thus appear to participate in the catalysis of cholesterol oxidase, whereas N318 does not. An analysis of the products of these mutant enzymes suggested that the previously proposed 6-hydroxylation reaction by cholesterol oxidase is actually autooxidation from 5-cholesten-3-one. Kinetic studies of the purified wild-type and mutant enzymes showed that the k(cat)/Km values for oxidation in E356D and for isomerization in N480A increased six- and threefold, respectively, over those in the wild-type. These mutational effects and the reaction mechanisms are discussed in terms of the three-dimensional structure of the enzyme constructed on the basis of homology modeling.

The importance of GLU361 position in the reaction catalyzed by cholesterol oxidase

Cholesterol oxidase stereospecifically isomerizes cholest-5-en-3-one to cholest-4-en-3-one. When the base catalyst for isomerization, Glu361, is mutated to Asp, the rate of deprotonation of cholest-5-en-3-one is not affected, but protonation of the dienolic intermediate becomes rate-limiting. This may be a consequence of the large distance between the catalytic base and carbon-6 of the intermediate in the mutant enzyme.

Assessment of the role of an omega loop of cholesterol oxidase: a truncated loop mutant has altered substrate specificity

The function of an active site loop (70-90) of cholesterol oxidase has been ascertained by deleting five contiguous residues (79-83) from the tip of the loop. From the crystal structure of the wild-type enzyme, it appears that this truncation will not significantly perturb the structure of the rest of the enzyme. The UV/vis and CD spectra of the mutant confirm that the enzyme is properly folded with FAD bound. The mutant enzyme still transfers 2H from the 4beta-carbon of the intermediate, cholest-5-en-3-one, to the 6beta-carbon of the product, cholest-4-en-3-one, during isomerization. The kcat/Km of the mutant is increased 6-fold with dehydroepiandrosterone as substrate. Thus, the enzyme is still catalytically active after deletion of the five loop-tip residues. With micellar cholesterol, the kcat/Km of the mutant is decreased 170-fold relative to wild type. This suggests that the tip of the loop is necessary for packing with the "tail" of cholesterol and is responsible for substrate specificity at C17. Increased release of intermediate cholest-5-en-3-one in the mutant-catalyzed reaction is not observed. Truncation of the loop, therefore, does not affect the grip of the enzyme on the intermediate. With lipid vesicle substrates (egg phosphatidylcholine/cholesterol, 1:1), the initial velocity of the mutant is reduced 3000-fold. The binding affinity for the vesicles, however, is only reduced 2-fold. Consequently, the loop is not the primary determinant of binding affinity for vesicles. It is concluded that the loop is important for movement of cholesterol from the lipid bilayer. The tip residues form a hydrophobic pathway between lipid membrane and active site to facilitate movement of substrate and product in to and out of the active site.

Increased expression of Brevibacterium sterolicum cholesterol oxidase in Escherichia coli by genetic modification

To improve expression of Brevibacterium sterolicum cholesterol oxidase in Escherichia coli, we utilized the T7lac promoter and modified the gene to encode the first 21 amino acids with high-expression E. coli codons. These changes resulted in a 60-fold improvement of expression level. N-terminal sequencing revealed that the E. coli produced cholesterol oxidase signal peptide is cleaved 6 amino acids closer to the N-terminus than in B. sterolicum. The recombinant E. coli produced protein is composed of 513 amino acids with a calculated Mr of 55,374. The kinetic rate constants of the recombinant protein and the B. sterolicum produced cholesterol oxidase are identical.

Investigation of membrane disruption in the reaction catalyzed by cholesterol oxidase

Dye leakage experiments were undertaken to investigate the membrane disruption properties of cholesterol oxidase. Inspection of the X-ray crystal structures of cholesterol oxidase suggested that an active-site "lid" opens in order to bind substrate [Li, J., Vrielink, A., Brick, P., & Blow, D. M. (1993) Biochemistry 32, 11507-11515]. We tested whether the interaction of the putative active-site lid with the membrane was sufficiently disruptive of the membrane structure to cause leakage or lysis of the cell membrane. Vesicles (100 nm) composed of egg phosphatidylcholine, 2-palmitoyl-3-oleoyl-1-sn-phosphatidylethanolamine, and 2-palmitoyl-3-oleoyl-1-sn-phosphatidylcholine were used in this study to mimic biomembranes. To separate the effects of membrane binding from conversion of cholesterol to cholest-4-en-3-one, the active-site mutant E361Q was utilized. In the reaction catalyzed by E361Q, isomerization of the cholest-5-en-3-one intermediate is suppressed and cholest-5-en-3-one is the major product isolated. Furthermore, E361Q produces cholest-5-en-3-one 20-fold more slowly than wild type produces cholest-4-en-3-one from cholesterol. Wild-type and E361Q cholesterol oxidases bind to vesicles with an apparent K(D) of approximately 25 microM, as measured by quenching of intrinsic tryptophan fluorescence, irrespective of headgroup size and cholesterol content. Membrane disruption was measured by leakage of the encapsulated marker carboxyfluorescein. Leakage was observed with cholesterol-containing vesicles and wild-type enzyme only; the rate of leakage was dependent on the rate of cholest-4-en-3-one production. E361Q did not induce membrane disruption, regardless of vesicle type tested. Thus, binding of cholesterol oxidase to the membrane and partitioning of cholesterol into the active site does not sufficiently perturb the bilayer to cause leakage of vesicle contents. Formation of the product cholest-4-en-3-one, however, does increase membrane permeability. Expansion of the lipid bilayer upon conversion of cholesterol to cholest-4-en-3-one is the likely cause of this increased permeability.

Improvement of thermal stability of Streptomyces cholesterol oxidase by random mutagenesis and a structural interpretation

Random mutagenesis was used to enhance the thermal stability of Streptomyces cholesterol oxidase. Four thermostable mutants were isolated and the following amino acid substitutions were identified: Ser103 to Thr (mutant S103T), Val121 to Ala (mutant V121A), Arg135 to His (mutant R135H) and Val145 to Glu (mutant V145E). The wild-type and mutant enzymes were purified and characterized. The properties of mutants S103T, V121A and R135H were similar to those of the wild type but they showed improved thermal stability. When the V145E mutation was introduced, the thermal stability of the enzyme was markedly increased and the optimum pH was desirably changed to encompass a broad range from acid to alkali. Analysis of multiple mutants constructed by site-directed mutagenesis showed that all the mutations except that of R135H had an additive influence on the other mutations. These mutational effects are discussed in terms of a three-dimensional structural model of the enzyme constructed on the basis of homology modelling.

Expression of Streptomyces melC and choA genes by a cloned Streptococcus thermophilus promoter STP2201

Streptococcus thermophilus (ST) chromosomal DNA (chr DNA) fragments having promoter activity were cloned and selected in Escherichia coli using a chloramphenicol acetyltransferase- (cat-) based promoter-probe vector pKK520-3. Insertion of a promoterless streptomycete melanin biosynthesis operon (melC) downstream from the promoters of the library further identified clone STP2201 as a strong promoter in E. coli. Subcloning of a STP2201-melC DNA fragment into the pMEU-series S. thermophilus-E. coli shuttle vectors yielded pEU5xML2201x plasmids that conferred Mel+ phenotype to E. coli. The pEU5aML2201a was further shown to afford a high level of tyrosinase pro-anti-tyrosinase antiserum in S. thermophilus. Substituting melC with a streptomycete cholesterol oxidase gene (choA) in the same orientation yielded pEU5aCH2201a that conferred ChoA activity to an E. coli transformant at a level of (1.06 +/- 0.15) x 10(-7) units mg-1 protein. Introduction of this plasmid into S. thermophilus by electrotransformation yielded ChoA+ transformant that produced the enzyme at about 25% of the level found in E. coli.

Expression of cho and melC operons by a Streptococcus thermophilus synthetic promoter in Escherichia coli

A 63-base-pair synthetic promoter, sP1, was synthesized on the basis of the nucleotide sequence of a putative Streptococcus thermophilus promoter. When inserted upstream from the Streptomyces cho operon in a recombinant plasmid, pUCO195P-36, sP1 activated the expression of the cho genes in Escherichia coli, as shown by the production of cholesterol oxidase by the transformants. The sP1-driven cholesterol oxidase production in pUCO195P-36-transformed cells was estimated to be 40% of that produced by P(lac)-mediated cho expression in a pUCO193-containing host. The recombinant pUCO195P-36 appeared to be segregationally less stable in E. coli DH5 alpha than in HB101. Its non-expressing counterpart, pUCO195P-1, was stable in both E. coli strains. The activity of sP1 was further demonstrated in E. coli by the expression of a Streptomyces melC operon. When placed upstream from the test operon in the pMCU22aPa construct, sP1 activated the melC expression as shown by the production of tyrosinase at (3.0 +/- 0.3) x 10(-3) U/mg and (16.0 +/- 1.0) x 10(-3) U/mg protein equivalent of cell extract in the absence and presence of isopropyl beta-D-thiogalactopyranoside, respectively. The presence of a counter-oriented P(lac) at the 3' end of the operon in the pMCU22bPa plasmid reduced the sP1-mediated tyrosinase production by about 85%.

Native promoter-plasmid vector system for heterologous cholesterol oxidase synthesis in Streptococcus thermophilus

The cholesterol oxidase gene (choA) of a streptomycete was used as a model for studying heterologous gene expression in Streptococcus thermophilus, an essential bacterium in dairy food fermentations. The vectors pER82 and pER82P were developed from the 2.2-kb indigenous plasmid (pER8) of S. thermophilus ST108, and sP1, a 51-bp synthetic promoter patterned after a chromosomal sequence of S. thermophilus. The presence of sP1 promoter in pER82PbCOb with the choA insert aligned with the cat gene was essential for the intracellular production of cholesterol oxidase. The pER82PbCOb was apparently stable in S. thermophilus with no detectable evidence of deletion mutational events.

Cloning of an insecticidal cholesterol oxidase gene and its expression in bacteria and in plant protoplasts

We cloned and sequenced structural gene choM, which encodes an insecticidally active cholesterol oxidase in Streptomyces sp. strain A19249. The primary translation product was predicted to be a 547-amino-acid protein whose first 43 amino acids constitute a secretory signal peptide. Expression of the gene with the signal sequence in Escherichia coli resulted in production of a protein that had enzymatic and insecticidal properties which were indistinguishable from those of the cholesterol oxidase secreted by Streptomyces sp. strain A19249. Expression of the gene with or without the signal sequence in tobacco protoplasts resulted in production of an enzymatically active cholesterol oxidase.

Cloning of the gene for cholesterol oxidase in Bacillus spp., Lactobacillus reuteri and its expression in Escherichia coli

The cloning of the cholesterol oxidase gene in several Gram-positive bacteria, including Lactobacillus reuteri of intestinal origin, was obtained. Only the transformants of Escherichia coli harbouring the recombinant plasmid pCHOA showed a good intracellular enzyme activity. The heterologous gene was stably maintained in Gram-positive transformants but no enzyme activity was detected.

Construction and characterization of shuttle plasmids for lactic acid bacteria and Escherichia coli

The chimeric plasmid pBN183 was first constructed in Escherichia coli by ligating the BamHI-digested E. coli plasmid pBR322 and a Bg/II-linearized streptococcal plasmid, pNZ18. The pBN183 transformed E. coli to ApR at a frequency of (8.2 +/- 1.2) x 10(5) colony forming units (CFU)/microgram DNA. Electrotransformation of Streptococcus thermophilus with pBN183 yielded CmR, ApS clones at a frequency of (2.6 +/- 0.3) x 10(1) CFU/microgram DNA. Plasmid screening with pBN183-transformed S. thermophilus clones revealed that ca. 70% of these transformants contained deleted plasmids. Plasmid pBN183A, a pBN183 deletion mutant lacking one copy of a tandemly arranged, highly homologous DNA sequence, was isolated for further study. It transformed E. coli to ApR and S. thermophilus to CmR with frequencies of (4.8 +/- 0.1) x 10(5) and (8.1 +/- 0.2) x 10(2) CFU/microgram DNA, respectively. Screening of S. thermophilus transformants did not show the presence of deleted plasmids. Based on the structure of pBN183A, a new shuttle plasmid, pDBN183, was constructed from pBN183 by removal of the small (1.2 kb) Sa/I fragment. Transformation frequencies of pDBN183 were (5.0 +/- 1.3) x 10(5) and (4.6 +/- 0.2) x 10(2) CFU/microgram DNA with E. coli and S. thermophilus, respectively. In contrast to the parent pBN183, only 17% of the pDBN183-transformed S. thermophilus contained deleted plasmids. Plasmid copy numbers of the three vectors in E. coli were estimated at 17-18 per chromosome. The three plasmids conferred ApR and CmR to E. coli, but only CmR to S. thermophilus. The insertion of a Streptomyces cholesterol oxidase gene (choA) into pDBN183 did not affect the plasmid's stability in Lactobacillus casei, but resulted in deletion of the recombinant DNA in S. thermophilus.

Expression of streptomycete cholesterol oxidase in Escherichia coli

A streptomycete gene coding for extracellular cholesterol oxidase (choA) was subcloned and expressed in Escherichia coli. The pUCO series recombinants were obtained by inserting the choA gene into the unique KpnI site of pUC19 vector. Expression was observed with pUCO192A and pUCO193 constructs in which the cloned gene(s) were aligned with the upstream lacZ promoter. Isopropyl beta-D-thioglucopyranoside (IPTG) enhanced this expression up to 2.5-fold. Specific Cho activity in the cell extracts of the stable pUCO193 transformant were 0.004 U and 0.007 U per mg protein without and with IPTG induction, respectively. Cho activity was detected in the spent medium of this culture, suggesting possible secretion of the enzyme.

Sequence of gene choB encoding cholesterol oxidase of Brevibacterium sterolicum: comparison with choA of streptomyces sp. SA-COO

The nucleotide (nt) sequence of the cholesterol oxidase (Cho)-encoding gene (choB) cloned from Brevibacterium sterolicum ATCC21387 was determined. The sequence contained an open reading frame with a G + C content of 64.9 mol% that would encode a protein of 552 amino acids (aa). Comparison of the nt sequence of choB and deduced aa sequence to those of the Cho-encoding gene (choA) of Streptomyces sp. strain SA-COO showed identities of 64% and 58%, respectively. N-terminal aa sequence analysis of the extracellular enzyme of B. sterolicum confirmed that the mature enzyme consisted of 507 aa with a predicted Mr of 54,902, and was preceded by a 45-aa signal sequence.

Transfer and expression of a Streptomyces cholesterol oxidase gene in Streptococcus thermophilus

The recombinant plasmid pNCO937 (8.1 kbp) containing a Streptomyces sp. cholesterol oxidase gene was introduced into Streptococcus thermophilus by electrotransformation. Transformation frequency was 7.2 x 10(5) colony forming units/micrograms of DNA. The presence of the cholesterol oxidase gene in S. thermophilus was confirmed with Southern blot analysis using a biotinylated probe. Thin-layer chromatographic analysis showed the expression of the Streptomyces cholesterol oxidase gene resulting in the oxidation of cholesterol to 4-cholesten-3-one. S. thermophilus may be a suitable host for the expression of other genes regulating prokaryotic cholesterol metabolism.

Isolation and identification of the gene of cholesterol oxidase from Brevibacterium sterolicum ATCC 21387, a widely used enzyme in clinical analysis

The gene coding cholesterol oxidase (CHOD) from Brevibacterium sterolicum, which is widely used in clinical analysis, has been selected from pUC19-based gene bank in E. coli MM294 by colony-hybridization using synthetic DNA as probe. The gene was identified to encode the protein having the same amino acid sequence as that determined from amino-acid sequence analysis. The expression of the CHOD gene in E. coli was not observed, probably due to the transcription failure. Attempts are being made to express it in various hosts including Streptomyces lividans, Corynebacterium glutamicum, and B. sterolicum itself.

An operon containing the genes for cholesterol oxidase and a cytochrome P-450-like protein from a Streptomyces sp

The nucleotide sequence of the promoter region of the gene for cholesterol oxidase (choA) from Streptomyces sp. strain SA-COO was determined. We found an open reading frame (choP) that is located between a potential promoter sequence and the structural gene for the ChoA protein. Deletion analysis showed that the promoter region for choP is essential for expression of the choA gene. Mappings of S1 nuclease and primer extension of transcripts generated in vivo suggested that the synthesis of mRNA starts at a site 41 bases upstream from the ATG initiation codon of the choP gene. By Northern (RNA) blot analysis of the transcripts, we found a 2.9-kilobase transcript that is identical in size to the total sequence of the choP and choA genes. These results suggest that the two genes, choP and choA, are transcribed polycistronically under the control of the promoter that is upstream from the structural gene for choP. The choP gene encodes a protein of 381 amino acids with a calculated Mr of 41,668. The nucleotide sequence of the choP gene has a high degree of similarity to the sequence of the genes for cytochrome P-450s from humans and Pseudomonas species. A region of homology with the cytochrome P-450s from various organisms was identified in the choP protein and may represent a region associated with a binding site for heme iron. Analysis of the CO difference spectrum of an extract of Streptomyces lividans cells that carry a plasmid which includes the choP gene revealed a unique peak, characteristic of cytochrome P-450, which is identical to that obtained with the parent strain.

Nucleotide sequence of the gene for cholesterol oxidase from a Streptomyces sp

The nucleotide sequence of a 2.1-kilobase-pair fragment containing the Streptomyces choA gene, which codes a secreted cholesterol oxidase, was determined. A single open reading frame encodes a mature cholesterol oxidase of 504 amino acids, with a calculated Mr of 54,913. The leader peptides extend over 42 amino acids and have the characteristics of a signal sequence, including basic amino acids near the amino terminus and a hydrophobic core near the signal cleavage site. Analyses of the total amino acid composition and amino acid sequencing of the first 21 amino acids from the N terminus of the purified extracellular enzyme agree with the values deduced from nucleotide sequencing data.

Cloning and expression of a Streptomyces cholesterol oxidase gene in Streptomyces lividans with plasmid pIJ702

The cholesterol oxidase gene (cho) of Streptomyces sp. was cloned into Streptomyces lividans with the vector pIJ702. Deletion analysis of the recombinant plasmid showed that entire coding sequence of the cho gene was located within a 2.5-kilobase segment of the chromosomal DNA obtained from the cholesterol oxidase-producing strain. When cloned cells of S. lividans were grown in an appropriate medium, the cells produced severalfold more cholesterol oxidase extracellularly than did the producing strain.