Characterization of 3-ketosteroid 9{alpha}-hydroxylase, a Rieske oxygenase in the cholesterol degradation pathway of Mycobacterium tuberculosis

Structural features in the KshA terminal oxygenase protein that determine substrate preference of 3-ketosteroid 9α-hydroxylase enzymes

Rieske nonheme monooxygenase 3-ketosteroid 9α-hydroxylase (KSH) enzymes play a central role in bacterial steroid catabolism. KSH is a two-component iron-sulfur-containing enzyme, with KshA representing the terminal oxygenase component and KshB the reductase component. We previously reported that the KshA1 and KshA5 homologues of Rhodococcus rhodochrous DSM43269 have clearly different substrate preferences. KshA protein sequence alignments and three-dimensional crystal structure information for KshA(H37Rv) of Mycobacterium tuberculosis H37Rv served to identify a variable region of 58 amino acids organized in a β sheet that is part of the so-called helix-grip fold of the predicted KshA substrate binding pocket. Exchange of the β sheets between KshA1 and KshA5 resulted in active chimeric enzymes with substrate preferences clearly resembling those of the donor enzymes. Exchange of smaller parts of the KshA1 and KshA5 β-sheet regions revealed that a highly variable loop region located at the entrance of the active site strongly contributes to KSH substrate preference. This loop region may be subject to conformational changes, thereby affecting binding of different substrates in the active site. This study provides novel insights into KshA structure-function relationships and shows that KSH monooxygenase enzymes are amenable to protein engineering for the development of biocatalysts with improved substrate specificities.

Activity of 3-ketosteroid 9α-hydroxylase (KshAB) indicates cholesterol side chain and ring degradation occur simultaneously in Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb), a significant global pathogen, contains a cholesterol catabolic pathway. Although the precise role of cholesterol catabolism in Mtb remains unclear, the Rieske monooxygenase in this pathway, 3-ketosteroid 9α-hydroxylase (KshAB), has been identified as a virulence factor. To investigate the physiological substrate of KshAB, a rhodococcal acyl-CoA synthetase was used to produce the coenzyme A thioesters of two cholesterol derivatives: 3-oxo-23,24-bisnorchol-4-en-22-oic acid (forming 4-BNC-CoA) and 3-oxo-23,24-bisnorchola-1,4-dien-22-oic acid (forming 1,4-BNC-CoA). The apparent specificity constant (k(cat)/K(m)) of KshAB for the CoA thioester substrates was 20-30 times that for the corresponding 17-keto compounds previously proposed as physiological substrates. The apparent K(m)(O(2)) was 90 ± 10 μM in the presence of 1,4-BNC-CoA, consistent with the value for two other cholesterol catabolic oxygenases. The Δ(1) ketosteroid dehydrogenase KstD acted with KshAB to cleave steroid ring B with a specific activity eight times greater for a CoA thioester than the corresponding ketone. Finally, modeling 1,4-BNC-CoA into the KshA crystal structure suggested that the CoA moiety binds in a pocket at the mouth of the active site channel and could contribute to substrate specificity. These results indicate that the physiological substrates of KshAB are CoA thioester intermediates of cholesterol side chain degradation and that side chain and ring degradation occur concurrently in Mtb. This finding has implications for steroid metabolites potentially released by the pathogen during infection and for the design of inhibitors for cholesterol-degrading enzymes. The methodologies and rhodococcal enzymes used to generate thioesters will facilitate the further study of cholesterol catabolism.

Cholesterol catabolism as a therapeutic target in Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) is an intracellular pathogen that infects 10 million people worldwide and kills 2 million people every year. The uptake and utilization of nutrients by Mtb within the host cell is still poorly understood, although lipids play an important role in Mtb persistence. The recent identification of a large regulon of cholesterol catabolic genes suggests that Mtb can use host sterol for infection and persistence. In this review, we report on recent progress in elucidation of the Mtb cholesterol catabolic reactions and their potential utility as targets for tuberculosis therapeutic agents.

Multiplicity of 3-Ketosteroid-9α-Hydroxylase enzymes in Rhodococcus rhodochrous DSM43269 for specific degradation of different classes of steroids

The well-known large catabolic potential of rhodococci is greatly facilitated by an impressive gene multiplicity. This study reports on the multiplicity of kshA, encoding the oxygenase component of 3-ketosteroid 9α-hydroxylase, a key enzyme in steroid catabolism. Five kshA homologues (kshA1 to kshA5) were previously identified in Rhodococcus rhodochrous DSM43269. These KshA(DSM43269) homologues are distributed over several phylogenetic groups. The involvement of these KshA homologues in the catabolism of different classes of steroids, i.e., sterols, pregnanes, androstenes, and bile acids, was investigated. Enzyme activity assays showed that all KSH enzymes with KshA(DSM43269) homologues are C-9 α-hydroxylases acting on a wide range of 3-ketosteroids, but not on 3-hydroxysteroids. KshA5 appeared to be the most versatile enzyme, with the broadest substrate range but without a clear substrate preference. In contrast, KshA1 was found to be dedicated to cholic acid catabolism. Transcriptional analysis and functional complementation studies revealed that kshA5 supported growth on any of the different classes of steroids tested, consistent with its broad expression induction pattern. The presence of multiple kshA genes in the R. rhodochrous DSM43269 genome, each displaying unique steroid induction patterns and substrate ranges, appears to facilitate a dynamic and fine-tuned steroid catabolism, with C-9 α-hydroxylation occurring at different levels during microbial steroid degradation.

The conserved Rieske oxygenase DAF-36/Neverland is a novel cholesterol-metabolizing enzyme

Steroid hormones play essential roles in a wide variety of biological processes in multicellular organisms. The principal steroid hormones in nematodes and arthropods are dafachronic acids and ecdysteroids, respectively, both of which are synthesized from cholesterol as an indispensable precursor. The first critical catalytic step in the biosynthesis of these ecdysozoan steroids is the conversion of cholesterol to 7-dehydrocholesterol. However, the enzymes responsible for cholesterol 7,8-dehydrogenation remain unclear at the molecular level. Here we report that the Rieske oxygenase DAF-36/Neverland (Nvd) is a cholesterol 7,8-dehydrogenase. The daf-36/nvd genes are evolutionarily conserved, not only in nematodes and insects but also in deuterostome species that do not produce dafachronic acids or ecdysteroids, including the sea urchin Hemicentrotus pulcherrimus, the sea squirt Ciona intestinalis, the fish Danio rerio, and the frog Xenopus laevis. An in vitro enzymatic assay system reveals that all DAF-36/Nvd proteins cloned so far have the ability to convert cholesterol to 7-dehydrocholesterol. Moreover, the lethality of loss of nvd function in the fruit fly Drosophila melanogaster is rescued by the expression of daf-36/nvd genes from the nematode Caenorhabditis elegans, the insect Bombyx mori, or the vertebrates D. rerio and X. laevis. These data suggest that daf-36/nvd genes are functionally orthologous across the bilaterian phylogeny. We propose that the daf-36/nvd family of proteins is a novel conserved player in cholesterol metabolism across the animal phyla.

Inhibition of the M. tuberculosis 3β-hydroxysteroid dehydrogenase by azasteroids

The cholesterol metabolism pathway in Mycobacterium tuberculosis (M. tb) is a potential source of energy as well as secondary metabolite production that is important for survival of M. tb in the host macrophage. Oxidation and isomerization of 3β-hydroxysterols to 4-en-3-ones is requisite for sterol metabolism and the reaction is catalyzed by 3β-hydroxysteroid dehydrogenase (Rv1106c). Three series of 6-azasteroids and 4-azasteroids were employed to define the substrate preferences of M. tb 3β-hydroxysteroid dehydrogenase. 6-Azasteroids with large, hydrophobic side chains at the C17 position are the most effective inhibitors. Substitutions at C1, C2, C4 and N6 were poorly tolerated. Our structure-activity studies indicate that the 6-aza version of cholesterol is the best and tightest binding competitive inhibitor (K(i)=100 nM) of the steroid substrate and are consistent with cholesterol being the preferred substrate of M. tb 3β-hydroxysteroid dehydrogenase.

Adventures inRhodococcus — from steroids to explosivesThis article is based on a presentation by Dr. Lindsay Eltis at the 60th Annual Meeting of the Canadian Society of Microbiologists in Hamilton, Ontario, 14 June 2010. Dr. Eltis was the recipient of the 2010 Norgen Biotek Corporation / CSM Award, an annual award sponsored by Norgen Biotek and the Canadian Society of Microbiologists intended to recognize outstanding scientific work in microbiology by a Canadian researcher

Rhodococcus is a genus of mycolic-acid-containing actinomycetes that utilize a remarkable variety of organic compounds as growth substrates. This degradation helps maintain the global carbon cycle and has increasing applications ranging from the biodegradation of pollutants to the biocatalytic production of drugs and hormones. We have been using Rhodococcus jostii RHA1 as a model organism to understand the catabolic versatility of Rhodococcus and related bacteria. Our approach is exemplified by the discovery of a cluster of genes specifying the catabolism of cholesterol. This degradation proceeds via β-oxidative degradation of the side chain and O2-dependent cleavage of steroid ring A in a process similar to bacterial degradation of aromatic compounds. The pathway is widespread in Actinobacteria and is critical to the pathogenesis of Mycobacterium tuberculosis , arguably the world’s most successful pathogen. The close similarity of some of these enzymes with biphenyl- and polychlorinated-biphenyl-degrading enzymes that we have characterized is facilitating inhibitor design. Our studies in RHA1 have also provided important insights into a number of novel metalloenzymes and their biosynthesis, such as acetonitrile hydratase (ANHase), a cobalt-containing enzyme with no significant sequence identity with characterized nitrile hydratases. Molecular genetic and biochemical studies have identified AnhE as a dimeric metallochaperone that delivers cobalt to ANHase, enabling its maturation in vivo. Other metalloenzymes we are characterizing include N-acetylmuramic acid hydroxylase, which catalyzes an unusual hydroxylation of the rhodococcal and mycobacterial peptidoglycan, and 2 RHA1 dye-decolorizing peroxidases. Using molecular genetic and biochemical approaches, we have demonstrated that one of these enzymes is involved in the degradation of lignin. Overall, our studies are providing fundamental insights into a range of catabolic processes that have a wide variety of applications.

Structural and biochemical characterization of Mycobacterium tuberculosis CYP142: evidence for multiple cholesterol 27-hydroxylase activities in a human pathogen

The Mycobacterium tuberculosis cytochrome P450 enzyme CYP142 is encoded in a large gene cluster involved in metabolism of host cholesterol. CYP142 was expressed and purified as a soluble, low spin P450 hemoprotein. CYP142 binds tightly to cholesterol and its oxidized derivative cholest-4-en-3-one, with extensive shift of the heme iron to the high spin state. High affinity for azole antibiotics was demonstrated, highlighting their therapeutic potential. CYP142 catalyzes either 27-hydroxylation of cholesterol/cholest-4-en-3-one or generates 5-cholestenoic acid/cholest-4-en-3-one-27-oic acid from these substrates by successive sterol oxidations, with the catalytic outcome dependent on the redox partner system used. The CYP142 crystal structure was solved to 1.6 Å, revealing a similar active site organization to the cholesterol-metabolizing M. tuberculosis CYP125, but having a near-identical organization of distal pocket residues to the branched fatty acid oxidizing M. tuberculosis CYP124. The cholesterol oxidizing activity of CYP142 provides an explanation for previous findings that ΔCYP125 strains of Mycobacterium bovis and M. bovis BCG cannot grow on cholesterol, because these strains have a defective CYP142 gene. CYP142 is revealed as a cholesterol 27-oxidase with likely roles in host response modulation and cholesterol metabolism.

Bacterial degradation of bile salts

Bile salts are surface-active steroid compounds. Their main physiological function is aiding the digestion of lipophilic nutrients in intestinal tracts of vertebrates. Many bacteria are capable of transforming and degrading bile salts in the digestive tract and in the environment. Bacterial bile salt transformation and degradation is of high ecological relevance and also essential for the biotechnological production of steroid drugs. While biotechnological aspects have been reviewed many times, the physiological, biochemical and genetic aspects of bacterial bile salt transformation have been neglected. This review provides an overview of the reaction sequence of bile salt degradation and on the respective enzymes and genes exemplified with the degradation pathway of the bile salt cholate. The physiological adaptations for coping with the toxic effects of bile salts, recent biotechnological applications and ecological aspects of bacterial bile salt metabolism are also addressed. As the pathway for bile salt degradation merges with metabolic pathways for bacterial transformation of other steroids, such as testosterone and cholesterol, this review provides helpful background information for metabolic engineering of steroid-transforming bacteria in general.

Characterization of a broad-specificity non-haem iron N-demethylase from Pseudomonas putida CBB5 capable of utilizing several purine alkaloids as sole carbon and nitrogen source

N-Demethylation of many xenobiotics and naturally occurring purine alkaloids such as caffeine and theobromine is primarily catalysed in higher organisms, ranging from fungi to mammals, by the well-studied membrane-associated cytochrome P450s. In contrast, there is no well-characterized enzyme for N-demethylation of purine alkaloids from bacteria, despite several reports on their utilization as sole source of carbon and nitrogen. Here, we provide what we believe to be the first detailed characterization of a purified N-demethylase from Pseudomonas putida CBB5. The soluble N-demethylase holoenzyme is composed of two components, a reductase component with cytochrome c reductase activity (Ccr) and a two-subunit N-demethylase component (Ndm). Ndm, with a native molecular mass of 240 kDa, is composed of NdmA (40 kDa) and NdmB (35 kDa). Ccr transfers reducing equivalents from NAD(P)H to Ndm, which catalyses an oxygen-dependent N-demethylation of methylxanthines to xanthine, formaldehyde and water. Paraxanthine and 7-methylxanthine were determined to be the best substrates, with apparent K(m) and k(cat) values of 50.4±6.8 μM and 16.2±0.6 min(-1), and 63.8±7.5 μM and 94.8±3.0 min(-1), respectively. Ndm also displayed activity towards caffeine, theobromine, theophylline and 3-methylxanthine, all of which are growth substrates for this organism. Ndm was deduced to be a Rieske [2Fe-2S]-domain-containing non-haem iron oxygenase based on (i) its distinct absorption spectrum and (ii) significant identity of the N-terminal sequences of NdmA and NdmB with the gene product of an uncharacterized caffeine demethylase in P. putida IF-3 and a hypothetical protein in Janthinobacterium sp. Marseille, both predicted to be Rieske non-haem iron oxygenases.

A flavin-dependent monooxygenase from Mycobacterium tuberculosis involved in cholesterol catabolism

Mycobacterium tuberculosis (Mtb) and Rhodococcus jostii RHA1 have similar cholesterol catabolic pathways. This pathway contributes to the pathogenicity of Mtb. The hsaAB cholesterol catabolic genes have been predicted to encode the oxygenase and reductase, respectively, of a flavin-dependent mono-oxygenase that hydroxylates 3-hydroxy-9,10-seconandrost-1,3,5(10)-triene-9,17-dione (3-HSA) to a catechol. An hsaA deletion mutant of RHA1 did not grow on cholesterol but transformed the latter to 3-HSA and related metabolites in which each of the two keto groups was reduced: 3,9-dihydroxy-9,10-seconandrost-1,3,5(10)-triene-17-one (3,9-DHSA) and 3,17-dihydroxy-9,10-seconandrost-1,3,5(10)-triene-9-one (3,17-DHSA). Purified 3-hydroxy-9,10-seconandrost-1,3,5(10)-triene-9,17-dione 4-hydroxylase (HsaAB) from Mtb had higher specificity for 3-HSA than for 3,17-DHSA (apparent k(cat)/K(m) = 1000 +/- 100 M(-1) s(-1) versus 700 +/- 100 M(-1) s(-1)). However, 3,9-DHSA was a poorer substrate than 3-hydroxybiphenyl (apparent k(cat)/K(m) = 80 +/- 40 M(-1) s(-1)). In the presence of 3-HSA the K(m)(app) for O(2) was 100 +/- 10 microM. The crystal structure of HsaA to 2.5-A resolution revealed that the enzyme has the same fold, flavin-binding site, and catalytic residues as p-hydroxyphenyl acetate hydroxylase. However, HsaA has a much larger phenol-binding site, consistent with the enzyme's substrate specificity. In addition, a second crystal form of HsaA revealed that a C-terminal flap (Val(367)-Val(394)) could adopt two conformations differing by a rigid body rotation of 25 degrees around Arg(366). This rotation appears to gate the likely flavin entrance to the active site. In docking studies with 3-HSA and flavin, the closed conformation provided a rationale for the enzyme's substrate specificity. Overall, the structural and functional data establish the physiological role of HsaAB and provide a basis to further investigate an important class of monooxygenases as well as the bacterial catabolism of steroids.

Cholesterol utilization in mycobacteria is controlled by two TetR-type transcriptional regulators: kstR and kstR2

Mycobacterium tuberculosis is able to use a variety of carbon sources in vivo and current knowledge suggests that cholesterol is used as a carbon source during infection. The catabolized cholesterol is used both as an energy source (ATP generation) and as a source of precursor molecules for the synthesis of complex methyl-branched fatty acids. In previous studies, we described a TetR-type transcriptional repressor, kstR, that controls the expression of a number of genes involved in cholesterol catabolism. In this study, we describe a second TetR-type repressor, which we call kstR2. We knocked this gene out in Mycobacterium smegmatis and used microarrays and quantitative RT-PCR to examine the effects on gene expression. We identified a palindromic regulatory motif for KstR2, showed that this motif is present in three promoter regions in mycobacteria and rhodococcus, and demonstrated binding of purified KstR2 to the motif. Using a combination of motif location analysis, gene expression analysis and the examination of gene conservation, we suggest that kstR2 controls the expression of a 15 gene regulon. Like kstR, kstR2 and the kstR2 regulon are highly conserved among the actinomycetes and studies in rhodococcus suggest a role for these genes in cholesterol catabolism. The functional significance of the regulon and implications for the control of cholesterol utilization are discussed.

Characterization of a carbon-carbon hydrolase from Mycobacterium tuberculosis involved in cholesterol metabolism

In the recently identified cholesterol catabolic pathway of Mycobacterium tuberculosis, 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoate hydrolase (HsaD) is proposed to catalyze the hydrolysis of a carbon-carbon bond in 4,5-9,10-diseco-3-hydroxy-5,9,17-tri-oxoandrosta-1(10),2-diene-4-oic acid (DSHA), the cholesterol meta-cleavage product (MCP) and has been implicated in the intracellular survival of the pathogen. Herein, purified HsaD demonstrated 4-33 times higher specificity for DSHA (k(cat)/K(m) = 3.3 +/- 0.3 x 10(4) m(-1) s(-1)) than for the biphenyl MCP 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoic acid (HOPDA) and the synthetic analogue 8-(2-chlorophenyl)-2-hydroxy-5-methyl-6-oxoocta-2,4-dienoic acid (HOPODA), respectively. The S114A variant of HsaD, in which the active site serine was substituted with alanine, was catalytically impaired and bound DSHA with a K(d) of 51 +/- 2 mum. The S114A.DSHA species absorbed maximally at 456 nm, 60 nm red-shifted versus the DSHA enolate. Crystal structures of the variant in complex with HOPDA, HOPODA, or DSHA to 1.8-1.9 Aindicate that this shift is due to the enzyme-induced strain of the enolate. These data indicate that the catalytic serine catalyzes tautomerization. A second role for this residue is suggested by a solvent molecule whose position in all structures is consistent with its activation by the serine for the nucleophilic attack of the substrate. Finally, the alpha-helical lid covering the active site displayed a ligand-dependent conformational change involving differences in side chain carbon positions of up to 6.7 A, supporting a two-conformation enzymatic mechanism. Overall, these results provide novel insights into the determinants of specificity in a mycobacterial cholesterol-degrading enzyme as well as into the mechanism of MCP hydrolases.

Mycobacterial cytochrome p450 125 (cyp125) catalyzes the terminal hydroxylation of c27 steroids

Cyp125 (Rv3545c), a cytochrome P450, is encoded as part of the cholesterol degradation gene cluster conserved among members of the Mycobacterium tuberculosis complex. This enzyme has been implicated in mycobacterial pathogenesis, and a homologue initiates cholesterol catabolism in the soil actinomycete Rhodococcus jostii RHA1. In Mycobacterium bovis BCG, cyp125 was up-regulated 7.1-fold with growth on cholesterol. A cyp125 deletion mutant of BCG did not grow on cholesterol and accumulated 4-cholesten-3-one when incubated in the presence of cholesterol. Wild-type BCG grew on this metabolite. By contrast, a parallel cyp125 deletion mutation of M. tuberculosis H37Rv did not affect growth on cholesterol. Purified Cyp125 from M. tuberculosis, heterologously produced in R. jostii RHA1, bound cholesterol and 4-cholesten-3-one with apparent dissociation constants of 0.20 +/- 0.02 microM and 0.27 +/- 0.05 microm, respectively. When reconstituted with KshB, the cognate reductase of the ketosteroid 9alpha-hydroxylase, Cyp125 catalyzed the hydroxylation of these steroids. MS and NMR analyses revealed that hydroxylation occurred at carbon 26 of the steroid side chain, allowing unambiguous classification of Cyp125 as a steroid C26-hydroxylase. This study establishes the catalytic function of Cyp125 and, in identifying an important difference in the catabolic potential of M. bovis and M. tuberculosis, suggests that Cyp125 may have an additional function in pathogenesis.

Cholesterol oxidase: physiological functions

An important aspect of catalysis performed by cholesterol oxidase (3beta-hydroxysteroid oxidase) concerns the nature of its association with the lipid bilayer that contains the sterol substrate. Efficient catalytic turnover is affected by the association of the protein with the membrane as well as the solubility of the substrate in the lipid bilayer. In this review, the binding of cholesterol oxidase to the lipid bilayer, its turnover of substrates presented in different physical environments, and how these conditions affect substrate specificity, are discussed. The physiological functions of the enzyme in bacterial metabolism, pathogenesis and macrolide biosynthesis are reviewed in this context.

A thiolase of Mycobacterium tuberculosis is required for virulence and production of androstenedione and androstadienedione from cholesterol

Mycobacterium tuberculosis, the causative agent of tuberculosis, is an intracellular pathogen that shifts to a lipid-based metabolism in the host. Moreover, metabolism of the host lipid cholesterol plays an important role in M. tuberculosis infection. We used transcriptional profiling to identify genes transcriptionally regulated by cholesterol and KstR (Rv3574), a TetR-like repressor. The fadA5 (Rv3546) gene, annotated as a lipid-metabolizing thiolase, the expression of which is upregulated by cholesterol and repressed by KstR, was deleted in M. tuberculosis H37Rv. We demonstrated that fadA5 is required for utilization of cholesterol as a sole carbon source in vitro and for full virulence of M. tuberculosis in the chronic stage of mouse lung infection. Cholesterol is not toxic to the fadA5 mutant strain, and, therefore, toxicity does not account for its attenuation. We show that the wild-type strain, H37Rv, metabolizes cholesterol to androst-4-ene-3,17-dione (AD) and androsta-1,4-diene-3,17-dione (ADD) and exports these metabolites into the medium, whereas the fadA5 mutant strain is defective for this activity. We demonstrate that FadA5 catalyzes the thiolysis of acetoacetyl-coenzyme A (CoA). This catalytic activity is consistent with a beta-ketoacyl-CoA thiolase function in cholesterol beta-oxidation that is required for the production of androsterones. We conclude that the attenuated phenotype of the fadA5 mutant is a consequence of disrupted cholesterol metabolism that is essential only in the persistent stage of M. tuberculosis infection and may be caused by the inability to produce AD/ADD from cholesterol.

Rhodococcus rhodochrous DSM 43269 3-ketosteroid 9alpha-hydroxylase, a two-component iron-sulfur-containing monooxygenase with subtle steroid substrate specificity

This paper reports the biochemical characterization of a purified and reconstituted two-component 3-ketosteroid 9alpha-hydroxylase (KSH). KSH of Rhodococcus rhodochrous DSM 43269, consisting of a ferredoxin reductase (KshB) and a terminal oxygenase (KshA), was heterologously expressed in Escherichia coli. E. coli cell cultures, expressing both KshA and KshB, converted 4-androstene-3,17-dione (AD) into 9alpha-hydroxy-4-AD (9OHAD) with a >60% molar yield over 48 h of incubation. Coexpression and copurification were critical to successfully obtain pure and active KSH. Biochemical analysis revealed that the flavoprotein KshB is an NADH-dependent reductase using flavin adenine dinucleotide as a cofactor. Reconstitution experiments confirmed that KshA, KshB, and NADH are essential for KSH activity with steroid substrates. KSH hydroxylation activity was inhibited by several divalent metal ions, especially by zinc. The reconstituted KSH displayed subtle steroid substrate specificity; a range of 3-ketosteroids, i.e., 5alpha-Eta, 5beta-Eta, Delta1, and Delta4 steroids, could act as KSH substrates, provided that they had a short side chain. The formation of 9OHAD from AD by KSH was confirmed by liquid chromatography-mass spectrometry analysis and by the specific enzymatic conversion of 9OHAD into 3-hydroxy-9,10-secoandrost-1,3,5(10)-triene-9,17-dione using 3-ketosteroid Delta1-dehydrogenase. Only a single KSH is encoded in the genome of the human pathogen Mycobacterium tuberculosis H37Rv, shown to be important for survival in macrophages. Since no human KSH homolog exists, the M. tuberculosis enzyme may provide a novel target for treatment of tuberculosis. Detailed knowledge about the biochemical properties of KSH thus is highly relevant in the research fields of biotechnology and medicine.

Cholesterol metabolism increases the metabolic pool of propionate in Mycobacterium tuberculosis

Mycobacterium tuberculosis can metabolize cholesterol to both acetate and propionate. The mass of isolated phthiocerol dimycoserate, a methyl-branched fatty acylated polyketide, was used as a reporter for intracellular propionate metabolic flux. When M. tuberculosis is grown using cholesterol as the only source of carbon, a 42 amu increase in average phthiocerol dimycoserate molecular weight is observed, consistent with the cellular pool of propionate and, thus, methylmalonyl CoA increasing upon cholesterol metabolism. In contrast, no shift in phthiocerol dimycoserate molecular weight is observed upon supplementation of medium containing glycerol and glucose with cholesterol. We conclude that cholesterol is a significant source of propionate only in the absence of sugar carbon sources.