Katja Kristan

Steroid-transforming enzymes in fungi.

Fungal species are a very important source of many different enzymes, and the ability of fungi to transform steroids has been used for several decades in the production of compounds with a sterane skeleton. Here, we review the characterised and/or purified enzymes for steroid transformations, dividing them into two groups: (i) enzymes of the ergosterol biosynthetic pathway, including data for, e.g. ERG11 (14α-demethylase), ERG6 (C-24 methyltransferase), ERG5 (C-22 desaturase) and ERG4 (C-24 reductase); and (ii) the other steroid-transforming enzymes, including different hydroxylases (7α-, 11α-, 11β-, 14α-hydroxylase), oxidoreductases (5α-reductase, 3β-hydroxysteroid dehydrogenase/isomerase, 17β-hydroxysteroid dehydrogenase, C-1/C-2 dehydrogenase) and C-17-C-20 lyase. The substrate specificities of these enzymes, their cellular localisation, their association with protein super-families, and their potential applications are discussed. Article from a special issue on steroids and microorganisms.

Cinnamates and cinnamamides inhibit fungal 17β-hydroxysteroid dehydrogenase

The 17beta-hydroxysteroid dehydrogenases (17beta-HSDs) have important roles in the regulation of steroid hormone actions through their catalysis of the oxidation or reduction of estrogens and androgens at position 17. Dysfunctions of the human 17beta-HSDs have been associated with reproduction disorders, neuronal diseases and the development of hormone-dependent forms of cancers. Therefore, these enzymes represent interesting targets for the development of new drugs. Here we present a series of new cinnamic acid esters and amides that inhibit the oxidative and reductive reaction catalyzed by 17beta-HSD from the fungus Cochliobolus lunatus, a model enzyme of the short-chain dehydrogenase/reductase superfamily. We found that esters of unsubstituted cinnamic acid were better inhibitors than esters of 3,4,5-trimethoxycinnamic acid. Cinnamates were also more potent inhibitors than structurally related cinnamamides. The compounds presented in this paper are potential leads for the development of inhibitors of human 17beta-HSD isoforms, which may prove to have different therapeutic applications.

The characterization of the human cell line Calu-3 under different culture conditions and its use as an optimized in vitro model to investigate bronchial epithelial function.

In this study we have investigated the effects of different cell culture conditions on the Calu-3 epithelial cell model. Calu-3 cells were cultured in media A-MEM at the air-liquid (A-L) or liquid-liquid (L-L) interface for one or three wks (weeks). Different cryomethods were tested and the cell line was characterized using histochemistry, immunofluorescence, transmission and scanning electron microscopy, transepithelial resistance (TEER) measurements, permeability studies, and gene profiling of 84 drug transporters. Cell culture was successful in A-MEM with only 2.5% FBS. Cell proliferation and viability depended on the cryopreservation method. All Calu-3 models expressed CK7, occludin, and E-cadherin. The A-L interface resulted in a more biomimetic native bronchial epithelium displaying pseudostratified columnar epithelium with more microvilli and secretory vesicles than at the L-L interface, where the epithelium was cuboidal, but exhibited higher TEER values and lower dextran permeabilities. Longer time in culture significantly decreased dextran permeability and increased the expression of specific drug transporters. Drug transporter expression was also notably influenced by the culture interface, where the A-L interface yielded a higher expression of drug transporter genes than the L-L interface. Since cell culture interface and time in culture affect Calu-3 cell differentiation, barrier integrity, permeability properties, and drug transporter expression, culture conditions need to be considered and standardized when using the Calu-3 cell line as an in vitro model for aerosol drug delivery and screening of bronchial drug candidates.

Coenzyme specificity in fungal 17β-hydroxysteroid dehydrogenase

The 17beta-hydroxysteroid dehydrogenase from the fungus Cochliobolus lunatus is an NADP(H)-dependent member of the short-chain dehydrogenase/reductase superfamily (SDR) that belongs to the cP1 classical subfamily. Here, we have created several mutants by site-directed mutagenesis, and through these we have studied the amino acid residues that are responsible for coenzyme binding and specificity. The Thr202Val and Thr202Ile mutants were inactive, thus confirming the importance of Thr202 for the appropriate orientation of the coenzyme that enables the hydride transfer. The Ala50Arg and Asn51Arg mutants had increased rates of NADPH dissociation, and thus an enhanced substrate oxidation with NADP+, while the Asn51Arg mutant also showed an increased rate of NADP+ dissociation, and thus an enhanced substrate reduction with NADPH. Addition of a negatively-charged amino acid residue at the first position after the second beta-strand (Tyr49Asp) affected the coenzyme specificity and turned the enzyme into an NAD+-dependent oxidase resembling the cD1d subfamily members.

Crystallization, X-ray diffraction analysis and phasing of 17β-hydroxysteroid dehydrogenase from the fungus Cochliobolus lunatus

17beta-Hydroxysteroid dehydrogenase from the filamentous fungus Cochliobolus lunatus (17beta-HSDcl) is an NADP(H)-dependent enzyme that preferentially catalyses the oxidoreduction of oestrogens and androgens. The enzyme belongs to the short-chain dehydrogenase/reductase superfamily and is the only fungal hydroxysteroid dehydrogenase known to date. 17beta-HSDcl has recently been characterized and cloned and has been the subject of several functional studies. Although several hypotheses on the physiological role of 17beta-HSDcl in fungal metabolism have been formulated, its function is still unclear. An X-ray crystallographic study has been undertaken and the optimal conditions for crystallization of 17beta-HSDcl (apo form) were established, resulting in well shaped crystals that diffracted to 1.7 A resolution. The space group was identified as I4(1)22, with unit-cell parameters a = b = 67.14, c = 266.77 A. Phasing was successfully performed by Patterson search techniques. A catalytic inactive mutant Tyr167Phe was also engineered, expressed, purified and crystallized for functional and structural studies.

Dimerization and enzymatic activity of fungal 17β-hydroxysteroid dehydrogenase from the short-chain dehydrogenase/reductase superfamily

Background17β-hydroxysteroid dehydrogenase from the fungus Cochliobolus lunatus (17β-HSDcl) is a member of the short-chain dehydrogenase/reductase (SDR) superfamily. SDR proteins usually function as dimers or tetramers and 17β-HSDcl is also a homodimer under native conditions.ResultsWe have investigated here which secondary structure elements are involved in the dimerization of 17β-HSDcl and examined the importance of dimerization for the enzyme activity. Sequence similarity with trihydroxynaphthalene reductase from Magnaporthe grisea indicated that Arg129 and His111 from the αE-helices interact with the Asp121, Glu117 and Asp187 residues from the αE and αF-helices of the neighbouring subunit. The Arg129Asp and His111Leu mutations both rendered 17β-HSDcl monomeric, while the mutant 17β-HSDcl-His111Ala was dimeric. Circular dichroism spectroscopy analysis confirmed the conservation of the secondary structure in both monomers. The three mutant proteins all bound coenzyme, as shown by fluorescence quenching in the presence of NADP+, but both monomers showed no enzymatic activity.ConclusionWe have shown by site-directed mutagenesis and structure/function analysis that 17β-HSDcl dimerization involves the αE and αF helices of both subunits. Neighbouring subunits are connected through hydrophobic interactions, H-bonds and salt bridges involving amino acid residues His111 and Arg129. Since the substitutions of these two amino acid residues lead to inactive monomers with conserved secondary structure, we suggest dimerization is a prerequisite for catalysis. A detailed understanding of this dimerization could lead to the development of compounds that will specifically prevent dimerization, thereby serving as a new type of inhibitor.

NMR and molecular dynamics study of the binding mode of naphthalene-N-sulfonyl-D-glutamic acid derivatives: novel MurD ligase inhibitors.

The presented series of naphthalene-N-sulfonyl-D-glutamic acid derivatives are novel MurD ligase inhibitors with moderate affinity that occupy the D-Glu binding site. We performed an NMR study including transfer NOE to determine the ligand bound conformation, as well as saturation transfer difference experiments to obtain ligand epitope maps. The difference in overall appearance of the epitope maps highlights the importance of hydrophobic interactions and shows the segments of molecular structure that are responsible for them. Transfer NOE experiments indicate the conformational flexibility of bound ligands, which were then further examined by unrestrained molecular dynamics calculations. The results revealed the differing degrees of ligand flexibility and their effect on particular ligand-enzyme contacts. Conformational flexibility not evident in the crystal structures may have an effect on ligand-binding site adaptability, and this is probably one of the important reasons for the only moderate activity of novel derivatives.

Rapid Exploration of Curing Process Design space for Production of Controlled‐Release Pellets

Time and cost are among the most often cited hurdles limiting the rate and extent of adoption of Quality by Design (QbD) and Process Analytical Technology. In this article, we demonstrate that, with appropriate techniques, a key QbD element can be achieved with amount of resources comparable to classical development approach. To control the dissolution rate of a highly soluble drug substance from latex polymer coated pellets, we have examined the effect of key variables affecting the curing process step by an experimental design study. To explore and characterize the Design Space, we have produced and tested 62 distinct pellet samples. To achieve this in a reasonable amount of time, we have developed a scaled-down automated dissolution method that demonstrated excellent correlation to the classical method. By careful planning of experimentation, we were able to obtain all samples from just two batches of pellet cores. The curing process Design Space was explored by statistical modeling of samples obtained from the first batch. Robustness and repeatability of the Design Space at the edge of failure was preliminarily investigated by analysis of selected samples from the second batch with encouraging results.

Novel inhibitors of β-ketoacyl-ACP reductase from Escherichia coli

Bacterial beta-ketoacyl-[acyl carrier protein] (beta-ketoacyl-ACP) reductase (FabG) is a highly conserved and ubiquitously expressed enzyme of the fatty-acid biosynthetic pathway of prokaryotic organisms that catalyzes NADPH-dependent reduction of beta-ketoacyl-ACP intermediates. Therefore, FabG represents an appealing target for the development of new antimicrobial agents. A number of trans-cinnamic acid derivatives were designed and screened for inhibitory activities against FabG from Escherichia coli. These inhibited FabG enzymatic activity with IC(50) values in the microM range, and were used as templates for the subsequent diversification of the chemotype. Introduction of an electron-withdrawing 4-cyano group to the phenol substituent showed improved inhibition over the non-substituted compound. The benzo-[1,3]-dioxol moiety also appeared to be essential for inhibitory activity of trans-cinnamic acid derivatives against FabG from E. coli. To explain the possible binding position, the best inhibitor from the present study was docked in the active site of FabG. The results for the best scoring conformers chosen by the docking programme revealed that cinnamic acid derivatives can be accommodated in the substrate-binding region of the active site, above the nicotinamide moiety of the NADPH cofactor. Additionally, a phage-displayed library of random linear 15-mer peptides was screened against FabG, to identify ligands with the common PPLTXY motif.

New High-Throughput Fluorimetric Assay for Discovering Inhibitors of UDP-N-Acetylmuramyl-l-Alanine: d-Glutamate (MurD) Ligase

A novel assay for monitoring the activity of the bacterial enzyme UDP-N-acetylmuramyl-L-alanine:D-glutamate ligase (MurD ligase) is presented. MurD, which belongs to an enzyme family of Mur ligases, is essential for the synthesis of bacterial peptidoglycan and therefore represents an attractive target for the discovery of novel antibacterial agents. The inhibition assay described in this article is amenable to high-throughput screening. It is based on the detection of the accumulation of adenosine 5′-diphosphate (ADP), a product of the reaction catalyzed by MurD ligase, by conversion to a fluorescent signal via a coupled enzyme system, using the ADP Quest™ assay kit from DiscoveRx. The novel assay has been validated by obtaining K M,app values for substrates D-Glu, UDP- N-acetylmuramyl-L-alanine (UMA) and ATP that are in agreement with the data reported in the literature. A counterscreen assay was introduced to eliminate false positives, and some of the known MurD inhibitors have been retested to compare the data measured with different methods. Moreover, a focused library of around 1000 compounds was screened for the inhibition of MurD to assess the performance and robustness of the assay. Finally, a novel MurD inhibitor belonging to a new structural class, with an IC50 value of 105 µM, was discovered. (Journal of Biomolecular Screening 2009:412-418)