A Bioinspired and Biocompatible ortho-sulfiliminyl phenol Synthesis

Synthetic methods inspired by Nature often offer unique advantages including mild conditions and biocompatibility with aqueous media. Inspired by an ergothioneine biosynthesis protein EgtB, a mononuclear non-haem iron enzyme capable of catalysing the C–S bond formation and sulfoxidation, herein, we discovered a mild and metal-free C–H sulfenylation/intramolecular rearrangement cascade reaction employing an internally oxidizing O–N bond as a directing group. Our strategy accommodates a variety of oxyamines with good site selectivity and intrinsic oxidative properties. Combining an O–N bond with an X–S bond generates a C–S bond and an S¼N bond rapidly. The newly discovered cascade reaction showed excellent chemoselectivity and a wide substrate scope for both oxyamines and sulfenylation reagents. We demonstrated the biocompatibility of the C–S bond coupling reaction by applying a coumarin-based fluorogenic probe in bacterial lysates. Finally, the C–S bond coupling reaction enabled the first fluorogenic formation of phospholipids, which self-assembled to fluorescent vesicles in situ

COMPUTATIONAL STUDY OF MOLECULAR STRUCTURES AND ANTIOXIDANT MECHANISM OF OVOTHIOLS

In this paper, the molecular structure and antioxidant activity of ovothiols (OSH) have beenstudied by using four DFT functionals, namely B3LYP, B3PW91, X3LYP, M06 with the basisset of 6-311++G(2df,2p). Two major antioxidant mechanisms, namely, hydrogen atom transfer(HAT) and stepwise electron transfer-proton transfer (SET-PT) have been investigated andapplied on three optimized conformations of ovothiols. Bond dissociation enthalpy (BDE),vertical ionization energy (IE), proton dissociation enthalpy (PDE), chemical potential (μ),chemical hardness (η) and global electrophilicity (ω), have been calculated and discussed in thegas phase

Single-step replacement of an unreactive C-H bond by a C-S bond using polysulfide as the direct sulfur source in anaerobic ergothioneine biosynthesis

Ergothioneine, a natural longevity vitamin and antioxidant, is a thiol-histidine derivative. Recently, two types of biosynthetic pathways were reported. In the aerobic ergothioneine biosynthesis, a non-heme iron enzyme incorporates a sulfoxide to an sp2 C-H bond in trimethyl-histidine (hercynine) through oxidation reactions. In contrast, in the anaerobic ergothioneine biosynthetic pathway in a green sulfur bacterium, Chlorobium limicola, a rhodanese domain containing protein (EanB) directly replaces this unreactive hercynine C-H bond with a C-S bond. Herein, we demonstrate that polysulfide (HSSnSR) is the direct sulfur-source in EanB-catalysis. After identifying EanB's substrates, X-ray crystallography of several intermediate states along with mass spectrometry results provide additional mechanistic details for this reaction. Further, quantum mechanics/molecular mechanics (QM/MM) calculations reveal that protonation of Nπ of hercynine by Tyr353 with the assistance of Thr414 is a key activation step for the hercynine sp2 C-H bond in this trans-sulfuration reaction.R01 GM106443 - NIGMS NIH HHS; R41 AT010878 - NCCIH NIH HHSAccepted manuscrip

Implications for an imidazol-2-yl carbene intermediate in the rhodanase-catalyzed C-S bond formation reaction of anaerobic ergothioneine biosynthesis

In the anaerobic ergothioneine biosynthetic pathway, a rhodanese domain containing enzyme (EanB) activates tne hercynine's sp2 ε-C-H Dona ana replaces it with a C-S bond to produce ergothioneine. The key intermediate for this trans-sulfuration reaction is the Cys412 persulfide. Substitution of the EanB-Cys412 persulfide with a Cys412 perselenide does not yield the selenium analog of ergothioneine, selenoneine. However, in deuterated buffer, the perselenide-modified EanB catalyzes the deuterium exchange between hercynine's sp2 ε-C-H bond and D2O. Results from QM/MM calculations suggest that the reaction involves a carbene intermediate and that Tyr353 plays a key role. We hypothesize that modulating the pKa of Tyr353 will affect the deuterium-exchange rate. Indeed, the 3,5-difluoro tyrosine containing EanB catalyzes the deuterium exchange reaction with k ex of ~10-fold greater than the wild-type EanB (EanBWT). With regards to potential mechanisms, these results support the involvement of a carbene intermediate in EanB-catalysis, rendering EanB as one of the few carbene-intermediate involving enzymatic systems.R01 GM106443 - NIGMS NIH HHSAccepted manuscrip

Metilsulfenilação de olefinas eletrofílicas

Organosulfur compounds are present in all living organisms, being essential to various biochemical systems, where important primary metabolites, such as lipoic acid and coenzymes A and B, exhibits sulfur in their molecular structures. Methylthiolation is an interesting method to access this class of compounds, since molecules containing -SMe groups with biological activity are highly desired. Traditionally, the most used reagent for this type of reaction is methanethiol, which in addition to being flammable has high toxicity and is difficult to handle. In this work, the methylthiolation methodology previously employed by our research group was revisited due to experimental evidence that the DMSO had a contradictory role compared with the initial mechanistic proposal. Thus, a new scope was developed, using sodium acetate and without the addition of methylthiomethyl ester (MTM), with acyl esters and MBH acetates as substrates. The methylthiolated products were accessed with good group tolerance and moderate to excellent yields, confirming the importance to understand the reaction mechanism. The investigation proceeded through several control experiments, as well as by theoretical calculations employing Density Functional Theory (DFT). The results strongly support that a sulfurane and a sulfonium ylide appear as key intermediates and that a Pummerer type rearrangement is also crucial for the formation of the reagent. Furthermore, the methylthiolation mechanism is likely to proceed through the nucleophilic attack of the reagent, followed by an entropically favoured step involving the acetate attack to the positively charged species, then releasing the product. This work was able to elucidate the participation of the MTM ester in the methylthiolation reaction. The reagent, which can be in situ generated or initially added, presents a viable alternative to the use of methanethiol and contributes to the scientific literature involving the formation of the C-S bond.Compostos organossulfurados estão presentes em todos os organismos vivos, sendo essenciais a variados sistemas bioquímicos, onde importantes metabólitos primários, como o ácido lipóico e as coenzimas A e B, apresentam enxofre em suas estruturas moleculares. A metilsulfenilação é uma interessante maneira de acessar essa classe de compostos, visto que moléculas contendo grupos -SMe com atividade biológica são altamente desejadas. O reagente mais usual para esse tipo de reação é o metanotiol, que além de inflamável apresenta elevada toxicidade e dificuldade de manuseio. Nesse trabalho, a metodologia de metilsulfenilação anteriormente empregada pelo nosso grupo de pesquisas foi revisitada devido aos indícios experimentais de que o DMSO apresentava papel incongruente com a proposta mecanística inicial. Assim, foi desenvolvido um novo escopo, empregando acetato de sódio e sem o incremento do éster de metil-tio-metil (MTM), tendo como substratos os derivados de adutos de MBH: acil ésteres e acetatos de MBH. Os produtos metilsulfenilados foram acessados com boa tolerância de grupo e rendimentos de moderado à excelente, ratificando a necessidade de compreender o mecanismo reacional. A investigação procedeu através de diversos experimentos controle e por cálculos teóricos empregando a metodologia da teoria do funcional da densidade (do inglês, DFT). Os resultados analisados suportam fortemente a hipótese de que o éster de MTM atua como reagente de metilsulfenilação. A espécie de enxofre hipervalente e o ilídeo de sulfônio aparecem como intermediários chaves para um rearranjo do tipo Pummerer para a formação in situ do reagente. Além disso, o mecanismo da metilsulfenilação é provável de proceder por meio do ataque nucleofílico do éster de MTM, seguido de uma etapa entropicamente favorável, envolvendo o ataque de uma molécula de acetato à espécie positivamente carregada, liberando o produto. Esse trabalho foi capaz de elucidar a participação do éster de MTM na reação de metilsulfenilação. O reagente, que pode ser originado in situ ou empregado inicialmente, apresenta uma alternativa viável a utilização de metanotiol e contribui para a literatura científica envolvendo a formação de ligação C-S

Mechanistic investigations of C-S bond formation in anaerobic ergothioneine biosynthesis and aerobic ovothiol biosynthesis

Ergothioneine and ovothiol A are naturally occurring thiol-histidine derivatives. Both of them are suggested to be beneficial to human health. Ergothioneine has anti-inflammation and anti-oxidative properties and ovothiol has anti-proliferative activities. Recently, ergothioneine has been suggested to be also linked to lifespan longevity. For these reasons, there is a need to investigate the mechanisms of ergothioneine and ovothiol biosynthesis is appealing. My thesis work has addressed this gap in knowledge, focusing on the mechanistic investigations of two C-S bond formation enzymes: EanB in anaerobic ergothioneine biosynthesis, and OvoA in ovothiol biosynthesis. Chapter 1 provides an overview of sulfur-containing metabolites, including the metabolism, potential biological functions, and biosynthesis of several key sulfur containing natural products. Chapter 2 contains my initial investigations into EanB catalysis, namely the original sulfur source for this enzyme. We demonstrated that the polysulfide (HSSnSR) is the direct sulfur source in EanB catalysis. With the discovery of the unique sulfur source, we then probed how EanB uses polysulfide for catalysis. A few reaction intermediate states were successfully characterized by X-ray crystallography and the proposed reaction mechanisms were further evaluated by QM/MM calculation. In Chapter 3, we evaluated the involvement of a proposed carbene intermediate involved in EanB catalysis by the deuterium exchange experiments with hercynine. In addition, using 3,5-difluoro-tyrosine containing EanB produced through amber suppressor method, we have also kinetically characterize the deuterium-exchange reaction. Chapter 4 reports the biochemical characterization of an OvoA homolog, OvoAMtht, from a mesophilic organism. OvoAMtht has dual activities: sulfoxide synthase and cysteine dioxygenase. In addition, I have demonstrated that both substrates and the active site iron’s secondary coordination shell residues exert exquisite control to OvoAMtht dual activities, which makes OvoAMtht an excellent system for future structure-function relationship studies for this class of enzymes. In summary, my thesis has laid the foundation for future detailed mechanistic investigations of the C-S bond formation reactions in both anaerobic ergothioneine biosynthetic and ovothiol aerobic biosynthetic pathways

Unprecedented sulfur transfer strategy in ergothioneine and ovothiol biosyntheses

Ergothioneine, a histidine-derived thiol, protects cells against reactive oxygen species and is emerging as a longevity vitamin. Ovothiol, another histidine-derived thiol, is also a potent antioxidant with therapeutic potential due to its anti-inflammatory and anti-proliferative activities. Despite these promising health benefits, the production of ergothioneine is limited by the underlying challenges of its only industrial synthetic method, while ovothiol is not commercially available. Due to these issues, the production of these thiols through metabolic engineering/synthetic biology approaches is appealing. The central steps in the ergothioneine and ovothiol biosynthetic pathways are the oxidative coupling C-S bond formation reaction mediated by non-heme iron sulfoxide synthases, and the pyridoxal-5'-phosphate (PLP)-dependent C-S lyases. This sulfur transfer strategy differs from all other pathways reported. Therefore, these trans-sulfuration reactions in ergothioneine and ovothiol biosyntheses are significant from both basic and translational research perspectives, hence, they were selected as my thesis project. This thesis comprises of five chapters. Sulfur metabolism and the biosynthesis of sulfur-containing natural products are presented in Chapter 1. The computational-guided protein engineering of a thermophilic sulfoxide synthase (EgtB) from Chloracidobacterium thermophiluim is covered in Chapter 2. Chapter 3 describes the mechanistic studies of the reductive C-S lyase (Egt2 from the Neurospora crassa’s ergothioneine biosynthesis), which revealed the involvement of a sulfenic acid intermediate in this reaction. In addition to reconstituting the ergothioneine biosynthetic pathway in vitro presented in Chapter 3, I fully reconstituted the in vitro ovothiol A biosynthetic pathway from Erwinia tasmaniensis, which is described in Chapter 4. In Chapter 5, the mechanistic studies of the ovothiol sulfoxide synthase OvoA using unnatural amino acid incorporation via amber-codon suppression are discussed. The success of this thesis work paves the way for the industrial production of ergothioneine and ovothiol through metabolic engineering/synthetic biology approaches. This study has also laid the foundation for future in-depth mechanistic characterization of these novel enzymes

Spectroscopic and analytical characterization of the distribution of iron in intact mitochondria from Saccharomyces cerevisiae

Electron paramagnetic resonance (EPR) and Mössbauer spectroscopy were used to examine the distribution of iron in mitochondria from Saccharomyces cerevisiae. These organelles were packed into EPR and Mössbauer cuvettes, affording spectra with unprecedented signal/noise ratios. EPR spectra of as-isolated intact mitochondria exhibited fourteen distinct signals, some of which were assigned according to previously reported g-values obtained using isolated proteins. Signals from adventitious manganese (II) and iron (III) were largely removed when mitochondria were isolated in buffers supplemented with the metal chelators EDTA or EGTA. Signals were simulated and intensities were quantified to afford spin concentrations and estimates of the concentration of EPR-active species in mitochondria. The effects of treating samples with chemical modifiers were examined. Packed samples were analyzed for protein and metal content, affording averaged values of 50 mg/mL [protein], 590 õM [Fe], 340 õM [Cu], and 17 õM [Mn]. 57Fe-enriched intact mitochondria isolated in the presence of metal chelators exhibited Mössbauer spectra dominated by three components. Approximately 60% of the 57Fe in the sample gave rise to a quadrupole doublet, most of which was diamagnetic. The parameters of this doublet are typical of S = 0 [4Fe-4S]2+ clusters and S = 0 ferrous heme groups. Spectra of samples reduced with dithionite, pH 8.5, suggested that at least half of this doublet arose from [4Fe-4S]2+ clusters. The second major component exhibited in the Mössbauer spectra arose from high-spin ferrous ions (10%-30%). The third major component (15%) came from iron exhibiting magnetic hyperfine interactions and is likely reflected in the Fe-containing species observed by EPR. The results presented here suggest that mitochondria contain ~ 600 õM of Fe overall, ~ 200 â 400 õM organized as [4Fe-4S]2+ clusters, with about 25 õM due to the [4Fe-4S]2+ cluster of aconitase. Approximately 60 õM â 200 õM of the Fe in mitochondria is high-spin ferrous ions, ~ 40 õM as the Rieske S = 1/2 [2Fe-2S]+ cluster of cytochrome bc1, and ~20 õM as the S = 1/2 [2Fe-2S]+ cluster of succinate dehydrogenase. The high-spin ferric hemes of the a3:CuB site of cytochrome oxidase and cytochrome c peroxidase each account for ~ 4 õM of Fe

Biosynthetic studies of thiosugar-containing natural products, BE-7585A and Lincomycin A

Sulfur is an essential element found ubiquitously in living systems. However, there exist only a few sulfur-containing sugars in nature and their biosyntheses have not been well understood. On the other hand, a wide variety of sugar derivatives commonly found in natural products are often vital components for the efficacy and specificity of their parent molecules. Elucidation of such unusual sugar biosyntheses is important both for understanding their intriguing chemical mechanisms and creating unnatural compounds by altering their biosynthetic machineries, which could potentially exhibit enhanced or novel biological activities. This dissertation describes biosynthetic studies of two thiosugar-containing natural products, BE-7585A and lincomycin A, produced by Amycolatopsis orientalis and Streptomyces lincolnensis, respectively. While the former possess a C-2-thiosugar-containing disaccharide moiety, the latter contains a C-1-thio substituent on a characteristic eight-carbon backbone sugar. The focus of this research is to characterize the biological pathways and mechanisms responsible for the sulfur incorporation and the unique sugar scaffolds. BE-7585A, an angucycline-type natural product, contains the rare C-2-thiosugar moiety. PCR-based screening of a cosmid library constructed from the genomic DNA of A. orientalis led to the identification of the BE-7585A biosynthetic gene cluster. A gene, bexX, was found to be a candidate for a thiosugar synthase with moderate sequence similarity to a thiazole synthase. The gene, bexX, and a glycosyltransferase homologue, bexG2, were heterologously expressed in Escherichia coli. A variety of biochemical experiments provided a wealth of evidence supporting the proposed biosynthetic pathway for the C-2-thiodisaccharide moiety. Finally, whole genome sequencing and a genome mining approach led to the identification of a sulfur carrier protein to accomplish the in vitro enzymatic synthesis of the C-2-thiosugar for the first time. Lincomycin A is a lincosamide antimicrobial natural product with a C-1 methylthio substituent. Although the lincomycin A biosynthetic gene cluster has been reported, biochemical verification of the biosynthetic pathway has remained elusive. In this dissertation, the complete methlthiolincosamide biosynthetic pathway including the potential C-1 sulfur incorporation mechanism was proposed. Furthermore, two early intermediates of the pathway were characterized for the first time by demonstrating the LmbR (transaldolase) and LmbN (isomerase) reactions in vitro.Chemistr