Thiocysteine lyases as polyketide synthase domains installing hydropersulfide into natural products and a hydropersulfide methyltransferase

Evolution-guided engineering of trans-acyltransferase polyketide synthases

Bacterial multimodular polyketide synthases (PKSs) are giant enzymes that generate a wide range of therapeutically important but synthetically challenging natural products. Diversification of polyketide structures can be achieved by engineering these enzymes. However, notwithstanding successes made with textbook cis-acyltransferase (cis-AT) PKSs, tailoring such large assembly lines remains challenging. Unlike textbook PKSs, trans-AT PKSs feature an extraordinary diversity of PKS modules and commonly evolve to form hybrid PKSs. In this study, we analyzed amino acid coevolution to identify a common module site that yields functional PKSs. We used this site to insert and delete diverse PKS parts and create 22 engineered trans-AT PKSs from various pathways and in two bacterial producers. The high success rates of our engineering approach highlight the broader applicability to generate complex designer polyketides.

Total Synthesis of the Guangnanmycin A Alcohol

Guangnanmycin A is a recently discovered congener of the well-known antitumor drug lead leinamycin; its macrolactam ring, however, is even more strained than that of the parent compound. The first synthetic foray towards this challenging target is reported, which relies on molybdenum-catalyzed macrocyclization by ring closing alkyne metathesis (RCAM) followed by ruthenium-catalyzed redox isomerization of the propargyl alcohol thus formed; the resulting enone enabled the introduction of the yet missing exo-methylene group by a modified Peterson olefination. The signature disulfide moiety of guangnanmycin A was installed by strain-driven thia-Michael addition followed by conversion of the thioether thus formed into an unsymmetric disulfide with the aid of (methylthio)dimethylsulfonium tetrafluoroborate and MeSSMe. While this sequence furnished racemic guangnanmycin A alcohol in good overall yield, the final oxidation to the corresponding acid failed, most likely because of the exceptional sensitivity of the strained scaffold.

Nickel-Catalyzed Acid Chlorides with Tetrasulfides for the Synthesis of Thioesters and Acyl Disulfides

Herein, we report a novel method for the synthesis of thioesters and acyl disulfides via nickel-catalyzed reductive cross-electrophile coupling of acid chlorides with tetrasulfides. This approach for the synthesis of thioesters and acyl disulfides is convenient and practical under mild reaction conditions, relying on easy availability. In addition, a wide range of thioesters and acyl disulfides were obtained in medium to good yields with good functional group tolerance. Moreover, thioesters and acyl disulfides can also be prepared at the gram scale, indicating that they have certain potential for industrial application.

How nature incorporates sulfur and selenium into bioactive natural products

Living organisms have evolved various strategies to incorporate sulfur and selenium into bioactive natural products. These chalcogen-containing compounds serve important and diverse biological functions for their producers and many of them are essential medicines against infectious diseases and cancer. We review recent advances in the biosynthesis of some sulfur/selenium-containing natural products with a focus on the formation or cleavage of C-S/C-Se bonds. We highlight unusual enzymes that catalyze these transformations, describe their proposed mechanisms, and discuss how understanding these enzymes may facilitate the discovery and synthesis of novel natural products containing sulfur or selenium.

The many facets of sulfur incorporation in natural product biosynthesis

Sulfur-containing natural products (S-containing NPs) exhibit diverse chemical structures and biosynthetic machineries. Unraveling the intricate chemistry of S-incorporation requires innovative and multidisciplinary approaches. In this review, we surveyed the landscape of S-containing NP biosynthetic machineries, classified the S-incorporation chemistry into four distinct classes, and highlighted each of the four classes with representative examples from recent studies. All highlighted chemistry has been correlated to the genes encoding the biosynthetic machineries of the S-containing NPs, which open new opportunities to discover S-containing NPs through genome mining. These examples should inspire the community to explore uncharted territories in NP research, promoting further advancements in both novel S-containing NP discovery and S-incorporation chemistry.

Cysteine hydropersulfide reduces lipid peroxidation and protects against myocardial ischaemia-reperfusion injury - Are endogenous persulfides mediators of ischaemic preconditioning?

Earlier studies revealed the presence of cysteine persulfide (CysSSH) and related polysulfide species in various mammalian tissues. CysSSH has both antioxidant and oxidant properties, modulates redox-dependent signal transduction and has been shown to mitigate oxidative stress. However, its functional relevance in the setting of myocardial ischaemia-reperfusion injury (IRI) remains unknown. The present study was undertaken to (1) study the dynamics of production and consumption of persulfides under normoxic and hypoxic conditions in the heart, and (2) determine whether exogenous administration of the CysSSH donor, cysteine trisulfide (Cys-SSS-Cys) at the onset of reperfusion rescues functional impairment and myocardial damage by interfering with lipid peroxidation. Utilising a well-established ex vivo Langendorff murine model, we here demonstrate that endogenous tissue concentrations of CysSSH are upregulated when oxygen supply is compromised (global myocardial ischaemia) and rapidly restored to baseline levels upon reperfusion, suggestive of active regulation. In a separate set of experiments, exogenous administration of Cys-SSS-Cys for 10 min at the onset of reperfusion was found to decrease malondialdehyde (MDA) concentrations, formation of 4-hydroxynonenal (4-HNE) protein adducts and rescue the heart from injury. Cys-SSS-Cys also restored post-ischaemic cardiac function, improving both coronary flow and left ventricular developed pressure (LVDP). Taken together, these results support the notion that endogenous CysSSH plays an important role as a "redox preconditioning" agent to combat the oxidative insult in myocardial IRI.

Understanding and Circumventing the Requirement for Native Thioester Substrates for α-Oxoamine Synthase Reactions

Many enzyme classes require thioester electrophiles such as acyl-carrier proteins and acyl-coenzyme A substrates. For in vitro applications, these substrates can render these chemical transformations impractical. To address this challenge, we have investigated the mechanism of coenzyme A in gating catalysis of one α-oxoamine synthase, SxtA AOS. Through investigating the reactivity of SxtA AOS and corresponding enzyme variants against a panel of substrates and coenzyme A mimics, we determined that activity is gated through the binding of the pantetheine arm and a phosphate group that hydrogen bonds to residue Lys154 that is predicted by an AlphaFold2 model to be located in a tunnel leading to the active site. To provide an economical solution for preparative-scale reactions, in situ transthioesterification was used with pantetheine and simple thioester substrate precursors, resulting in productive reactions. These findings outline a strategy for employing ACP- and CoA-dependent enzymes that are inaccessible through other means without the need for cost-prohibitive coenzyme A or carrier protein-activated substrates.

Decatungstate-catalyzed radical disulfuration through direct C-H functionalization for the preparation of unsymmetrical disulfides

Unsymmetrical disulfides are widely found in the areas of food chemistry, pharmaceutical industry, chemical biology and polymer science. Due the importance of such disulfides in various fields, general methods for the nondirected intermolecular disulfuration of C-H bonds are highly desirable. In this work, the conversion of aliphatic C(sp³)-H bonds and aldehydic C(sp²)-H bonds into the corresponding C-SS bonds with tetrasulfides (RSSSSR) as radical disulfuration reagents is reported. The decatungstate anion ([W₁₀O₃₂]⁴⁻) as photocatalyst is used for C-radical generation via intermolecular hydrogen atom transfer in combination with cheap sodium persulfate (Na₂S₂O₈) as oxidant. Herein a series of valuable acyl alkyl disulfides, important precursors for the generation of RSS-anions, and unsymmetrical dialkyl disulfides are synthesized using this direct approach. To demonstrate the potential of the method for late-stage functionalization, approved drugs and natural products were successfully C-H functionalized.

Despite the importance of unsymmetrical disulfides in various fields such as food chemistry, pharmaceutical industry, and polymer science, the nondirected intermolecular disulfuration of C-H bonds remains challenging. Here, the authors report the conversion of aliphatic C(sp³)-H bonds and aldehydic C(sp²)-H bonds into the corresponding C-SS bonds with tetrasulfides (RSSSSR) as radical disulfuration reagents.