The structural basis of an NADP⁺-independent dithiol oxidase in FK228 biosynthesis

Convergent evolution in toxin detection and resistance provides evidence for conserved bacterial-fungal interactions

Microbes rarely exist in isolation and instead form complex polymicrobial communities. As a result, microbes have developed intricate offensive and defensive strategies that enhance their fitness in these complex communities. Thus, identifying and understanding the molecular mechanisms controlling polymicrobial interactions is critical for understanding the function of microbial communities. In this study, we show that the gram-negative opportunistic human pathogen Pseudomonas aeruginosa, which frequently causes infection alongside a plethora of other microbes including fungi, encodes a genetic network which can detect and defend against gliotoxin, a potent, disulfide-containing antimicrobial produced by the ubiquitous filamentous fungus Aspergillus fumigatus. We show that gliotoxin exposure disrupts P. aeruginosa zinc homeostasis, leading to transcriptional activation of a gene encoding a previously uncharacterized dithiol oxidase (herein named as DnoP), which detoxifies gliotoxin and structurally related toxins. Despite sharing little homology to the A. fumigatus gliotoxin resistance protein (GliT), the enzymatic mechanism of DnoP from P. aeruginosa appears to be identical that used by A. fumigatus. Thus, DnoP and its transcriptional induction by low zinc represent a rare example of both convergent evolution of toxin defense and environmental cue sensing across kingdoms. Collectively, these data provide compelling evidence that P. aeruginosa has evolved to survive exposure to an A. fumigatus disulfide-containing toxin in the natural environment.

Evaluation of Antioxidant Therapeutic Value of ACE Inhibitor as Adjunct Therapy on Type 2 Diabetes Mellitus Patients with Cardiovascular Disease

Diabetes mellitus (DM) is believed to promote oxidative stress, which potentially provokes and accelerates complications in conditions such as atherosclerotic cardiovascular, peripheral arterial, and cerebrovascular diseases. In this study, we evaluated the antioxidant therapeutic value of adding an angiotensin-converting enzyme (ACE) inhibitor-a low dose of captopril-as adjunct therapy to the treatment regimen of Type 2 diabetes mellitus (T2DM). Participants were distributed among two different groups: control and treated. T2DM patients in the treated group (group 2) were given a supplement of the ACE inhibitor capotopril, 12.5 mg/day, in addition to standard treatment. All subjects were interviewed for clinical examination. All patients in group 2 were re-examined monthly for 3 months to evaluate FBS, HbA1c, MDA, total GSH, reduced GSH, GSSG, and ox-LDL. All parameters were repeated for patients in group 2 after 1 and 3 months. The study showed improvements in the glycemic and oxidative stress status with the addition of a low dose of captopril-not very prominent but statistically significant. Reduced GSH decreased by 73.6% (P = 0.016) and the TBARS level was decreased by 58.3% (P = 0.018) after 3 months of treatment, while ox-LDL was decreased by 26.4% (P = 0.036) at the end of treatment. In summary, the clinical improvements in the disease indices toward normal levels make the use of low doses of ACE inhibitors as adjunct therapy in T2DM worth pursuing. Thus, investigations directed at preventing or protecting against oxidative damage may open a new window for treatment of diabetes mellitus.

Analogues of desferrioxamine B (DFOB) with new properties and new functions generated using precursor-directed biosynthesis

Desferrioxamine B (DFOB) is a siderophore native to Streptomyces pilosus biosynthesised by the DesABCD enzyme cluster as a high affinity Fe(III) chelator. Although DFOB has a long clinical history for the treatment of chronic iron overload, limitations encourage the development of new analogues. This review describes a recent body of work that has used precursor-directed biosynthesis (PDB) to access new DFOB analogues. PDB exploits the native biosynthetic machinery of a producing organism in culture medium augmented with non-native substrates that compete against native substrates during metabolite assembly. The method allows access to analogues of natural products using benign methods, compared to multistep organic synthesis. The disadvantages of PDB are the production of metabolites in low yield and the need to purify complex mixtures. Streptomyces pilosus medium was supplemented with different types of non-native diamine substrates to compete against native 1,5-diaminopentane to generate DFOB analogues containing alkene bonds, fluorine atoms, ether or thioether functional groups, or a disulfide bond. All analogues retained function as Fe(III) chelators and have properties that could broaden the utility of DFOB. These PDB studies have also added knowledge to the understanding of DFOB biosynthesis.

Engineering a cleavable disulfide bond into a natural product siderophore using precursor-directed biosynthesis

An analogue of the bacterial siderophore desferrioxamine B (DFOB) containing a disulfide motif in the backbone was produced from Streptomyces pilosus cultures supplemented with cystamine.

Unprecedented pathway of reducing equivalents in a diflavin-linked disulfide oxidoreductase

Flavoproteins participate in a wide variety of physiologically relevant processes that typically involve redox reactions. Within this protein superfamily, there exists a group that is able to transfer reducing equivalents from FAD to a redox-active disulfide bridge, which further reduces disulfide bridges in target proteins to regulate their structure and function. We have identified a previously undescribed type of flavin enzyme that is exclusive to oxygenic photosynthetic prokaryotes and that is based on the primary sequence that had been assigned as an NADPH-dependent thioredoxin reductase (NTR). However, our experimental data show that the protein does not transfer reducing equivalents from flavins to disulfides as in NTRs but functions in the opposite direction. High-resolution structures of the protein from Gloeobacter violaceus and Synechocystis sp. PCC6803 obtained by X-ray crystallography showed two juxtaposed FAD molecules per monomer in redox communication with an active disulfide bridge in a variant of the fold adopted by NTRs. We have tentatively named the flavoprotein "DDOR" (diflavin-linked disulfide oxidoreductase) and propose that its activity is linked to a thiol-based transfer of reducing equivalents in bacterial membranes. These findings expand the structural and mechanistic repertoire of flavoenzymes with oxidoreductase activity and pave the way to explore new protein engineering approaches aimed at designing redox-active proteins for diverse biotechnological applications.

Chemistry and Enzymology of Disulfide Cross-Linking in Proteins

Cysteine thiols are among the most reactive functional groups in proteins, and their pairing in disulfide linkages is a common post-translational modification in proteins entering the secretory pathway. This modest amino acid alteration, the mere removal of a pair of hydrogen atoms from juxtaposed cysteine residues, contrasts with the substantial changes that characterize most other post-translational reactions. However, the wide variety of proteins that contain disulfides, the profound impact of cross-linking on the behavior of the protein polymer, the numerous and diverse players in intracellular pathways for disulfide formation, and the distinct biological settings in which disulfide bond formation can take place belie the simplicity of the process. Here we lay the groundwork for appreciating the mechanisms and consequences of disulfide bond formation in vivo by reviewing chemical principles underlying cysteine pairing and oxidation. We then show how enzymes tune redox-active cofactors and recruit oxidants to improve the specificity and efficiency of disulfide formation. Finally, we discuss disulfide bond formation in a cellular context and identify important principles that contribute to productive thiol oxidation in complex, crowded, dynamic environments.

Heteroatom-Heteroatom Bond Formation in Natural Product Biosynthesis

Natural products that contain functional groups with heteroatom-heteroatom linkages (X-X, where X = N, O, S, and P) are a small yet intriguing group of metabolites. The reactivity and diversity of these structural motifs has captured the interest of synthetic and biological chemists alike. Functional groups containing X-X bonds are found in all major classes of natural products and often impart significant biological activity. This review presents our current understanding of the biosynthetic logic and enzymatic chemistry involved in the construction of X-X bond containing functional groups within natural products. Elucidating and characterizing biosynthetic pathways that generate X-X bonds could both provide tools for biocatalysis and synthetic biology, as well as guide efforts to uncover new natural products containing these structural features.

Enzymatic Carbon-Sulfur Bond Formation in Natural Product Biosynthesis

Sulfur plays a critical role for the development and maintenance of life on earth, which is reflected by the wealth of primary metabolites, macromolecules, and cofactors bearing this element. Whereas a large body of knowledge has existed for sulfur trafficking in primary metabolism, the secondary metabolism involving sulfur has long been neglected. Yet, diverse sulfur functionalities have a major impact on the biological activities of natural products. Recent research at the genetic, biochemical, and chemical levels has unearthed a broad range of enzymes, sulfur shuttles, and chemical mechanisms for generating carbon-sulfur bonds. This Review will give the first systematic overview on enzymes catalyzing the formation of organosulfur natural products.

Generate a bioactive natural product library by mining bacterial cytochrome P450 patterns

The increased number of annotated bacterial genomes provides a vast resource for genome mining. Several bacterial natural products with epoxide groups have been identified as pre-mRNA spliceosome inhibitors and antitumor compounds through genome mining. These epoxide-containing natural products feature a common biosynthetic characteristic that cytochrome P450s (CYPs) and its patterns such as epoxidases are employed in the tailoring reactions. The tailoring enzyme patterns are essential to both biological activities and structural diversity of natural products, and can be used for enzyme pattern-based genome mining. Recent development of direct cloning, heterologous expression, manipulation of the biosynthetic pathways and the CRISPR-CAS9 system have provided molecular biology tools to turn on or pull out nascent biosynthetic gene clusters to generate a microbial natural product library. This review focuses on a library of epoxide-containing natural products and their associated CYPs, with the intention to provide strategies on diversifying the structures of CYP-catalyzed bioactive natural products. It is conceivable that a library of diversified bioactive natural products will be created by pattern-based genome mining, direct cloning and heterologous expression as well as the genomic manipulation.