A Remarkable Oxidative Cascade That Replaces the Riboflavin C8 Methyl with an Amino Group during Roseoflavin Biosynthesis

Superb Silk Hydrogels with High Adaptability, Bioactivity, and Versatility Enabled by Photo-Cross-Linking

The exceptional biocompatibility and adaptability of hydrogels have garnered significant interest in the biomedical field for the fabrication of biomedical devices. However, conventional synthetic hydrogels still exhibit relatively weak and fragile properties. Drawing inspiration from the photosynthesis process, we developed a facile approach to achieve a harmonious combination of superior mechanical properties and efficient preparation of silk fibroin hydrogel through photo-cross-linking technology, accomplished within 60 s. The utilization of riboflavin and H2O2 enabled a sustainable cyclic photo-cross-linking reaction, facilitating the transformation from tyrosine to dityrosine and ultimately contributing to the formation of highly cross-linked hydrogels. These photo-cross-linking hydrogels exhibited excellent elasticity and restorability even after undergoing 1000 cycles of compression. Importantly, our findings presented that hydrogel-encapsulated adipose stem cells possess the ability to stimulate cell proliferation along with stem cell stemness. This was evidenced by the continuous high expression levels of OCT4 and SOX2 over 21 days. Additionally, the utilization of photo-cross-linking hydrogels can be extended to various material molding platforms, including microneedles, microcarriers, and bone screws. Consequently, this study offered a significant approach to fabricating biomedical hydrogels capable of facilitating real-time cell delivery, thereby introducing an innovative avenue for designing silk devices with exceptional machinability and adaptability in biomedical applications.

A single hydrogen bond that tunes flavin redox reactivity and activates it for modification

Electron bifurcation produces high-energy products based on less energetic reagents. This feat enables biological systems to exploit abundant mediocre fuel to drive vital but demanding reactions, including nitrogen fixation and CO2 capture. Thus, there is great interest in understanding principles that can be portable to man-made devices. Bifurcating electron transfer flavoproteins (Bf ETFs) employ two flavins with contrasting reactivities to acquire pairs of electrons from a modest reductant, NADH. The bifurcating flavin then dispatches the electrons individually to a high and a low reduction midpoint potential (E°) acceptor, the latter of which captures most of the energy. Maximum efficiency requires that only one electron accesses the exergonic path that will 'pay for' the production of the low-E° product. It is therefore critical that one of the flavins, the 'electron transfer' (ET) flavin, is tuned to execute single-electron (1e-) chemistry only. To learn how, and extract fundamental principles, we systematically altered interactions with the ET-flavin O2 position. Removal of a single hydrogen bond (H-bond) disfavored the formation of the flavin anionic semiquinone (ASQ) relative to the oxidized (OX) state, lowering by 150 mV and retuning the flavin's tendency for 1e-vs. 2e- reactivity. This was achieved by replacing conserved His 290 with Phe, while also replacing the supporting Tyr 279 with Ile. Although this variant binds oxidized FADs at 90% the WT level, the ASQ state of the ET-flavin is not stable in the absence of H290's H-bond, and dissociates, in contrast to the WT. Removal of this H-bond also altered the ET-flavin's covalent chemistry. While the WT ETF accumulates modified flavins whose formation is believed to rely on an anionic paraquinone methide intermediate, the FADs of the H-bond lacking variant remain unchanged over weeks. Hence the variant that destabilizes the anionic semiquinone also suppresses the anionic intermediate in flavin modification, verifying electronic similarities between these two species. These correlations suggest that the H-bond that stabilizes the crucial flavin ASQ also promotes flavin modification. The two effects may indeed be inseparable, as a Jekyll and Hydrogen bond.

Isolation, characterisation and description of the roseoflavin producer Streptomyces berlinensis sp. nov

The Gram-positive bacteria Streptomyces davaonensis and Streptomyces cinnabarinus have been the only organisms known to produce roseoflavin, a riboflavin (vitamin B2) derived red antibiotic. Using a selective growth medium and a phenotypic screening, we were able to isolate a novel roseoflavin producer from a German soil sample. The isolation procedure was repeated twice, that is, the same strain could be isolated from the same location in Berlin 6 months and 12 months after its first isolation. Whole genome sequencing of the novel roseoflavin producer revealed an unusual chromosomal arrangement and the deposited genome sequence of the new isolate (G + C content of 71.47%) contains 897 genes per inverted terminal repeat, 6190 genes in the core and 107 genes located on an illegitimate terminal end. We identified the roseoflavin biosynthetic genes rosA, rosB and rosC and an unusually high number of riboflavin biosynthetic genes. Overexpression of rosA, rosB and rosC in Escherichia coli and enzyme assays confirmed their predicted functions in roseoflavin biosynthesis. A full taxonomic analysis revealed that the isolate represents a previously unknown Streptomyces species and we propose the name Streptomyces berlinensis sp. nov. for this roseoflavin producer.

Unusual reactivity of a flavin in a bifurcating electron-transferring flavoprotein leads to flavin modification and a charge-transfer complex

From the outset, canonical electron transferring flavoproteins (ETFs) earned a reputation for containing modified flavin. We now show that modification occurs in the recently recognized bifurcating (Bf) ETFs as well. In Bf ETFs, the 'electron transfer' (ET) flavin mediates single electron transfer via a stable anionic semiquinone state, akin to the FAD of canonical ETFs, whereas a second flavin mediates bifurcation (the Bf FAD). We demonstrate that the ET FAD undergoes transformation to two different modified flavins by a sequence of protein-catalyzed reactions that occurs specifically in the ET site, when the enzyme is maintained at pH 9 in an amine-based buffer. Our optical and mass spectrometric characterizations identify 8-formyl flavin early in the process and 8-amino flavins (8AFs) at later times. The latter have not previously been documented in an ETF to our knowledge. Mass spectrometry of flavin products formed in Tris or bis-tris-aminopropane solutions demonstrates that the source of the amine adduct is the buffer. Stepwise reduction of the 8AF demonstrates that it can explain a charge transfer band observed near 726 nm in Bf ETF, as a complex involving the hydroquinone state of the 8AF in the ET site with the oxidized state of unmodified flavin in the Bf site. This supports the possibility that Bf ETF can populate a conformation enabling direct electron transfer between its two flavins, as has been proposed for cofactors brought together in complexes between ETF and its partner proteins.

Noncanonical Functions of Enzyme Cofactors as Building Blocks in Natural Product Biosynthesis

Enzymes involved in secondary metabolite biosynthetic pathways have typically evolutionarily diverged from their counterparts functioning in primary metabolism. They often catalyze diverse and complex chemical transformations and are thus a treasure trove for the discovery of unique enzyme-mediated chemistries. Besides major natural product classes, such as terpenoids, polyketides, and ribosomally or nonribosomally synthesized peptides, biosynthetic investigations of noncanonical natural product biosynthetic pathways often reveal functionally distinct enzyme chemistries. In this Perspective, we aim to highlight challenges and opportunities of biosynthetic investigations on noncanonical natural product pathways that utilize primary metabolites as building blocks, otherwise generally considered as enzyme cofactors. A focus is made on the discovered chemical and enzymological novelties.

Structures and function of a tailoring oxidase in complex with a nonribosomal peptide synthetase module

Nonribosomal peptide synthetases (NRPSs) are large modular enzymes that synthesize secondary metabolites and natural product therapeutics. Most NRPS biosynthetic pathways include an NRPS and additional proteins that introduce chemical modifications before, during or after assembly-line synthesis. The bacillamide biosynthetic pathway is a common, three-protein system, with a decarboxylase that prepares an NRPS substrate, an NRPS, and an oxidase. Here, the pathway is reconstituted in vitro. The oxidase is shown to perform dehydrogenation of the thiazoline in the peptide intermediate while it is covalently attached to the NRPS, as the penultimate step in bacillamide D synthesis. Structural analysis of the oxidase reveals a dimeric, two-lobed architecture with a remnant RiPP recognition element and a dramatic wrapping loop. The oxidase forms a stable complex with the NRPS and dimerizes it. We visualized co-complexes of the oxidase bound to the elongation module of the NRPS using X-ray crystallography and cryo-EM. The three active sites (for adenylation, condensation/cyclization, and oxidation) form an elegant arc to facilitate substrate delivery. The structures enabled a proof-of-principle bioengineering experiment in which the BmdC oxidase domain is embedded into the NRPS.

A biosynthetic pathway to aromatic amines that uses glycyl-tRNA as nitrogen donor

Aromatic amines in nature are typically installed with Glu or Gln as the nitrogen donor. Here we report a pathway that features glycyl-tRNA instead. During the biosynthesis of pyrroloiminoquinone-type natural products such as ammosamides, peptide-aminoacyl tRNA ligases append amino acids to the C-terminus of a ribosomally synthesized peptide. First, [Formula: see text] adds Trp in a Trp-tRNA-dependent reaction and the flavoprotein AmmC₁ then carries out three hydroxylations of the indole ring of Trp. After oxidation to the corresponding ortho-hydroxy para-quinone, [Formula: see text] attaches Gly to the indole ring in a Gly-tRNA dependent fashion. Subsequent decarboxylation and hydrolysis results in an amino-substituted indole. Similar transformations are catalysed by orthologous enzymes from Bacillus halodurans. This pathway features three previously unknown biochemical processes using a ribosomally synthesized peptide as scaffold for non-ribosomal peptide extension and chemical modification to generate an amino acid-derived natural product.

Tuning the Quantum Chemical Properties of Flavins via Modification at C8

Flavins are central to countless enzymes but display different reactivities depending on their environments. This is understood to reflect modulation of the flavin electronic structure. To understand changes in orbital natures, energies, and correlation over the ring system, we begin by comparing seven flavin variants differing at C8, exploiting their different electronic spectra to validate quantum chemical calculations. Ground state calculations replicate a Hammett trend and reveal the significance of the flavin π-system. Comparison of higher-level theories establishes CC2 and ACD(2) as methods of choice for characterization of electronic transitions. Charge transfer character and electron correlation prove responsive to the identity of the substituent at C8. Indeed, bond length alternation analysis demonstrates extensive conjugation and delocalization from the C8 position throughout the ring system. Moreover, we succeed in replicating a particularly challenging UV/Vis spectrum by implementing hybrid QM/MM in explicit solvents. Our calculations reveal that the presence of nonbonding lone pairs correlates with the change in the UV/Vis spectrum observed when the 8-methyl is replaced by NH₂, OH, or SH. Thus, our computations offer routes to understanding the spectra of flavins with different modifications. This is a first step toward understanding how the same is accomplished by different binding environments.

The novel phosphatase RosC catalyzes the last unknown step of roseoflavin biosynthesis in Streptomyces davaonensis

The bacterium Streptomyces davaonensis produces the antibiotic roseoflavin, which is a riboflavin (vitamin B₂ ) analog. The key enzyme of roseoflavin biosynthesis is the 8-demethyl-8-amino-riboflavin-5'-phosphate (AFP) synthase RosB which synthesizes AFP from riboflavin-5'-phosphate. AFP is not a substrate for the last enzyme of roseoflavin biosynthesis the N, N-dimethyltransferase RosA, which generates roseoflavin from 8-demethyl-8-amino-riboflavin (AF). Consequently, the roseoflavin biosynthetic pathway depends on a phosphatase, which dephosphorylates AFP to AF. Here, we report on the identification and characterization of such an AFP phosphatase which we named RosC. The gene rosC is located immediately downstream of rosA and both genes are part of a cluster comprising 10 genes. Deletion of rosC from the chromosome of S. davaonensis led to reduced roseoflavin levels in the corresponding recombinant strain. In contrast to wild-type S. davaonensis, cell-free extracts of the rosC deletion strain did not catalyze dephosphorylation of AFP. RosC was purified from an overproducing Escherichia coli strain. RosC is the fastest enzyme of roseoflavin biosynthesis (k_cat 31.3 ± 1.4 min^-1 ). The apparent K_M for the substrate AFP was 34.5 µM. Roseoflavin biosynthesis is now completely understood--it takes three enzymes (RosB, RosC, and RosA) to convert the flavin cofactor riboflavin-5'-phosphate into a potent antibiotic.

The microenvironment surrounding FAD mediates its conversion to 8-formyl-FAD in Aspergillus oryzae RIB40 formate oxidase

Aspergillus oryzae RIB40 formate oxidase has Arg87 and Arg554 near the formyl group and O(4) atom of 8-formyl-flavin adenine dinucleotide (FAD), respectively, with Asp396 neighbouring Arg554. Herein, we probed the roles of these three residues in modification of FAD to 8-formyl-FAD. Replacement of Arg87 or Arg554 with Lys or Ala decreased and abolished the modification, respectively. Replacement of Asp396 with Ala or Asn lowered the modification rate. The observation of unusual effects of maintaining pH 7.0 on the modification in R87K, R554K and D396 variants indicates initial and subsequent processes with different pH dependencies. Comparison of the initial process at pH 4.5 and 7.0 suggests that the microenvironment around Arg87 and the protonation state of Asp396 affect the initial process in the native enzyme. Comparison of the crystal structures of native and R554 variants showed that the replacements had minimal effect on catalytic site structure. The positively charged Arg87 might contribute to the formation of an anionic quinone-methide tautomer intermediate, while the positively charged Arg554, in collaboration with the negatively charged Asp396, might stabilize this intermediate and form a hydrogen bonding network with the N(5)/O(4) region, thereby facilitating efficient FAD modification.

The hidden enzymology of bacterial natural product biosynthesis

Bacterial natural products display astounding structural diversity, which, in turn, endows them with a remarkable range of biological activities that are of significant value to modern society. Such structural features are generated by biosynthetic enzymes that construct core scaffolds or perform peripheral modifications, and can thus define natural product families, introduce pharmacophores and permit metabolic diversification. Modern genomics approaches have greatly enhanced our ability to access and characterize natural product pathways via sequence-similarity-based bioinformatics discovery strategies. However, many biosynthetic enzymes catalyse exceptional, unprecedented transformations that continue to defy functional prediction and remain hidden from us in bacterial (meta)genomic sequence data. In this Review, we highlight exciting examples of unusual enzymology that have been uncovered recently in the context of natural product biosynthesis. These suggest that much of the natural product diversity, including entire substance classes, awaits discovery. New approaches to lift the veil on the cryptic chemistries of the natural product universe are also discussed.

Structure-Guided Analyses of a Key Enzyme Involved in the Biosynthesis of an Antivitamin

RosB catalyzes the formation of 8-aminoriboflavin 5'-phosphate (AFP), the key intermediate in roseoflavin biosynthesis, from the metabolic precursors riboflavin 5'-phosphate (RP, also known as FMN) and glutamate. The conversion of the aromatic methyl group at position 8 in RP into the aromatic amine in AFP occurs via two intermediates, namely, the aldehyde 8-formyl-RP and the acid 8-carboxy-RP. To gain insights into the mechanism for this chemically challenging transformation, we utilized a structure-based approach to identify active site variants of RosB that stall the reaction at various points along the reaction coordinate. Crystal structures of individual variants in complex with different reaction intermediates, identified via mass spectroscopic analysis, illuminate conformational changes that occur at the active site during multistep conversion. These studies provide a plausible route for the progression of the reaction and a molecular rationale for the mechanism of this unusual biocatalyst.

Oxidation of the FAD cofactor to the 8-formyl-derivative in human electron-transferring flavoprotein

The heterodimeric human (h) electron-transferring flavoprotein (ETF) transfers electrons from at least 13 different flavin dehydrogenases to the mitochondrial respiratory chain through a non-covalently bound FAD cofactor. Here, we describe the discovery of an irreversible and pH-dependent oxidation of the 8α-methyl group to 8-formyl-FAD (8f-FAD), which represents a unique chemical modification of a flavin cofactor in the human flavoproteome. Furthermore, a set of hETF variants revealed that several conserved amino acid residues in the FAD-binding pocket of electron-transferring flavoproteins are required for the conversion to the formyl group. Two of the variants generated in our study, namely αR249C and αT266M, cause glutaric aciduria type II, a severe inherited disease. Both of the variants showed impaired formation of 8f-FAD shedding new light on the potential molecular cause of disease development. Interestingly, the conversion of FAD to 8f-FAD yields a very stable flavin semiquinone that exhibited slightly lower rates of electron transfer in an artificial assay system than hETF containing FAD. In contrast, the formation of 8f-FAD enhanced the affinity to human dimethylglycine dehydrogenase 5-fold, indicating that formation of 8f-FAD modulates the interaction of hETF with client enzymes in the mitochondrial matrix. Thus, we hypothesize that the FAD cofactor bound to hETF is subject to oxidation in the alkaline (pH 8) environment of the mitochondrial matrix, which may modulate electron transport between client dehydrogenases and the respiratory chain. This discovery challenges the current concepts of electron transfer processes in mitochondria.

Hot off the press

A personal selection of 32 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as chrysamide A from a deep-sea fungus Penicillium chrysogenum.