Tryptophan synthase: the workings of a channeling nanomachine

Porous protein crystals: synthesis and applications

Large-pore protein crystals (LPCs) are an emerging class of biomaterials. The inherent diversity of proteins translates to a diversity of crystal lattice structures, many of which display large pores and solvent channels. These pores can, in turn, be functionalized via directed evolution and rational redesign based on the known crystal structures. LPCs possess extremely high solvent content, as well as extremely high surface area to volume ratios. Because of these characteristics, LPCs continue to be explored in diverse applications including catalysis, targeted therapeutic delivery, templating of nanostructures, structural biology. This Feature review article will describe several of the existing platforms in detail, with particular focus on LPC synthesis approaches and reported applications.

Neutron diffraction from a microgravity-grown crystal reveals the active site hydrogens of the internal aldimine form of tryptophan synthase

Pyridoxal 5'-phosphate (PLP), the biologically active form of vitamin B6, is an essential cofactor in many biosynthetic pathways. The emergence of PLP-dependent enzymes as drug targets and biocatalysts, such as tryptophan synthase (TS), has underlined the demand to understand PLP-dependent catalysis and reaction specificity. The ability of neutron diffraction to resolve the positions of hydrogen atoms makes it an ideal technique to understand how the electrostatic environment and selective protonation of PLP regulates PLP-dependent activities. Facilitated by microgravity crystallization of TS with the Toledo Crystallization Box, we report the 2.1 Å joint X-ray/neutron (XN) structure of TS with PLP in the internal aldimine form. Positions of hydrogens were directly determined in both the α- and β-active sites, including PLP cofactor. The joint XN structure thus provides insight into the selective protonation of the internal aldimine and the electrostatic environment of TS necessary to understand the overall catalytic mechanism.

Fungi Tryptophan Synthases: What Is the Role of the Linker Connecting the α and β Structural Domains in Hemileia vastatrix TRPS? A Molecular Dynamics Investigation

Tryptophan synthase (TRPS) is a complex enzyme responsible for tryptophan biosynthesis. It occurs in bacteria, plants, and fungi as an αββα heterotetramer. Although encoded by independent genes in bacteria and plants, in fungi, TRPS is generated by a single gene that concurrently expresses the α and β entities, which are linked by an elongated peculiar segment. We conducted 1 µs all-atom molecular dynamics simulations on Hemileia vastatrix TRPS to address two questions: (i) the role of the linker segment and (ii) the comparative mode of action. Since there is not an experimental structure, we started our simulations with homology modeling. Based on the results, it seems that TRPS makes use of an already-existing tunnel that can spontaneously move the indole moiety from the α catalytic pocket to the β one. Such behavior was completely disrupted in the simulation without the linker. In light of these results and the αβ dimer's low stability, the full-working TRPS single genes might be the result of a particular evolution. Considering the significant losses that Hemileia vastatrix causes to coffee plantations, our next course of action will be to use the TRPS to look for substances that can block tryptophan production and therefore control the disease.

Mycobacterium tuberculosis: Pathogenesis and therapeutic targets

Tuberculosis (TB) remains a significant public health concern in the 21st century, especially due to drug resistance, coinfection with diseases like immunodeficiency syndrome (AIDS) and coronavirus disease 2019, and the lengthy and costly treatment protocols. In this review, we summarize the pathogenesis of TB infection, therapeutic targets, and corresponding modulators, including first-line medications, current clinical trial drugs and molecules in preclinical assessment. Understanding the mechanisms of Mycobacterium tuberculosis (Mtb) infection and important biological targets can lead to innovative treatments. While most antitubercular agents target pathogen-related processes, host-directed therapy (HDT) modalities addressing immune defense, survival mechanisms, and immunopathology also hold promise. Mtb's adaptation to the human host involves manipulating host cellular mechanisms, and HDT aims to disrupt this manipulation to enhance treatment effectiveness. Our review provides valuable insights for future anti-TB drug development efforts.

Allostery, engineering and inhibition of tryptophan synthase

The final two steps of tryptophan biosynthesis are catalyzed by the enzyme tryptophan synthase (TS), composed of alpha (αTS) and beta (βTS) subunits. Recently, experimental and computational methods have mapped "allosteric networks" that connect the αTS and βTS active sites. In αTS, allosteric networks change across the catalytic cycle, which might help drive the conformational changes associated with its function. Directed evolution studies to increase catalytic function and expand the substrate profile of stand-alone βTS have also revealed the importance of αTS in modulating the conformational changes in βTS. These studies also serve as a foundation for the development of TS inhibitors, which can find utility against Mycobacterium tuberculosis and other bacterial pathogens.

Millisecond cryo-trapping by the spitrobot crystal plunger simplifies time-resolved crystallography

We introduce the spitrobot, a protein crystal plunger, enabling reaction quenching via cryo-trapping with a time-resolution in the millisecond range. Protein crystals are mounted on canonical micromeshes on an electropneumatic piston, where the crystals are kept in a humidity and temperature-controlled environment, then reactions are initiated via the liquid application method (LAMA) and plunging into liquid nitrogen is initiated after an electronically set delay time to cryo-trap intermediate states. High-magnification images are automatically recorded before and after droplet deposition, prior to plunging. The SPINE-standard sample holder is directly plunged into a storage puck, enabling compatibility with high-throughput infrastructure. Here we demonstrate binding of glucose and 2,3-butanediol in microcrystals of xylose isomerase, and of avibactam and ampicillin in microcrystals of the extended spectrum beta-lactamase CTX-M-14. We also trap reaction intermediates and conformational changes in macroscopic crystals of tryptophan synthase to demonstrate that the spitrobot enables insight into catalytic events.

Engineering Saccharomyces cerevisiae for the de novo Production of Halogenated Tryptophan and Tryptamine Derivatives

The indole scaffold is a recurring structure in multiple bioactive heterocycles and natural products. Substituted indoles like the amino acid tryptophan serve as a precursor for a wide range of natural products with pharmaceutical or agrochemical applications. Inspired by the versatility of these compounds, medicinal chemists have for decades exploited indole as a core structure in the drug discovery process. With the aim of tuning the properties of lead drug candidates, regioselective halogenation of the indole scaffold is a common strategy. However, chemical halogenation is generally expensive, has a poor atom economy, lacks regioselectivity, and generates hazardous waste streams. As an alternative, in this work we engineer the industrial workhorse Saccharomyces cerevisiae for the de novo production of halogenated tryptophan and tryptamine derivatives. Functional expression of bacterial tryptophan halogenases together with a partner flavin reductase and a tryptophan decarboxylase resulted in the production of halogenated tryptophan and tryptamine with chlorine or bromine. Furthermore, by combining tryptophan halogenases, production of di-halogenated molecules was also achieved. Overall, this works paves the road for the production of new-to-nature halogenated natural products in yeast.

Expanding the Toolbox for Functional Genomics in Fonsecaea pedrosoi: The Use of Split-Marker and Biolistic Transformation for Inactivation of Tryptophan Synthase (trpB) Gene

Chromoblastomycosis (CBM) is a disease caused by several dematiaceous fungi from different genera, and Fonsecaea is the most common which has been clinically isolated. Genetic transformation methods have recently been described; however, molecular tools for the functional study of genes have been scarcely reported for those fungi. In this work, we demonstrated that gene deletion and generation of the null mutant by homologous recombination are achievable for Fonsecaea pedrosoi by the use of two approaches: use of double-joint PCR for cassette construction, followed by delivery of the split-marker by biolistic transformation. Through in silico analyses, we identified that F. pedrosoi presents the complete enzymatic apparatus required for tryptophan (trp) biosynthesis. The gene encoding a tryptophan synthase trpB -which converts chorismate to trp-was disrupted. The ΔtrpB auxotrophic mutant can grow with external trp supply, but germination, viability of conidia, and radial growth are defective compared to the wild-type and reconstituted strains. The use of 5-FAA for selection of trp^- phenotypes and for counter-selection of strains carrying the trp gene was also demonstrated. The molecular tools for the functional study of genes, allied to the genetic information from genomic databases, significantly boost our understanding of the biology and pathogenicity of CBM causative agents.

Co-Immobilizing Two Glycosidases Based on Cross-Linked Enzyme Aggregates to Enhance Enzymatic Properties for Achieving High Titer Icaritin Biosynthesis

Icaritin is a rare and high-value isopentane flavonoid compound with remarkable activities. Increasing yields while reducing cost has been a great challenge in icaritin production. Herein, we first reported a high titer icaritin biosynthesis strategy from epimedin C through co-immobilizing α-l-rhamnosidase (Rha1) and β-glucosidase (Glu4) using cross-linked enzyme aggregates (CLEAs). The created CLEAs exhibited excellent performances in terms of catalytic activity, thermal stability, pH stability, and reusability. Notably, Rha1-CLEAs (K_i: 1 M) and Glu4-CLEAs (K_i: 0.1 M) were more tolerant to sugars (glucose or rhamnose) than free enzymes (0.1 M for Rha1 and 0.007 M for Glu4) by immobilization, achieving the highest icaritin productivity under the highest substrate concentration ever reported. Finally, about 34.24 g/L icaritin could be obtained from 100 g/L epimedin C within 8 h, indicating the great potential for industrialization. This study also provides a promising strategy for the low-cost production of other high-value aglycone compounds by solving poor stability and sugar inhibition of glycosidase.

Allosteric regulation of substrate channeling: Salmonella typhimurium tryptophan synthase

The regulation of the synthesis of L-tryptophan (L-Trp) in enteric bacteria begins at the level of gene expression where the cellular concentration of L-Trp tightly controls expression of the five enzymes of the Trp operon responsible for the synthesis of L-Trp. Two of these enzymes, trpA and trpB, form an αββα bienzyme complex, designated as tryptophan synthase (TS). TS carries out the last two enzymatic processes comprising the synthesis of L-Trp. The TS α-subunits catalyze the cleavage of 3-indole D-glyceraldehyde 3′-phosphate to indole and D-glyceraldehyde 3-phosphate; the pyridoxal phosphate-requiring β-subunits catalyze a nine-step reaction sequence to replace the L-Ser hydroxyl by indole giving L-Trp and a water molecule. Within αβ dimeric units of the αββα bienzyme complex, the common intermediate indole is channeled from the α site to the β site via an interconnecting 25 Å-long tunnel. The TS system provides an unusual example of allosteric control wherein the structures of the nine different covalent intermediates along the β-reaction catalytic path and substrate binding to the α-site provide the allosteric triggers for switching the αββα system between the open (T) and closed (R) allosteric states. This triggering provides a linkage that couples the allosteric conformational coordinate to the covalent chemical reaction coordinates at the α- and β-sites. This coupling drives the α- and β-sites between T and R conformations to achieve regulation of substrate binding and/or product release, modulation of the α- and β-site catalytic activities, prevention of indole escape from the confines of the active sites and the interconnecting tunnel, and synchronization of the α- and β-site catalytic activities. Here we review recent advances in the understanding of the relationships between structure, function, and allosteric regulation of the complex found in Salmonella typhimurium.

3D Printing: An Emerging Technology for Biocatalyst Immobilization

Employment of enzymes as biocatalysts offers immense benefits across diverse sectors in the context of green chemistry, biodegradability, and sustainability. When compared to free enzymes in solution, enzyme immobilization proposes an effective means of improving functional efficiency and operational stability. The advance of printable and functional materials utilized in additive manufacturing, coupled with the capability to produce bespoke geometries, has sparked great interest towards the 3D printing of immobilized enzymes. Printable biocatalysts represent a new generation of enzyme immobilization in a more customizable and adaptable manner, unleashing their potential functionalities for countless applications in industrial biotechnology. This review provides an overview of enzyme immobilization techniques and 3D printing technologies, followed by illustrations of the latest 3D printed enzyme-immobilized industrial and clinical applications. The unique advantages of harnessing 3D printing as an enzyme immobilization technique will be presented, alongside a discussion on its potential limitations. Finally, the future perspectives of integrating 3D printing with enzyme immobilization will be considered, highlighting the endless possibilities that are achievable in both research and industry. This article is protected by copyright. All rights reserved.

Microgravity crystallization of perdeuterated tryptophan synthase for neutron diffraction

Biologically active vitamin B6-derivative pyridoxal 5'-phosphate (PLP) is an essential cofactor in amino acid metabolic pathways. PLP-dependent enzymes catalyze a multitude of chemical reactions but, how reaction diversity of PLP-dependent enzymes is achieved is still not well understood. Such comprehension requires atomic-level structural studies of PLP-dependent enzymes. Neutron diffraction affords the ability to directly observe hydrogen positions and therefore assign protonation states to the PLP cofactor and key active site residues. The low fluxes of neutron beamlines require large crystals (≥0.5 mm3). Tryptophan synthase (TS), a Fold Type II PLP-dependent enzyme, crystallizes in unit gravity with inclusions and high mosaicity, resulting in poor diffraction. Microgravity offers the opportunity to grow large, well-ordered crystals by reducing gravity-driven convection currents that impede crystal growth. We developed the Toledo Crystallization Box (TCB), a membrane-barrier capillary-dialysis device, to grow neutron diffraction-quality crystals of perdeuterated TS in microgravity. Here, we present the design of the TCB and its implementation on Center for Advancement of Science in Space (CASIS) supported International Space Station (ISS) Missions Protein Crystal Growth (PCG)-8 and PCG-15. The TCB demonstrated the ability to improve X-ray diffraction and mosaicity on PCG-8. In comparison to ground control crystals of the same size, microgravity-grown crystals from PCG-15 produced higher quality neutron diffraction data. Neutron diffraction data to a resolution of 2.1 Å has been collected using microgravity-grown perdeuterated TS crystals from PCG-15.

Evidence Supporting Substrate Channeling between Domains of Human PAICS: A Time-Course Analysis of 13C-Bicarbonate Incorporation

Human phosphoribosylaminoimidazole carboxylase phosphoribosylaminoimdiazole succinocarboxamide synthetase (PAICS) is a dual activity enzyme catalyzing two consecutive reactions in de novo purine nucleotide synthesis. Crystallographic structures of recombinant human PAICS suggested the channeling of 4-carboxy-5-aminoimidazole-1-ribose-5'-phosphate (CAIR) between two active sites of PAICS, while a prior work of an avian PAICS suggested otherwise. Here, we present time-course mass spectrometric data supporting the channeling of CAIR between domains of recombinant human PAICS. Time-course mass spectral analysis showed that CAIR added to the bulk solution (CAIR^bulk) is decarboxylated and re-carboxylated before the accumulation of succinyl-5-aminoimidazole-4-carboxamide-1-ribose-5'-phosphate (SAICAR). An experiment with ¹³C-bicarbonate showed that SAICAR production was proportional to re-carboxylated CAIR instead of total CAIR or CAIR^bulk. This result indicates that the SAICAR synthase domain selectively uses enzyme-made CAIR over CAIR^bulk, which is consistent with the channeling model. This channeling between PAICS domains may be a part of a larger channeling process in de novo purine nucleotide synthesis.

Discovery of antimicrobial agent targeting tryptophan synthase

Antibiotic resistance is a continually growing challenge in the treatment of various bacterial infections worldwide. New drugs and new drug targets are necessary to curb the threat of infectious diseases caused by multidrug-resistant pathogens. The tryptophan biosynthesis pathway is essential for bacterial growth but is absent in higher animals and humans. Drugs that can inhibit the bacterial biosynthesis of tryptophan offer a new class of antibiotics. In this work, we combined a structure-based strategy using in silico docking screening and molecular dynamics (MD) simulations to identify compounds targeting the α subunit of tryptophan synthase with experimental methods involving the whole-cell minimum inhibitory concentration (MIC) test, solution state NMR, and crystallography to confirm the inhibition of L-tryptophan biosynthesis. Screening 1,800 compounds from the National Cancer Institute Diversity Set I against α subunit revealed 28 compounds for experimental validation; four of the 28 hit compounds showed promising activity in MIC testing. We performed solution state NMR experiments to demonstrate that a one successful inhibitor, 3-amino-3-imino-2-phenyldiazenylpropanamide (Compound 1) binds to the α subunit. We also report a crystal structure of Salmonella enterica serotype Typhimurium tryptophan synthase in complex with Compound 1 which revealed a binding site at the αβ interface of the dimeric enzyme. MD simulations were carried out to examine two binding sites for the compound. Our results show that this small molecule inhibitor could be a promising lead for future drug development.

Imaging active site chemistry and protonation states: NMR crystallography of the tryptophan synthase α-aminoacrylate intermediate

NMR-assisted crystallography-the integrated application of solid-state NMR, X-ray crystallography, and first-principles computational chemistry-holds significant promise for mechanistic enzymology: by providing atomic-resolution characterization of stable intermediates in enzyme active sites, including hydrogen atom locations and tautomeric equilibria, NMR crystallography offers insight into both structure and chemical dynamics. Here, this integrated approach is used to characterize the tryptophan synthase α-aminoacrylate intermediate, a defining species for pyridoxal-5'-phosphate-dependent enzymes that catalyze β-elimination and replacement reactions. For this intermediate, NMR-assisted crystallography is able to identify the protonation states of the ionizable sites on the cofactor, substrate, and catalytic side chains as well as the location and orientation of crystallographic waters within the active site. Most notable is the water molecule immediately adjacent to the substrate β-carbon, which serves as a hydrogen bond donor to the ε-amino group of the acid-base catalytic residue βLys87. From this analysis, a detailed three-dimensional picture of structure and reactivity emerges, highlighting the fate of the L-serine hydroxyl leaving group and the reaction pathway back to the preceding transition state. Reaction of the α-aminoacrylate intermediate with benzimidazole, an isostere of the natural substrate indole, shows benzimidazole bound in the active site and poised for, but unable to initiate, the subsequent bond formation step. When modeled into the benzimidazole position, indole is positioned with C3 in contact with the α-aminoacrylate Cβ and aligned for nucleophilic attack. Here, the chemically detailed, three-dimensional structure from NMR-assisted crystallography is key to understanding why benzimidazole does not react, while indole does.

Structural Characterization of Multienzyme Assemblies: An Overview

Multienzyme assemblies have attracted significant attention in recent years for use in industrial applications instead of single enzymes. Owing to their ability to catalyze cascade reactions, multienzyme assemblies have become inspirational tools for the in vitro construction of multienzyme molecular machines. The use of such molecular machines could offer several advantages such as fewer side reactions, a high product yield, a fast reaction speed, easy product separation, a tolerable toxic environment, and robust system operability compared to current microbial cell catalytic systems. Besides, they can provide all the benefits found in the use of enzymes, including reusability, catalytic efficiency, and specificity. Similar to single enzymes, multienzyme assemblies could offer economical and environmentally friendly alternatives to conventional catalysts and play a central role as biocatalysts in green chemistry applications. However, detailed characterization of multienzyme assemblies and a full understanding of their mechanistic details are required for their efficient use in industrial biotransformations. Since the determination of the first enzyme structure in 1965, structural information has played a pivotal role in the characterization of enzymes and elucidation of their structure-function relationship. Among the structural biology techniques, X-ray crystallography has provided key mechanistic details into multienzyme assemblies. Here, the structural characterization of multienzyme assemblies is reviewed and several examples are provided.

Probing the function of a ligand-modulated dynamic tunnel in bifunctional proline utilization A (PutA)

In many bacteria, the reactions of proline catabolism are catalyzed by the bifunctional enzyme known as proline utilization A (PutA). PutA catalyzes the two-step oxidation of l-proline to l-glutamate using distinct proline dehydrogenase (PRODH) and l-glutamate-γ-semialdehyde dehydrogenase (GSALDH) active sites, which are separated by over 40 Å and connected by a complex tunnel system. The tunnel system consists of a main tunnel that connects the two active sites and functions in substrate channeling, plus six ancillary tunnels whose functions are unknown. Here we used tunnel-blocking mutagenesis to probe the role of a dynamic ancillary tunnel (tunnel 2a) whose shape is modulated by ligand binding to the PRODH active site. The 1.90 Å resolution crystal structure of Geobacter sulfurreducens PutA variant A206W verified that the side chain of Trp206 cleanly blocks tunnel 2a without perturbing the surrounding structure. Steady-state kinetic measurements indicate the mutation impaired PRODH activity without affecting the GSALDH activity. Single-turnover experiments corroborated a severe impairment of PRODH activity with flavin reduction decreased by nearly 600-fold in A206W relative to wild-type. Substrate channeling is also significantly impacted as A206W exhibited a 3000-fold lower catalytic efficiency in coupled PRODH-GSALDH activity assays, which measure NADH formation as a function of proline. The structure suggests that Trp206 inhibits binding of the substrate l-proline by preventing the formation of a conserved glutamate-arginine ion pair and closure of the PRODH active site. Our data are consistent with tunnel 2a serving as an open space through which the glutamate of the ion pair travels during the opening and closing of the active site in response to binding l-proline. These results confirm the essentiality of the conserved ion pair in binding l-proline and support the hypothesis that the ion pair functions as a gate that controls access to the PRODH active site.

Mutation of βGln114 to Ala Alters the Stabilities of Allosteric States in Tryptophan Synthase Catalysis

The tryptophan synthase (TS) bienzyme complexes found in bacteria, yeasts, and molds are pyridoxal 5'-phosphate (PLP)-requiring enzymes that synthesize l-Trp. In the TS catalytic cycle, switching between the open and closed states of the α- and β-subunits via allosteric interactions is key to the efficient conversion of 3-indole-d-glycerol-3'-phosphate and l-Ser to l-Trp. In this process, the roles played by β-site residues proximal to the PLP cofactor have not yet been fully established. βGln114 is one such residue. To explore the roles played by βQ114, we conducted a detailed investigation of the βQ114A mutation on the structure and function of tryptophan synthase. Initial steady-state kinetic and static ultraviolet-visible spectroscopic analyses showed the Q to A mutation impairs catalytic activity and alters the stabilities of intermediates in the β-reaction. Therefore, we conducted X-ray structural and solid-state nuclear magnetic resonance spectroscopic studies to compare the wild-type and βQ114A mutant enzymes. These comparisons establish that the protein structural changes are limited to the Gln to Ala replacement, the loss of hydrogen bonds among the side chains of βGln114, βAsn145, and βArg148, and the inclusion of waters in the cavity created by substitution of the smaller Ala side chain. Because the conformations of the open and closed allosteric states are not changed by the mutation, we hypothesize that the altered properties arise from the lost hydrogen bonds that alter the relative stabilities of the open (βT state) and closed (βR state) conformations of the β-subunit and consequently alter the distribution of intermediates along the β-subunit catalytic path.

Engineered and Artificial Metalloenzymes for Selective C-H Functionalization

The direct functionalization of C-H bonds constitutes a powerful strategy to construct and diversify organic molecules. However, controlling the chemo- and site-selectivity of this transformation in particularly complex molecular settings represents a significant challenge. Metalloenzymes are ideal platforms for achieving catalyst-controlled selective C-H bond functionalization as their reactivities can be tuned by protein engineering and/or redesign of their cofactor environment. In this review, we highlight recent progress in the development of engineered and artificial metalloenzymes for C-H functionalization, with a focus on biocatalytic strategies for selective C-H oxyfunctionalization and halogenation as well as C-H amination and C-H carbene insertion via abiological nitrene and carbene transfer chemistries. Engineered heme- and non-heme iron dependent enzymes have emerged as promising scaffolds for executing these transformations with high chemo-, regio- and stereocontrol as well as tunable selectivity. These emerging systems and methodologies have expanded the toolbox of sustainable strategies for organic synthesis and created new opportunities for the generation of chiral building blocks, the late-stage C-H functionalization of complex molecules, and the total synthesis of natural products.

Construction of Multienzyme Co-immobilized Hybrid Nanoflowers for an Efficient Conversion of Cellulose into Glucose in a Cascade Reaction

Today, we are seeking an efficient biotransformation of cellulosic material into sustainable biochemical products to meet the increasing global energy demand. Herein, we report the fabrication of multienzyme hybrid nanoflowers (ECG-NFs) by co-immobilizing three recombinant enzymes (cellobiohydrolase (CBH), endo-glucanase (EG), and β-glucosidase (BG)) integrating a binary tag composed of elastin-like polypeptide (ELP) and His-tag to act as a tri-enzyme biocatalyst, which catalyzes the hydrolysis of cellulose into glucose. The prepared ECG-NFs exhibited excellent performance in terms of pH stability, thermal stability, storage stability, and catalytic efficiency compared to free multienzyme system. Notably, ECG-NFs could be recycled for up to eight consecutive runs. The K_m and k_cat/K_m values for ECG-NFs were 9.33 g L^-1 and 0.0051 L min^-1 g^-1, respectively, which were better than those of the free multienzyme system, indicating a better substrate affinity. Finally, the overall enzyme activity of ECG-NFs increased by 1.12 times and the degradation efficiency of ECG-NFs was superior to the free multienzyme system, which revealed that ECG-NFs could facilitate an effective one-pot hydrolysis of cellulose into glucose.

Construction and characterization of novel bifunctional fusion proteins composed of alcohol dehydrogenase and NADH oxidase with efficient oxidized cofactor regeneration

To tune the efficiency of oxidized cofactor recycling between alcohol dehydrogenase (ADH) and NADH oxidase (NOX) for the production of aromatic chiral alcohols, we designed and constructed four novel bifunctional fusion proteins composed of thermostable ADH and NOX from Thermococcus kodakarensis KOD1. ADH was linked to the N- or C-terminus of NOX with a typical rigid linker (EAAAK)₃ and a flexible linker (GGGGS)₃ , respectively. Compared with the parental enzymes, the NOX moieties in the four fusion proteins exhibited higher specific activities (141%-282%), while the ADH moieties exhibited varying levels of specific activity (69%-167%). All fusion proteins showed decreased affinities toward the cofactors, with increased K_m values toward NADH (159%-406%) and NAD⁺ (202%-372%). In the enantioselective oxidation of (RS)-1-phenylethanol coupled with cofactor regeneration, the four fusion proteins displayed different positive and negative effects on the recycling efficiency of the oxidized cofactor. The two fusion proteins composed of NOX at the N-terminus exhibited higher total turnover numbers than the corresponding mixtures of individual enzymes with equal activities, particularly at low cofactor concentrations. These findings suggest high cofactor recycling efficiencies of the fusion proteins with appropriate design and their potential application in the biosynthesis of chiral alcohols.

Cascaded Biocatalysis and Bioelectrocatalysis: Overview and Recent Advances

Enzyme cascades are plentiful in nature, but they also have potential in artificial applications due to the possibility of using the target substrate in biofuel cells, electrosynthesis, and biosensors. Cascade reactions from enzymes or hybrid bioorganic catalyst systems exhibit extended substrate range, reaction depth, and increased overall performance. This review addresses the strategies of cascade biocatalysis and bioelectrocatalysis for ( a) CO2 fixation, ( b) high value-added product formation, ( c) sustainable energy sources via deep oxidation, and ( d) cascaded electrochemical enzymatic biosensors. These recent updates in the field provide fundamental concepts, designs of artificial electrocatalytic oxidation-reduction pathways (using a flexible setup involving organic catalysts and engineered enzymes), and advances in hybrid cascaded sensors for sensitive analyte detection.

Multi 'Omics Analysis of Intestinal Tissue in Ankylosing Spondylitis Identifies Alterations in the Tryptophan Metabolism Pathway

Intestinal microbial dysbiosis, intestinal inflammation, and Th17 immunity are all linked to the pathophysiology of spondyloarthritis (SpA); however, the mechanisms linking them remain unknown. One potential hypothesis suggests that the dysbiotic gut microbiome as a whole produces metabolites that influence human immune cells. To identify potential disease-relevant, microbiome-produced metabolites, we performed metabolomics screening and shotgun metagenomics on paired colon biopsies and fecal samples, respectively, from subjects with axial SpA (axSpA, N=21), Crohn's disease (CD, N=27), and Crohn's-axSpA overlap (CD-axSpA, N=12), as well as controls (HC, N=24). Using LC-MS based metabolomics of 4 non-inflamed pinch biopsies of the distal colon from subjects, we identified significant alterations in tryptophan pathway metabolites, including an expansion of indole-3-acetate (IAA) in axSpA and CD-axSpA compared to HC and CD and indole-3-acetaldehyde (I3Ald) in axSpA and CD-axSpA but not CD compared to HC, suggesting possible specificity to the development of axSpA. We then performed shotgun metagenomics of fecal samples to characterize gut microbial dysbiosis across these disease states. In spite of no significant differences in alpha-diversity among the 4 groups, our results confirmed differences in gene abundances of numerous enzymes involved in tryptophan metabolism. Specifically, gene abundance of indolepyruvate decarboxylase, which generates IAA and I3Ald, was significantly elevated in individuals with axSpA while gene abundances in HC demonstrated a propensity towards tryptophan synthesis. Such genetic changes were not observed in CD, again suggesting disease specificity for axSpA. Given the emerging role of tryptophan and its metabolites in immune function, altogether these data indicate that tryptophan metabolism into I3Ald and then IAA is one mechanism by which the gut microbiome potentially influences the development of axSpA.

Research progress and the biotechnological applications of multienzyme complex

The multienzyme complex system has become a research focus in synthetic biology due to its highly efficient overall catalytic ability and has been applied to various fields. Multienzyme complexes are formed by cascading complexes, which are multiple functionally related enzymes that continuously and efficiently catalyze the production of substrates. Compared with current mainstream microbial cell catalytic systems, in vitro multienzyme molecular machines have many advantages, such as fewer side reactions, a high product yield, a fast reaction speed, easy product separation, a tolerable toxic environment, and robust system operability, showing increasing competitiveness in the field of biomanufacturing. In this review, the research progress of multienzyme complexes in nature and multienzyme cascades in vivo or in vitro will be introduced, and the discovered enzyme cascades concerning scaffolding proteins will also be discussed. This review is expected to provide a more theoretical basis for the modification of multienzyme complexes and broaden their application in the field of synthetic biology. KEY POINTS: • The cascade reactions of some natural multienzyme complexes are reviewed. • The main approaches of constructing artificial multienzyme complexes are summarized. • The structure and application of cellulosomes are discussed and prospected.

Towards Photochromic Azobenzene-Based Inhibitors for Tryptophan Synthase

Light regulation of drug molecules has gained growing interest in biochemical and pharmacological research in recent years. In addition, a serious need for novel molecular targets of antibiotics has emerged presently. Herein, the development of a photocontrollable, azobenzene-based antibiotic precursor towards tryptophan synthase (TS), an essential metabolic multienzyme complex in bacteria, is presented. The compound exhibited moderately strong inhibition of TS in its E configuration and five times lower inhibition strength in its Z configuration. A combination of biochemical, crystallographic, and computational analyses was used to characterize the inhibition mode of this compound. Remarkably, binding of the inhibitor to a hitherto-unconsidered cavity results in an unproductive conformation of TS leading to noncompetitive inhibition of tryptophan production. In conclusion, we created a promising lead compound for combatting bacterial diseases, which targets an essential metabolic enzyme, and whose inhibition strength can be controlled with light.

Structural Basis of the Stereochemistry of Inhibition of Tryptophan Synthase by Tryptophan and Derivatives

We have examined the reaction of Salmonella enterica serovar typhimurium tryptophan (Trp) synthase α₂β₂ complex with l-Trp, d-Trp, oxindolyl-l-alanine (OIA), and dioxindolyl-l-alanine (DOA) in the presence of disodium (dl)-α-glycerol phosphate (GP), using stopped-flow spectrophotometry and X-ray crystallography. All structures contained the d-isomer of GP bound at the α-active site. (3S)-OIA reacts with the pyridoxal-5'-phosphate (PLP) of Trp synthase to form a mixture of external aldimine and quinonoid complexes. The α-carboxylate of OIA rotates about 90° to become planar with the PLP when the quinonoid complex is formed, resulting in a conformational change in the loop of residues 110-115. The COMM domain of the Trp synthase-OIA complex is found as a mixture of two conformations. The (3R)-diastereomer of DOA binds about 5-fold more tightly than (3S)-OIA and also forms a mixture of aldimine and quinonoid complexes. DOA forms an additional H-bond between the 3-OH of DOA and βLys-87. l-Trp does not form a covalent complex with the PLP of Trp synthase. However, d-Trp forms a mixture of two external aldimine complexes which differ in the orientation of the α-carboxylate. In one conformation, the α-carboxylate is in the plane of the PLP, while in the other conformation, the α-carboxylate is perpendicular to the PLP plane. These results confirm that the stereochemistry of the transient indolenine quinonoid intermediate in the mechanism of Trp synthase is (3S) and demonstrate the linkage between aldimine and quinonoid reaction intermediates in the β-active site and allosteric communications with the α-active site.

Construction and characterization of a novel glucose dehydrogenase-leucine dehydrogenase fusion enzyme for the biosynthesis of L-tert-leucine

Background

Biosynthesis of l-tert-leucine (l-tle), a significant pharmaceutical intermediate, by a cofactor regeneration system friendly and efficiently is a worthful goal all the time. The cofactor regeneration system of leucine dehydrogenase (LeuDH) and glucose dehydrogenase (GDH) has showed great coupling catalytic efficiency in the synthesis of l-tle, however the multi-enzyme complex of GDH and LeuDH has never been constructed successfully.

Results

In this work, a novel fusion enzyme (GDH–R3–LeuDH) for the efficient biosynthesis of l-tle was constructed by the fusion of LeuDH and GDH mediated with a rigid peptide linker. Compared with the free enzymes, both the environmental tolerance and thermal stability of GDH–R3–LeuDH had a great improved since the fusion structure. The fusion structure also accelerated the cofactor regeneration rate and maintained the enzyme activity, so the productivity and yield of l-tle by GDH–R3–LeuDH was all enhanced by twofold. Finally, the space–time yield of l-tle catalyzing by GDH–R3–LeuDH whole cells could achieve 2136 g/L/day in a 200 mL scale system under the optimal catalysis conditions (pH 9.0, 30 °C, 0.4 mM of NAD⁺ and 500 mM of a substrate including trimethylpyruvic acid and glucose).

Conclusions

It is the first report about the fusion of GDH and LeuDH as the multi-enzyme complex to synthesize l-tle and reach the highest space–time yield up to now. These results demonstrated the great potential of the GDH–R3–LeuDH fusion enzyme for the efficient biosynthesis of l-tle.

Short-term effects of the allelochemical umbelliferone on Triticum durum L. metabolism through GC-MS based untargeted metabolomics

The present study used untargeted metabolomics to investigate the short-term metabolic changes induced in wheat seedlings by the specialized metabolite umbelliferone, an allelochemical. We used 10 day-old wheat seedlings treated with 104 μM umbelliferone over a time course experiment covering 6 time points (0 h, 6 h, 12 h, 24 h, 48 h, and 96 h), and compared the metabolomic changes to control (mock-treated) plants. Using gas chromatography mass spectrometry (GCMS)-based metabolomics, we obtained quantitative data on 177 metabolites that were derivatized (either derivatized singly or multiple times) or not, representing 139 non-redundant (unique) metabolites. Of these 139 metabolites, 118 were associated with a unique Human Metabolome Database (HMDB) identifier, while 113 were associated with a Kyoto Encyclopedia of Genes and Genomes (KEGG) identifier. Relative quantification of these metabolites across the time-course of umbelliferone treatment revealed 22 compounds (sugars, fatty acids, secondary metabolites, organic acids, and amino acids) that changed significantly (repeated measures ANOVA, P-value < 0.05) over time. Using multivariate partial least squares discriminant analysis (PLS-DA), we showed the grouping of samples based on time-course across the control and umbelliferone-treated plants, whereas the metabolite-metabolite Pearson correlations revealed tightly formed clusters of umbelliferone-derived metabolites, fatty acids, amino acids, and carbohydrates. Also, the time-course umbelliferone treatment revealed that phospho-l-serine, maltose, and dehydroquinic acid were the top three metabolites showing highest importance in discrimination among the time-points. Overall, the biochemical changes converge towards a mechanistic explanation of the plant metabolic responses induced by umbelliferone. In particular, the perturbation of metabolites involved in tryptophan metabolism, as well as the imbalance of the shikimate pathways, which are strictly interconnected, were significantly altered by the treatment, suggesting a possible mechanism of action of this natural compound.

Pyridoxal 5'-Phosphate-Dependent Enzymes at the Crossroads of Host-Microbe Tryptophan Metabolism

The chemical processes taking place in humans intersects the myriad of metabolic pathways occurring in commensal microorganisms that colonize the body to generate a complex biochemical network that regulates multiple aspects of human life. The role of tryptophan (Trp) metabolism at the intersection between the host and microbes is increasingly being recognized, and multiple pathways of Trp utilization in either direction have been identified with the production of a wide range of bioactive products. It comes that a dysregulation of Trp metabolism in either the host or the microbes may unbalance the production of metabolites with potential pathological consequences. The ability to redirect the Trp flux to restore a homeostatic production of Trp metabolites may represent a valid therapeutic strategy for a variety of pathological conditions, but identifying metabolic checkpoints that could be exploited to manipulate the Trp metabolic network is still an unmet need. In this review, we put forward the hypothesis that pyridoxal 5′-phosphate (PLP)-dependent enzymes, which regulate multiple pathways of Trp metabolism in both the host and in microbes, might represent critical nodes and that modulating the levels of vitamin B6, from which PLP is derived, might represent a metabolic checkpoint to re-orienteer Trp flux for therapeutic purposes.

Backbone assignments and conformational dynamics in the S. typhimurium tryptophan synthase α-subunit from solution-state NMR

Backbone assignments for the isolated α-subunit of Salmonella typhimurium tryptophan synthase (TS) are reported based on triple resonance solution-state NMR experiments on a uniformly 2H,13C,15N-labeled sample. From the backbone chemical shifts, secondary structure and random coil index order parameters (RCI-S2) are predicted. Titration with the 3-indole-D-glycerol 3'-phosphate analog, N-(4'-trifluoromethoxybenzenesulfonyl)-2-aminoethyl phosphate (F9), leads to chemical shift perturbations indicative of conformational changes from which an estimate of the dissociation constant is obtained. Comparisons of the backbone chemical-shifts, RCI-S2 values, and site-specific relaxation times with and without F9 reveal allosteric changes including modulation in secondary structures and loop rigidity induced upon ligand binding. A comparison is made to the X-ray crystal structure of the α-subunit in the full TS αββα bi-enzyme complex and to two new X-ray crystal structures of the isolated TS α-subunit reported in this work.

Five enzymes of the Arg/N-degron pathway form a targeting complex: The concept of superchanneling

The Arg/N-degron pathway targets proteins for degradation by recognizing their N-terminal (Nt) residues. If a substrate bears, for example, Nt-Asn, its targeting involves deamidation of Nt-Asn, arginylation of resulting Nt-Asp, binding of resulting (conjugated) Nt-Arg to the UBR1-RAD6 E3-E2 ubiquitin ligase, ligase-mediated synthesis of a substrate-linked polyubiquitin chain, its capture by the proteasome, and substrate's degradation. We discovered that the human Nt-Asn-specific Nt-amidase NTAN1, Nt-Gln-specific Nt-amidase NTAQ1, arginyltransferase ATE1, and the ubiquitin ligase UBR1-UBE2A/B (or UBR2-UBE2A/B) form a complex in which NTAN1 Nt-amidase binds to NTAQ1, ATE1, and UBR1/UBR2. In addition, NTAQ1 Nt-amidase and ATE1 arginyltransferase also bind to UBR1/UBR2. In the yeast Saccharomyces cerevisiae, the Nt-amidase, arginyltransferase, and the double-E3 ubiquitin ligase UBR1-RAD6/UFD4-UBC4/5 are shown to form an analogous targeting complex. These complexes may enable substrate channeling, in which a substrate bearing, for example, Nt-Asn, would be captured by a complex-bound Nt-amidase, followed by sequential Nt modifications of the substrate and its polyubiquitylation at an internal Lys residue without substrate's dissociation into the bulk solution. At least in yeast, the UBR1/UFD4 ubiquitin ligase interacts with the 26S proteasome, suggesting an even larger Arg/N-degron-targeting complex that contains the proteasome as well. In addition, specific features of protein-sized Arg/N-degron substrates, including their partly sequential and partly nonsequential enzymatic modifications, led us to a verifiable concept termed "superchanneling." In superchanneling, the synthesis of a substrate-linked poly-Ub chain can occur not only after a substrate's sequential Nt modifications, but also before them, through a skipping of either some or all of these modifications within a targeting complex.

Construction of a Multienzymatic Cascade Reaction System of Coimmobilized Hybrid Nanoflowers for Efficient Conversion of Starch into Gluconic Acid

Introducing an efficient method for the rapid conversion of starch into gluconic acid is desirable to solve the current problems existing in traditional gluconic acid preparation processes. In this study, a robust and easy-to-use multienzymatic cascade reaction system of coimmobilized GA@GOx hybrid nanoflowers with a specific spatial distribution of enzymes by compartmentalization was constructed and applied to catalyze starch to gluconic acid in one pot. In the preparation processes, the glucose oxidase (GOx) hybrid nanoflowers were first synthesized via a self-assembly mechanism, and then, glucoamylase (GA) was adsorbed on the surface of GOx hybrid nanoflowers through the interaction of Cu²⁺ and amino acids of GA. The optimum preparation conditions and reaction parameters of the GA@GOx hybrid nanoflowers had been investigated. In addition, the morphology, composition, and crystallization of the GA@GOx hybrid nanoflowers had been fully studied. Based on the lower K_m, the GA@GOx hybrid nanoflowers with compartmentalization had a better effect of the substrate channeling on the catalytic efficiency. The final results indicated that the overall enzyme activity of the GA@GOx hybrid nanoflowers increased by 1.5 times, and the conversion efficiency was 92.12% within 80 min significantly superior to the free multienzyme system, which showed the outstanding conversion of starch into gluconic acid in one pot.

Allosteric inhibitors of Mycobacterium tuberculosis tryptophan synthase

Global dispersion of multidrug resistant bacteria is very common and evolution of antibiotic-resistance is occurring at an alarming rate, presenting a formidable challenge for humanity. The development of new therapeuthics with novel molecular targets is urgently needed. Current drugs primarily affect protein, nucleic acid, and cell wall synthesis. Metabolic pathways, including those involved in amino acid biosynthesis, have recently sparked interest in the drug discovery community as potential reservoirs of such novel targets. Tryptophan biosynthesis, utilized by bacteria but absent in humans, represents one of the currently studied processes with a therapeutic focus. It has been shown that tryptophan synthase (TrpAB) is required for survival of Mycobacterium tuberculosis in macrophages and for evading host defense, and therefore is a promising drug target. Here we present crystal structures of TrpAB with two allosteric inhibitors of M. tuberculosis tryptophan synthase that belong to sulfolane and indole-5-sulfonamide chemical scaffolds. We compare our results with previously reported structural and biochemical studies of another, azetidine-containing M. tuberculosis tryptophan synthase inhibitor. This work shows how structurally distinct ligands can occupy the same allosteric site and make specific interactions. It also highlights the potential benefit of targeting more variable allosteric sites of important metabolic enzymes.

Detecting Protein-Protein Interaction Based on Protein Fragment Complementation Assay

Proteins are the most critical executive molecules by responding to the instructions stored in the genetic materials in any form of life. More frequently, proteins do their jobs by acting as a roleplayer that interacts with other protein(s), which is more evident when the function of a protein is examined in the real context of a cell. Identifying the interactions between (or amongst) proteins is very crucial for the biochemistry investigation of an individual protein and for the attempts aiming to draw a holo-picture for the interacting members at the scale of proteomics (or protein-protein interactions mapping). Here, we introduced the currently available reporting systems that can be used to probe the interaction between candidate protein pairs based on the fragment complementation of some particular proteins. Emphasis was put on the principles and details of experimental design. These systems are dihydrofolate reductase (DHFR), β-lactamase, tobacco etch virus (TEV) protease, luciferase, β- galactosidase, GAL4, horseradish peroxidase (HRP), focal adhesion kinase (FAK), green fluorescent protein (GFP), and ubiquitin.

3D domain swapping in the TIM barrel of the α subunit of Streptococcus pneumoniae tryptophan synthase

Tryptophan synthase catalyzes the last two steps of tryptophan biosynthesis in plants, fungi and bacteria. It consists of two protein chains, designated α and β, encoded by trpA and trpB genes, that function as an αββα complex. Structural and functional features of tryptophan synthase have been extensively studied, explaining the roles of individual residues in the two active sites in catalysis and allosteric regulation. TrpA serves as a model for protein-folding studies. In 1969, Jackson and Yanofsky observed that the typically monomeric TrpA forms a small population of dimers. Dimerization was postulated to take place through an exchange of structural elements of the monomeric chains, a phenomenon later termed 3D domain swapping. The structural details of the TrpA dimer have remained unknown. Here, the crystal structure of the Streptococcus pneumoniae TrpA homodimer is reported, demonstrating 3D domain swapping in a TIM-barrel fold for the first time. The N-terminal domain comprising the H0-S1-H1-S2 elements is exchanged, while the hinge region corresponds to loop L2 linking strand S2 to helix H2'. The structural elements S2 and L2 carry the catalytic residues Glu52 and Asp63. As the S2 element is part of the swapped domain, the architecture of the catalytic apparatus in the dimer is recreated from two protein chains. The homodimer interface overlaps with the α-β interface of the tryptophan synthase αββα heterotetramer, suggesting that the 3D domain-swapped dimer cannot form a complex with the β subunit. In the crystal, the dimers assemble into a decamer comprising two pentameric rings.

Comparative Genomic Analysis of Soil Dwelling Bacteria Utilizing a Combinational Codon Usage and Molecular Phylogenetic Approach Accentuating on Key Housekeeping Genes

Soil is a diversified and complex ecological niche, home to a myriad of microorganisms particularly bacteria. The physico-chemical complexities of soil results in a plethora of physiological variations to exist within the different types of soil dwelling bacteria, giving rise to a wide variation in genome structure and complexity. This serves as an attractive proposition to analyze and compare the genome of a large number soil bacteria to comprehend their genome complexity and evolution. In this study a combination of codon usage and molecular phylogenetics of the whole genome and key housekeeping genes like infB (translation initiation factor 2), trpB (tryptophan synthase, beta subunit), atpD (ATP synthase, beta subunit), and rpoB (RNA polymerase, beta subunit) of 92 soil bacterial species spread across the entire eubacterial domain and residing in different soil types was performed. The results indicated the direct relationship of genome size with codon bias and coding frequency in the studied bacteria. The codon usage profile demonstrated by the gene trpB was found to be relatively different from the rest of the housekeeping genes with a large number of bacteria having a greater percentage of genes with Nc values less than the Nc of trpB. The results from the overall codon usage bias profile also depicted that the codon usage bias in the key housekeeping genes of soil bacteria was majorly due to selectional pressure and not mutation. The analysis of hydrophobicity of the gene product encoded by the rpoB coding sequences demonstrated tight clustering across all the soil bacteria suggesting conservation of protein structure for maintenance of form and function. The phylogenetic affinities inferred using 16S rRNA gene and the housekeeping genes demonstrated conflicting signals with trpB gene being the noisiest one. The housekeeping gene atpD was found to depict the least amount of evolutionary change in the soil bacteria considered in this study except in two Clostridium species. The phylogenetic and codon usage analysis of the soil bacteria consistently demonstrated the relatedness of Azotobacter chroococcum with different species of the genus Pseudomonas.

Synthetic Protein Scaffolding at Biological Membranes

Protein scaffolding is a natural phenomenon whereby proteins colocalize into macromolecular complexes via specific protein-protein interactions. In the case of metabolic enzymes, protein scaffolding drives metabolic flux through specific pathways by colocalizing enzyme active sites. Synthetic protein scaffolding is increasingly used as a mechanism to improve product specificity and yields in metabolic engineering projects. To date, synthetic scaffolding has focused primarily on soluble enzyme systems, but many metabolic pathways for high-value secondary metabolites depend on membrane-bound enzymes. The compositional diversity of biological membranes and general challenges associated with modifying membrane proteins complicate scaffolding with membrane-requiring enzymes. Several recent studies have introduced new approaches to protein scaffolding at membrane surfaces, with notable success in improving product yields from specific metabolic pathways.

Metagenomic analysis of relationships between the denitrification process and carbon metabolism in a bioaugmented full-scale tannery wastewater treatment plant

The goal of this study was to investigate the relationship between the denitrification process and carbon metabolism in a full-scale tannery wastewater treatment plant bioaugmented with the microbial consortium BM-S-1. The metagenomic analysis of the microbial community showed that Brachymonas denitrificans, a known denitrifier, was present at a high level in the treatment stages of buffering (B), primary aeration (PA), and sludge digestion (SD). The occurrences of the amino acid-degrading enzymes alpha ketoglutarate dehydrogenase (α-KGDH) and tryptophan synthase were highly correlated with the presence of denitrification genes, such as napA, narG, nosZ and norB. The occurrence of glutamate dehydrogenase (GDH) was also highly paralleled with the occurrence of denitrification genes such as napA, narG, and norZ. The denitrification genes (nosZ, narG, napA, norB and nrfA) and amino acid degradation enzymes (tryptophan synthase, α-KGDH and pyridoxal phosphate dependent enzymes) were observed at high levels in B. This indicates that degradation of amino acids and denitrification of nitrate may potentially occur in B. The high concentrations of the fatty acid degradation enzyme groups (enoyl-CoA hydratase, 3-hydroxyacyl-CoA dehydrogenase and β-ketothiolase) were observed together with the denitrification genes, such as napA, narG and nosZ. Phospholipase/carboxylesterase, enoyl-CoA hydratase/isomerase, acyl-CoA dehydrogenase, phenylacetate degradation enzyme and 3-hydroxyacyl-CoA dehydrogenase 2 were also dominant in B. All these results clearly indicate that the denitrification pathways are potentially linked to the degradation pathways of amino acids and fatty acids whose degradation products go through the TCA cycle, generating the NADH that is used as electron donors for denitrification.

Nitroalkanes as Versatile Nucleophiles for Enzymatic Synthesis of Noncanonical Amino Acids

C-C bond-forming reactions often require nucleophilic carbon species rarely compatible with aqueous reaction media, thus restricting their appearance in biocatalysis. Here we report the use of nitroalkanes as a structurally versatile class of nucleophilic substrates for C-C bond formation catalyzed by variants of the β-subunit of tryptophan synthase (TrpB). The enzymes accept a wide range of nitroalkanes to form noncanonical amino acids, here the nitro group can serve as a handle for further modification. Using nitroalkane nucleophiles greatly expands the scope of compounds made by TrpB variants and establishes nitroalkanes as a valuable substrate class for biocatalytic C-C bond formation.

Conservation of the structure and function of bacterial tryptophan synthases

Tryptophan biosynthesis is one of the most characterized processes in bacteria, in which the enzymes from Salmonella typhimurium and Escherichia coli serve as model systems. Tryptophan synthase (TrpAB) catalyzes the final two steps of tryptophan biosynthesis in plants, fungi and bacteria. This pyridoxal 5'-phosphate (PLP)-dependent enzyme consists of two protein chains, α (TrpA) and β (TrpB), functioning as a linear αββα heterotetrameric complex containing two TrpAB units. The reaction has a complicated, multistep mechanism resulting in the β-replacement of the hydroxyl group of l-serine with an indole moiety. Recent studies have shown that functional TrpAB is required for the survival of pathogenic bacteria in macrophages and for evading host defense. Therefore, TrpAB is a promising target for drug discovery, as its orthologs include enzymes from the important human pathogens Streptococcus pneumoniae, Legionella pneumophila and Francisella tularensis, the causative agents of pneumonia, legionnaires' disease and tularemia, respectively. However, specific biochemical and structural properties of the TrpABs from these organisms have not been investigated. To fill the important phylogenetic gaps in the understanding of TrpABs and to uncover unique features of TrpAB orthologs to spearhead future drug-discovery efforts, the TrpABs from L. pneumophila, F. tularensis and S. pneumoniae have been characterized. In addition to kinetic properties and inhibitor-sensitivity data, structural information gathered using X-ray crystallo-graphy is presented. The enzymes show remarkable structural conservation, but at the same time display local differences in both their catalytic and allosteric sites that may be responsible for the observed differences in catalysis and inhibitor binding. This functional dissimilarity may be exploited in the design of species-specific enzyme inhibitors.

Ammonia generation by tryptophan synthase drives a key genetic difference between genital and ocular Chlamydia trachomatis isolates

A striking difference between genital and ocular clinical isolates of Chlamydia trachomatis is that only the former express a functional tryptophan synthase and therefore can synthesize tryptophan by indole salvage. Ocular isolates uniformly cannot use indole due to inactivating mutations within tryptophan synthase, indicating a selection against maintaining this enzyme in the ocular environment. Here, we demonstrate that this selection occurs in two steps. First, specific indole derivatives, produced by the human gut microbiome and present in serum, rapidly induce expression of C. trachomatis tryptophan synthase, even under conditions of tryptophan sufficiency. We demonstrate that these indole derivatives function by acting as de-repressors of C. trachomatis TrpR. Second, trp operon de-repression is profoundly deleterious when infected cells are in an indole-deficient environment, because in the absence of indole, tryptophan synthase deaminates serine to pyruvate and ammonia. We have used biochemical and genetic approaches to demonstrate that expression of wild-type tryptophan synthase is required for the bactericidal production of ammonia. Pertinently, although these indole derivatives de-repress the trpRBA operon of C. trachomatis strains with trpA or trpB mutations, no ammonia is produced, and no deleterious effects are observed. Our studies demonstrate that tryptophan synthase can catalyze the ammonia-generating β-elimination reaction within any live bacterium. Our results also likely explain previous observations demonstrating that the same indole derivatives inhibit the growth of other pathogenic bacterial species, and why high serum levels of these indole derivatives are favorable for the prognosis of diseased conditions associated with bacterial dysbiosis.

Production Options for Psilocybin: Making of the Magic

The fungal genus Psilocybe and other genera comprise numerous mushroom species that biosynthesize psilocybin (4-phosphoryloxy-N,N-dimethyltryptamine). It represents the prodrug to its dephosphorylated psychotropic analogue, psilocin. The colloquial term "magic mushrooms" for these fungi alludes to their hallucinogenic effects and to their use as recreational drugs. However, clinical trials have recognized psilocybin as a valuable candidate to be developed into a medication against depression and anxiety. We here highlight its recently elucidated biosynthesis, the concurrently developed concept of enzymatic in vitro and heterologous in vivo production, along with previous synthetic routes. The prospect of psilocybin as a promising therapeutic may entail an increased demand, which can be met by biotechnological production. Therefore, we also briefly touch on psilocybin's therapeutic relevance and pharmacology.

Enzymatic complexes across scales

An unprecedented opportunity to integrate ~100 years of meticulous in vitro biomolecular research is currently provided in the light of recent advances in methods to visualize closer-to-native architectures of biomolecular machines, and metabolic enzymes in particular. Traditional views of enzymes, namely biomolecular machines, only partially explain their role, organization and kinetics in the cellular milieu. Enzymes self- or hetero-associate, form fibers, may bind to membranes or cytoskeletal elements, have regulatory roles, associate into higher order assemblies (metabolons) or even actively participate in phase-separated membraneless organelles, and all the above in a transient, temporal and spatial manner in response to environmental changes or structural/functional changes of their assemblies. Here, we focus on traditional and emerging concepts in cellular biochemistry and discuss new opportunities in bridging structural, molecular and cellular analyses for metabolic pathways, accumulated over the years, highlighting functional aspects of enzymatic complexes discussed across different levels of spatial resolution.

Evolutionary Morphing of Tryptophan Synthase: Functional Mechanisms for the Enzymatic Channeling of Indole

Tryptophan synthase (TrpS) is a heterotetrameric αββα enzyme that exhibits complex substrate channeling and allosteric mechanisms and is a model system in enzymology. In this work, we characterize proposed early and late evolutionary states of TrpS and show that they have distinct quaternary structures caused by insertions-deletions of sequence segments (indels) in the β-subunit. Remarkably, indole hydrophobic channels that connect α and β active sites have re-emerged in both TrpS types, yet they follow different paths through the β-subunit fold. Also, both TrpS geometries activate the α-subunit through the rearrangement of loops flanking the active site. Our results link evolutionary sequence changes in the enzyme subunits with channeling and allostery in the TrpS enzymes. The findings demonstrate that indels allow protein quaternary architectures to escape "minima" in the evolutionary landscape, thereby overcoming the conservational constraints imposed by existing functional interfaces and being free to morph into new mechanistic enzymes.