Mechanism of 4-(beta-D-ribofuranosyl)aminobenzene 5'-phosphate synthase, a key enzyme in the methanopterin biosynthetic pathway

Biocatalytic cascade transformations for the synthesis of C-nucleosides and N-nucleoside analogs

Nucleosides and their analogs, including those that feature substitution of the canonical N-glycosidic by a C-glycosidic linkage, provide access to potent antiviral, antibacterial, and antitumor drugs. Furthermore, they are key building blocks of m-RNA vaccines and play a crucial role for vaccine therapeutic effectiveness. As the medicinal applications of nucleosides increase in number and importance, there is a growing need for efficiency-enhanced routes of nucleoside synthesis. Cascade biocatalysis, that is, the application of natural or evolved enzymes promoting complex transformations in multiple steps in one pot and without the need of intermediate purification, emerges as a powerful tool to obtain nucleosides from readily available starting materials. Recent efforts in enzyme discovery and protein engineering expand the toolbox of catalysts active toward nucleosides or nucleotides. In this review, we highlight recent applications, and discuss challenges, of cascade biocatalysis for nucleoside synthesis. We focus on C-nucleosides and important analogs of the canonical N-nucleosides.

Open Issues for Protein Function Assignment in Haloferax volcanii and Other Halophilic Archaea

Background: Annotation ambiguities and annotation errors are a general challenge in genomics. While a reliable protein function assignment can be obtained by experimental characterization, this is expensive and time-consuming, and the number of such Gold Standard Proteins (GSP) with experimental support remains very low compared to proteins annotated by sequence homology, usually through automated pipelines. Even a GSP may give a misleading assignment when used as a reference: the homolog may be close enough to support isofunctionality, but the substrate of the GSP is absent from the species being annotated. In such cases, the enzymes cannot be isofunctional. Here, we examined a variety of such issues in halophilic archaea (class Halobacteria), with a strong focus on the model haloarchaeon Haloferax volcanii. Results: Annotated proteins of Hfx. volcanii were identified for which public databases tend to assign a function that is probably incorrect. In some cases, an alternative, probably correct, function can be predicted or inferred from the available evidence, but this has not been adopted by public databases because experimental validation is lacking. In other cases, a probably invalid specific function is predicted by homology, and while there is evidence that this assigned function is unlikely, the true function remains elusive. We listed 50 of those cases, each with detailed background information, so that a conclusion about the most likely biological function can be drawn. For reasons of brevity and comprehension, only the key aspects are listed in the main text, with detailed information being provided in a corresponding section of the Supplementary Materials. Conclusions: Compiling, describing and summarizing these open annotation issues and functional predictions will benefit the scientific community in the general effort to improve the evaluation of protein function assignments and more thoroughly detail them. By highlighting the gaps and likely annotation errors currently in the databases, we hope this study will provide a framework for experimentalists to systematically confirm (or disprove) our function predictions or to uncover yet more unexpected functions.

Uncovering the chemistry of C-C bond formation in C-nucleoside biosynthesis: crystal structure of a C-glycoside synthase/PRPP complex

The enzyme ForT catalyzes C-C bond formation between 5'-phosphoribosyl-1'-pyrophosphate (PRPP) and 4-amino-1H-pyrazole-3,5-dicarboxylate to make a key intermediate in the biosynthesis of formycin A 5'-phosphate by Streptomyces kaniharaensis. We report the 2.5 Å resolution structure of the ForT/PRPP complex and locate active site residues critical for PRPP recognition and catalysis.

Identification of the C-Glycoside Synthases during Biosynthesis of the Pyrazole-C-Nucleosides Formycin and Pyrazofurin

C-Nucleosides are characterized by a C-C rather than a C-N linkage between the heterocyclic base and the ribofuranose ring. While the biosynthesis of pseudouridine-C-nucleosides has been studied, less is known about the pyrazole-C-nucleosides such as the formycins and pyrazofurin. Herein, genome screening of Streptomyces candidus NRRL 3601 led to the discovery of the pyrazofurin biosynthetic gene cluster pyf. In vitro characterization of gene product PyfQ demonstrated that it is able to catalyze formation of the C-glycoside carboxyhydroxypyrazole ribonucleotide (CHPR) from 4-hydroxy-1H-pyrazole-3,5-dicarboxylic acid and phosphoribosyl pyrophosphate (PRPP). Similarly, ForT, the PyfQ homologue in the formycin pathway, can catalyze the coupling of 4-amino-1H-pyrazole-3,5-dicarboxylic acid and PRPP to form carboxyaminopyrazole ribonucleotide. Finally, PyfP and PyfT are shown to catalyze amidation of CHPR to pyrazofurin 5'-phosphate thereby establishing the latter stages of both pyrazofurin and formycin biosynthesis.

Recent advances in the biosynthesis of nucleoside antibiotics

Nucleoside antibiotics are a diverse class of natural products with promising biomedical activities. These compounds contain a saccharide core and a nucleobase. Despite the large number of nucleoside antibiotics that have been reported, biosynthetic studies on these compounds have been limited compared with those on other types of natural products such as polyketides, peptides, and terpenoids. Due to recent advances in genome sequencing technology, the biosynthesis of nucleoside antibiotics has rapidly been clarified. This review covering 2009-2019 focuses on recent advances in the biosynthesis of nucleoside antibiotics.

Purification, kinetic characterization, and site-directed mutagenesis of Methanothermobacter thermautotrophicus RFAP Synthase Produced in Escherichia coli

Methane-producing archaea are among a select group of microorganisms that utilize tetrahydromethanopterin (H₄MPT) as a one-carbon carrier instead of tetrahydrofolate. In H₄MPT biosynthesis, β-ribofuranosylaminobenzene 5'-phosphate (RFAP) synthase catalyzes the production of RFAP, CO₂, and pyrophosphate from p-aminobenzoic acid (pABA) and phosphoribosyl-pyrophosphate (PRPP). In this work, to gain insight into amino acid residues required for substrate binding, RFAP synthase from Methanothermobacter thermautotrophicus was produced in Escherichia coli, and site-directed mutagenesis was used to alter arginine 26 (R26) and aspartic acid 19 (D19), located in a conserved sequence of amino acids resembling the pABA binding site of dihydropteroate synthase. Replacement of R26 with lysine increased the K_M for pABA by an order of magnitude relative to wild-type enzyme without substantially altering the K_M for PRPP. Although replacement of D19 with alanine produced inactive enzyme, asparagine substitution allowed retention of some activity, and the K _M for pABA increased about threefold relative to wild-type enzyme. A molecular model developed by threading RFAP synthase onto the crystal structure of homoserine kinase places R26 in the proposed active site. In the static model, D19 is located close to the active site, yet appears too far away to influence ligand binding directly. This may be indicative of the protein conformational change predicted previously in the Bi-Ter kinetic mechanism and/or formation of the active site at the interface of two subunits. Due to the vital role of RFAP synthase in H₄MPT biosynthesis, insights into the mode of substrate binding and mechanism could be beneficial for developing RFAP synthase inhibitors designed to reduce the production of methane as a greenhouse gas.

Identification of the Formycin A Biosynthetic Gene Cluster from Streptomyces kaniharaensis Illustrates the Interplay between Biological Pyrazolopyrimidine Formation and de Novo Purine Biosynthesis

Formycin A is a potent purine nucleoside antibiotic with a C-glycosidic linkage between the ribosyl moiety and the pyrazolopyrimidine base. Herein, a cosmid is identified from the Streptomyces kaniharaensis genome library that contains the for gene cluster responsible for the biosynthesis of formycin. Subsequent gene deletion experiments and in vitro characterization of the forBCH gene products established their catalytic functions in formycin biosynthesis. Results also demonstrated that PurH from de novo purine biosynthesis plays a key role in pyrazolopyrimidine formation during biosynthesis of formycin A. The participation of PurH in both pathways represents a good example of how primary and secondary metabolism are interlinked.

Identification and Characterization of Enzymes Catalyzing Pyrazolopyrimidine Formation in the Biosynthesis of Formycin A

Genome scanning of Streptomyces kaniharaensis, the producer of formycin A, reveals two sets of purA, purB, purC, and purH genes. The Pur enzymes catalyze pyrimidine assembly of purine nucleobases. To test whether enzymes encoded by the second set of pur genes catalyze analogous transformations in formycin biosynthesis, formycin B 5'-phosphate was synthesized and shown to be converted by ForA and ForB to formycin A 5'-phosphate. These results support that For enzymes are responsible for formycin formation.

Phosphoribosyl Diphosphate (PRPP): Biosynthesis, Enzymology, Utilization, and Metabolic Significance

Phosphoribosyl diphosphate (PRPP) is an important intermediate in cellular metabolism. PRPP is synthesized by PRPP synthase, as follows: ribose 5-phosphate + ATP → PRPP + AMP. PRPP is ubiquitously found in living organisms and is used in substitution reactions with the formation of glycosidic bonds. PRPP is utilized in the biosynthesis of purine and pyrimidine nucleotides, the amino acids histidine and tryptophan, the cofactors NAD and tetrahydromethanopterin, arabinosyl monophosphodecaprenol, and certain aminoglycoside antibiotics. The participation of PRPP in each of these metabolic pathways is reviewed. Central to the metabolism of PRPP is PRPP synthase, which has been studied from all kingdoms of life by classical mechanistic procedures. The results of these analyses are unified with recent progress in molecular enzymology and the elucidation of the three-dimensional structures of PRPP synthases from eubacteria, archaea, and humans. The structures and mechanisms of catalysis of the five diphosphoryltransferases are compared, as are those of selected enzymes of diphosphoryl transfer, phosphoryl transfer, and nucleotidyl transfer reactions. PRPP is used as a substrate by a large number phosphoribosyltransferases. The protein structures and reaction mechanisms of these phosphoribosyltransferases vary and demonstrate the versatility of PRPP as an intermediate in cellular physiology. PRPP synthases appear to have originated from a phosphoribosyltransferase during evolution, as demonstrated by phylogenetic analysis. PRPP, furthermore, is an effector molecule of purine and pyrimidine nucleotide biosynthesis, either by binding to PurR or PyrR regulatory proteins or as an allosteric activator of carbamoylphosphate synthetase. Genetic analyses have disclosed a number of mutants altered in the PRPP synthase-specifying genes in humans as well as bacterial species.

Biochemical fossils of the ancient transition from geoenergetics to bioenergetics in prokaryotic one carbon compound metabolism

The deep dichotomy of archaea and bacteria is evident in many basic traits including ribosomal protein composition, membrane lipid synthesis, cell wall constituents, and flagellar composition. Here we explore that deep dichotomy further by examining the distribution of genes for the synthesis of the central carriers of one carbon units, tetrahydrofolate (H4F) and tetrahydromethanopterin (H4MPT), in bacteria and archaea. The enzymes underlying those distinct biosynthetic routes are broadly unrelated across the bacterial-archaeal divide, indicating that the corresponding pathways arose independently. That deep divergence in one carbon metabolism is mirrored in the structurally unrelated enzymes and different organic cofactors that methanogens (archaea) and acetogens (bacteria) use to perform methyl synthesis in their H4F- and H4MPT-dependent versions, respectively, of the acetyl-CoA pathway. By contrast, acetyl synthesis in the acetyl-CoA pathway - from a methyl group, CO2 and reduced ferredoxin - is simpler, uniform and conserved across acetogens and methanogens, and involves only transition metals as catalysts. The data suggest that the acetyl-CoA pathway, while being the most ancient of known CO2 assimilation pathways, reflects two phases in early evolution: an ancient phase in a geochemically confined and non-free-living universal common ancestor, in which acetyl thioester synthesis proceeded spontaneously with the help of geochemically supplied methyl groups, and a later phase that reflects the primordial divergence of the bacterial and archaeal stem groups, which independently invented genetically-encoded means to synthesize methyl groups via enzymatic reactions. This article is part of a Special Issue entitled: 18th European Bioenergetic Conference.

Discovery and characterization of the first archaeal dihydromethanopterin reductase, an iron-sulfur flavoprotein from Methanosarcina mazei

The microbial production of methane by methanogenic archaea is dependent on the synthesis of the pterin-containing cofactor tetrahydromethanopterin (H4MPT). The enzyme catalyzing the last step of H4MPT biosynthesis (dihydromethanopterin reductase) has not previously been identified in methane-producing microorganisms. Previous complementation studies with the methylotrophic bacterium Methylobacterium extorquens have indicated that an uncharacterized archaeal-flavoprotein-like flavoprotein (AfpA) from Methylobacillus flagellatus or Burkholderia xenovorans can replace the activity of a phylogenetically unrelated bacterial dihydromethanopterin reductase (DmrA). We propose that MM1854, a homolog of AfpA from Methanosarcina mazei, catalyzes the last step of H4MPT biosynthesis in methane-producing microorganisms. To test this hypothesis, a six-histidine (His6)-tagged version of MM1854 was produced. Bioinformatic analysis revealed the presence of one flavin mononucleotide (FMN)-binding site and two iron-sulfur cluster sites, consistent with an oxidoreductase enzyme. Purified His6-MM1854 occurred as a homodimer of 29-kDa subunits, and the UV-visible spectrum of the purified protein showed absorbance peaks at 380 and 460 nm, characteristic of oxidized FMN. NAD(P)H was incapable of directly reducing the flavin cofactor, but dithionite eliminated the FMN peaks, indicating successful electron transfer to MM1854. An electron transfer system of NADPH, spinach NADPH-ferredoxin oxidoreductase, and ferredoxin could also reduce the FMN peaks. A newly developed assay indicated that dithiothreitol-reduced MM1854 could transfer electrons to dihydromethanopterin. This assay was also effective with a heat-stable DmrX analog from Methanocaldococcus jannaschii (MJ0208). These results provide the first biochemical evidence that MM1854 and MJ0208 function as archaeal dihydromethanopterin reductases (DmrX) and that ferredoxin may serve as an electron donor.

Pseudouridine monophosphate glycosidase: a new glycosidase mechanism

Pseudouridine (Ψ), the most abundant modification in RNA, is synthesized in situ using Ψ synthase. Recently, a pathway for the degradation of Ψ was described [Preumont, A., Snoussi, K., Stroobant, V., Collet, J. F., and Van Schaftingen, E. (2008) J. Biol. Chem. 283, 25238-25246]. In this pathway, Ψ is first converted to Ψ 5'-monophosphate (ΨMP) by Ψ kinase and then ΨMP is degraded by ΨMP glycosidase to uracil and ribose 5-phosphate. ΨMP glycosidase is the first example of a mechanistically characterized enzyme that cleaves a C-C glycosidic bond. Here we report X-ray crystal structures of Escherichia coli ΨMP glycosidase and a complex of the K166A mutant with ΨMP. We also report the structures of a ring-opened ribose 5-phosphate adduct and a ring-opened ribose ΨMP adduct. These structures provide four snapshots along the reaction coordinate. The structural studies suggested that the reaction utilizes a Lys166 adduct during catalysis. Biochemical and mass spectrometry data further confirmed the existence of a lysine adduct. We used site-directed mutagenesis combined with kinetic analysis to identify roles for specific active site residues. Together, these data suggest that ΨMP glycosidase catalyzes the cleavage of the C-C glycosidic bond through a novel ribose ring-opening mechanism.

The conversion of a phenol to an aniline occurs in the biochemical formation of the 1-(4-aminophenyl)-1-deoxy-D-ribitol moiety in methanopterin

Recent work has demonstrated that 4-hydroxybenzoic acid is the in vivo precursor to the 1-(4-aminophenyl)-1-deoxy-D-ribitol (APDR) moiety present in the C(1) carrier coenzyme methanopterin present in the methanogenic archaea. For this transformation to occur, the hydroxyl group of the 4-hydroxybenzoic acid must be replaced with an amino group at some point in the biosynthetic pathway. Using stable isotopically labeled precursors and liquid chromatography with electrospray-ionization mass spectroscopy, the first step of this transformation in Methanocaldococcus jannaschii occurs by the reaction of 4-hydroxybenzoic acid with phosphoribosyl pyrophosphate (PRPP) to form 4-(β-d-ribofuranosyl)hydroxybenzene 5'-phosphate (β-RAH-P). The β-RAH-P then condenses with l-aspartate in the presence of ATP to form 4-(β-d-ribofuranosyl)-N-succinylaminobenzene 5'-phosphate (β-RFSA-P). Elimination of fumarate from β-RFSA-P produces 4-(β-D-ribofuranosyl)aminobenzene 5'-phosphate (β-RFA-P), the known precursor to the APDR moiety of methanopterin [White, R. H. (1996) Biochemistry 35, 3447-3456]. This work represents the first biochemical example of the conversion of a phenol to an aniline.

Proteome and transcriptional analysis of ethanol-grown Sulfolobus solfataricus P2 reveals ADH2, a potential alcohol dehydrogenase

Sulfolobus solfataricus P2 was shown to survive on ethanol at various concentrations (0.08-3.97% w/v) as the sole carbon source. The highest ethanol consumption rate was 15.1 mg/L/hr (via GC-MS analysis) in cultures grown on 0.79% w/v ethanol. In vivo metabolic labeling, using 13C universally labeled ethanol, provided evidence for both ethanol uptake and metabolic utilization. Results obtained from isobaric mass tag-facilitated shotgun proteomics (iTRAQ) indicate that on average, 21 and 31% of the 284 proteins identified (with > or = 2 MS/MS) are increased and decreased expression in ethanol cultures compared to glucose control cultures. Preliminary analysis shows >2-fold increase of the zinc-dependent alcohol dehydrogenase, ADH-10 (SSO2536), and the putative ADH-2 (SSO0764) in both translational and transcriptional data (using quantitative RT-PCR), suggesting both proteins are integral to ethanol metabolism. Evidence that ethanol was catabolised into central metabolism via acetyl-CoA intermediates was further indicated by another >2-fold increase in protein expression levels of various acetyl-CoA synthetases. The decreased expression (>2-fold) of isocitrate dehydrogenase at the protein level suggests that the ethanol grown cultures shifted toward the glyoxylate cycle. Subsequently, the activity of ADH-2 was confirmed by overexpression in Escherichia coli, with the resultant purified in vitro enzyme exhibiting an activity that increased with temperature up to 95 degrees C, and giving a specific activity of 1.05 U/mg.