Identification of the Missing Links in Prokaryotic Pentose Oxidation Pathways

Evolving Nonphosphorylative Metabolism for Improving Production of 2-Oxoglutarate Derivatives

The bioconversion of lignocellulosic biomass into value-added products provides an alternative solution to environmental and economic challenges. Nonphosphorylative metabolism can convert pentoses and d-galacturonate into 2-oxoglutarate (2-KG) in a few steps, facilitating the production of 2-KG derivatives. However, the efficiency of the Weimberg pathway from Caulobacter crescentus, a type of nonphosphorylative metabolism, is constrained by the low activity of CcXylX, 2-keto-3-deoxy-d-xylonate dehydratase. To overcome this limitation, we engineered CcXylX through directed evolution. A resulting CcXylX mutant exhibited a 3-fold higher kcat value and notably enhanced the production of 2-KG derivatives from d-xylose, a major component of lignocellulosic hydrolysates, including a 32% increase in l-glutamate titer (8.3 g/L) and a 79% increase in l-proline titer (4.3 g/L) compared with the wild-type CcXylX. This research holds promise for advancing lignocellulosic biotechnology and provides insights into economically viable production of other 2-KG derivatives besides l-glutamate and l-proline.

From Nontargeted to Targeted Analysis: Feature Selection in the Differentiation of Truffle Species (Tuber spp.) Using 1H NMR Spectroscopy and Support Vector Machine

The price of different truffle types varies according to their culinary value, sometimes by more than a factor of 10. Nonprofessionals can hardly distinguish visually the species within the white or black truffles, making the possibility of food fraud very easy. Therefore, the identification of different truffle species (Tuber spp.) is an analytical task that could be solved in this study. The polar extract from a total of 80 truffle samples was analyzed by 1H NMR spectroscopy in combination with chemometric methods covering five commercially relevant species. All classification models were validated applying a repeated nested cross-validation. In direct comparison, the two very similar looking and closely related black representatives Tuber melanosporum and Tuber indicum could be classified 100% correctly. The most expensive truffle Tuber magnatum could be distinguished 100% from the other relevant white truffle Tuber borchii. In addition, signals for a potential Tuber borchii and a potential Tuber melanosporum marker for targeted approaches could be detected, and the corresponding molecules were identified as betaine and ribonate. A model covering all five truffle species Tuber aestivum, Tuber borchii, Tuber indicum, Tuber magnatum, and Tuber melanosporum was able to correctly discriminate between each of the species.

Structure and function of aldopentose catabolism enzymes involved in oxidative non-phosphorylative pathways

Platform chemicals and polymer precursors can be produced via enzymatic pathways starting from lignocellulosic waste materials. The hemicellulose fraction of lignocellulose contains aldopentose sugars, such as D-xylose and L-arabinose, which can be enzymatically converted into various biobased products by microbial non-phosphorylated oxidative pathways. The Weimberg and Dahms pathways convert pentose sugars into α-ketoglutarate, or pyruvate and glycolaldehyde, respectively, which then serve as precursors for further conversion into a wide range of industrial products. In this review, we summarize the known three-dimensional structures of the enzymes involved in oxidative non-phosphorylative pathways of pentose catabolism. Key structural features and reaction mechanisms of a diverse set of enzymes responsible for the catalytic steps in the reactions are analysed and discussed.

The Biosynthesis of D-1,2,4-Butanetriol From d-Arabinose With an Engineered Escherichia coli

D-1,2,4-Butanetriol (BT) has attracted much attention for its various applications in energetic materials and the pharmaceutical industry. Here, a synthetic pathway for the biosynthesis of BT from d-arabinose was constructed and optimized in Escherichia coli. First, E. coli Trans1-T1 was selected for the synthesis of BT. Considering the different performance of the enzymes from different organisms when expressed in E. coli, the synthetic pathway was optimized. After screening two d-arabinose dehydrogenases (ARAs), two d-arabinonate dehydratases (ADs), four 2-keto acid decarboxylases (ADXs), and three aldehyde reductases (ALRs), ADG from Burkholderia sp., AraD from Sulfolobus solfataricus, KivD from Lactococcus lactis IFPL730, and AdhP from E. coli were selected for the bio-production of BT. After 48 h of catalysis, 0.88 g/L BT was produced by the recombinant strain BT5. Once the enzymes were selected for the pathway, metabolic engineering strategy was conducted for further improvement. The final strain BT5ΔyiaEΔycdWΔyagE produced 1.13 g/L BT after catalyzing for 48 h. Finally, the fermentation conditions and characteristics of BT5ΔyiaEΔycdWΔyagE were also evaluated, and then 2.24 g/L BT was obtained after 48 h of catalysis under the optimized conditions. Our work was the first report on the biosynthesis of BT from d-arabinose which provided a potential for the large-scale production of d-glucose-based BT.

Workflows for optimization of enzyme cascades and whole cell catalysis based on enzyme kinetic characterization and pathway modelling

To move towards a circular bioeconomy, sustainable strategies for the utilization of renewable, non-food biomass wastes such as lignocellulose, are needed. To this end, an efficient bioconversion of d-xylose - after d-glucose the most abundant sugar in lignocellulose - is highly desirable. Most standard organisms used in biotechnology are limited in metabolising d-xylose, and also in vitro enzymatic strategies for its conversion have not been very successful. We herein discuss that bioconversion of d-xylose is mostly hampered by missing knowledge on the kinetic properties of the enzymes involved in its metabolism. We propose a combination of classical enzyme characterizations and mathematical modelling approaches as a workflow for rational, model-based design to optimize enzyme cascades and/or whole cell biocatalysts for efficient d-xylose metabolism.

Achievements and Perspectives in 1,4-Butanediol Production from Engineered Microorganisms

1,4-Butanediol (1,4-BDO), a significant commodity chemical, is currently manufactured exclusively from a host of energy-intensive processes, accompanied by severe environmental issues, such as the greenhouse effect and air pollution. As a result of the ever-increasing global market demands and increasing applications of 1,4-BDO, attention has turned to the sustainable bioproduction of 1,4-BDO, and several bio-based approaches for 1,4-BDO production have been successfully established in engineered Escherichia coli, including de novo biosynthesis and biocatalysis. Recent achievements in enhancing the accumulation of 1,4-BDO have been achieved by metabolic engineering strategies, such as improving precursor supply, enhancing activities of critical enzymes, and fewer byproduct synthesis. Here, we summarize the primary advances of the biological pathway for 1,4-BDO synthesis and put forward the future development prospect of bio-based 1,4-BDO production.

The biology of thermoacidophilic archaea from the order Sulfolobales

Thermoacidophilic archaea belonging to the order Sulfolobales thrive in extreme biotopes, such as sulfuric hot springs and ore deposits. These microorganisms have been model systems for understanding life in extreme environments, as well as for probing the evolution of both molecular genetic processes and central metabolic pathways. Thermoacidophiles, such as the Sulfolobales, use typical microbial responses to persist in hot acid (e.g. motility, stress response, biofilm formation), albeit with some unusual twists. They also exhibit unique physiological features, including iron and sulfur chemolithoautotrophy, that differentiate them from much of the microbial world. Although first discovered >50 years ago, it was not until recently that genome sequence data and facile genetic tools have been developed for species in the Sulfolobales. These advances have not only opened up ways to further probe novel features of these microbes but also paved the way for their potential biotechnological applications. Discussed here are the nuances of the thermoacidophilic lifestyle of the Sulfolobales, including their evolutionary placement, cell biology, survival strategies, genetic tools, metabolic processes and physiological attributes together with how these characteristics make thermoacidophiles ideal platforms for specialized industrial processes.

Identification of the Genes Related to the Glycogen Metabolism in Hyperthermophilic Archaeon, Sulfolobus acidocaldarius

Glycogen is a polysaccharide that comprises α-1,4-linked glucose backbone and α-1,6-linked glucose polymers at the branching points. It is widely found in organisms ranging from bacteria to eukaryotes. The physiological role of glycogen is not confined to being an energy reservoir and carbon source but varies depending on organisms. Sulfolobus acidocaldarius, a thermoacidophilic archaeon, was observed to accumulate granular glycogen in the cell. However, the role of glycogen and genes that are responsible for glycogen metabolism in S. acidocaldarius has not been identified clearly. The objective of this study is to identify the gene cluster, which is composed of enzymes that are predicted to be involved in the glycogen metabolism, and confirm the role of each of these genes by constructing deletion mutants. This study also compares the glycogen content of mutant and wild type and elucidates the role of glycogen in this archaeon. The glycogen content of S. acidocaldarius MR31, which is used as a parent strain for constructing the deletion mutant in this study, was increased in the early and middle exponential growth phases and decreased during the late exponential and stationary growth phases. The pattern of the accumulated glycogen was independent to the type of supplemented sugar. In the comparison of the glycogen content between the gene deletion mutant and MR31, glycogen synthase (GlgA) and α-amylase (AmyA) were shown to be responsible for the synthesis of glycogen, whereas glycogen debranching enzyme (GlgX) and glucoamylase (Gaa) appeared to affect the degradation of glycogen. The expressions of glgC-gaa-glgX and amyA-glgA were detected by the promoter assay. This result suggests that the gradual decrease of glycogen content in the late exponential and stationary phases occurs due to the increase in the gene expression of glgC-gaa-glgX. When the death rate in nutrient limited condition was compared among the wild type strain, the glycogen deficient strain and the strain with increased glycogen content, the death rate of the glycogen deficient strain was found to be higher than any other strain, thereby suggesting that the glycogen in S. acidocaldarius supports cell maintenance in harsh conditions.

Utilization of Phenol as Carbon Source by the Thermoacidophilic Archaeon Saccharolobus solfataricus P2 Is Limited by Oxygen Supply and the Cellular Stress Response

Present in many industrial effluents and as common degradation product of organic matter, phenol is a widespread compound which may cause serious environmental problems, due to its toxicity to animals and humans. Degradation of phenol from the environment by mesophilic bacteria has been studied extensively over the past decades, but only little is known about phenol biodegradation at high temperatures or low pH. In this work we studied phenol degradation in the thermoacidophilic archaeon Saccharolobus solfataricus P2 (basonym: Sulfolobus solfataricus) under extreme conditions (80°C, pH 3.5). We combined metabolomics and transcriptomics together with metabolic modeling to elucidate the organism’s response to growth with phenol as sole carbon source. Although S. solfataricus is able to utilize phenol for biomass production, the carbon source induces profound stress reactions, including genome rearrangement as well as a strong intracellular accumulation of polyamines. Furthermore, computational modeling revealed a 40% higher oxygen demand for substrate oxidation, compared to growth on glucose. However, only 16.5% of oxygen is used for oxidation of phenol to catechol, resulting in a less efficient integration of carbon into the biomass. Finally, our data underlines the importance of the phenol meta-degradation pathway in S. solfataricus and enables us to predict enzyme candidates involved in the degradation processes downstream of 2-hydroxymucconic acid.

Pentose metabolism and conversion to biofuels and high-value chemicals in yeasts

Pentose sugars are widespread in nature and two of them, D-xylose and L-arabinose belong to the most abundant sugars being the second and third by abundance sugars in dry plant biomass (lignocellulose) and in general on planet. Therefore, it is not surprising that metabolism and bioconversion of these pentoses attract much attention. Several different pathways of D-xylose and L-arabinose catabolism in bacteria and yeasts are known. There are even more common and really ubiquitous though not so abundant pentoses, D-ribose and 2-deoxy-D-ribose, the constituents of all living cells. Thus, ribose metabolism is example of endogenous metabolism whereas metabolism of other pentoses, including xylose and L-arabinose, represents examples of the metabolism of foreign exogenous compounds which normally are not constituents of yeast cells. As a rule, pentose degradation by the wild-type strains of microorganisms does not lead to accumulation of high amounts of valuable substances; however, productive strains have been obtained by random selection and metabolic engineering. There are numerous reviews on xylose and (less) L-arabinose metabolism and conversion to high value substances; however, they mostly are devoted to bacteria or the yeast Saccharomyces cerevisiae. This review is devoted to reviewing pentose metabolism and bioconversion mostly in non-conventional yeasts, which naturally metabolize xylose. Pentose metabolism in the recombinant strains of S. cerevisiae is also considered for comparison. The available data on ribose, xylose, L-arabinose transport, metabolism, regulation of these processes, interaction with glucose catabolism and construction of the productive strains of high-value chemicals or pentose (ribose) itself are described. In addition, genome studies of the natural xylose metabolizing yeasts and available tools for their molecular research are reviewed. Metabolism of other pentoses (2-deoxyribose, D-arabinose, lyxose) is briefly reviewed.

Enzymology of Alternative Carbohydrate Catabolic Pathways

The Embden–Meyerhof–Parnas (EMP) and Entner–Doudoroff (ED) pathways are considered the most abundant catabolic pathways found in microorganisms, and ED enzymes have been shown to also be widespread in cyanobacteria, algae and plants. In a large number of organisms, especially common strains used in molecular biology, these pathways account for the catabolism of glucose. The existence of pathways for other carbohydrates that are relevant to biomass utilization has been recognized as new strains have been characterized among thermophilic bacteria and Archaea that are able to transform simple polysaccharides from biomass to more complex and potentially valuable precursors for industrial microbiology. Many of the variants of the ED pathway have the key dehydratase enzyme involved in the oxidation of sugar derived from different families such as the enolase, IlvD/EDD and xylose-isomerase-like superfamilies. There are the variations in structure of proteins that have the same specificity and generally greater-than-expected substrate promiscuity. Typical biomass lignocellulose has an abundance of xylan, and four different pathways have been described, which include the Weimberg and Dahms pathways initially oxidizing xylose to xylono-gamma-lactone/xylonic acid, as well as the major xylose isomerase pathway. The recent realization that xylan constitutes a large proportion of biomass has generated interest in exploiting the compound for value-added precursors, but few chassis microorganisms can grow on xylose. Arabinose is part of lignocellulose biomass and can be metabolized with similar pathways to xylose, as well as an oxidative pathway. Like enzymes in many non-phosphorylative carbohydrate pathways, enzymes involved in L-arabinose pathways from bacteria and Archaea show metabolic and substrate promiscuity. A similar multiplicity of pathways was observed for other biomass-derived sugars such as L-rhamnose and L-fucose, but D-mannose appears to be distinct in that a non-phosphorylative version of the ED pathway has not been reported. Many bacteria and Archaea are able to grow on mannose but, as with other minor sugars, much of the information has been derived from whole cell studies with additional enzyme proteins being incorporated, and so far, only one synthetic pathway has been described. There appears to be a need for further discovery studies to clarify the general ability of many microorganisms to grow on the rarer sugars, as well as evaluation of the many gene copies displayed by marine bacteria.

Pentose degradation in archaea: Halorhabdus species degrade D-xylose, L-arabinose and D-ribose via bacterial-type pathways

The degradation of the pentoses D-xylose, L-arabinose and D-ribose in the domain of archaea, in Haloferax volcanii and in Haloarcula and Sulfolobus species, has been shown to proceed via oxidative pathways to generate α-ketoglutarate. Here, we report that the haloarchaeal Halorhabdus species utilize the bacterial-type non-oxidative degradation pathways for pentoses generating xylulose-5-phosphate. The genes of these pathways are each clustered and were constitutively expressed. Selected enzymes involved in D-xylose degradation, xylose isomerase and xylulokinase, and those involved in L-arabinose degradation, arabinose isomerase and ribulokinase, were characterized. Further, D-ribose degradation in Halorhabdus species involves ribokinase, ribose-5-phosphate isomerase and D-ribulose-5-phosphate-3-epimerase. Ribokinase of Halorhabdus tiamatea and ribose-5-phosphate isomerase of Halorhabdus utahensis were characterized. This is the first report of pentose degradation via the bacterial-type pathways in archaea, in Halorhabdus species that likely acquired these pathways from bacteria. The utilization of bacterial-type pathways of pentose degradation rather than the archaeal oxidative pathways generating α-ketoglutarate might be explained by an incomplete gluconeogenesis in Halorhabdus species preventing the utilization of α-ketoglutarate in the anabolism.

Characterization of highly active 2-keto-3-deoxy-L-arabinonate and 2-keto-3-deoxy-D-xylonate dehydratases in terms of the biotransformation of hemicellulose sugars to chemicals

2-keto-3-L-arabinonate dehydratase (L-KdpD) and 2-keto-3-D-xylonate dehydratase (D-KdpD) are the third enzymes in the Weimberg pathway catalyzing the dehydration of respective 2-keto-3-deoxy sugar acids (KDP) to α-ketoglutaric semialdehyde (KGSA). The Weimberg pathway has been explored recently with respect to the synthesis of chemicals from L-arabinose and D-xylose. However, only limited work has been done toward characterizing these two enzymes. In this work, several new L-KdpDs and D-KdpDs were cloned and heterologously expressed in Escherichia coli. Following kinetic characterizations and kinetic stability studies, the L-KdpD from Cupriavidus necator (CnL-KdpD) and D-KdpD from Pseudomonas putida (PpD-KdpD) appeared to be the most promising variants from each enzyme class. Magnesium had no effect on CnL-KdpD, whereas increased activity and stability were observed for PpD-KdpD in the presence of Mg2+. Furthermore, CnL-KdpD was not inhibited in the presence of L-arabinose and L-arabinonate, whereas PpD-KdpD was inhibited with D-xylonate (I50 of 75 mM), but not with D-xylose. Both enzymes were shown to be highly active in the one-step conversions of L-KDP and D-KDP. CnL-KdpD converted > 95% of 500 mM L-KDP to KGSA in the first 2 h while PpD-KdpD converted > 90% of 500 mM D-KDP after 4 h. Both enzymes in combination were able to convert 83% of a racemic mixture of D,L-KDP (500 mM) after 4 h, with both enzymes being specific toward the respective stereoisomer. Key points • L-KdpDs and D-KdpDs are specific toward L- and D-KDP, respectively. • Mg2+affected activity and stabilities of D-KdpDs, but not of L-KdpDs. • CnL-KdpD and PpD-KdpD converted 0.5 M of each KDP isomer reaching 95 and 90% yield. • Both enzymes in combination converted 0.5 M racemic D,L-KDP reaching 83% yield.

Alternative carbohydrate pathways - enzymes, functions and engineering

Metabolic engineering is crucial in the development of production strains for platform chemicals, pharmaceuticals and biomaterials from renewable resources. The central carbon metabolism (CCM) of heterotrophs plays an essential role in the conversion of biomass to the cellular building blocks required for growth. Yet, engineering the CCM ultimately aims toward a maximization of flux toward products of interest. The most abundant dissimilative carbohydrate pathways amongst prokaryotes (and eukaryotes) are the Embden-Meyerhof-Parnas (EMP) and the Entner-Doudoroff (ED) pathways, which build the basics for heterotrophic metabolic chassis strains. Although the EMP is regarded as the textbook example of a carbohydrate pathway owing to its central role in production strains like Escherichia coli, Saccharomyces cerevisiae and Bacillus subtilis, it is either modified, complemented or even replaced by alternative carbohydrate pathways in different organisms. The ED pathway also plays key roles in biotechnological relevant bacteria, like Zymomonas mobilis and Pseudomonas putida, and its importance was recently discovered in photoautotrophs and marine microorganisms. In contrast to the EMP, the ED pathway and its variations are not evolutionary optimized for high ATP production and it differs in key principles such as protein cost, energetics and thermodynamics, which can be exploited in the construction of unique metabolic designs. Single ED pathway enzymes and complete ED pathway modules have been used to rewire carbon metabolisms in production strains and for the construction of cell-free enzymatic pathways. This review focuses on the differences of the ED and EMP pathways including their variations and discusses the use of alternative pathway strategies for in vivo and cell-free metabolic engineering.

Identification of XylR, the Activator of Arabinose/Xylose Inducible Regulon in Sulfolobus acidocaldarius and Its Application for Homologous Protein Expression

The thermophilic archaeon Sulfolobus acidocaldarius can use different carbon sources for growth, including the pentoses D-xylose and L-arabinose. In this study, we identified the activator XylR (saci_2116) responsible for the transcriptional regulation of the pentose transporter and pentose metabolizing genes in S. acidocaldarius. A xylR deletion mutant showed growth retardation on D-xylose/L-arabinose containing media and the lack of transcription of the respective ABC transporter. In contrast to so far used promoters for expression in S. acidocaldarius, the xylR responsive promoters have a very low background activity. Finally, two XylR dependent promoters next to the long-established maltose inducible promotor were used to construct a high-throughput expression vector system for S. acidocaldarius to efficiently clone and express proteins in S. acidocaldarius.

The pentose phosphate pathway of cellulolytic clostridia relies on 6-phosphofructokinase instead of transaldolase

The genomes of most cellulolytic clostridia do not contain genes annotated as transaldolase. Therefore, for assimilating pentose sugars or for generating C5 precursors (such as ribose) during growth on other (non-C5) substrates, they must possess a pathway that connects pentose metabolism with the rest of metabolism. Here we provide evidence that for this connection cellulolytic clostridia rely on the sedoheptulose 1,7-bisphosphate (SBP) pathway, using pyrophosphate-dependent phosphofructokinase (PPi-PFK) instead of transaldolase. In this reversible pathway, PFK converts sedoheptulose 7-phosphate (S7P) to SBP, after which fructose-bisphosphate aldolase cleaves SBP into dihydroxyacetone phosphate and erythrose 4-phosphate. We show that PPi-PFKs of Clostridium thermosuccinogenes and Clostridium thermocellum indeed can convert S7P to SBP, and have similar affinities for S7P and the canonical substrate fructose 6-phosphate (F6P). By contrast, (ATP-dependent) PfkA of Escherichia coli, which does rely on transaldolase, had a very poor affinity for S7P. This indicates that the PPi-PFK of cellulolytic clostridia has evolved the use of S7P. We further show that C. thermosuccinogenes contains a significant SBP pool, an unusual metabolite that is elevated during growth on xylose, demonstrating its relevance for pentose assimilation. Last, we demonstrate that a second PFK of C. thermosuccinogenes that operates with ATP and GTP exhibits unusual kinetics toward F6P, as it appears to have an extremely high degree of cooperative binding, resulting in a virtual on/off switch for substrate concentrations near its K½ value. In summary, our results confirm the existence of an SBP pathway for pentose assimilation in cellulolytic clostridia.

d-Ribose Catabolism in Archaea: Discovery of a Novel Oxidative Pathway in Haloarcula Species

The Haloarcula species H. marismortui and H. hispanica were found to grow on d-ribose, d-xylose, and l-arabinose. Here, we report the discovery of a novel promiscuous oxidative pathway of pentose degradation based on genome analysis, identification and characterization of enzymes, transcriptional analysis, and growth experiments with knockout mutants. Together, the data indicate that in Haloarcula spp., d-ribose, d-xylose, and l-arabinose were degraded to α-ketoglutarate involving the following enzymes: (i) a promiscuous pentose dehydrogenase that catalyzed the oxidation of d-ribose, d-xylose, and l-arabinose; (ii) a promiscuous pentonolactonase that was involved in the hydrolysis of ribonolactone, xylonolactone, and arabinolactone; (iii) a highly specific dehydratase, ribonate dehydratase, which catalyzed the dehydration of ribonate, and a second enzyme, a promiscuous xylonate/gluconate dehydratase, which was involved in the conversion of xylonate, arabinonate, and gluconate. Phylogenetic analysis indicated that the highly specific ribonate dehydratase constitutes a novel sugar acid dehydratase family within the enolase superfamily; and (iv) finally, 2-keto-3-deoxypentanonate dehydratase and α-ketoglutarate semialdehyde dehydrogenase catalyzed the conversion of 2-keto-3-deoxypentanonate to α-ketoglutarate via α-ketoglutarate semialdehyde. We conclude that the expanded substrate specificities of the pentose dehydrogenase and pentonolactonase toward d-ribose and ribonolactone, respectively, and the presence of a highly specific ribonate dehydratase are prerequisites of the oxidative degradation of d-ribose in Haloarcula spp. This is the first characterization of an oxidative degradation pathway of d-ribose to α-ketoglutarate in archaea.IMPORTANCE The utilization and degradation of d-ribose in archaea, the third domain of life, have not been analyzed so far. We show that Haloarcula species utilize d-ribose, which is degraded to α-ketoglutarate via a novel oxidative pathway. Evidence is presented that the oxidative degradation of d-ribose involves novel promiscuous enzymes, pentose dehydrogenase and pentonolactonase, and a novel sugar acid dehydratase highly specific for ribonate. This is the first report of an oxidative degradation pathway of d-ribose in archaea, which differs from the canonical nonoxidative pathway of d-ribose degradation reported for most bacteria. The data contribute to our understanding of the unusual sugar degradation pathways and enzymes in archaea.

The gastrointestinal pathogen Campylobacter jejuni metabolizes sugars with potential help from commensal Bacteroides vulgatus

Although the gastrointestinal pathogen Campylobacter jejuni was considered asaccharolytic, >50% of sequenced isolates possess an operon for L-fucose utilization. In C. jejuni NCTC11168, this pathway confers L-fucose chemotaxis and competitive colonization advantages in the piglet diarrhea model, but the catabolic steps remain unknown. Here we solved the putative dehydrogenase structure, resembling FabG of Burkholderia multivorans. The C. jejuni enzyme, FucX, reduces L-fucose and D-arabinose in vitro and both sugars are catabolized by fuc-operon encoded enzymes. This enzyme alone confers chemotaxis to both sugars in a non-carbohydrate-utilizing C. jejuni strain. Although C. jejuni lacks fucosidases, the organism exhibits enhanced growth in vitro when co-cultured with Bacteroides vulgatus, suggesting scavenging may occur. Yet, when excess amino acids are available, C. jejuni prefers them to carbohydrates, indicating a metabolic hierarchy exists. Overall this study increases understanding of nutrient metabolism by this pathogen, and identifies interactions with other gut microbes.

Engineering microbial pathways for production of bio-based chemicals from lignocellulosic sugars: current status and perspectives

Lignocellulose is the most abundant biomass on earth with an annual production of about 2 × 10¹¹ tons. It is an inedible renewable carbonaceous resource that is very rich in pentose and hexose sugars. The ability of microorganisms to use lignocellulosic sugars can be exploited for the production of biofuels and chemicals, and their concurrent biotechnological processes could advantageously replace petrochemicals' processes in a medium to long term, sustaining the emerging of a new economy based on bio-based products from renewable carbon sources. One of the major issues to reach this objective is to rewire the microbial metabolism to optimally configure conversion of these lignocellulosic-derived sugars into bio-based products in a sustainable and competitive manner. Systems' metabolic engineering encompassing synthetic biology and evolutionary engineering appears to be the most promising scientific and technological approaches to meet this challenge. In this review, we examine the most recent advances and strategies to redesign natural and to implement non-natural pathways in microbial metabolic framework for the assimilation and conversion of pentose and hexose sugars derived from lignocellulosic material into industrial relevant chemical compounds leading to maximal yield, titer and productivity. These include glycolic, glutaric, mesaconic and 3,4-dihydroxybutyric acid as organic acids, monoethylene glycol, 1,4-butanediol and 1,2,4-butanetriol, as alcohols. We also discuss the big challenges that still remain to enable microbial processes to become industrially attractive and economically profitable.

Characterization of l-2-keto-3-deoxyfuconate aldolases in a nonphosphorylating l-fucose metabolism pathway in anaerobic bacteria

The genetic context in bacterial genomes and screening for potential substrates can help identify the biochemical functions of bacterial enzymes. The Gram-negative, strictly anaerobic bacterium Veillonella ratti possesses a gene cluster that appears to be related to l-fucose metabolism and contains a putative dihydrodipicolinate synthase/N-acetylneuraminate lyase protein (FucH). Here, screening of a library of 2-keto-3-deoxysugar acids with this protein and biochemical characterization of neighboring genes revealed that this gene cluster encodes enzymes in a previously unknown "route I" nonphosphorylating l-fucose pathway. Previous studies of other aldolases in the dihydrodipicolinate synthase/N-acetylneuraminate lyase protein superfamily used only limited numbers of compounds, and the approach reported here enabled elucidation of the substrate specificities and stereochemical selectivities of these aldolases and comparison of them with those of FucH. According to the aldol cleavage reaction, the aldolases were specific for (R)- and (S)-stereospecific groups at the C4 position of 2-keto-3-deoxysugar acid but had no structural specificity or preference of methyl groups at the C5 and C6 positions, respectively. This categorization corresponded to the (Re)- or (Si)-facial selectivity of the pyruvate enamine on the (glycer)aldehyde carbonyl in the aldol-condensation reaction. These properties are commonly determined by whether a serine or threonine residue is positioned at the equivalent position close to the active site(s), and site-directed mutagenesis markedly modified C4-OH preference and selective formation of a diastereomer. I propose that substrate specificity of 2-keto-3-deoxysugar acid aldolases was convergently acquired during evolution and report the discovery of another l-2-keto-3-deoxyfuconate aldolase involved in the same nonphosphorylating l-fucose pathway in Campylobacter jejuni.

Substrate and metabolic promiscuities of d-altronate dehydratase family proteins involved in non-phosphorylative d-arabinose, sugar acid, l-galactose and l-fucose pathways from bacteria

The gene context in microorganism genomes is of considerable help for identifying potential substrates. The C785_RS13685 gene in Herbaspirillum huttiense IAM 15032 is a member of the d-altronate dehydratase protein family, and which functions as a d-arabinonate dehydratase in vitro, is clustered with genes related to putative pentose metabolism. In the present study, further biochemical characterization and gene expression analyses revealed that l-xylonate is a physiological substrate that is ultimately converted to α-ketoglutarate via so-called Route II of a non-phosphorylative pathway. Several hexonates, including d-altronate, d-idonate and l-gluconate, which are also substrates of C785_RS13685, also significantly up-regulated the gene cluster containing C785_RS13685, suggesting a possibility that pyruvate and d- or l-glycerate were ultimately produced (novel Route III). On the contrary, ACAV_RS08155 of Acidovorax avenae ATCC 19860, a homologous gene to C785_RS13685, functioned as a d-altronate dehydratase in a novel l-galactose pathway, through which l-galactonate was epimerized at the C5 position by the sequential activity of two dehydrogenases, resulting in d-altronate. Furthermore, this pathway completely overlapped with Route III of the non-phosphorylative l-fucose pathway. The 'substrate promiscuity' of d-altronate dehydratase protein(s) is significantly expanded to 'metabolic promiscuity' in the d-arabinose, sugar acid, l-fucose and l-galactose pathways.

In vitro reconstitution and characterisation of the oxidative D-xylose pathway for production of organic acids and alcohols

The oxidative d-xylose pathway, i.e. Dahms pathway, can be utilised to produce from cheap biomass raw material useful chemical intermediates. In vitro metabolic pathways offer a fast way to study the rate-limiting steps and find the most suitable enzymes for each reaction. We have constructed here in vitro multi-enzyme cascades leading from d-xylose or d-xylonolactone to ethylene glycol, glycolic acid and lactic acid, and use simple spectrophotometric assays for the read-out of the efficiency of these pathways. Based on our earlier results, we focussed particularly on the less studied xylonolactone ring opening (hydrolysis) reaction. The bacterial Caulobacter crescentus lactonase (Cc XylC), was shown to be a metal-dependent enzyme clearly improving the formation of d-xylonic acid at pH range from 6 to 8. The following dehydration reaction by the ILVD/EDD family d-xylonate dehydratase is a rate-limiting step in the pathway, and an effort was made to screen for novel enolase family d-xylonate dehydratases, however, no suitable replacing enzymes were found for this reaction. Concerning the oxidation of glycolaldehyde to glycolic acid, several enzyme candidates were also tested. Both Escherichia coli aldehyde dehydrogenase (Ec AldA) and Azospirillum brasilense α-ketoglutarate semialdehyde dehydrogenase (Ab AraE) proved to be suitable enzymes for this reaction.

Uptake of D-xylose and L-arabinose in Haloferax volcanii involves an ABC transporter of the CUT1 subfamily

Haloferax volcanii degrades D-xylose and L-arabinose via an oxidative pathway to α-ketoglutarate as an intermediate. The enzymes of this pathway are encoded by the xac gene cluster (xylose and arabinose catabolism) which also contains genes (xacGHIJK) that encode all components of a putative ABC transporter. The xacGHIJK genes encode one substrate binding protein, two transmembrane domains and two nucleotide binding domains. It is shown here, that xacGHIJK is upregulated by both D-xylose and L-arabinose mediated by the transcriptional regulator XacR, the general regulator of xac genes. Knock-out mutants of xacG and of xacGHIJK resulted in a reduced growth rate on both pentoses; wild type growth could be recovered by complementation in trans. Together, the data indicate that uptake of xylose and arabinose in H. volcanii is mediated by this ABC transporter. Pentose specific ABC transporters, homologous to that of H. volcanii, were identified in other haloarchaea suggesting a similar function in pentose uptake in these archaea. Sequence analyses attribute the haloarchaeal pentose ABC transporter to the CUT1 (carbohydrate uptake transporter 1) subfamily.

l-Rhamnose catabolism in archaea

The halophilic archaeon Haloferax volcanii utilizes l-rhamnose as a sole carbon and energy source. It is shown that l-rhamnose is taken up by an ABC transporter and is oxidatively degraded to pyruvate and l-lactate via the diketo-hydrolase pathway. The genes involved in l-rhamnose uptake and degradation form a l-rhamnose catabolism (rhc) gene cluster. The rhc cluster also contains a gene, rhcR, that encodes the transcriptional regulator RhcR which was characterized as an activator of all rhc genes. 2-keto-3-deoxy-l-rhamnonate, a metabolic intermediate of l-rhamnose degradation, was identified as inducer molecule of RhcR. The essential function of rhc genes for uptake and degradation of l-rhamnose was proven by the respective knockout mutants. Enzymes of the diketo-hydrolase pathway, including l-rhamnose dehydrogenase, l-rhamnonolactonase, l-rhamnonate dehydratase, 2-keto-3-deoxy-l-rhamnonate dehydrogenase and 2,4-diketo-3-deoxy-l-rhamnonate hydrolase, were characterized. Further, genes of the diketo-hydrolase pathway were also identified in the hyperthermophilic crenarchaeota Vulcanisaeta distributa and Sulfolobus solfataricus and selected enzymes were characterized, indicating the presence of the diketo-hydrolase pathway in these archaea. Together, this is the first comprehensive description of l-rhamnose catabolism in the domain of archaea.

Novel non-phosphorylative pathway of pentose metabolism from bacteria

Pentoses, including D-xylose, L-arabinose, and D-arabinose, are generally phosphorylated to D-xylulose 5-phosphate in bacteria and fungi. However, in non-phosphorylative pathways analogous to the Entner-Dodoroff pathway in bacteria and archaea, such pentoses can be converted to pyruvate and glycolaldehyde (Route I) or α-ketoglutarate (Route II) via a 2-keto-3-deoxypentonate (KDP) intermediate. Putative gene clusters related to these metabolic pathways were identified on the genome of Herbaspirillum huttiense IAM 15032 using a bioinformatic analysis. The biochemical characterization of C785_RS13685, one of the components encoded to D-arabinonate dehydratase, differed from the known acid-sugar dehydratases. The biochemical characterization of the remaining components and a genetic expression analysis revealed that D- and L-KDP were converted not only to α-ketoglutarate, but also pyruvate and glycolate through the participation of dehydrogenase and hydrolase (Route III). Further analyses revealed that the Route II pathway of D-arabinose metabolism was not evolutionally related to the analogous pathway from archaea.

Everyone loves an underdog: metabolic engineering of the xylose oxidative pathway in recombinant microorganisms

The D-xylose oxidative pathway (XOP) has recently been employed in several recombinant microorganisms for growth or for the production of several valuable compounds. The XOP is initiated by D-xylose oxidation to D-xylonolactone, which is then hydrolyzed into D-xylonic acid. D-Xylonic acid is then dehydrated to form 2-keto-3-deoxy-D-xylonic acid, which may be further dehydrated then oxidized into α-ketoglutarate or undergo aldol cleavage to form pyruvate and glycolaldehyde. This review introduces a brief discussion about XOP and its discovery in bacteria and archaea, such as Caulobacter crescentus and Haloferax volcanii. Furthermore, the current advances in the metabolic engineering of recombinant strains employing the XOP are discussed. This includes utilization of XOP for the production of diols, triols, and short-chain organic acids in Escherichia coli, Saccharomyces cerevisiae, and Corynebacterium glutamicum. Improving the D-xylose uptake, growth yields, and product titer through several metabolic engineering techniques bring some of these recombinant strains close to industrial viability. However, more developments are still needed to optimize the XOP pathway in the host strains, particularly in the minimization of by-product formation.

Mutant phenotypes for thousands of bacterial genes of unknown function

One-third of all protein-coding genes from bacterial genomes cannot be annotated with a function. Here, to investigate the functions of these genes, we present genome-wide mutant fitness data from 32 diverse bacteria across dozens of growth conditions. We identified mutant phenotypes for 11,779 protein-coding genes that had not been annotated with a specific function. Many genes could be associated with a specific condition because the gene affected fitness only in that condition, or with another gene in the same bacterium because they had similar mutant phenotypes. Of the poorly annotated genes, 2,316 had associations that have high confidence because they are conserved in other bacteria. By combining these conserved associations with comparative genomics, we identified putative DNA repair proteins; in addition, we propose specific functions for poorly annotated enzymes and transporters and for uncharacterized protein families. Our study demonstrates the scalability of microbial genetics and its utility for improving gene annotations.

Characterization of a pentonolactonase involved in D-xylose and L-arabinose catabolism in the haloarchaeon Haloferax volcanii

Haloferax volcanii degrades the pentoses D-xylose and L-arabinose via an oxidative pathway to α-ketoglutarate as an intermediate. The initial dehydrogenases of the pathway, D-xylose dehydrogenase (XDH) and L-arabinose dehydrogenase (L-AraDH) catalyze the NADP+ dependent D-xylose and L-arabinose oxidation. It is shown here that the pentoses are oxidized to the corresponding lactones, D-xylono-γ-lactone and L-arabino-γ-lactone, rather than to the respective sugar acids. A putative lactonase gene, xacC, located in genomic vicinity of XDH and L-AraDH, was found to be transcriptionally upregulated by both D-xylose and L-arabinose mediated by the pentose-specific regulator XacR. The recombinant lactonase catalyzed the hydrolysis of D-xylono-γ-lactone and L-arabino-γ-lactone. This is the first report of a functional lactonase involved in sugar catabolism in the domain of archaea.

Sulfolobus acidocaldarius Transports Pentoses via a Carbohydrate Uptake Transporter 2 (CUT2)-Type ABC Transporter and Metabolizes Them through the Aldolase-Independent Weimberg Pathway

Sulfolobus spp. possess a great metabolic versatility and grow heterotrophically on various carbon sources, such as different sugars and peptides. Known sugar transporters in Archaea predominantly belong to ABC transport systems. Although several ABC transporters for sugar uptake have been characterized in the crenarchaeon Sulfolobus solfataricus, only one homologue of these transporters, the maltose/maltooligomer transporter, could be identified in the closely related Sulfolobus acidocaldarius Comparison of the transcriptome of S. acidocaldarius MW001 grown on peptides alone and peptides in the presence of d-xylose allowed for the identification of the ABC transporter for d-xylose and l-arabinose transport and the gaining of deeper insights into pentose catabolism under the respective growth conditions. The d-xylose/l-arabinose substrate binding protein (SBP) (Saci_2122) of the ABC transporter is unique in Archaea and shares more similarity to bacterial SBPs of the carbohydrate uptake transporter-2 (CUT2) family than to any characterized archaeal one. The identified pentose transporter is the first CUT2 family ABC transporter analyzed in the domain of Archaea Single-gene deletion mutants of the ABC transporter subunits exemplified the importance of the transport system for d-xylose and l-arabinose uptake. Next to the transporter operon, enzymes of the aldolase-independent pentose catabolism branch were found to be upregulated in N-Z-Amine and d-xylose medium. The α-ketoglutarate semialdehyde dehydrogenase (KGSADH; Saci_1938) seemed not to be essential for growth on pentoses. However, the deletion mutant of the 2-keto-3-deoxyarabinoate/xylonate dehydratase (KDXD [also known as KDAD]; Saci_1939) was no longer able to catabolize d-xylose or l-arabinose, suggesting the absence of the aldolase-dependent branch in S. acidocaldarius IMPORTANCE Thermoacidophilic microorganisms are emerging model organisms for biotechnological applications, as their optimal growth conditions resemble conditions used in certain biotechnologies such as industrial plant waste degradation. Because of its high genome stability, Sulfolobus acidocaldarius is especially suited as a platform organism for such applications. For use in (ligno)cellulose degradation, it was important to understand pentose uptake and metabolism in S. acidocaldarius This study revealed that only the aldolase-independent Weimberg pathway is required for growth of S. acidocaldarius MW001 on d-xylose and l-arabinose. Moreover, S. acidocaldarius employs a CUT2 ABC transporter for pentose uptake, which is more similar to bacterial than to archaeal ABC transporters. The identification of pentose-inducible promoters will expedite the metabolic engineering of S. acidocaldarius for its development into a platform organism for (ligno)cellulose degradation.

Sulfolobus - A Potential Key Organism in Future Biotechnology

Extremophilic organisms represent a potentially valuable resource for the development of novel bioprocesses. They can act as a source for stable enzymes and unique biomaterials. Extremophiles are capable of carrying out microbial processes and biotransformations under extremely hostile conditions. Extreme thermoacidophilic members of the well-characterized genus Sulfolobus are outstanding in their ability to thrive at both high temperatures and low pH. This review gives an overview of the biological system Sulfolobus including its central carbon metabolism and the development of tools for its genetic manipulation. We highlight findings of commercial relevance and focus on potential industrial applications. Finally, the current state of bioreactor cultivations is summarized and we discuss the use of Sulfolobus species in biorefinery applications.

Crystal structure analysis of 3,6-anhydro-l-galactonate cycloisomerase suggests emergence of novel substrate specificity in the enolase superfamily

3,6-Anydro-l-galatonate cycloisomerase (ACI) catalyzes the cycloisomerization of a 3,6-anhydro-l-galactonic acid known as a novel metabolite in agarolytic bacteria. Here, we present 3-D structures of ACI from Vibrio sp. strain EJY3 (VejACI) in native and mutant forms at 2.2 Å and 2.6 Å resolutions, respectively. The enzyme belongs to the mandelate racemase subgroup of the enolase superfamily catalyzing common β-elimination reactions by α-carbon deprotonation of substrates. The structure of VejACI revealed a notable 20s loop region in the capping domain, which can be a highly conserved structural motif in ACI homologs of agar metabolism. By comparing mutant (mVejAC/H300 N) and native VejACI structures, we identified a conformational change of Ile142 in VejACI that causes spatial expansion in the binding pocket. These observations imply that Ile142 and the 20s loop play important roles in enzymatic reactivity and substrate specificity. The structural phylogenetic analysis of the enolase superfamily including ACIs revealed sequential, structural, and functional relationships related to the emergence of novel substrate specificity.

The Crystal Structure of a Bacterial l-Arabinonate Dehydratase Contains a [2Fe-2S] Cluster

We present a novel crystal structure of the IlvD/EDD family enzyme, l-arabinonate dehydratase from Rhizobium leguminosarum bv. trifolii (RlArDHT, EC 4.2.1.25), which catalyzes the conversion of l-arabinonate to 2-dehydro-3-deoxy-l-arabinonate. The enzyme is a tetramer consisting of a dimer of dimers, where each monomer is composed of two domains. The active site contains a catalytically important [2Fe-2S] cluster and Mg²⁺ ion and is buried between two domains, and also at the dimer interface. The active site Lys129 was found to be carbamylated. Ser480 and Thr482 were shown to be essential residues for catalysis, and the S480A mutant structure showed an unexpected open conformation in which the active site was more accessible for the substrate. This structure showed the partial binding of l-arabinonate, which allowed us to suggest that the alkoxide ion form of the Ser480 side chain functions as a base and the [2Fe-2S] cluster functions as a Lewis acid in the elimination reaction.

Oxidative Stickland reactions in an obligate aerobic organism - amino acid catabolism in the Crenarchaeon Sulfolobus solfataricus

The thermoacidophilic Crenarchaeon Sulfolobus solfataricus is a model organism for archaeal adaptation to extreme environments and renowned for its ability to degrade a broad variety of substrates. It has been well characterised concerning the utilisation of numerous carbohydrates as carbon source. However, its amino acid metabolism, especially the degradation of single amino acids, is not as well understood. In this work, we performed metabolic modelling as well as metabolome, transcriptome and proteome analysis on cells grown on caseinhydrolysate as carbon source in order to draw a comprehensive picture of amino acid metabolism in S. solfataricus P2. We found that 10 out of 16 detectable amino acids are imported from the growth medium. Overall, uptake of glutamate, methionine, leucine, phenylalanine and isoleucine was the highest of all observed amino acids. Our simulations predict an incomplete degradation of leucine and tyrosine to organic acids, and in accordance with this, we detected the export of branched-chain and aromatic organic acids as well as amino acids, ammonium and trehalose into the culture supernatants. The branched-chain amino acids as well as phenylalanine and tyrosine are degraded to organic acids via oxidative Stickland reactions. Such reactions are known for prokaryotes capable of anaerobic growth, but so far have never been observed in an obligate aerobe. Also, 3-methyl-2-butenoate and 2-methyl-2-butenoate are for the first time found as products of modified Stickland reactions for the degradation of branched-chain amino acids. This work presents the first detailed description of branched-chain and aromatic amino acid catabolism in S. solfataricus.

A conserved hexanucleotide motif is important in UV-inducible promoters in Sulfolobus acidocaldarius

Upon DNA damage, Sulfolobales exhibit a global gene regulatory response resulting in the expression of DNA transfer and repair proteins and the repression of the cell division machinery. Because the archaeal DNA damage response is still poorly understood, we investigated the promoters of the highly induced ups operon. Ups pili are involved in cellular aggregation and DNA exchange between cells. With LacS reporter gene assays we identified a conserved, non-palindromic hexanucleotide motif upstream of the ups core promoter elements to be essential for promoter activity. Substitution of this cis regulatory motif in the ups promoters resulted in abolishment of cellular aggregation and reduced DNA transfer. By screening the Sulfolobus acidocaldarius genome we identified a total of 214 genes harbouring the hexanucleotide motif in their respective promoter regions. Many of these genes were previously found to be regulated upon UV light treatment. Given the fact that the identified motif is conserved among S. acidocaldarius and Sulfolobus tokodaii promoters, we speculate that a common regulatory mechanism is present in these two species in response to DNA-damaging conditions.

Identification and characterization of d-arabinose reductase and d-arabinose transporters from Pichia stipitis

d-xylose and l-arabinose are the major constituents of plant lignocelluloses, and the related fungal metabolic pathways have been extensively examined. Although Pichia stipitis CBS 6054 grows using d-arabinose as the sole carbon source, the hypothetical pathway has not yet been clarified at the molecular level. We herein purified NAD(P)H-dependent d-arabinose reductase from cells grown on d-arabinose, and found that the enzyme was identical to the known d-xylose reductase (XR). The enzyme activity of XR with d-arabinose was previously reported to be only 1% that with d-xylose. The kcat/Km value with d-arabinose (1.27 min−1 mM−1), which was determined using the recombinant enzyme, was 13.6- and 10.5-fold lower than those with l-arabinose and d-xylose, respectively. Among the 34 putative sugar transporters from P. stipitis, only seven genes exhibited uptake ability not only for d-arabinose, but also for d-glucose and other pentose sugars including d-xylose and l-arabinose in Saccharomyces cerevisiae.

A systems biology approach reveals major metabolic changes in the thermoacidophilic archaeon Sulfolobus solfataricus in response to the carbon source L-fucose versus D-glucose

Archaea are characterised by a complex metabolism with many unique enzymes that differ from their bacterial and eukaryotic counterparts. The thermoacidophilic archaeon Sulfolobus solfataricus is known for its metabolic versatility and is able to utilize a great variety of different carbon sources. However, the underlying degradation pathways and their regulation are often unknown. In this work, the growth on different carbon sources was analysed, using an integrated systems biology approach. The comparison of growth on L-fucose and D-glucose allows first insights into the genome-wide changes in response to the two carbon sources and revealed a new pathway for L-fucose degradation in S. solfataricus. During growth on L-fucose major changes in the central carbon metabolic network, as well as an increased activity of the glyoxylate bypass and the 3-hydroxypropionate/4-hydroxybutyrate cycle were observed. Within the newly discovered pathway for L-fucose degradation the following key reactions were identified: (i) L-fucose oxidation to L-fuconate via a dehydrogenase, (ii) dehydration to 2-keto-3-deoxy-L-fuconate via dehydratase, (iii) 2-keto-3-deoxy-L-fuconate cleavage to pyruvate and L-lactaldehyde via aldolase and (iv) L-lactaldehyde conversion to L-lactate via aldehyde dehydrogenase. This pathway as well as L-fucose transport shows interesting overlaps to the D-arabinose pathway, representing another example for pathway promiscuity in Sulfolobus species.

Crystallization and X-ray diffraction analysis of an L-arabinonate dehydratase from Rhizobium leguminosarum bv. trifolii and a D-xylonate dehydratase from Caulobacter crescentus

l-Arabinonate dehydratase and d-xylonate dehydratase from the IlvD/EDD family were crystallized by the vapour-diffusion method. Diffraction data sets were collected to resolutions of 2.40 and 2.66 Å from crystals of l-arabinonate dehydratase and d-xylonate dehydratase, respectively.

l-Arabinonate dehydratase (EC 4.2.1.25) and d-xylonate dehydratase (EC 4.2.1.82) are two enzymes that are involved in a nonphosphorylative oxidation pathway of pentose sugars. l-Arabinonate dehydratase converts l-arabinonate into 2-dehydro-3-deoxy-l-arabinonate, and d-xylonate dehydratase catalyzes the dehydration of d-xylonate to 2-dehydro-3-deoxy-d-xylonate. l-Arabinonate and d-xylonate dehydratases belong to the IlvD/EDD family, together with 6-phosphogluconate dehydratases and dihydroxyacid dehydratases. No crystal structure of any l-arabinonate or d-xylonate dehydratase is available in the PDB. In this study, recombinant l-arabinonate dehydratase from Rhizobium leguminosarum bv. trifolii (RlArDHT) and d-xylonate dehydratase from Caulobacter crescentus (CcXyDHT) were heterologously expressed in Escherichia coli and purified by the use of affinity chromatography followed by gel-filtration chromatography. The purified proteins were crystallized using the hanging-drop vapour-diffusion method at 293 K. Crystals of RlArDHT that diffracted to 2.40 Å resolution were obtained using sodium formate as a precipitating agent. They belonged to space group P2₁, with unit-cell parameters a = 106.07, b = 208.61, c = 147.09 Å, β = 90.43°. Eight RlArDHT molecules (two tetramers) in the asymmetric unit give a V_M value of 3.2 ų Da⁻¹ and a solvent content of 62%. Crystals of CcXyDHT that diffracted to 2.66 Å resolution were obtained using sodium formate and polyethylene glycol 3350. They belonged to space group C2, with unit-cell parameters a = 270.42, b = 236.13, c = 65.17 Å, β = 97.38°. Four CcXyDHT molecules (a tetramer) in the asymmetric unit give a V_M value of 4.0 ų Da⁻¹ and a solvent content of 69%.

Characterization and mutagenesis of two novel iron-sulphur cluster pentonate dehydratases

We describe here the identification and characterization of two novel enzymes belonging to the IlvD/EDD protein family, the D-xylonate dehydratase from Caulobacter crescentus, Cc XyDHT, (EC 4.2.1.82), and the L-arabonate dehydratase from Rhizobium leguminosarum bv. trifolii, Rl ArDHT (EC 4.2.1.25), that produce the corresponding 2-keto-3-deoxy-sugar acids. There is only a very limited amount of characterization data available on pentonate dehydratases, even though the enzymes from these oxidative pathways have potential applications with plant biomass pentose sugars. The two bacterial enzymes share 41 % amino acid sequence identity and were expressed and purified from Escherichia coli as homotetrameric proteins. Both dehydratases were shown to accept pentonate and hexonate sugar acids as their substrates and require Mg(2+) for their activity. Cc XyDHT displayed the highest activity on D-xylonate and D-gluconate, while Rl ArDHT functioned best on D-fuconate, L-arabonate and D-galactonate. The configuration of the OH groups at C2 and C3 position of the sugar acid were shown to be critical, and the C4 configuration also contributed substantially to the substrate recognition. The two enzymes were also shown to contain an iron-sulphur [Fe-S] cluster. Our phylogenetic analysis and mutagenesis studies demonstrated that the three conserved cysteine residues in the aldonic acid dehydratase group of IlvD/EDD family members, those of C60, C128 and C201 in Cc XyDHT, and of C59, C127 and C200 in Rl ArDHT, are needed for coordination of the [Fe-S] cluster. The iron-sulphur cluster was shown to be crucial for the catalytic activity (kcat) but not for the substrate binding (Km) of the two pentonate dehydratases.

L-Hydroxyproline and d-Proline Catabolism in Sinorhizobium meliloti

Sinorhizobium meliloti forms N2-fixing root nodules on alfalfa, and as a free-living bacterium, it can grow on a very broad range of substrates, including l-proline and several related compounds, such as proline betaine, trans-4-hydroxy-l-proline (trans-4-l-Hyp), and cis-4-hydroxy-d-proline (cis-4-d-Hyp). Fourteen hyp genes are induced upon growth of S. meliloti on trans-4-l-Hyp, and of those, hypMNPQ encodes an ABC-type trans-4-l-Hyp transporter and hypRE encodes an epimerase that converts trans-4-l-Hyp to cis-4-d-Hyp in the bacterial cytoplasm. Here, we present evidence that the HypO, HypD, and HypH proteins catalyze the remaining steps in which cis-4-d-Hyp is converted to α-ketoglutarate. The HypO protein functions as a d-amino acid dehydrogenase, converting cis-4-d-Hyp to Δ(1)-pyrroline-4-hydroxy-2-carboxylate, which is deaminated by HypD to α-ketoglutarate semialdehyde and then converted to α-ketoglutarate by HypH. The crystal structure of HypD revealed it to be a member of the N-acetylneuraminate lyase subfamily of the (α/β)8 protein family and is consistent with the known enzymatic mechanism for other members of the group. It was also shown that S. meliloti can catabolize d-proline as both a carbon and a nitrogen source, that d-proline can complement l-proline auxotrophy, and that the catabolism of d-proline is dependent on the hyp cluster. Transport of d-proline involves the HypMNPQ transporter, following which d-proline is converted to Δ(1)-pyrroline-2-carboxylate (P2C) largely via HypO. The P2C is converted to l-proline through the NADPH-dependent reduction of P2C by the previously uncharacterized HypS protein. Thus, overall, we have now completed detailed genetic and/or biochemical characterization of 9 of the 14 hyp genes.

IMPORTANCE

Hydroxyproline is abundant in proteins in animal and plant tissues and serves as a carbon and a nitrogen source for bacteria in diverse environments, including the rhizosphere, compost, and the mammalian gut. While the main biochemical features of bacterial hydroxyproline catabolism were elucidated in the 1960s, the genetic and molecular details have only recently been determined. Elucidating the genetics of hydroxyproline catabolism will aid in the annotation of these genes in other genomes and metagenomic libraries. This will facilitate an improved understanding of the importance of this pathway and may assist in determining the prevalence of hydroxyproline in a particular environment.

XacR - a novel transcriptional regulator of D-xylose and L-arabinose catabolism in the haloarchaeon Haloferax volcanii

The haloarchaeon Haloferax volcanii degrades D-xylose and L-arabinose via oxidative pathways to α-ketoglutarate. The genes involved in these pathways are clustered and were transcriptionally upregulated by both D-xylose and L-arabinose suggesting a common regulator. Adjacent to the gene cluster, a putative IclR-like transcriptional regulator, HVO_B0040, was identified. It is shown that HVO_B0040, designated xacR, encodes an activator of both D-xylose and L-arabinose catabolism: in ΔxacR cells, transcripts of genes involved in pentose catabolism could not be detected; transcript formation could be recovered by complementation, indicating XacR dependent transcriptional activation. Upstream activation promoter regions and nucleotide sequences that were essential for XacR-mediated activation of pentose-specific genes were identified by in vivo deletion and scanning mutagenesis. Besides its activator function XacR acted as repressor of its own synthesis: xacR deletion resulted in an increase of xacR promoter activity. A palindromic sequence was identified at the operator site of xacR promoter, and mutation of this sequence also resulted in an increase and thus derepression of xacR promoter activity. It is concluded that the palindromic sequence represents the binding site of XacR as repressor. This is the first report of a transcriptional regulator of pentose catabolism in the domain of archaea.

Carbohydrate metabolism in Archaea: current insights into unusual enzymes and pathways and their regulation

The metabolism of Archaea, the third domain of life, resembles in its complexity those of Bacteria and lower Eukarya. However, this metabolic complexity in Archaea is accompanied by the absence of many "classical" pathways, particularly in central carbohydrate metabolism. Instead, Archaea are characterized by the presence of unique, modified variants of classical pathways such as the Embden-Meyerhof-Parnas (EMP) pathway and the Entner-Doudoroff (ED) pathway. The pentose phosphate pathway is only partly present (if at all), and pentose degradation also significantly differs from that known for bacterial model organisms. These modifications are accompanied by the invention of "new," unusual enzymes which cause fundamental consequences for the underlying regulatory principles, and classical allosteric regulation sites well established in Bacteria and Eukarya are lost. The aim of this review is to present the current understanding of central carbohydrate metabolic pathways and their regulation in Archaea. In order to give an overview of their complexity, pathway modifications are discussed with respect to unusual archaeal biocatalysts, their structural and mechanistic characteristics, and their regulatory properties in comparison to their classic counterparts from Bacteria and Eukarya. Furthermore, an overview focusing on hexose metabolic, i.e., glycolytic as well as gluconeogenic, pathways identified in archaeal model organisms is given. Their energy gain is discussed, and new insights into different levels of regulation that have been observed so far, including the transcript and protein levels (e.g., gene regulation, known transcription regulators, and posttranslational modification via reversible protein phosphorylation), are presented.