Mitochondrial alternative oxidase pathway accelerates non-motile cell germination by enhancing respiratory carbon metabolism and maintaining redox poise in Haematococcus pluvialis.

Low efficiency of the cultivation process is a major obstacle in the commercial production of Haematococcus pluvialis. Germination of red, non-motile cells is an efficient strategy for rapid acquisition of zoospores. However, the regulatory mechanisms associated with germination remain unexplored. In the present study, it was confirmed that the mitochondrial alternative oxidase (AOX) pathway accelerates H. pluvialis cell germination, and the regulatory mechanisms were clarified. When the AOX pathway was inhibited, the transcriptomic and metabonomic data revealed a downregulation in respiratory carbon metabolism and nucleotide synthesis due to NADH accumulation. This observation suggested that AOX promoted the rapid consumption of NADH, which accelerated carbohydrate and lipid catabolism, thereby producing carbon skeletons for DNA replication through respiratory metabolism. Moreover, AOX could potentially enhance germination by disturbing the abscisic acid signaling pathway. These findings provide novel insights for developing industrial cultivation models based on red-cell-germination for achieving rapid proliferation of H. pluvialis.

Reductive stress in cancer.

Reductive stress is defined as a condition characterized by excess accumulation of reducing equivalents (e.g., NADH, NADPH, GSH), surpassing the activity of endogenous oxidoreductases. Excessive reducing equivalents can perturb cell signaling pathways, change the formation of disulfide bonding in proteins, disturb mitochondrial homeostasis or decrease metabolism. Reductive stress is influenced by cellular antioxidant load, its flux and a subverted homeostasis that paradoxically can result in excess ROS induction. Balanced reducing equivalents and antioxidant enzymes that contribute to reductive stress can be regulated by Nrf2, typically considered as an oxidative stress induced transcription factor. Cancer cells may coordinate distinct pools of redox couples under reductive stress and these may link to biological consequences from both molecular and translational standpoints. In cancer, there is recent interest in understanding how selective induction of reductive stress may influence therapeutic management and disease progression.

Carbon Monoxide Induced Metabolic Shift in the Carboxydotrophic Parageobacillus thermoglucosidasius DSM 6285.

Parageobacillus thermoglucosidasius is known to catalyse the biological water gas shift (WGS) reaction, a pathway that serves as a source of alternative energy and carbon to a wide variety of bacteria. Despite increasing interest in this bacterium due to its ability to produce biological hydrogen through carbon monoxide (CO) oxidation, there are no data on the effect of toxic CO gas on its physiology. Due to its general requirement of O2, the organism is often grown aerobically to generate biomass. Here, we show that carbon monoxide (CO) induces metabolic changes linked to distortion of redox balance, evidenced by increased accumulation of organic acids such as acetate and lactate. This suggests that P. thermoglucosidasius survives by expressing several alternative pathways, including conversion of pyruvate to lactate, which balances reducing equivalents (oxidation of NADH to NAD+), and acetyl-CoA to acetate, which directly generates energy, while CO is binding terminal oxidases. The data also revealed clearly that P. thermoglucosidasius gained energy and grew during the WGS reaction. Combined, the data provide critical information essential for further development of the biotechnological potential of P. thermoglucosidasius.

The lipid biochemistry of eukaryotic algae.

Algal lipid metabolism fascinates both scientists and entrepreneurs due to the large diversity of fatty acyl structures that algae produce. Algae have therefore long been studied as sources of genes for novel fatty acids; and, due to their superior biomass productivity, algae are also considered a potential feedstock for biofuels. However, a major issue in a commercially viable "algal oil-to-biofuel" industry is the high production cost, because most algal species only produce large amounts of oils after being exposed to stress conditions. Recent studies have therefore focused on the identification of factors involved in TAG metabolism, on the subcellular organization of lipid pathways, and on interactions between organelles. This has been accompanied by the development of genetic/genomic and synthetic biological tools not only for the reference green alga Chlamydomonas reinhardtii but also for Nannochloropsis spp. and Phaeodactylum tricornutum. Advances in our understanding of enzymes and regulatory proteins of acyl lipid biosynthesis and turnover are described herein with a focus on carbon and energetic aspects. We also summarize how changes in environmental factors can impact lipid metabolism and describe present and potential industrial uses of algal lipids.

Understanding lipogenesis by dynamically profiling transcriptional activity of lipogenic promoters in Yarrowia lipolytica.

Lipogenesis is a complicated process involving global transcriptional reprogramming of lipogenic pathways. It is commonly believed that nitrogen starvation triggers a metabolic shift that reroutes carbon flux from Krebs cycles to lipogenesis. In this study, we systematically surveyed and dynamically profiled the transcriptional activity of 22 lipogenic promoters aiming to delineate a picture how nitrogen starvation regulates lipogenesis in Y. lipolytica. These lipogenic promoters drive the expression of critical pathways that are responsible for the generation of reducing equivalents (NADPH), carbon backbones (acetyl-CoA, malonyl-CoA, DHAP, etc.), synthesis and degradation of fatty acids. Specifically, our investigated promoters span across an array of metabolic pathways, including glycolysis, Krebs cycle, pentose phosphate pathway, mannitol cycle, glutamine-GABA cycle, fatty acid and lipid synthesis, glyoxylate, ß-oxidation, and POM (pyruvate-oxaloacetate-malate) cycle. Our work provides evidences that mannitol cycle, glutamine-GABA cycle and amino acid degradation, pyruvate oxidation, and acetate assimilation pathways are lipogenesis-related steps involved in generating cytosolic NADPH and acetyl-CoA precursors. This systematic investigation and dynamic profiling of lipogenic promoters may help us better understand lipogenesis, facilitate the formulation of structure-based kinetic models, as well as develop efficient cell factories for fuels and chemical production in oleaginous species.

A 2D tank test on remediation of nitrobenzene-contaminated aquifer using in-situ reactive zone with emulsified nanoscale zero-valent iron.

Nitrobenzene (NB) is one of the most challenging pollutants for groundwater remediation due to its great harm and recalcitrance. Emulsified nanoscale zero-valent iron (EZVI) is considered as a promising agent for in-situ remediation of contaminated groundwater for its high reactivity, good durability and low cost. In this paper, 2D tank experiment was conducted to evaluate the effectiveness of enhanced remediation of NB-contaminated groundwater with EZVI. 9 L of EZVI solution was injected into aquifer to establish in-situ reactive zone (IRZ) before 40 d of NB contamination. Results indicate that injection of EZVI leads to 90% reduction of total NB, which is mainly converted to aniline (AN). NB concentration decreases along the flow path in the tank. Fe2+ is generated from Fe0 oxidation. Significant acetate and bicarbonate are released due to emulsified oil decomposition during the whole operation time. Groundwater pH maintains in neutral value (6.6-8.2) owing to the balance between organic acids and OH- released after iron oxidation. Drastic decrease of ORP and DO indicates the transformation from oxidizing to reducing condition, leading to the reduction of oxidative species (e.g. sulfate, nitrate) in subsurface. Calculation of reducing equivalents suggests that microbial breakdown of emulsified oil provides more electrons than Fe0 oxidation does to the system. Both biotic and abiotic processes are involved in the enhanced degradation of NB.

Modular Engineering Intracellular NADH Regeneration Boosts Extracellular Electron Transfer of Shewanella oneidensis MR-1.

Efficient extracellular electron transfer (EET) of exoelectrogens is essentially for practical applications of versatile bioelectrochemical systems. Intracellular electrons flow from NADH to extracellular electron acceptors via EET pathways. However, it was yet established how the manipulation of intracellular NADH impacted the EET efficiency. Strengthening NADH regeneration from NAD+, as a feasible approach for cofactor engineering, has been used in regulating the intracellular NADH pool and the redox state (NADH/NAD+ ratio) of cells. Herein, we first adopted a modular metabolic engineering strategy to engineer and drive the metabolic flux toward the enhancement of intracellular NADH regeneration. We systematically studied 16 genes related to the NAD+-dependent oxidation reactions for strengthening NADH regeneration in the four metabolic modules of S. oneidensis MR-1, i.e., glycolysis, C1 metabolism, pyruvate fermentation, and tricarboxylic acid cycle. Among them, three endogenous genes mostly responsible for increasing NADH regeneration were identified, namely gapA2 encoding a NAD+-dependent glyceraldehyde-3-phosphate dehydrogenase in the glycolysis module, mdh encoding a NAD+-dependent malate dehydrogenase in the TCA cycle, and pflB encoding a pyruvate-formate lyase that converted pyruvate to formate in the pyruvate fermentation module. An exogenous gene fdh* from Candida boidinii encoding a NAD+-dependent formate dehydrogenase to increase NADH regeneration in the pyruvate fermentation module was further identified. Upon assembling these four genes in S. oneidensis MR-1, ∼4.3-fold increase in NADH/NAD+ ratio, and ∼1.2-fold increase in intracellular NADH pool were obtained under anaerobic conditions without discharge, which elicited ∼3.0-fold increase in the maximum power output in microbial fuel cells, from 26.2 ± 2.8 (wild-type) to 105.8 ± 4.1 mW/m2 (recombinant S. oneidensis), suggesting a boost in the EET efficiency. This modular engineering method in controlling the intracellular reducing equivalents would be a general approach in tuning the EET efficiency of exoelectrogens.

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

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

Reductive Stress in Inflammation-Associated Diseases and the Pro-Oxidant Effect of Antioxidant Agents.

Abstract: Reductive stress (RS) is the counterpart oxidative stress (OS), and can occur in response to conditions that shift the redox balance of important biological redox couples, such as the NAD⁺/NADH, NADP⁺/NADPH, and GSH/GSSG, to a more reducing state. Overexpression of antioxidant enzymatic systems leads to excess reducing equivalents that can deplete reactive oxidative species, driving the cells to RS. A feedback regulation is established in which chronic RS induces OS, which in turn, stimulates again RS. Excess reducing equivalents may regulate cellular signaling pathways, modify transcriptional activity, induce alterations in the formation of disulfide bonds in proteins, reduce mitochondrial function, decrease cellular metabolism, and thus, contribute to the development of some diseases in which NF-κB, a redox-sensitive transcription factor, participates. Here, we described the diseases in which an inflammatory condition is associated to RS, and where delayed folding, disordered transport, failed oxidation, and aggregation are found. Some of these diseases are aggregation protein cardiomyopathy, hypertrophic cardiomyopathy, muscular dystrophy, pulmonary hypertension, rheumatoid arthritis, Alzheimer's disease, and metabolic syndrome, among others. Moreover, chronic consumption of antioxidant supplements, such as vitamins and/or flavonoids, may have pro-oxidant effects that may alter the redox cellular equilibrium and contribute to RS, even diminishing life expectancy.

Zebra chip disease enhances respiration and oxidative stress of potato tubers (Solanum tuberosum L.).

MAIN CONCLUSION: The physiological phenotype of potato tubers afflicted by zebra chip disease is characterized by increased oxidative stress metabolism and upregulation of systems for its mitigation. Starch catabolism and extensive buildup of reducing sugars render potatoes infected with zebra chip (ZC) pathogen (Candidatus Liberibacter solanacearum) unsuitable for fresh market and processing into chips/fries. Here we show that the disease inflicts considerable oxidative stress, which likely constitutes a substantial sink for metabolic energy, resulting in increased respiration rate of afflicted tubers. In contrast to healthy tubers, tissue from diseased tubers had greater ability to reduce 2,3,5-triphenyl-tetrazolium chloride to formazan, indicating enhanced dehydrogenase activity, probable disease-induced changes in cellular redox potential, and increased respiratory activity. The respiration rate of diseased tubers (cv. Atlantic) was 2.4-fold higher than healthy tubers and this correlated with increased activities of glucose-6-phosphate and 6-phosphogluconate dehydrogenases, key enzymes responsible for synthesis of cytosolic reducing equivalents. Wound-induced NADPH oxidase activity was greater for ZC than healthy tubers, but the resulting superoxide was rapidly catabolized by higher superoxide dismutase activity in ZC tubers. Peroxidase, catalase, glutathione reductase and ascorbate free radical reductase activities were also higher in diseased tubers, as was malondialdehyde, a biomarker of peroxidative damage and oxidative stress. Upregulation of the glutathione-ascorbate pathway is a direct response to (and indicator of) oxidative stress, which consumes reducing equivalents (NADPH) to catabolize reactive oxygen species and maintain cellular redox homeostasis. ZC disease substantially altered the oxidative metabolism of tubers, resulting in a physiological phenotype defined by metabolic changes directed toward mitigating oxidative stress. Paradoxically, the increased respiration rate of ZC tubers, which fuels the metabolic pathways responsible for attenuating oxidative stress, likely also contributes to oxidative stress.

Reconstruction of a genome-scale metabolic model and in silico analysis of the polymalic acid producer Aureobasidium pullulans CCTCC M2012223.

Aureobasidium pullulans is a yeast-like fungus used for producing biopolymers e.g. polymalic acid (PMA) and pullulan. A high PMA producing strain, A. pullulans CCTCC M2012223, was isolated and sequenced in our previous study. To understand its metabolic performance, a genome-scale metabolic model, iZX637, consisting of 637 genes, 1347 reactions and 1133 metabolites, was reconstructed based on genome annotation and literature mining studies. The iZX637 model was validated by simulating cell growth, utilization of carbon and nitrogen sources, and gene essentiality analysis in A. pullulans. We further validated our model, designed a simulation program for the prediction of PMA production using experimental data, and further analyzed the carbon flux distribution and change with increasing PMA synthesis rates. Through the calculated flux distribution, NADPH- and NADH-dependent methylenetetrahydrofolate dehydrogenase (MTHFD) were found to be associated with the transfer of reducing equivalents from NADPH to NADH for supplementing NADH in the metabolic network. Furthermore, under the high PMA synthesis rate, a large amount of carbon flux was through pyruvate into malic acid via the reductive TCA cycle. Thus, pyruvate carboxylase, which can convert pyruvate to oxaloacetate with CO2 fixation, may also be an important target for PMA synthesis. These results illustrated that the model iZX637 was a powerful tool for optimization of A. pullulans metabolism and identification of targets for guiding metabolic engineering.

Enhanced fatty acid accumulation in Isochrysis galbana by inhibition of the mitochondrial alternative oxidase pathway under nitrogen deprivation.

The purpose of this study was to clarify the interrelation between the mitochondrial alternative oxidase (AOX) pathway and fatty acid accumulation in marine microalga Isochrysis galbana. Under normal conditions, the activity of the AOX pathway was maintained at a low level in I. galbana. Compared with the normal condition, nitrogen deprivation significantly increased the AOX pathway activity and fatty acid accumulation. Under nitrogen deprivation, the inhibition of the AOX pathway by salicylhydroxamic acid caused the accumulation of reducing equivalents and the over-reduction of chloroplasts in I. galbana cells, leading to a decrease in the photosynthetic O2 evolution rate. The over-production of reducing equivalents due to the inhibition of the AOX pathway under nitrogen deprivation further enhanced the accumulation of fatty acids in I. galbana cells.

Impaired cytosolic NADH shuttling and elevated UCP3 contribute to inefficient citric acid cycle flux support of postischemic cardiac work in diabetic hearts.

Diabetic hearts are subject to more extensive ischemia/reperfusion (ISC/REP) damage. This study examined the efficiency of citric acid cycle (CAC) flux and the transfer of cytosolic reducing equivalents into the mitochondria for oxidative support of cardiac work following ISC/REP in hearts of c57bl/6 (NORM) and type 2 diabetic, db/db mouse hearts. Flux through the CAC and malate-aspartate shuttle (MA) were monitored via dynamic (13)C NMR of isolated hearts perfused with (13)C palmitate+glucose. MA flux was lower in db/db than NORM. Oxoglutarate malate carrier (OMC) was elevated in the db/db heart, suggesting a compensatory response to low NADHc. Baseline CAC flux per unit work (rate-pressure-product, RPP) was similar between NORM and db/db, but ISC/REP reduced the efficiency of CAC flux/RPP by 20% in db/db. ISC/REP also increased UCP3 transcription, indicating potential for greater uncoupling. Therefore, ISC/REP induces inefficient carbon utilization through the CAC in hearts of diabetic mice due to the combined inefficiencies in NADHc transfer per OMC content and increased uncoupling via UCP3. Ischemia and reperfusion exacerbated pre-existing mitochondrial defects and metabolic limitations in the cytosol of diabetic hearts. These limitations and defects render diabetic hearts more susceptible to inefficient carbon fuel utilization for oxidative energy metabolism.