Increasing NADH oxidation reduces overflow metabolism in Saccharomyces cerevisiae

Respiratory metabolism plays an important role in energy production in the form of ATP in all aerobically growing cells. However, a limitation in respiratory capacity results in overflow metabolism, leading to the formation of byproducts, a phenomenon known as "overflow metabolism" or "the Crabtree effect." The yeast Saccharomyces cerevisiae has served as an important model organism for studying the Crabtree effect. When subjected to increasing glycolytic fluxes under aerobic conditions, there is a threshold value of the glucose uptake rate at which the metabolism shifts from purely respiratory to mixed respiratory and fermentative. It is well known that glucose repression of respiratory pathways occurs at high glycolytic fluxes, resulting in a decrease in respiratory capacity. Despite many years of detailed studies on this subject, it is not known whether the onset of the Crabtree effect is due to limited respiratory capacity or is caused by glucose-mediated repression of respiration. When respiration in S. cerevisiae was increased by introducing a heterologous alternative oxidase, we observed reduced aerobic ethanol formation. In contrast, increasing nonrespiratory NADH oxidation by overexpression of a water-forming NADH oxidase reduced aerobic glycerol formation. The metabolic response to elevated alternative oxidase occurred predominantly in the mitochondria, whereas NADH oxidase affected genes that catalyze cytosolic reactions. Moreover, NADH oxidase restored the deficiency of cytosolic NADH dehydrogenases in S. cerevisiae. These results indicate that NADH oxidase localizes in the cytosol, whereas alternative oxidase is directed to the mitochondria.

Homolactate fermentation by metabolically engineered Escherichia coli strains

We report the homofermentative production of lactate in Escherichia coli strains containing mutations in the aceEF, pfl, poxB, and pps genes, which encode the pyruvate dehydrogenase complex, pyruvate formate lyase, pyruvate oxidase, and phosphoenolpyruvate synthase, respectively. The process uses a defined medium and two distinct fermentation phases: aerobic growth to an optical density of about 30, followed by nongrowth, anaerobic production. Strain YYC202 (aceEF pfl poxB pps) generated 90 g/liter lactate in 16 h during the anaerobic phase (with a yield of 0.95 g/g and a productivity of 5.6 g/liter . h). Ca(OH)(2) was found to be superior to NaOH for pH control, and interestingly, significant succinate also accumulated (over 7 g/liter) despite the use of N(2) for maintaining anaerobic conditions. Strain ALS961 (YYC202 ppc) prevented succinate accumulation, but growth was very poor. Strain ALS974 (YYC202 frdABCD) reduced succinate formation by 70% to less than 3 g/liter. (13)C nuclear magnetic resonance analysis using uniformly labeled acetate demonstrated that succinate formation by ALS974 was biochemically derived from acetate in the medium. The absence of uniformly labeled succinate, however, demonstrated that glyoxylate did not reenter the tricarboxylic acid cycle via oxaloacetate. By minimizing the residual acetate at the time that the production phase commenced, the process with ALS974 achieved 138 g/liter lactate (1.55 M, 97% of the carbon products), with a yield of 0.99 g/g and a productivity of 6.3 g/liter . h during the anaerobic phase.

Increased recombinant protein production inEscherichia coli strains with overexpressed water-forming NADH oxidase and a deleted ArcA regulatory protein

Glycolytic flux is increased and acetate production is reduced in Escherichia coli by the expression of heterologous NADH oxidase (NOX) from Streptococcus pneumoniae coupled with the deletion of the arcA gene, which encodes the ArcA regulatory protein. In this study, we examined the overproduction of a model recombinant protein in strains of E. coli expressing NOX with or without an arcA mutation. The presence of NOX or the absence of ArcA reduced acetate by about 50% and increased beta-galactosidase production by 10-20%. The presence of NOX in the arcA strain eliminated acetate production entirely in batch fermentations and resulted in a 120% increase in beta-galactosidase production.

Overflow metabolism in Escherichia coli during steady-state growth: transcriptional regulation and effect of the redox ratio

Overflow metabolism in the form of aerobic acetate excretion by Escherichia coli is an important physiological characteristic of this common industrial microorganism. Although acetate formation occurs under conditions of high glucose consumption, the genetic mechanisms that trigger this phenomenon are not clearly understood. We report on the role of the NADH/NAD ratio (redox ratio) in overflow metabolism. We modulated the redox ratio in E. coli through the expression of Streptococcus pneumoniae (water-forming) NADH oxidase. Using steady-state chemostat cultures, we demonstrated a strong correlation between acetate formation and this redox ratio. We furthermore completed genome-wide transcription analyses of a control E. coli strain and an E. coli strain overexpressing NADH oxidase. The transcription results showed that in the control strain, several genes involved in the tricarboxylic acid (TCA) cycle and respiration were repressed as the glucose consumption rate increased. Moreover, the relative repression of these genes was alleviated by expression of NADH oxidase and the resulting reduced redox ratio. Analysis of a promoter binding site upstream of the genes which correlated with redox ratio revealed a degenerate sequence with strong homology with the binding site for ArcA. Deletion of arcA resulted in acetate reduction and increased the biomass yield due to the increased capacities of the TCA cycle and respiration. Acetate formation was completely eliminated by reducing the redox ratio through expression of NADH oxidase in the arcA mutant, even at a very high glucose consumption rate. The results provide a basis for studying new regulatory mechanisms prevalent at reduced NADH/NAD ratios, as well as for designing more efficient bioprocesses.

Physiological response of central metabolism inEscherichia coli to deletion of pyruvate oxidase and introduction of heterologous pyruvate carboxylase

We studied the physiological response of Escherichia coli central metabolism to the expression of heterologous pyruvate carboxylase (PYC) in the presence and absence of pyruvate oxidase (POX). These studies were complemented with expression analysis of central and intermediary metabolic genes and conventional in vitro enzyme assays to evaluate glucose metabolism at steady-state growth conditions (chemostats). The absence of POX activity reduced nongrowth-related energy metabolism (maintenance coefficient) and increased the maximum specific rate of oxygen consumption. The presence of PYC activity (i.e., with POX activity) increased the biomass yield coefficient and reduced the maximum specific oxygen consumption rate compared to the wildtype. The presence of PYC in a poxB mutant resulted in a 42% lower maintenance coefficient and a 42% greater biomass yield compared to the wildtype. Providing E. coli with PYC or removing POX increased the threshold specific growth rate at which acetate accumulation began, with an 80% reduction in acetate accumulation observed at a specific growth rate of 0.4 h-1 in the poxB-pyc+ strain. Gene expression analysis suggests utilization of energetically less favorable glucose metabolism via glucokinase and the Entner-Doudoroff pathway in the absence of functional POX, while the upregulation of the phosphotransferase glucose uptake system and several amino acid biosynthetic pathways occurs in the presence of PYC. The physiological and expression changes resulting from these genetic perturbations demonstrate the importance of the pyruvate node in respiration and its impact on acetate overflow during aerobic growth.

Production of 5-aminolevulinic acid by an Escherichia coli aminolevulinate dehydratase mutant that overproduces Rhodobacter sphaeroides aminolevulinate synthase

Two Escherichia coli strains, a wild-type strain and a hemB mutant, which contained the hemA gene from Rhodobacter sphaeroides produced aminolevulinate at 4 g l(-1). The hemB mutant did not increase the accumulation of aminolevulinate, an observation attributed to its 50% lower aminolevulinate synthase activity in than the wild-type. Growth was limited for both strains probably due to the accumulation of 0.5 g aminoacetone l(-1).

Aerobic production of alanine by Escherichia coli aceF ldhA mutants expressing the Bacillus sphaericus alaD gene

Alanine was produced from glucose in an Escherichia coli aceF ldhA double mutant strain that contained the pTrc99A- alaD plasmid expressing Bacillus sphaericus alanine dehydrogenase. The aceF gene encodes one of the proteins of the pyruvate dehydrogenase complex, and therefore this strain required acetate as an additional carbon source. The ldhA gene encodes fermentative lactate dehydrogenase, a competitor of alanine dehydrogenase for the substrate pyruvate. Fermentations included an oxygenated growth phase followed by an oxygen-limited alanine production phase. The lowest value for the mass transfer coefficient ( k(L)a) studied during the production phase yielded the greatest alanine. With feeding of glucose and NH(4)Cl, 32 g/l alanine accumulated in 27 h with a yield of 0.63 g alanine generated per gram glucose consumed.

Optimization of recombinant aminolevulinate synthase production in Escherichia coli using factorial design

The production of recombinant Rhodobacter sphaeroides aminolevulinate (ALA) synthase was optimized in two strains of Escherichia coli: the wild-type strain MG1655, and a ptsG mutant AFP111. The effects of initial succinate, glucose and isopropyl-beta-d-thiogalactopyranoside (IPTG) concentrations and the time of induction on enzyme activity were studied. One-way analysis was used to approximate the optimal ranges for these factors, followed by a full factorial design to quantify the effects of each factor and the interactions between the factors. Initial succinate, glucose, and IPTG concentration were observed to be the key factors affecting ALA synthase activity with the optimal levels determined to be above 6 g/l succinate, 0 g/l glucose, and 0.10 mM IPTG. ALA synthase activity was generally lower with AFP111 than with MG1655, and the effect of these three key factors was also lower with AFP111 than with MG1655. Based on the full factorial design results, a fermentation was completed that yielded 296 mU/mg protein with a final ALA concentration of 5.2 g/l (39 mM).

Effect of redox potential on stationary-phase xylitol fermentations using Candida tropicalis

Redox potential was used to develop a stationary-phase fermentation of Candida tropicalis that resulted in non-growth conditions with a limited decline in cell viability, a xylitol yield of 0.87 g g(-1) (95% of the theoretical value), and a high maximum specific production rate (0.67 g g(-1) h(-1)). A redox potential of -100 mV was found to be optimum for xylitol production over the range 0-150 mV [correction]. A shift from ethanol to xylitol production occurred when the redox potential was reduced from 50 mV to 100 mV as cumulative ethanol (Y(ethanol)) decreased from 0.34 g g(-1) to 0.025 g g(-1) and Y(xylitol) increased from 0.15 g g(-1) to 0.87 g g(-1) (alpha=0.05). Reducing the redox potential to 150 mV did not improve the fermentation. Instead, the xylitol yield and productivity decreased to 0.63 g g(-1) and 0.58 g g(-1) h(-1) respectively and cell viability declined. The viable, stationary-phase fermentation could be used to develop a continuous fermentation process, significantly increasing volumetric productivity and reducing downstream separation costs, potentially by the use of a membrane cell-recycle reactor.

The effect of acetate pathway mutations on the production of pyruvate in Escherichia coli

We compared pyruvate accumulation in six strains of Escherichia coli and their corresponding ppc mutants. Each strain contained a mutation of a gene involved in the pathway to acetate synthesis. Strains with mutations in genes encoding the pyruvate dehydrogenase complex generally exhibited the greatest pyruvate accumulation of which CGSC6162 (an aceF mutant) and CGSC6162 Delta ppc were studied in greater detail in controlled fermenters. Both CGSC6162 and CGSC6162 Delta ppc accumulated greater than 35 g/l pyruvate in a medium supplemented with acetate. We observed pyruvate mass yields from glucose of 0.72 in CGSC6162, with volumetric productivities above 1.5 g l(-1) h(-1). For CGSC6162 Delta ppc, we observed pyruvate yields of 0.78 and volumetric productivities above 1.2 g l(-1) h(-1). CGSC6162 consumed all initially supplied acetate, while CGSC6162 Delta ppc first consumed and then generated acetate during the course of a 36 h fermentation. Acetate generation and pyruvate oxidase activity was pH- and temperature-dependent, with a pH of 7.0 and the lowest temperature studied (32 degrees C) favoring the greatest pyruvate generation. Lactate was an unexpected by-product even though measured lactate dehydrogenase (LDH) activity was very low.

Expression of an anaplerotic enzyme, pyruvate carboxylase, improves recombinant protein production in Escherichia coli

Anaplerotic enzyme reactions are those which replenish tricarboxylic acid intermediates that are withdrawn for the synthesis of biomass. In this study, we examined recombinant protein production in Escherichia coli containing activity in an additional anaplerotic enzyme, pyruvate carboxylase. In batch fermentations, the presence of pyruvate carboxylase resulted in 68% greater production of the model protein, beta-galactosidase, 41% greater cell yield, and 57% lower acetate concentration. We discuss why these results indicate that acetate concentration does not limit cell growth and protein synthesis, as predicted by other researchers, and suggest instead that the rate of acetate formation represents an inefficient consumption of glucose carbon, which is reduced by the presence of pyruvate carboxylase.

Succinate production in dual-phase Escherichia coli fermentations depends on the time of transition from aerobic to anaerobic conditions

We examined succinic acid production in Escherichia coli AFP111 using dual-phase fermentations, which comprise an initial aerobic growth phase followed by an anaerobic production phase. AFP111 has mutations in the pfl, ldhA, and ptsG genes, and we additionally transformed this strain with the pyc gene (AFP111/pTrc99A-pyc) to provide metabolic flexibility at the pyruvate node. Aerobic fermentations with these two strains were completed to catalog physiological states during aerobic growth that might influence succinate generation in the anaerobic phase. Activities of six key enzymes were also determined for these aerobic fermentations. From these results, six transition times based on physiological states were selected for studying dual-phase fermentations. The final succinate yield and productivity depend greatly on the physiological state of the cells at the time of transition. Using the best transition time, fermentations achieved a final succinic acid concentration of 99.2 g/l with an overall yield of 110% and productivity of 1.3 g/l h.

Effects of growth mode and pyruvate carboxylase on succinic acid production by metabolically engineered strains of Escherichia coli

Escherichia coli NZN111, which lacks activities for pyruvate-formate lyase and lactate dehydrogenase, and AFP111, a derivative which contains an additional mutation in ptsG (a gene encoding an enzyme of the glucose phophotransferase system), accumulate significant levels of succinic acid (succinate) under anaerobic conditions. Plasmid pTrc99A-pyc, which expresses the Rhizobium etli pyruvate carboxylase enzyme, was introduced into both strains. We compared growth, substrate consumption, product formation, and activities of seven key enzymes (acetate kinase, fumarate reductase, glucokinase, isocitrate dehydrogenase, isocitrate lyase, phosphoenolpyruvate carboxylase, and pyruvate carboxylase) from glucose for NZN111, NZN111/pTrc99A-pyc, AFP111, and AFP111/pTrc99A-pyc under both exclusively anaerobic and dual-phase conditions (an aerobic growth phase followed by an anaerobic production phase). The highest succinate mass yield was attained with AFP111/pTrc99A-pyc under dual-phase conditions with low pyruvate carboxylase activity. Dual-phase conditions led to significant isocitrate lyase activity in both NZN111 and AFP111, while under exclusively anaerobic conditions, an absence of isocitrate lyase activity resulted in significant pyruvate accumulation. Enzyme assays indicated that under dual-phase conditions, carbon flows not only through the reductive arm of the tricarboxylic acid cycle for succinate generation but also through the glyoxylate shunt and thus provides the cells with metabolic flexibility in the formation of succinate. Significant glucokinase activity in AFP111 compared to NZN111 similarly permits increased metabolic flexibility of AFP111. The differences between the strains and the benefit of pyruvate carboxylase under both exclusively anaerobic and dual-phase conditions are discussed in light of the cellular constraint for a redox balance.

Solid state fermentation of broiler litter for production of biocontrol agents

Several varieties of heat-sterilized broiler litter with 60% (wet basis, wb) moisture content were substrate in solid-state fermentations to produce biocontrol agents. Litter varieties included litter produced by one flock of broilers from medicated and non-medicated controlled rations, and litter produced by two flocks and four flocks on a single application of bedding material from medicated commercial sources. Litter preparations were inoculated with monocultures of Bacillus thuringiensis serovar japonensis strain Buibui, a pathogen of Japanese beetle larvae (Popillia japonica), or Pseudomonas fluorescens 2-79. B. thuringiensis did not grow in unextracted 1-flock litter nor in water extracted litter, but grew in methanol extracted litter to 5 x 10(10) cell forming units (CFU)/g litter (dry weight, dw) and a spore count of 1 x 10(10) CFU/g litter (dw). B. thuringiensis also grew in unprocessed 2-flock and 4-flock litter, achieving cell counts of 3 x 10(9) and 1 x 10(9) CFU/g litter (dw), respectively, and spore counts of 1 x 10(9) CFU/g litter (dw). P. fluorescens grew in medicated 1-flock litter with no extraction to a cell density greater than 4 x 10(11) CFU/g litter (dw). Bioassays in soil containing over 0.5% (db) litter fermented with B. thuringiensis resulted in over 90% mortality in 21 days for first instars of Japanese beetle when compared to a control treatment using compost without fermented litter. The investigations demonstrate that bacterial biocontrol agents produced via solid substrate fermentations using broiler poultry litter have potential in biocontrol applications in the soil environment.

The physiological effects and metabolic alterations caused by the expression of Rhizobium etli pyruvate carboxylase in Escherichia coli

Oxaloacetate (OAA) plays an important role in the tricarboxylic acid cycle and for the biosynthesis of a variety of cellular compounds. Some microorganisms, such as Rhizobium etli and Corynebacterium glutamicum, are able to synthesize OAA during growth on glucose via either of the enzymes pyruvate carboxylase (PYC) or phosphoenolpyruvate carboxylase (PPC). Other microorganisms, including Escherichia coli, synthesize OAA during growth on glucose only via PPC because they lack PYC. In this study we have examined the effect that the R. etli PYC has on the physiology of E. coli. The expressed R. etli PYC was biotinylated by the native biotin holoenzyme synthase of E. coli and displayed kinetic properties similar to those reported for alpha4 PYC enzymes from other sources. R. etli PYC was able to restore the growth of an E. coli ppc null mutant in minimal glucose medium, and PYC expression caused increased carbon flow towards OAA in wild-type E. coli cells without affecting the glucose uptake rate or the growth rate. During aerobic glucose metabolism, expression of PYC resulted in a 56% increase in biomass yield and a 43% decrease in acetate yield. During anaerobic glucose metabolism, expression of PYC caused a 2.7-fold increase in succinate concentration, making it the major product by mass. The increase in succinate came mainly at the expense of lactate formation. However, in a mutant lacking lactate dehydrogenase activity, expression of PYC resulted in only a 1.7-fold increase in succinate concentration. The decreased enhancement of succinate formation in the /dh mutant was hypothesized to be due to accumulation of pyruvate and NADH, metabolites that affect the interconversion of the active and inactive form of the enzyme pyruvate formate-lyase.

Metabolic flux analysis of Clostridium thermosuccinogenes: effects of pH and culture redox potential

Clostridium thermosuccinogenes are anaerobic thermophilic bacteria that ferment various carbohydrates to succinate and acetate as major products and formate, lactate, and ethanol as minor products. Metabolic carbon flux analysis was used to evaluate the effect of pH and redox potential on the batch fermentation of C. thermosuccinogenes. In a first study, the effects of four pH values (6.50, 6.75, 7.00, and 7.25) on intracellular carbon flux at a constant redox potential of -275 mV were compared. The flux of carbon toward succinate and formate increased whereas the flux to lactate decreased significantly with a pH increase from 6.50 to 7.25. Both specific growth rate and specific rate of glucose consumption were unaffected by changes in pH. The fraction of carbon flux at the phosphoenolpyruvate (PEP) node flowing to oxaloacetate increased with an increase in pH. At the pyruvate node, the fraction of flux to formate increased with increasing pH. At the acetyl CoA node, the fraction of flux to acetate increased significantly with an increase in pH. A second study elucidated the effect of four controlled culture redox potentials (-225, -250, -275, and -310 mV) on metabolic carbon flux at a constant pH of 7.25. Lower values of culture redox potential were correlated with increased succinate, acetate, and formate fluxes and decreased ethanol and hydrogen fluxes in C. thermosuccinogenes. Lactate formation was not significantly influenced by redox potential. At the PEP node, the fraction of carbon to oxaloacetate increased with a decrease in redox potential. At the pyruvate node, the fraction of carbon to formate increased, while at the acetyl CoA node, the fraction of carbon flux to acetate increased with reduced redox potential. The presence of hydrogen in the headspace or the addition of nicotinic acid to the growth media resulted in increased hydrogen and ethanol fluxes and decreased succinate, acetate, formate, and lactate fluxes.

Metabolic analysis of Escherichia coli in the presence and absence of the carboxylating enzymes phosphoenolpyruvate carboxylase and pyruvate carboxylase

Fermentation patterns of Escherichia coli with and without the phosphoenolpyruvate carboxylase (PPC) and pyruvate carboxylase (PYC) enzymes were compared under anaerobic conditions with glucose as a carbon source. Time profiles of glucose and fermentation product concentrations were determined and used to calculate metabolic fluxes through central carbon pathways during exponential cell growth. The presence of the Rhizobium etli pyc gene in E. coli (JCL1242/pTrc99A-pyc) restored the succinate producing ability of E. coli ppc null mutants (JCL1242), with PYC competing favorably with both pyruvate formate lyase and lactate dehydrogenase. Succinate formation was slightly greater by JCL1242/pTrc99A-pyc than by cells which overproduced PPC (JCL1242/pPC201, ppc(+)), even though PPC activity in cell extracts of JCL1242/pPC201 (ppc(+)) was 40-fold greater than PYC activity in extracts of JCL1242/pTrc99a-pyc. Flux calculations indicate that during anaerobic metabolism the pyc(+) strain had a 34% greater specific glucose consumption rate, a 37% greater specific rate of ATP formation, and a 6% greater specific growth rate compared to the ppc(+) strain. In light of the important position of pyruvate at the juncture of NADH-generating pathways and NADH-dissimilating branches, the results show that when PPC or PYC is expressed, the metabolic network adapts by altering the flux to lactate and the molar ratio of ethanol to acetate formation.

Comparison of solvents for removing pesticides from skin using an in vitro porcine model

This study compared four solvents (1-propanol, polyethylene glycol [avg. MW 400], 10% Ivory Liquid and water, and D-TAM) for their ability to remove selected pesticides from an in vitro porcine skin model using a solvent-moistened wipe. Wipes were performed 90 min after pesticide was applied to the skin. The four pesticides selected (glyphosate, alachlor, methyl parathion, and trifluralin) were chosen because of their differences in water solubility. This study also determined whether pretreatment of skin with a solvent prior to pesticide application would either increase or decrease recovery of the pesticide. Recovery efficiencies for all solvents and pesticides were affected by the amount of contaminant on the skin. Although pesticide recoveries from all four solvents were similar (range: 45-57%), on average 1-propanol had significantly higher recoveries, followed by soap and water. There was no significant difference between polyethylene glycol, and D-TAM. When skin was pretreated with any of the four solvents before pesticide application, the recoveries of the more water soluble compounds, glyphosate and alachlor, decreased. When pretreatment with solvent preceded application of trifluralin, the pesticide with the lowest water solubility, recoveries increased. 1-Propanol or soap and water were more effective in removing pesticides from skin than polyethylene glycol or D-TAM, but the amount of pesticide recovered from skin was affected by the chemical characteristics of the pesticide (such as water solubility) and the amount of pesticide originally on the skin. This study provides information useful to the interpretation of skin wipe sample results collected in field studies.

Elucidation of enzymes in fermentation pathways used by Clostridium thermosuccinogenes growing on inulin

Based on the presence and absence of enzyme activities, the biochemical pathways for the fermentation of inulin by Clostridium thermosuccinogenes DSM 5809 are proposed. Activities of nine enzymes (lactate dehydrogenase, phosphoenolpyruvate carboxylase, malate dehydrogenase, fumarase, fumarate reductase, phosphotransacetylase, acetate kinase, pyruvate kinase, and alcohol dehydrogenase) were measured at four temperatures (37, 47, 58, and 70 degrees C). Each of the enzymes increased 1.5 to 2.0-fold in activity between 37 and 58 degrees C, but only lactate dehydrogenase, fumarate reductase, malate dehydrogenase, and fumarase increased at a similar rate between 58 and 70 degrees C. No acetate kinase activity was observed at 70 degrees C. Arrhenius energies were calculated for each of these nine enzymes and were in the range of 9.8 to 25.6 kcal/mol. To determine if a relationship existed between product formation and enzyme activity, serum bottle fermentations were completed at the four temperatures. Maximum yields (in moles per mole hexose unit) for succinate (0.23) and acetate (0.79) and for biomass (29.5 g/mol hexose unit) occurred at 58 degrees C, whereas the maximum yields for lactate (0.19) and hydrogen (0.25) and the lowest yields for acetate (0.03) and biomass (19.2 g/mol hexose unit) were observed at 70 degrees C. The ratio of oxidized products to reduced products changed significantly, from 0.52 to 0.65, with an increase in temperature from 58 to 70 degrees C, and there was an unexplained detection of increased reduced products (ethanol, lactate, and hydrogen) with a concomitant decrease in oxidized-product formation at the higher temperature.

Production of succinate from glucose, cellobiose, and various cellulosic materials by the ruminal anaerobic bacteria Fibrobacter succinogenes and Ruminococcus flavefaciens

The production of organic acids by two anaerobic ruminal bacteria Fibrobacter succinogenes S85 and Ruminococcus flavefaciens FD-1, was compared with glucose, cellobiose, microcrystalline cellulose, Walseth cellulose (acid swollen cellulose), pulped paper, and steam-exploded yellow poplar as substrates. The major end product produced by F. succinogenes from each of these substrates was succinate (69.5-83%), the principal secondary product was acetate (16-30.5%). Maximum succinate productivity ranged from 14.1 mg/L.h for steam-exploded yellow Poplar to 59.7 mg/L.h for pulped paper. For R. flavefaciens, the major end product from cellobiose, microcrystalline cellulose, and acid-swollen Walseth cellulose was acetate (39-46%), pulped paper and steam-exploded yellow poplar yielded succinate (42-54%) as the major product. Maximum succinate productivity by R. flavefaciens ranged from 9.21 mg/L.h for cellobiose to 43.1 mg/L.h for pulped paper. In general, much less succinate was produced at a lower maximum productivity by R. flavefaciens than by F. succinogenes under similar fermentation conditions. The maximum succinate productivities by these two organisms are comparable to the previously reported value of 59 mg/L.h for Anderobiospirillum succiniciproducens grown on glucose and corn steep liquor.

Toxicity of Glucosinolates and Their Enzymatic Decomposition Products to Caenorhabditis elegans

An aquatic 24-hour lethality test using Caenorhabditis elegans was used to assess toxicity of glucosinolates and their enzymatic breakdown products. In the absence of the enzyme thioglucosidase (myrosinase), allyl glucosinolate (sinigrin) was found to be nontoxic at all concentrations tested, while a freeze-dried, dialyzed water extract of Crambe abyssinica containing 26% 2-hydroxyl 3-butenyl glucosinolate (epi-progoitrin) had a 50% lethal concentration (LC) of 18.5 g/liter. Addition of the enzyme increased the toxicity (LC value) of sinigrin to 0.5 g/liter, but the enzyme had no effect on the toxicity of the C. abyssinica extract. Allyl isothiocyanate and allyl cyanide, two possible breakdown products of sinigrin, had an LC value of 0.04 g/liter and approximately 3 g/liter, respectively. Liquid chromatographic studies showed that a portion of the sinigrin decomposed into allyl isothiocyanate. The results indicated that allyl isothiocyanate is nearly three orders of magnitude more toxic to C. elegans than the corresponding glncosinolate, suggesting isothiocyanate formation would improve nematode control from application of glucosinolates.

A mathematical model to predict the partitioning of peptides and peptide-modified proteins in aqueous two-phase systems

A mathematical procedure was developed to predict the partition coefficients of the peptides AIIP, AWWP, AIIPAIIP and AWWPAWWP in poly(ethylene glycol) (PEG)/phosphate aqueous two-phase systems from amino acid hydrophobicities. In general, peptides containing tryptophan partition more into the PEG-enriched upper phase than analogous peptides containing isoleucine. Specifically, as the PEG concentration difference between the phases increased in a PEG/potassium phosphate aqueous two-phase system, the peptide AIIP was observed to have a partition coefficient ranging from 1.2 to 1.6, AIIPAIIP from 2.4 to 5.7, AWWP from 13.5 to 32.2, and AWWPAWWP from 43 to 170. The model was extended to predict the partitioning of a staphylococcal protein A derivative (ZZ) modified with these four peptides. As predicted, the protein modified with isoleucine-containing peptides had lower partition coefficients than the protein modified with tryptophan-containing peptides. The partition coefficient of the ZZ protein ranged from 0.35 to 0.20, that of ZZAIIPAIIP from 0.58 to 0.48, and that of ZZAWWPAWWP from 3.5 to 5.3 in these systems. The results show that short peptide handles can significantly enhance the partitioning of proteins in aqueous two-phase systems. The relationship between the model and the surface exposure of peptide handles and the utility of the model to aid in the design of such handles to enhance purifications are also discussed.

Partition of isomeric dipeptides in poly(ethylene glycol)/magnesium sulfate aqueous two-phase systems

A series of isomeric dipeptides, i.e., those containing identical residues but in different order such as Trp-Gly versus Gly-Trp, was partitioned in a poly(ethylene glycol) (PEG)/magnesium sulfate (MgSO4) aqueous two-phase system. Dipeptides having a more hydrophobic character favored the upper (PEG) phase. Moreover, the partition coefficients for isomeric dipeptides are different, with the partition coefficients for dipeptides containing the more hydrophobic residue in the C-terminal position being, in general, greater than the partition coefficients for corresponding isomers which contain the more hydrophobic residue in the N-terminal position. These observations can be attributed to the different interactions that the isomers have with specific two-phase systems.

A model for the prediction of partition coefficients in aqueous two-phase systems

A mathematical model has been developed to predict partition coefficients for aqueous two-phase systems. The model is based on a previously-developed equation for partitioning which arises from an osmotic pressure viral expansion. The model suggests that the properties of importance are the concentration difference of one of the phase-forming components, such as a polymer, and the hydrophobicity of the solute relative to the hydrophobic-hydrophilic difference between the two phases. Several two-phase systems have been studied, with a particular emphasis on the poly(ethylene glycol)/magnesium sulfate system. Numerous solutes, including peptides, were used in this system and their partition coefficients show good agreement with the model.

Peptide hydrophobicity and partitioning in poly(ethylene glycol)/magnesium sulfate aqueous two-phase systems

Several amino acids and peptides were partitioned in poly(ethylene glycol) (PEG)/magnesium sulfate (MgSO4) aqueous two-phase systems. The partition coefficients measured for amino acids and peptides were proportional to the difference in PEG concentration between the phases. The partitioning data were used to calculate the relative hydrophobicities of individual amino acids, which were then used to estimate the hydrophobicities of peptides. The partition coefficients of several dipeptides were predicted from these estimated hydrophobicities. A series of peptide fragments that compose the pentapeptide leucine enkephalin was also partitioned in the PEG/MgSO4 system. Again, the partitioning depended upon the hydrophobicities of the individual exposed amino acids.