Enzymatic and Microbial Preparation of
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            -Xylulose from
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            -Xylose

Chiang, Lin-Chang; Hsiao, Humg-Yu; Ueng, Peter P.; Tsao, George T.

doi:10.1128/aem.42.1.66-69.1981

Cited by 34 publications

(12 citation statements)

References 14 publications

(9 reference statements)

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“…Table 2 shows the derived slopes of the flux time profiles for emission in the fully radiative uncooled and cooled regimes When g < g r , which happens when the incoming energy is not fully radiated -and which must be the case in view of the fact that there are still some nonthermal electrons which are radiating energy -the slopes are less negative than given here, so that the profiles decay more slowly. A numerical calculation (Chiang & Dermer 1997b) is required to determine the slopes of the time profiles in this regime. In the extreme non-radiative regime, where g = g a = g r /2, the amount of radiated energy is negligible, and the slopes of the time profiles are harder by c 0 · c 1 = g/(2g + 1) units.…”

Section: Analytic Interpretationmentioning

confidence: 99%

“…Against the apparent successes of the relativistic blast wave model in explaining the slopes of the afterglow time profiles, several difficulties must be mentioned: (1) The characteristic GRB spectral shape cannot be reproduced with the model presented here, which gives a 1/2 power break due to incomplete synchrotron cooling rather than a more typical break of about one unit from the hard X-rays to the soft gamma rays. This problem can be ameliorated by the inclusion of relativistic shock effects which produce time-varying equipartition magnetic fields and shock-heated low-energy cutoffs in the electron momentum distribution, and will be dealt with elsewhere (Chiang & Dermer 1997b). (2) The short time-scale variability and the large diversity of GRB time profiles are not accounted for here, and probably cannot result from variations in the external medium (Fenimore, Madras, & Nayakshin 1996;Sari & Piran 1995), though internal shocks may suffice (see Kobayashi, Piran, & Sari 1997).…”

Section: Application To Gamma Ray Burstsmentioning

confidence: 99%

“…We summarize in §9. A detailed numerical treatment is presented in a companion paper (Chiang & Dermer 1997b).…”

Section: Introductionmentioning

confidence: 99%

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Electron acceleration and synchrotron radiation in decelerating plasmoids

1998

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An equation is derived to calculate the dynamics of relativistic magnetized plasma which decelerates by sweeping up matter from the ISM. Reduction to the non-radiative and radiative regimes is demonstrated for the general case of a collimated plasmoid whose area increases as a power of distance x from the explosion, and in the specific case of a blast-wave geometry where the area increases quadratically with x. An equation for the evolution of the electron momentum distribution function in the comoving fluid frame is used to calculate the observed synchrotron radiation spectrum. The central uncertainty involves the mechanism for transfering energy from the nonthermal protons to the electrons. The simplest prescription is to assume that a fixed fraction of the comoving proton power is instantaneously transformed into a power-law, "shock"-like electron distribution function. This permits an analytic solution for the case of a relativistic plasmoid with a constant internal magnetic field. Effects of parameter changes are presented. Breaks in the temporal behavior of the primary burst emission and long wavelength afterglows occur on two time scales for external ISM density distributions n ext ∝ x −η . The first, as emphasized by Mészáros & Rees, represents the time when the outflow sweeps up ≈ M th /Γ 0 of material from the ISM, where M th is the mass in the outflow ejecta and Γ 0 is the initial Lorentz factor of the plasmoid. The second represents the time when synchrotron cooling begins strongly to regulate the number of nonthemal electrons that are producing radiation which is observed at a given energy.Results are applied to GRB and blazar variability. The slopes of the time profiles of the X-ray and optical light cuves of the afterglows of GRB 970228 and GRB 970508 are explained by injection of hard electron spectra with indices s < 2 in a deceleration regime near the radiative limit. We also propose that blazar flares are due to the transformation of the directed kinetic energy of the plasmoid when interacting with the external medium, as would occur if relativistic outflows pass through clouds orbiting the central supermassive black hole.

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Section: Analytic Interpretationmentioning

confidence: 99%

Section: Application To Gamma Ray Burstsmentioning

confidence: 99%

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Electron acceleration and synchrotron radiation in decelerating plasmoids

1998

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“…The effects of Xylulose-xylose substrate. For the production of xylulosexylose solutions, the method previously described by Chiang et al (10,11) was used with the following modifications. Xylose, purchased either from Fluka AG, Buchs, Switzerland or from Sigma Chemical Co., St. Louis, Mo.…”

mentioning

confidence: 99%

Intermediary Metabolite Concentrations in Xylulose- and Glucose-Fermenting Saccharomyces cerevisiae Cells

Senac

Hahn‐Hägerdal

1990

Appl Environ Microbiol

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Glucose and xylulose fermentation and product formation by Saccharomyces cerevisiae were compared in batch culture under anaerobic conditions. In both cases the main product was ethanol, with glycerol, xylitol, and arabitol produced as by-products. During glucose and xylulose fermentation, 0.74 and 0.37 g of cell mass liter-', respectively, were formed. In glucose-fermenting cells, the carbon balance could be closed, whereas in xylulose-fermenting cells, about 25% of the consumed sugar carbon could not be accounted for. The rate of sugar consumption was 3.94 mmol g of initial biomass-' h-1 for glucose and 0.39 mmol g of initial biomass-' h-' for xylulose. Concentrations of the intermediary metabolites fructose-1,6-diphosphate (FDP), pyruvate (PYR), sedoheptulose 7-phosphate (S7P), erytrose 4-phosphate, citrate (CIT), fumarate, and malate were compared for both types of cells. Levels of FDP, PYR, and CIT were lower, and levels of S7P were higher in xylulose-fermenting cells. After normalization to the carbon consumption rate, the levels of FDP were approximately the same, whereas there was a significant accumulation of S7P, PYR, CIT, and malate, especially of S7P, in xylulose-fermenting cells compared with in glucose-fermenting cells. In the presence of 15 ,iM iodoacetate, an inhibitor of the enzyme glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.12), FDP levels increased and S7P levels decreased in xylulose-assimilating cells compared with in the absence of the inhibitor, whereas fermentation was slightly slowed down. The specific activity of transaldolase (EC 2.2.1.2), the pentose phosphate pathway enzyme reacting with S7P and glyceraldehyde-3-phosphate, was essentially the same for both glucoseand xylulose-fermenting cells. It was, however, several orders of magnitude lower than that reported for a Torula yeast and Candida utilis. The presence of iodoacetate did not influence the activity of transaldolase in xylulose-fermenting cells. The results are discussed in terms of a competition between the pentose phosphate pathway and glycolysis for the common metabolite, glyceraldehyde-3-phosphate, which would explain the low rates of xylulose assimilation and ethanol production from xylulose by S. cerevisiae. * Corresponding author. iodoacetate (IA), an inhibitor of glyceraldehyde-3-phosphate (G3P) dehydrogenase (EC 1.2.1.12) (4, 6, 36), on metabolite levels and the specific activity of the PPP enzyme transaldolase (EC 2.2.1.2) were studied in order to elucidate the importance of the pool of G3P on the rate of xylulose fermentation in S. cerevisiae. MATERIALS AND METHODS Organism. S. cerevisiae ATCC 24860 was maintained at 4°C on slants containing yeast extract (3 g liter-1; Difco Laboratories, Detroit, Mich.), Bacto-Peptone (5 g liter-1; Difco), agar (20 g liter-1; Difco), malt extract (3 g liter'; Difco), and glucose or xylose (10 g liter-').

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“…Utilization of xylulose (an intermediate) as a sole carbon source, though is rare for naturally growing yeasts, but has been observed in some instances. 29 Briefly, the xylulose is transported by the hexose transporter family into the cells, which is then phosphorylated and through the pentose phosphate pathway (PPP) moves in to the central metabolism system. 30 Similar to glucose, xylulose may also be utilized for respiration or fermentation.…”

Section: Carbon Sourcesmentioning

confidence: 99%

Influence of reactors, microbial carbohydrate uptake, and metabolic pathways on ethanol production from grass biomass: A review

et al. 2018

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Summary Grasses are considered to be potential lignocellulosic feedstock for renewable and sustainable biofuels such as bioethanol. However, the process involved, ie, pretreatment, enzymatic saccharification, and fermentation in conversion of these lignocellulosic biomass to bioethanol, remains expensive and at present is not affordable for industrial production. Thus, the present review assesses the influence of the recent technologies that can be employed for the bio‐refinery based pretreatment and enzymatic hydrolysis of grass biomass using advanced bioreactors. Since plant extracts have been seen to enhance the glucose uptake, an experiment was implemented to elucidate the role of plant extracts (bark extracts of Xylocarpus granatum) on glucose uptake capacity of microorganisms like Saccharomyces cerevisiae, Pichia sp., and Zymomonas mobilis and their subsequent ethanol production capability from glucose and xylose sugars. The results of these experiments indicated that supplementation of plant extracts promoted both glucose and xylose uptake in S. cerevisiae and Pichia sp. as compared with the control and Z. mobilis strain. Further, as Pichia sp. exhibited good uptake ability for both glucose and xylose, a model was proposed focusing on the gene silencing and operon concept in Pichia sp. for preferential pentose utilization during the fermentation of grass biomass to bioethanol.

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Enzymatic and Microbial Preparation of d -Xylulose from d -Xylose

Cited by 34 publications

References 14 publications

Electron acceleration and synchrotron radiation in decelerating plasmoids

Electron acceleration and synchrotron radiation in decelerating plasmoids

Intermediary Metabolite Concentrations in Xylulose- and Glucose-Fermenting Saccharomyces cerevisiae Cells

Influence of reactors, microbial carbohydrate uptake, and metabolic pathways on ethanol production from grass biomass: A review

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