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Pentose utilization in yeasts: Physiology and biochemistry

Jeppsson, Helena (1996)
Abstract (Swedish)
Popular Abstract in Swedish

Etanol, dvs vanlig alkohol, från biomassa såsom energiskog, jordbruksavfallsrester och skogsavverkningsrester, är tänkt att användas som motorbränsle och ersätta fossila bränslen som bensin och diesel. Etanol är dyrare än bensin, men etanol är ett mera miljövänligt alternativ. Koldioxiden som bildas vid förbränningen av etanol tillverkad av biomassa ökar ej den totala koldioxidmängden i atmosfären och bidrar därmed ej till den sk växthuseffekten, eftersom återplanterad biomassa konsumerar koldioxid när den växer, det bildas ett kretslopp. Etanol kan produceras från inhemska råvaror vilket gör att man minskar beroendet av importerad olja.



Energiskog, jordbruksavfall och... (More)
Popular Abstract in Swedish

Etanol, dvs vanlig alkohol, från biomassa såsom energiskog, jordbruksavfallsrester och skogsavverkningsrester, är tänkt att användas som motorbränsle och ersätta fossila bränslen som bensin och diesel. Etanol är dyrare än bensin, men etanol är ett mera miljövänligt alternativ. Koldioxiden som bildas vid förbränningen av etanol tillverkad av biomassa ökar ej den totala koldioxidmängden i atmosfären och bidrar därmed ej till den sk växthuseffekten, eftersom återplanterad biomassa konsumerar koldioxid när den växer, det bildas ett kretslopp. Etanol kan produceras från inhemska råvaror vilket gör att man minskar beroendet av importerad olja.



Energiskog, jordbruksavfall och skogsavfall innehåller bla sockerarterna glukos (en hexossockerart) och xylos (en pentossockerart). Beroende på råmaterialet utgör xylos en betydande del av råvaran.



Mikroorganismer förjäser sockerarter till etanol. Innan sockerarterna kan förjäsas till etanol måste de frigöras ur biomassan, man får ett sk hydrolysat. Vid nedbrytningen av biomassan bildas ämnen som verkar hämmande på mikroorganismerna som användes vid förjäsningen av sockerarterna. För att etanolen skall bli så billig som möjligt måste alla steg i processen vara så effektiva som möjligt vilket bla innebär att alla sockerarterna i biomassan måste förjäsas till etanol. Förjäsning av glukos till etanol är en väl beprövad teknik för framställning av konsumptionssprit från spannmål, potatis och frukter och den sker med jästen Saccharomyces cerevisiae, samma jäst som användes vid bakning. S. cerevisiae jäser emellertid inte xylos till etanol.



Denna avhandling behandlar förjäsning av xylos till etanol. Jäsningsförmågan hos bakterier, jästar och filamentösa svampar jämfördes i ett xylosrikt hydrolysat. Den filamentösa svampen Fusarium oxysporum och den naturligt xylosjäsande jästen Pichia stipitis var mycket känsliga för de hämmande ämnena i hydrolysatet. Den bästa jäsningsförmågan hade den genetiskt förändrade bakterien Escherichia coli. För bästa resultat var man emellertid tvungen att ta bort de hämmande ämnena i hydrolysatet vilket kraftigt höjer etanolpriset.



Syrets reglerande inverkan på sockermetabolismen och etanolbildningen hos P. stipitis studerades. S. cerevisiae är en jäst som i jämförelse med andra mikroorganismer väl tål de hämmande ämnena i hydrolysat. S. cerevisiae kan ej jäsa xylos men den kan jäsa ett socker som liknar xylos-xylulos. Förjäsningen av xylulos till etanol studerades hos S. cerevisiae för att utveckla gentekniska strategier för att förändra jästen så att den effektivt jäser xylos till etanol. (Less)
Abstract
The fermentative performance of bacteria, yeasts, and filamentous fungi was investigated in a pentose (xylose)-rich lignocellulosic hydrolyzate. The filamentous fungus Fusarium oxysporum and the xylose-fermenting yeast Pichia stipitis were found to be very sensitive to the inhibiting hydrolyzate. Recombinant xylose-utilizing Saccharomyces cerevisiae showed very poor ethanol formation from xylose; xylitol being the major product formed. The highest ethanol yields were obtained with recombinant Escherichia coli KO11, however, for maximal ethanol yield detoxification of the hydrolyzate was required.



The influence of oxygen on the regulation of carbohydrate metabolism in the xylose-fermenting yeast P. stipitis CBS 6054 was... (More)
The fermentative performance of bacteria, yeasts, and filamentous fungi was investigated in a pentose (xylose)-rich lignocellulosic hydrolyzate. The filamentous fungus Fusarium oxysporum and the xylose-fermenting yeast Pichia stipitis were found to be very sensitive to the inhibiting hydrolyzate. Recombinant xylose-utilizing Saccharomyces cerevisiae showed very poor ethanol formation from xylose; xylitol being the major product formed. The highest ethanol yields were obtained with recombinant Escherichia coli KO11, however, for maximal ethanol yield detoxification of the hydrolyzate was required.



The influence of oxygen on the regulation of carbohydrate metabolism in the xylose-fermenting yeast P. stipitis CBS 6054 was investigated. A low and well-controlled level of oxygenation has been found to be required for efficient ethanol formation from xylose by the xylose-fermenting yeasts. The requirement of oxygen is frequently ascribed to the apparent redox imbalance which develops under anaerobic conditions due to the difference in co-factor utilization of the two first enzymes in the xylose metabolism, further reflected in xylitol excretion. However, a low and well controlled level of oxygenation for maximal ethanol production from glucose was also demonstrated, suggesting that the oxygen requirement is not only due to the dual co-factor utilization, but also serves other purposes. Cyanide-insensitive and salicyl hydroxamic acid-sensitive respiration (CIR) was found in P. stipitis. CIR is suggested to act as a redox sink preventing xylitol formation in P. stipitis under oxygen-limited xylose fermentations. Xylitol metabolism by P. stipitis CBS 6054 was strictly respiratory and ethanol was not formed under any conditions. The absence of ethanol formation was not due to a lack of fermentative enzymes, since the addition of glucose to xylitol-pregrown cells resulted in ethanol formation. Xylitol was not metabolized under anaerobic conditions, whereas ethanol was formed both from xylose and glucose. The in vitro pyruvate decarboxylase activity was shown to increase with decreasing oxygenation.



Xylulose fermentation by four strains of S. cerevisiae, P. stipitis CBS 6054 and Candida shehatae NJ 23, was compared under anaerobic batch cultivations. In addition to ethanol, polyols such as xylitol and arabinitol were formed by all yeasts. The S. cerevisiae strains showed xylulose consumption rates 20- to 60-fold lower than the glucose consumption rates. Co-utilization of xylulose and glucose by S. cerevisiae ATCC 24860 in chemostat culture decreased the ethanol yield compared with utilization of glucose alone, due to increased acetate and arabinitol formation. The formation of acetate and arabinitol involves reduction and oxidation of co-factors and indicates an altered redox flux during xylulose fermentation. (Less)
Please use this url to cite or link to this publication:
author
opponent
  • Prof. van Dam, Karel, E.C. Slater Institute, The Netherlands
publishing date
type
Thesis
publication status
published
subject
keywords
mycology, Mikrobiologi, virology, bacteriology, Microbiology, oxygenation, lignocellulosic hydrolyzate, Pentose fermentation, Pichia stipitis, Saccharomyces cerevisiae, xylulose, xylose, xylitol, cyanide-insensitive respiration, glucose, bakteriologi, virologi, mykologi
pages
54 pages
publisher
Helena Jeppsson, Dept. of Applied Microbiology, Lund University, P.O. Box 124, S-221 00 Lund, Sweden
defense location
K:C, Center for Chemistry and Chemical Engineering, Lund, Sweden
defense date
1996-06-03 10:15
external identifiers
  • Other:LUTKDH/(TKMB-1022)/125(1996)
language
English
LU publication?
no
id
c6ff1d82-22e9-4dab-b415-495b6cf59889 (old id 28588)
date added to LUP
2007-06-13 14:30:32
date last changed
2016-09-19 08:45:13
@phdthesis{c6ff1d82-22e9-4dab-b415-495b6cf59889,
  abstract     = {The fermentative performance of bacteria, yeasts, and filamentous fungi was investigated in a pentose (xylose)-rich lignocellulosic hydrolyzate. The filamentous fungus Fusarium oxysporum and the xylose-fermenting yeast Pichia stipitis were found to be very sensitive to the inhibiting hydrolyzate. Recombinant xylose-utilizing Saccharomyces cerevisiae showed very poor ethanol formation from xylose; xylitol being the major product formed. The highest ethanol yields were obtained with recombinant Escherichia coli KO11, however, for maximal ethanol yield detoxification of the hydrolyzate was required.<br/><br>
<br/><br>
The influence of oxygen on the regulation of carbohydrate metabolism in the xylose-fermenting yeast P. stipitis CBS 6054 was investigated. A low and well-controlled level of oxygenation has been found to be required for efficient ethanol formation from xylose by the xylose-fermenting yeasts. The requirement of oxygen is frequently ascribed to the apparent redox imbalance which develops under anaerobic conditions due to the difference in co-factor utilization of the two first enzymes in the xylose metabolism, further reflected in xylitol excretion. However, a low and well controlled level of oxygenation for maximal ethanol production from glucose was also demonstrated, suggesting that the oxygen requirement is not only due to the dual co-factor utilization, but also serves other purposes. Cyanide-insensitive and salicyl hydroxamic acid-sensitive respiration (CIR) was found in P. stipitis. CIR is suggested to act as a redox sink preventing xylitol formation in P. stipitis under oxygen-limited xylose fermentations. Xylitol metabolism by P. stipitis CBS 6054 was strictly respiratory and ethanol was not formed under any conditions. The absence of ethanol formation was not due to a lack of fermentative enzymes, since the addition of glucose to xylitol-pregrown cells resulted in ethanol formation. Xylitol was not metabolized under anaerobic conditions, whereas ethanol was formed both from xylose and glucose. The in vitro pyruvate decarboxylase activity was shown to increase with decreasing oxygenation.<br/><br>
<br/><br>
Xylulose fermentation by four strains of S. cerevisiae, P. stipitis CBS 6054 and Candida shehatae NJ 23, was compared under anaerobic batch cultivations. In addition to ethanol, polyols such as xylitol and arabinitol were formed by all yeasts. The S. cerevisiae strains showed xylulose consumption rates 20- to 60-fold lower than the glucose consumption rates. Co-utilization of xylulose and glucose by S. cerevisiae ATCC 24860 in chemostat culture decreased the ethanol yield compared with utilization of glucose alone, due to increased acetate and arabinitol formation. The formation of acetate and arabinitol involves reduction and oxidation of co-factors and indicates an altered redox flux during xylulose fermentation.},
  author       = {Jeppsson, Helena},
  keyword      = {mycology,Mikrobiologi,virology,bacteriology,Microbiology,oxygenation,lignocellulosic hydrolyzate,Pentose fermentation,Pichia stipitis,Saccharomyces cerevisiae,xylulose,xylose,xylitol,cyanide-insensitive respiration,glucose,bakteriologi,virologi,mykologi},
  language     = {eng},
  pages        = {54},
  publisher    = {Helena Jeppsson, Dept. of Applied Microbiology, Lund University, P.O. Box 124, S-221 00 Lund, Sweden},
  title        = {Pentose utilization in yeasts: Physiology and biochemistry},
  year         = {1996},
}