Xylose Fermentation (xylose + fermentation)

Distribution by Scientific Domains


Selected Abstracts


Effect of the reversal of coenzyme specificity by expression of mutated Pichia stipitis xylitol dehydrogenase in recombinant Saccharomyces cerevisiae

LETTERS IN APPLIED MICROBIOLOGY, Issue 2 2007
J. Hou
Abstract Aims:, To determine the effects on xylitol accumulation and ethanol yield of expression of mutated Pichia stipitis xylitol dehydrogenase (XDH) with reversal of coenzyme specificity in recombinant Saccharomyces cerevisiae. Methods and Results:, The genes XYL2 (D207A/I208R/F209S) and XYL2 (S96C/S99C/Y102C/D207A/I208R/F209S) were introduced into S. cerevisiae, which already contained the P. stipitis XYL1 gene (encoding xylose reductase, XR) and the endogenously overexpressed XKS1 gene (encoding xylulokinase, XK). The specific activities of mutated XDH in both strains showed a distinct increase in NADP+ -dependent activity in both strains with mutated XDH, reaching 0782 and 0698 U mg,1. In xylose fermentation, the strain with XDH (D207A/I208R/F209S) had a large decrease in xylitol and glycerol yield, while the xylose consumption and ethanol yield were decreased. In the strain with XDH (S96C/S99C/Y102C/D207A/I208R/F209S), the xylose consumption and ethanol yield were also decreased, and the xylitol yield was increased, because of low XDH activity. Conclusions:, Changing XDH coenzyme specificity was a sufficient method for reducing the production of xylitol, but high activity of XDH was also required for improved ethanol formation. Significance and Impact of the Study:, The difference in coenzyme specificity was a vital parameter controlling ethanolic xylose fermentation but the XDH/XR ratio was also important. [source]


An economic comparison of different fermentation configurations to convert corn stover to ethanol using Z. mobilis and Saccharomyces

BIOTECHNOLOGY PROGRESS, Issue 1 2010
Abhijit Dutta
Abstract Numerous routes are being explored to lower the cost of cellulosic ethanol production and enable large-scale production. One critical area is the development of robust cofermentative organisms to convert the multiple, mixed sugars found in biomass feedstocks to ethanol at high yields and titers without the need for processing to remove inhibitors. Until such microorganisms are commercialized, the challenge is to design processes that exploit the current microorganisms' strengths. This study explored various process configurations tailored to take advantage of the specific capabilities of three microorganisms, Z. mobilis 8b, S. cerevisiae, and S. pastorianus. A technoeconomic study, based on bench-scale experimental data generated by integrated process testing, was completed to understand the resulting costs of the different process configurations. The configurations included whole slurry fermentation with a coculture, and separate cellulose simultaneous saccharification and fermentation (SSF) and xylose fermentations with none, some or all of the water to the SSF replaced with the fermented liquor from the xylose fermentation. The difference between the highest and lowest ethanol cost for the different experimental process configurations studied was $0.27 per gallon ethanol. Separate fermentation of solid and liquor streams with recycle of fermented liquor to dilute the solids gave the lowest ethanol cost, primarily because this option achieved the highest concentrations of ethanol after fermentation. Further studies, using methods similar to ones employed here, can help understand and improve the performance and hence the economics of integrated processes involving enzymes and fermentative microorganisms. 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2010 [source]


Global Gene Expression Differences Associated with Changes in Glycolytic Flux and Growth Rate in Escherichia coli during the Fermentation of Glucose and Xylose

BIOTECHNOLOGY PROGRESS, Issue 1 2002
Ramon Gonzalez
The simplicity of the fermentation process (anaerobic with pH, temperature, and agitation control) in ethanologenic Escherichia coli KO11 and LY01 makes this an attractive system to investigate the utility of gene arrays for biotechnology applications. By using this system, gene expression, glycolytic flux, and growth rate have been compared in glucose-grown and xylose-grown cells. Although the initial metabolic steps differ, ethanol yields from both sugars were essentially identical on a weight basis, and little carbon was diverted to biosynthesis. Expression of only 27 genes changed by more than 2-fold in both strains. These included induction of xylose-specific operons ( xylE, xylFGHR, and xylAB) regulated by XylR and the cyclic AMP,CRP system and repression of Mlc-regulated genes encoding glucose uptake ( ptsHIcrr, ptsG) and mannose uptake ( manXYZ) during growth on xylose. However, expression of genes encoding central carbon metabolism and biosynthesis differed by less than 2-fold. Simple statistical methods were used to investigate these more subtle changes. The reproducibility (coefficient of variation of 12%) of expression measurements (mRNA as cDNA) was found to be similar to that typically observed for in vitro measurements of enzyme activities. Using Studentapos;s t test, many smaller but significant sugar-dependent changes were identified ( p < 0.05 in both strains). A total of 276 genes were more highly expressed during growth on xylose; 307 genes were more highly expressed with glucose. Slower growth (lower ATP yield) on xylose was accompanied by decreased expression of 62 genes concerned with the biosynthesis of small molecules (amino acids, nucleotides, cofactors, and lipids), transcription, and translation; 5 such genes were expressed at a higher level. In xylose-grown cells, 90 genes associated with the transport, catabolism, and regulation of pathways for alternative carbon sources were expressed at higher levels than in glucose-grown cells, consistent with a relaxation of control by the cyclic AMP,CRP regulatory system. Changes in expression of genes encoding the Embden,Meyerhof,Parnas (EMP) pathway were in excellent agreement with calculated changes in flux for individual metabolites. Flux through all but one step, pyruvate kinase, was predicted to be higher during glucose fermentation. Expression levels (glucose/xylose) were higher in glucose-grown cells for all EMP genes except the isoenzymes encoding pyruvate kinase ( pykA and pykF). Expression of both isoenzymes was generally higher during xylose fermentation but statistically higher in both strains only for pykF encoding the isoenzyme activated by fructose-6-phosphate, a key metabolite connecting pentose metabolism to the EMP pathway. The coordinated changes in expression of genes encoding the EMP pathway suggest the presence of a common regulatory system and that flux control within the EMP pathway may be broadly distributed. In contrast, expression levels for genes encoding the Pentose,Phosphate pathway did not differ significantly between glucose-grown and xylose-grown cells. [source]


An economic comparison of different fermentation configurations to convert corn stover to ethanol using Z. mobilis and Saccharomyces

BIOTECHNOLOGY PROGRESS, Issue 1 2010
Abhijit Dutta
Abstract Numerous routes are being explored to lower the cost of cellulosic ethanol production and enable large-scale production. One critical area is the development of robust cofermentative organisms to convert the multiple, mixed sugars found in biomass feedstocks to ethanol at high yields and titers without the need for processing to remove inhibitors. Until such microorganisms are commercialized, the challenge is to design processes that exploit the current microorganisms' strengths. This study explored various process configurations tailored to take advantage of the specific capabilities of three microorganisms, Z. mobilis 8b, S. cerevisiae, and S. pastorianus. A technoeconomic study, based on bench-scale experimental data generated by integrated process testing, was completed to understand the resulting costs of the different process configurations. The configurations included whole slurry fermentation with a coculture, and separate cellulose simultaneous saccharification and fermentation (SSF) and xylose fermentations with none, some or all of the water to the SSF replaced with the fermented liquor from the xylose fermentation. The difference between the highest and lowest ethanol cost for the different experimental process configurations studied was $0.27 per gallon ethanol. Separate fermentation of solid and liquor streams with recycle of fermented liquor to dilute the solids gave the lowest ethanol cost, primarily because this option achieved the highest concentrations of ethanol after fermentation. Further studies, using methods similar to ones employed here, can help understand and improve the performance and hence the economics of integrated processes involving enzymes and fermentative microorganisms. 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2010 [source]