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Xylose Fermentation to Ethanol by Schizosaccharomyces Pombe Clones with Xylose Isomerase Gene." Biotechnology Letters (8:4); Pp

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Xylose Fermentation to Ethanol by Schizosaccharomyces Pombe Clones with Xylose Isomerase Gene. NREL!TP-421-4944 • UC Category: 246 • DE93000067 l I Xylose Fermenta to Ethanol: A R ew '.) i I, -- , ) )I' J. D. McMillan I ' J.( .!i �/ .6' ....� .T u�.•ls:l ., �-- • National Renewable Energy Laboratory II 'J 1617 Cole Boulevard Golden, Colorado 80401-3393 A Division of Midwest Research Institute Operated for the U.S. Department of Energy under Contract No. DE-AC02-83CH10093 Prepared under task no. BF223732 January 1993 NOTICE This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, com­ pleteness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily con­ stitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof. Printed in the United States of America Available from: National Technical Information Service U.S. Department of Commerce 5285 Port Royal Road Springfield, VA22161 Price: Microfiche A01 Printed Copy A03 Codes are used for pricing all publications. The code is determined by the number of pages in the publication. Information pertaining to the pricing codes can be found in the current issue of the following publications which are generally available in most libraries: Energy Research Abstracts (ERA); Govern­ ment Reports Announcements and Index ( GRA and I); Scientific and Technical Abstract Reports(STAR); and publication NTIS-PR-360 available from NTIS at the above address. l ! Summary The past several years have seen tremendous progress in the understanding of xylose metabolism and in the identification, characterization, and development of strains with improved xylose fermentation characteristics. A survey of thenumerous microorganisms capableof directlyfermenting xylose to ethanol J indicatesthat wild-type yeast and recombinant bacteriaoffer the best overall performance in termsof high yield, final ethanolconcentration, and volumetricproductivity. The best performing bacteria, yeast, and fungi can achieve yields greater than 0.4 gig and final ethanol concentrations approaching 5% (w/v). Productivitiesremain low for most yeast and particularly for fungi,but volumetricproductivities exceeding 1.0 g/L-hhave been reported for xylose-fermentingbacteria. In termsof wild-type microorganisms, strains of the yeast Pichia stipitis show the most promisein theshort termfor direct high-yield fermentation of xylose without byproductformation. Of the recombinant xylose-fermenting microorganisms developed, recombinant E. coli AITC 11303 (pLOI297) exhibits the most favorable performance characteristics reported to date. Severalfungi and thermophilic bacteriaoffer long-term promise and should continueto be examined. There are disadvantages to the use of eitherP. stipitis or recombinantE. coli. The primary disadvantage with P. stipitis is that very low levels of aeration are necessary to achieve optimal performance. RecombinantE. coli AITC 11303 (pLOI297) suffers from the fact that its genetic stability is unproven in prolonged culture,there is a higher probability of contamination becauseit must be cultivated at nearly neutral pH, and regulatory obstaclesmay hinder its large-scale use. Developments in xylose conversion are progressing rapidly, although many questions remain regarding I ' key aspects of xylose metabolism in bacteria, yeast, and fungi. A large body of literature already exists j concerningthe performance of xylose-fermentingyeast. Data on theperformance of recombinantbacteria are accumulating quickly. I I' The construction of recombinant bacterial and yeast strains that exhibit improved xylose fermentation performance characteristics represents the most significant recent development in xylose conversion research. Geneticengineering work aimed at constructinga recombinant Saccharomyces cerevisiae strain capable of high-yield xylose fermentation is nearing fruition. Progress is being made on developing improved recombinant bacterial strains. Given the rapid pace of advances being made in metabolic engineeringof improved xylose-fermentingorganisms, thedevelopment of recombinant organisms capable of much improved performance seems imminent. Substantialprogress is being made in achieving efficienthigh-yield xylose fermentation and the successful . '} development of large-scale processes appears likely within the decade. j i J i ·�J Contents !I - T1 Summ ary .............................................................. i _j Introduction . .... ... ... ............. 1 �-_j Background and His to ric al Overview . 1 Microorganisms . 1 Met abolism . 3 Tr anspo rt . 5 Xylulose-5-® Formation . 6 Energetics . 6 Cofactor Requirements of Xylose Reductase . 6 Regul ati on of Xylose Utilization . 8 Ethanol and Coproduct Formation . 9 Genetic Engineering Advances . 11 Bacteria . 13 Ye as t . 15 Fermentation Studies . ......... ....... ..... 16 Ye as t ..... ................ ............. ... · · · · · · · · · · · · · · · · · 16 Medium Composition . 17 Xylose Co ncentration . 17 Vi tamins andTrace Miner als . 17 Nitrogen Source . 20 Cultivation Conditions . 20 Aer ation ............................................. 20 pH ................................................. 24 Temperature . ..... .... 24 Bacte ria ......................................................... 25 Thermophiles . ...... .......... .... ........... .............. 25 Recombinants ............................................... 27 Fungi 27 Di scussion . ........; . 30 r '! Factors Influencing thePerform ance of Xylose-fermenti ng Yeast . 30 'I 'I Comparison of "Best to-- date" Performance Re su tsl . 32 �. J Adv an tages and Di sadvantages of Bacteria-based andYe as t-b as ed Processes . 33 ·. l Ke y Issues . 34 Nutrient Requirements (Including Aer ati on) . 34 Performance on Real Hydrolyzates . 34 Performance and Stability during a Prolonged Culture . 34 r.i. '! De velopment of Im proved Xylose-fermenting Microorganisms usingRecombinant ._j DN A Technology . 35 De ve lopment of Improved Xylose-fermenting Microorganisms by Adaptation . 35 Isolationof Im provedXylose -fermenting Org an isms fromNature . 35 ii Techniques fo r Providing Micro ae ration at Large Scales . .. .. .. .. .. 36 Potential for Cell Imm obilization an d/o r Cell Recycling Techniques to Imp ro ve Productivity . .. .. .. .. .. .. .. .. .. .. .. .. 36 Fun dame ntal Aspects of D-xylose Met abolism . .. .. .. .. .. .. .. 36 \ Research at NREL . .. .. .. .. .. .. .. .. .. .. .. .. 36 \ Conclusions . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 37 References . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 38 \ Tables Table 1. Genera Capable of Di rect Fermentation of Xylose to Ethanol . .. .. .. .. .. .. .. .. .. .. .. 2 Table 2. Nongaseous Coproducts of Ethanolic Xylose Fermentation . .. .. .. .. .. 10 Table 3. "Best-to-d ate" Performance Results for Yeast . .. .. .. .. .. .. .. 18 Table 4. Performance Results for Yeast under Ana erobic or "Microaerophilic" Conditions . .. 23 t Table 5. "Best-to-date" Performance of Thermophilic Bacteria .. .. .. .. .. 26 Table 6. "Best-to-date" Performance Results for Recombin an t Bacteria . .. .. .. .. .. 28 :l Table 7. "Bestto-date" - Performance Results for Fungi . .... .... .. .... .. ... ........ .. 29 Table 8. Summ ary of "Best-to-d ate" Performance Results .. .. .. .. .. .. .. .. 33 iii ,, .l\I 1. I ' Introduction Because the pentose sugar D-xylose comprises about one-third of the total carbohydrate sugars in lignocellulosic biomass, favorable economics for large-scale production of ethanol from lignocellulosic materials require efficient conversion of xylose. The U.S. Department of Energy (DOE)/National Renewable Energy Laboratory (NREL) Ethanol from Biomass Program is evaluating a variety of Jl fermentation-based bioconversion processes for converting xylose (and other pentose sugars) to ethanol at an industrial scale. An economic analysis of xylose fermentation conducted by Hinman et al. (1989) concludes that for fixed substrate costs, ethanol yield and fmal concentration most influence ethanol production costs, with ethanol production rate of secondary importance. Thus, efforts are focused on determining the microorganisms that are most capable of carrying out high-yield xylose fermentation to high ethanol concentration. The past several years have seen tremendous progress in the understanding of xylose metabolism and in the identification, characterization, and development of strains with improved xylose fermentation characteristics. Thisreport reviews the current stateof xylose fermentation technology, focusing on recent improvements in the direct fermentation of xylose (developments involving xylanases and simultaneous isomerization and fermentationare not discussed). Anticipated advantages anddisa dvantages of processes currently being considered for large-scale development are discussed and recommendations are made concerning directions for future xylose conversion research. Background and Historical Overview Microorganisms Microorganisms that rapidly
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