Conversion of cellulosic materials to ethanol

Surface Modified Activated Carbons: Sustainable Bio-Based Materials for Environmental Remediation

Global warming and water/air contamination caused by human activities are major challenges in environmental pollution and climate change. The improper discharge of a large amount of agro-forest byproduct is accelerating these issues mainly in developing countries. The burning of agricultural byproducts causes global warming, whereas their improper waste management causes water/air pollution. The conversion of these waste materials into effective smart materials can be considered as a promising strategy in waste management and environmental remediation. Over the past decades, activated carbons (ACs) have been prepared from various agricultural wastes and extensively used as adsorbents. The adsorption capacity of ACs is linked to a well-developed porous structure, large specific surface area, and rich surface functional moieties. Activated carbon needs to increase their adsorption capacity, especially for specific adsorbates, making them suitable for specific applications, and this is possible by surface modifications of their surface chemistry. The modifications of surface chemistry involve the introduction of surface functional groups which can be carried out by various methods such as acid treatment, alkaline treatment, impregnation, ozone treatment, plasma treatment, and so on. Depending on the treatment methods, surface modification mainly affects surface chemistry. In this review, we summarized several modification methods for agricultural-waste-based ACs. In addition, the applications of AC for the adsorption of various pollutants are highlighted.

Selective radical depolymerization of cellulose to glucose induced by high frequency ultrasound

The depolymerization of cellulose to glucose is a challenging reaction and often constitutes a scientific obstacle in the synthesis of downstream bio-based products. Here, we show that cellulose can be selectively depolymerized to glucose by ultrasonic irradiation in water at a high frequency (525 kHz). The concept of this work is based on the generation of H˙ and ˙OH radicals, formed by homolytic dissociation of water inside the cavitation bubbles, which induce the cleavage of the glycosidic bonds. The transfer of radicals on the cellulose particle surfaces prevents the side degradation of released glucose into the bulk solution, allowing maintaining the selectivity to glucose close to 100%. This work is distinguished from previous technologies in that (i) no catalyst is needed, (ii) no external source of heating is required, and (iii) the complete depolymerization of cellulose is achieved in a selective fashion. The addition of specific radical scavengers coupled to different gaseous atmospheres and ˙OH radical dosimetry experiments suggested that H˙ radicals are more likely to be responsible for the depolymerisation of cellulose.

The emergence of adaptive laboratory evolution as an efficient tool for biological discovery and industrial biotechnology

Harnessing the process of natural selection to obtain and understand new microbial phenotypes has become increasingly possible due to advances in culturing techniques, DNA sequencing, bioinformatics, and genetic engineering. Accordingly, Adaptive Laboratory Evolution (ALE) experiments represent a powerful approach both to investigate the evolutionary forces influencing strain phenotypes, performance, and stability, and to acquire production strains that contain beneficial mutations. In this review, we summarize and categorize the applications of ALE to various aspects of microbial physiology pertinent to industrial bioproduction by collecting case studies that highlight the multitude of ways in which evolution can facilitate the strain construction process. Further, we discuss principles that inform experimental design, complementary approaches such as computational modeling that help maximize utility, and the future of ALE as an efficient strain design and build tool driven by growing adoption and improvements in automation.

Efficient and repeated production of succinic acid by turning sugarcane bagasse into sugar and support

Here we reported an endeavor in making full use of sugarcane bagasse for biological production of succinic acid. Through NaOH pre-treatment and multi-enzyme hydrolysis, a reducing sugar solution mainly composed of glucose and xylose was obtained from the sugarcane bagasse. By optimizing portions of cellulase, xylanase, β-glucanase and pectinase in the multi-enzyme "cocktail", the hydrolysis percentage of the total cellulose in pre-treated sugarcane bagasse can be as high as 88.5%. A. succinogenes CCTCC M2012036 was used for converting reducing sugars into succinic acid in a 3-L bioreactor with a sugar-fed strategy to prevent cell growth limitation. Importantly, cells were found to be adaptive on the sugarcane bagasse residue, offering possibilities of repeated batch fermentation and replacement for MgCO3 with soluble NaHCO3 in pH modulation. Three cycles of fermentation without activity loss were realized with the average succinic acid yield and productivity to be 80.5% and 1.65g·L(-1)·h(-1).

Lignocellulosic agriculture wastes as biomass feedstocks for second-generation bioethanol production: concepts and recent developments

Production of liquid biofuels, such as bioethanol, has been advocated as a sustainable option to tackle the problems associated with rising crude oil prices, global warming and diminishing petroleum reserves. Second-generation bioethanol is produced from lignocellulosic feedstock by its saccharification, followed by microbial fermentation and product recovery. Agricultural residues generated as wastes during or after processing of agricultural crops are one of such renewable and lignocellulose-rich biomass resources available in huge amounts for bioethanol production. These agricultural residues are converted to bioethanol in several steps which are described here. This review enlightens various steps involved in production of the second-generation bioethanol. Mechanisms and recent advances in pretreatment, cellulases production and second-generation ethanol production processes are described here.

Bioconversion of xylose, hexoses and biomass to ethanol by a new isolate of the white rot basidiomycete Trametes versicolor

Second-generation bioethanol production requires the development of economically feasible and sustainable processes that use renewable lignocellulosic biomass as a starting material. However, the microbial fermentation of xylose, which is the principal pentose sugar in hemicellulose, is a limiting factor in developing such processes. Here, a strain of the white rot basidiomycete Trametes versicolor that was capable of efficiently fermenting xylose was newly isolated and characterized. This strain, designated KT9427, was capable of assimilating and converting xylose to ethanol under anaerobic conditions with a yield of 0.44 g ethanol per 1 g of sugar consumed. In culture medium containing low yeast extract concentrations, xylose consumption and ethanol productivity were enhanced. Adjusting the initial pH between 3.0 and 5.0 did not markedly influence xylose fermentation. T. versicolor KT9427 also produced ethanol from glucose, mannose, fructose, cellobiose and maltose at yields ranging from 0.45 to 0.49 g ethanol per 1 g of sugar consumed. In addition, strain KT9427 exhibited favourable conversion of non-pretreated starch, cellulose, xylan, wheat bran and rice straw into ethanol compared to common recombinant yeast strains. Taken together, the present findings suggest that T. versicolor KT9427 is a promising candidate for environmentally friendly ethanol production directly from lignocellulosic biomass.

Pretreatment of banana agricultural waste for bio-ethanol production: individual and interactive effects of acid and alkali pretreatments with autoclaving, microwave heating and ultrasonication

Banana agricultural waste is one of the potential lignocellulosic substrates which are mostly un-utilized but sufficiently available in many parts of the world. In the present study, suitability of banana waste for biofuel production with respect to pretreatment and reducing sugar yield was assessed. The effectiveness of both acid and alkali pretreatments along with autoclaving, microwave heating and ultrasonication on different morphological parts of banana (BMPs) was studied. The data were statistically analyzed using ANOVA and numerical point prediction tool of MINITAB RELEASE 14. Accordingly, the optimum cumulative conditions for maximum recovery of reducing sugar through acid pretreatment are: leaf (LF) as the substrate with 25 min of reaction time and 180°C of reaction temperature using microwave. Whereas, the optimum conditions for alkaline pretreatments are: pith (PH) as the substrate with 51 min of reaction time and 50°C of reaction temperature using ultrasonication (US).

Metabolic engineering of biosynthetic pathway for production of renewable biofuels

Metabolic engineering is an important area of research that involves editing genetic networks to overproduce a certain substance by the cells. Using a combination of genetic, metabolic, and modeling methods, useful substances have been synthesized in the past at industrial scale and in a cost-effective manner. Currently, metabolic engineering is being used to produce sufficient, economical, and eco-friendly biofuels. In the recent past, a number of efforts have been made towards engineering biosynthetic pathways for large scale and efficient production of biofuels from biomass. Given the adoption of metabolic engineering approaches by the biofuel industry, this paper reviews various approaches towards the production and enhancement of renewable biofuels such as ethanol, butanol, isopropanol, hydrogen, and biodiesel. We have also identified specific areas where more work needs to be done in the future.

D-Xylulose kinase from Saccharomyces cerevisiae: isolation and characterization of the highly unstable enzyme, recombinantly produced in Escherichia coli

The Saccharomyces cerevisiae gene encoding xylulose kinase (XKS1) was over-expressed to an abundance of ≥ 10% intracellular protein in Escherichia coli. Instability of XKS1, not pointed out in previous reports of the enzyme, prevented isolation of active enzyme in native or "tagged" form under a wide range of purification conditions. A fusion protein haboring C-terminal Strep-tag II (XKS1-Strep) displayed activity (∼20 U/mg) as isolated. However, the half-life time of purified XKS1-Strep was only ∼1.5h at 4°C and could not be enhanced substantially by an assortment of extrinsic stabilizers (osmolytes, protein, substrates). Peptide mass mapping and N-terminal sequencing showed that the recombinant protein was structurally intact, ruling out proteolytic processing and chemical modifications as possible factors to compromise the stability of the enzyme as isolated. Partial functional complementation of a largely inactive XKS1 preparation by the high-molecular mass fraction (≥ 10kDa) of cell extract prepared from an E. coli BL21 (DE3) expression host suggests a possible role for heterotropic protein-XKS1 interactions in conferring activity/stability to the enzyme. Michaelis-Menten constants of XKS1-Strep were determined: d-xylulose (210 ± 40 μM) and Mg(2+)-ATP (1.70 ± 0.10 mM).

Purification and characterization of novel glucanases from Trichoderma harzianum ETS 323

Trichoderma harzianum ETS 323 secretes two glucanases, a 23.5 kDa endoglucanase (EG Th1) and a 61 kDa exoglucanase (ExG Th1). They were identified by their hydrolysis products and were purified to homogeneity. The optimal temperature and pH for both EG Th1 (7.3-fold purification, 5.0% yield) and ExG Th1 (33.7-fold purification, 0.15% yield) were 50 °C and pH 4.5, respectively. The kinetic parameters of EG Th1 (K(m) = 23 mg mL(-1), V(max) = 294 μM min(-1), specific activity = 7.4 U mg(-1)) and ExG Th1 (K(m) = 85 mg mL(-1), V(max) = 385 μM min(-1), specific activity = 24.6 U mg(-1)) toward carboxymethyl cellulose were determined. Both enzymes favored CMC and maintained 100% activity for 10 days at 38 °C. KCl, MgCl(2), HgCl(2), and FeCl(3) showed approximately 30% inhibition against EG Th1 but not ExG Th1. They catalyzed transglycosylation of glucose in the presence of cellobiose, but ExG Th1 exhibited better activity and higher product diversity.

Simultaneous saccharification and fermentation and partial saccharification and co-fermentation of lignocellulosic biomass for ethanol production

Ethanol production by fermentation of lignocellulosic biomass-derived sugars involves a fairly ancient art and an ever-evolving science. Production of ethanol from lignocellulosic biomass is not avant-garde, and wood ethanol plants have been in existence since at least 1915. Most current ethanol production relies on starch- and sugar-based crops as the substrate; however, limitations of these materials and competing value for human and animal feeds is renewing interest in lignocellulose conversion. Herein, we describe methods for both simultaneous saccharification and fermentation (SSF) and a similar but separate process for partial saccharification and cofermentation (PSCF) of lignocellulosic biomass for ethanol production using yeasts or pentose-fermenting engineered bacteria. These methods are applicable for small-scale preliminary evaluations of ethanol production from a variety of biomass sources.

Structural features affecting biomass enzymatic digestibility

The rate and extent of enzymatic hydrolysis of lignocellulosic biomass highly depend on enzyme loadings, hydrolysis periods, and structural features resulting from pretreatments. Furthermore, the influence of one structural feature on biomass digestibility varies with the changes in enzyme loading, hydrolysis period and other structural features as well. In this paper, the effects of lignin content, acetyl content, and biomass crystallinity on the 1-, 6-, and 72-h digestibilities with various enzyme loadings were investigated. To eliminate the cross effects among structural features, selective pretreatment techniques were employed to vary one particular structural feature during a pretreatment, while the other two structural features remained unchanged. The digestibility results showed that lignin content and biomass crystallinity dominated digestibility whereas acetyl content had a lesser effect. Lignin removal greatly enhanced the ultimate hydrolysis extent. Crystallinity reduction, however, tremendously increased the initial hydrolysis rate and reduced the hydrolysis time or the amount of enzyme required to attain high digestibility. To some extent, the effects of structural features on digestibility were interrelated. At short hydrolysis periods, lignin content was not important to digestibility when crystallinity was low. Similarly, at long hydrolysis periods, crystallinity was not important to digestibility when lignin content was low.

Potential of agricultural residues and hay for bioethanol production

Production of bioethanol from agricultural residues and hays (wheat, barley, and triticale straws, and barley, triticale, pearl millet, and sweet sorghum hays) through a series of chemical pretreatment, enzymatic hydrolysis, and fermentation processes was investigated in this study. Composition analysis suggested that the agricultural straws and hays studied contained approximately 28.62-38.58% glucan, 11.19-20.78% xylan, and 22.01-27.57% lignin, making them good candidates for bioethanol production. Chemical pretreatment with sulfuric acid or sodium hydroxide at concentrations of 0.5, 1.0, and 2.0% indicated that concentration and treatment agent play a significant role during pretreatment. After 2.0% sulfuric acid pretreatment at 121 degrees C/15 psi for 60 min, 78.10-81.27% of the xylan in untreated feedstocks was solubilized, while 75.09-84.52% of the lignin was reduced after 2.0% sodium hydroxide pretreatment under similar conditions. Enzymatic hydrolysis of chemically pretreated (2.0% NaOH or H2SO4) solids with Celluclast 1.5 L-Novozym 188 (cellobiase) enzyme combination resulted in equal or higher glucan and xylan conversion than with Spezyme(R) CP- xylanase combination. The glucan and xylan conversions during hydrolysis with Celluclast 1.5 L-cellobiase at 40 FPU/g glucan were 78.09 to 100.36% and 74.03 to 84.89%, respectively. Increasing the enzyme loading from 40 to 60 FPU/g glucan did not significantly increase sugar yield. The ethanol yield after fermentation of the hydrolyzate from different feedstocks with Saccharomyces cerevisiae ranged from 0.27 to 0.34 g/g glucose or 52.00-65.82% of the theoretical maximum ethanol yield.

A comparison of chemical pretreatment methods for improving saccharification of cotton stalks

The effectiveness of sulfuric acid (H(2)SO(4)), sodium hydroxide (NaOH), hydrogen peroxide (H(2)O(2)), and ozone pretreatments for conversion of cotton stalks to ethanol was investigated. Ground cotton stalks at a solid loading of 10% (w/v) were pretreated with H(2)SO(4), NaOH, and H(2)O(2) at concentrations of 0.5%, 1%, and 2% (w/v). Treatment temperatures of 90 degrees C and 121 degrees C at 15 psi were investigated for residence times of 30, 60, and 90 min. Ozone pretreatment was performed at 4 degrees C with constant sparging of stalks in water. Solids from H(2)SO(4), NaOH, and H(2)O(2) pretreatments (at 2%, 60 min, 121 degrees C/15 psi) showed significant lignin degradation and/or high sugar availability and hence were hydrolyzed by Celluclast 1.5L and Novozym 188 at 50 degrees C. Sulfuric acid pretreatment resulted in the highest xylan reduction (95.23% for 2% acid, 90 min, 121 degrees C/15 psi) but the lowest cellulose to glucose conversion during hydrolysis (23.85%). Sodium hydroxide pretreatment resulted in the highest level of delignification (65.63% for 2% NaOH, 90 min, 121 degrees C/15 psi) and cellulose conversion (60.8%). Hydrogen peroxide pretreatment resulted in significantly lower (p<or=0.05) delignification (maximum of 29.51% for 2%, 30 min, 121 degrees C/15 psi) and cellulose conversion (49.8%) than sodium hydroxide pretreatment, but had a higher (p<or=0.05) cellulose conversion than sulfuric acid pretreatment. Ozone did not cause any significant changes in lignin, xylan, or glucan contents over time. Quadratic models using time, temperature, and concentration as continuous variables were developed to predict xylan and lignin reduction, respectively for sulfuric acid and sodium hydroxide pretreatments. In addition, a modified severity parameter (log M(0)) was constructed and explained most of the variation in xylan or lignin reduction through simple linear regressions.

Biofuels from microbes

Today, biomass covers about 10% of the world's primary energy demand. Against a backdrop of rising crude oil prices, depletion of resources, political instability in producing countries and environmental challenges, besides efficiency and intelligent use, only biomass has the potential to replace the supply of an energy hungry civilisation. Plant biomass is an abundant and renewable source of energy-rich carbohydrates which can be efficiently converted by microbes into biofuels, of which, only bioethanol is produced on an industrial scale today. Biomethane is produced on a large scale, but is not yet utilised for transportation. Biobutanol is on the agenda of several companies and may be used in the near future as a supplement for gasoline, diesel and kerosene, as well as contributing to the partially biological production of butyl-t-butylether, BTBE as does bioethanol today with ETBE. Biohydrogen, biomethanol and microbially made biodiesel still require further development. This paper reviews microbially made biofuels which have potential to replace our present day fuels, either alone, by blending, or by chemical conversion. It also summarises the history of biofuels and provides insight into the actual production in various countries, reviewing their policies and adaptivity to the energy challenges of foreseeable future.

Fuel ethanol production: process design trends and integration opportunities

Current fuel ethanol research and development deals with process engineering trends for improving biotechnological production of ethanol. In this work, the key role that process design plays during the development of cost-effective technologies is recognized through the analysis of major trends in process synthesis, modeling, simulation and optimization related to ethanol production. Main directions in techno-economical evaluation of fuel ethanol processes are described as well as some prospecting configurations. The most promising alternatives for compensating ethanol production costs by the generation of valuable co-products are analyzed. Opportunities for integration of fuel ethanol production processes and their implications are underlined. Main ways of process intensification through reaction-reaction, reaction-separation and separation-separation processes are analyzed in the case of bioethanol production. Some examples of energy integration during ethanol production are also highlighted. Finally, some concluding considerations on current and future research tendencies in fuel ethanol production regarding process design and integration are presented.

Ethanol fermentation from biomass resources: current state and prospects

In recent years, growing attention has been devoted to the conversion of biomass into fuel ethanol, considered the cleanest liquid fuel alternative to fossil fuels. Significant advances have been made towards the technology of ethanol fermentation. This review provides practical examples and gives a broad overview of the current status of ethanol fermentation including biomass resources, microorganisms, and technology. Also, the promising prospects of ethanol fermentation are especially introduced. The prospects included are fermentation technology converting xylose to ethanol, cellulase enzyme utilized in the hydrolysis of lignocellulosic materials, immobilization of the microorganism in large systems, simultaneous saccharification and fermentation, and sugar conversion into ethanol.

Glycolysis and Flux Control

Central metabolism of carbohydrates uses the Embden-Meyerhof-Parnas (EMP), pentose phosphate (PP), and Entner-Doudoroff (ED) pathways. This review reviews the biological roles of the enzymes and genes of these three pathways of E. coli. Glucose, pentoses, and gluconate are primarily discussed as the initial substrates of the three pathways, respectively. The genetic and allosteric regulatory mechanisms of glycolysis and the factors that affect metabolic flux through the pathways are considered here. Despite the fact that a lot of information on each of the reaction steps has been accumulated over the years for E. coli, surprisingly little quantitative information has been integrated to analyze glycolysis as a system. Therefore, the review presents a detailed description of each of the catalytic steps by a systemic approach. It considers both structural and kinetic aspects. Models that include kinetic information of the reaction steps will always contain the reaction stoichiometry and therefore follow the structural constraints, but in addition to these also kinetic rate laws must be fulfilled. The kinetic information obtained on isolated enzymes can be integrated using computer models to simulate behavior of the reaction network formed by these enzymes. Successful examples of such approaches are the modeling of glycolysis in S. cerevisiae, the parasite Trypanosoma brucei, and the red blood cell. With the rapid developments in the field of Systems Biology many new methods have been and will be developed, for experimental and theoretical approaches, and the authors expect that these will be applied to E. coli glycolysis in the near future.

Genomic diversity of Erwinia carotovora subsp. carotovora and its correlation with virulence

We used genetic and biochemical methods to examine the genomic diversity of the enterobacterial plant pathogen Erwinia carotovora subsp. carotovora. The results obtained with each method showed that E. carotovora subsp. carotovora strains isolated from one ecological niche, potato plants, are surprisingly diverse compared to related pathogens. A comparison of 23 partial mdh sequences revealed a maximum pairwise difference of 10.49% and an average pairwise difference of 2.13%, values which are much greater than the maximum variation (1.81%) and average variation (0.75%) previously reported for Escherichia coli. Pulsed-field gel electrophoresis analysis of I-CeuI-digested genomic DNA revealed seven rrn operons in all E. carotovora subsp. carotovora strains examined except strain WPP17, which had only six copies. We identified 26 I-CeuI restriction fragment length polymorphism patterns and observed significant polymorphism in fragment sizes ranging from 100 to 450 kb for all strains. We detected large plasmids in two strains, including the model strain E. carotovora subsp. carotovora 71. The two least virulent strains had an unusual chromosomal structure, suggesting that a particular pulsotype is correlated with virulence. To compare chromosomal organization of multiple enterobacterial genomes, several genes were mapped onto I-CeuI fragments. We identified portions of the genome that appear to be conserved across enterobacteria and portions that have undergone genome rearrangements. We found that the least virulent strain, WPP17, failed to oxidize cellobiose and was missing several hrp and hrc genes. The unexpected variability among isolates obtained from clonal hosts in one region and in one season suggests that factors other than the host plant, potato, drive the evolution of this common environmental bacterium and key plant pathogen.

Altered regulation of pyruvate kinase or co-overexpression of phosphofructokinase increases glycolytic fluxes in resting Escherichia coli

Glycolytic fluxes in resting Escherichia coli were enhanced by overexpression of heterologous pyruvate kinases (Pyk) from Bacillus stearothermophilus and Zymomonas mobilis, but not homologous Pyk. Compared to the control, an increase of 10% in specific glucose consumption and of 15% in specific ethanol production rates was found in anaerobic resting cells, expressing the heterologous Pyks, that were harvested from exponentially growing aerobic cultures. A further increase in glycolytic flux was achieved by simultaneous overexpression of E. coli phosphofructokinase (Pfk) and Pyk with specific glucose consumption and ethanol production rates of 25% and 35% greater, respectively, than the control. Fluxes to lactate were not significantly affected, contrary to previous observations with resting cells harvested from anaerobically growing cultures. To correlate the physiology of resting cells with the physiology of cells prior to harvest, we determined the relevant growth parameters from aerobic and anaerobic precultures. We conclude that glycolytic fluxes in E. coli with submaximal (aerobic) metabolic activity can be increased by overexpression of pyruvate kinases which do not require allosteric activation or co-overexpression with Pfk. However, such improvements require more extensive engineering in E. coli with near maximal (anaerobic) metabolic activity.

Application of mathematical tools for metabolic design of microbial ethanol production

Many attempts to engineer cellular metabolism have failed due to the complexity of cellular functions. Mathematical and computational methods are needed that can organize the available experimental information, and provide insight and guidance for successful metabolic engineering. Two such methods are reviewed here. Both methods employ a (log)linear kinetic model of metabolism that is constructed based on enzyme kinetics characteristics. The first method allows the description of the dynamic responses of metabolic systems subject to spatiotemporal variations in their parameters. The second method considers the product-oriented, constrained optimization of metabolic reaction networks using mixed-integer linear programming methods. The optimization framework is used in order to identify the combinations of the metabolic characteristics of the glycolytic enzymes from yeast and bacteria that will maximize ethanol production. The methods are also applied to the design of microbial ethanol production metabolism. The results of the calculations are in qualitative agreement with experimental data presented here. Experiments and calculations suggest that, in resting Escherichia coli cells, ethanol production and glucose uptake rates can be increased by 30% and 20%, respectively, by overexpression of a deregulated pyruvate kinase, while increase in phosphofructokinase expression levels has no effect on ethanol production and glucose uptake rates.

Metabolic engineering of bacteria for ethanol production

Technologies are available which will allow the conversion of lignocellulose into fuel ethanol using genetically engineered bacteria. Assembling these into a cost-effective process remains a challenge. Our work has focused primarily on the genetic engineering of enteric bacteria using a portable ethanol production pathway. Genes encoding Zymomonas mobilis pyruvate decarboxylase and alcohol dehydrogenase have been integrated into the chromosome of Escherichia coli B to produce strain KO11 for the fermentation of hemicellulose-derived syrups. This organism can efficiently ferment all hexose and pentose sugars present in the polymers of hemicellulose. Klebsiella oxytoca M5A1 has been genetically engineered in a similar manner to produce strain P2 for ethanol production from cellulose. This organism has the native ability to ferment cellobiose and cellotriose, eliminating the need for one class of cellulase enzymes. The optimal pH for cellulose fermentation with this organism (pH 5.0-5.5) is near that of fungal cellulases. The general approach for the genetic engineering of new biocatalysts has been most successful with enteric bacteria thus far. However, this approach may also prove useful with Gram-positive bacteria which have other important traits for lignocellulose conversion. Many opportunities remain for further improvements in the biomass to ethanol processes. These include the development of enzyme-based systems which eliminate the need for dilute acid hydrolysis or other pretreatments, improvements in existing pretreatments for enzymatic hydrolysis, process improvements to increase the effective use of cellulase and hemicellulase enzymes, improvements in rates of ethanol production, decreased nutrient costs, increases in ethanol concentrations achieved in biomass beers, increased resistance of the biocatalysts to lignocellulosic-derived toxins, etc. To be useful, each of these improvements must result in a decrease in the cost for ethanol production. Copyright 1998 John Wiley & Sons, Inc.

Thermostable variants of Zymomonas mobilis alcohol dehydrogenase obtained using PCR-mediated random mutagenesis

Using a random mutagenesis technique, the ferrous-ion-activated alcohol dehydrogenase of Zymomonas mobilis has been altered to produce more thermally stable variants. After three rounds of mutation, a variant over 10 degrees C more stable at pH 8, with essentially unaltered kinetic characteristics, was produced. However, the pH profile of thermostability of this variant was much altered compared with the wild-type, with a relatively small increase (4 degrees C) at pH 6. Sequencing of the variants indicated five amino acids changes which contributed to thermostability: F9S, M13I, K31R, F90L, and G250D. Four of these were contained in the final stable variant, and the changes were partially additive, with individual mutations causing between 2 and 3.5 degrees C stability increases (at pH 7.5). It is estimated that the most stable variant would have a half-life under physiological conditions at 70 degrees C of 15 min.