The effect of acetic acid on fuel ethanol production byZymomonas

Production, Characterization, Kinetics, and Thermodynamics Analysis of Amyloglucosidase from Fungal Consortium

The current study aimed to produce an amyloglucosidase enzyme from the fungal consortium. The best amylolytic fungal consortia were identified as Alternaria alternata and Aspergillus niger through the 18S rDNA technique. Fermentation kinetics and various nutritional and cultural parameters were analyzed. Maximum production was obtained in M4 media, pH 5.5, 30 °C, and 4 mL inoculum at 150 rpm after 72 h of incubation. Along with that, sodium nitrate at 2.5%, maltose, beef extract 1%, zinc sulfate (0.1%), and Tween 80 (0.1%) supported the maximum amyloglucosidase production. Amyloglucosidase was partially purified up to 1.6 purification fold with a specific activity of 1.84 Umg-1 in a stepwise manner by ammonium sulfate purification, dialysis, and ion exchange chromatography. The AMG enzyme also revealed maximum activity at 50 °C with 5.0 pH. Upon the kinetic analysis, the specific yield coefficient Yp/x and volumetric rates Qp and Qx were also found to be significant in the above optimized conditions. The Km value 0.33 mg mL-1 and Vmax 26.31 U mL-1 were obtained at 1% soluble starch substrate. Thermodynamic parameters for soluble starch hydrolysis were as follows: ΔH = 48.78 kJ mol-1, (Ea) =  - 46.0 kJ mol-1, and ΔS =  - 43.10 J mol-1 K-1. This finding indicates the indigenously isolated fungal consortium can be the best candidate for industrial applications.

Proteomic and metabolomic analysis of the cellular biomarkers related to inhibitors tolerance in Zymomonas mobilis ZM4

BACKGROUND

Toxic compounds present in both the hydrolysate and pyrolysate of lignocellulosic biomass severely hinder the further conversion of lignocellulose-derived fermentable sugars into useful chemicals by common biocatalysts like Zymomonas mobilis, which has remarkable advantages over yeast. Although the extra detoxification treatment prior to fermentation process can help biocatalysts to eliminate the inhibitory environment, it is not environment friendly and cost effective for industrial application. As also reported by previous studies, an ideal and holistic approach to solve this issue is to develop microbial strains with inhibitor tolerance. However, previously engineered strains had the limitation that they could not cope well with the synergistic effect of multiple inhibitors as they are resistant only to a single inhibitor. Hence, understanding the universal cellular responses of Z. mobilis to various inhibitors may guide the designing of rational strategies to obtain more robust engineered strains for biofuel production from lignocellulosic biomass.

RESULTS

Quantitative proteomics and metabolomics approaches were used to determine the cellular responses of Z. mobilis ZM4 to representative biomass-derived inhibitors like formic acid, acetic acid, furfural, 5-hydroxymethylfurfural, and phenol. The differentially expressed proteins identified under the challenge of single and combined inhibitors were involved in cell wall/membrane biogenesis, energy production, DNA replication, DNA recombination, DNA repair, DNA transcription, RNA translation, posttranslational modification, biosynthesis of amino acids, central carbon metabolism, etc. Metabolomics analysis showed that the up- or down-regulation pattern of metabolites was changed consistently with that of relevant proteins.

CONCLUSION

Fifteen up-regulated proteins (e.g., Isopropylmalate isomerase LeuC, transcription-repair-coupling factor Mfd, and phosphoglucose isomerase PGI) and thirteen down-regulated proteins (e.g., TonB-dependent transporter ZMO1522, transcription termination factor Rho, and S1/P1 nuclease ZMO0127) were identified as candidate proteins related to all the stress conditions, implying that these proteins are potential biomarkers for the improvement of Z. mobilis ZM4 to resist complex biomass-derived inhibitors. These data can be used to generate a database of inhibitor-tolerance biomarkers, which could provide a basis for engineering Z. mobilis that would be able to grow in the presence of multiple inhibitors and directly ferment the biomass-derived sugars into biofuels.

Paradigm for industrial strain improvement identifies sodium acetate tolerance loci in Zymomonas mobilis and Saccharomyces cerevisiae

The application of systems biology tools holds promise for rational industrial microbial strain development. Here, we characterize a Zymomonas mobilis mutant (AcR) demonstrating sodium acetate tolerance that has potential importance in biofuel development. The genome changes associated with AcR are determined using microarray comparative genome sequencing (CGS) and 454-pyrosequencing. Sanger sequencing analysis is employed to validate genomic differences and to investigate CGS and 454-pyrosequencing limitations. Transcriptomics, genetic data and growth studies indicate that over-expression of the sodium-proton antiporter gene nhaA confers the elevated AcR sodium acetate tolerance phenotype. nhaA over-expression mostly confers enhanced sodium (Na(+)) tolerance and not acetate (Ac(-)) tolerance, unless both ions are present in sufficient quantities. NaAc is more inhibitory than potassium and ammonium acetate for Z. mobilis and the combination of elevated Na(+) and Ac(-) ions exerts a synergistic inhibitory effect for strain ZM4. A structural model for the NhaA sodium-proton antiporter is constructed to provide mechanistic insights. We demonstrate that Saccharomyces cerevisiae sodium-proton antiporter genes also contribute to sodium acetate, potassium acetate, and ammonium acetate tolerances. The present combination of classical and systems biology tools is a paradigm for accelerated industrial strain improvement and combines benefits of few a priori assumptions with detailed, rapid, mechanistic studies.

Effect of acetic acid and pH on the cofermentation of glucose and xylose to ethanol by a genetically engineered strain of Saccharomyces cerevisiae

A current challenge of the cellulosic ethanol industry is the effect of inhibitors present in biomass hydrolysates. Acetic acid is an example of one such inhibitor that is released during the pretreatment of hemicellulose. This study examined the effect of acetic acid on the cofermentation of glucose and xylose under controlled pH conditions by Saccharomyces cerevisiae 424A(LNH-ST), a genetically engineered industrial yeast strain. Acetic acid concentrations of 7.5 and 15 g L(-1), representing the range of concentrations expected in actual biomass hydrolysates, were tested under controlled pH conditions of 5, 5.5, and 6. The presence of acetic acid in the fermentation media led to a significant decrease in the observed maximum cell biomass concentration. Glucose- and xylose-specific consumption rates decreased as the acetic acid concentration increased, with the inhibitory effect being more severe for xylose consumption. The ethanol production rates also decreased when acetic acid was present, but ethanol metabolic yields increased under the same conditions. The results also revealed that the inhibitory effect of acetic acid could be reduced by increasing media pH, thus confirming that the undissociated form of acetic acid is the inhibitory form of the molecule.

Zymomonas mobilis for fuel ethanol and higher value products

High oil prices, increasing focus on renewable carbohydrate-based feedstocks for fuels and chemicals, and the recent publication of its genome sequence, have provided continuing stimulus for studies on Zymomonas mobilis. However, despite its apparent advantages of higher yields and faster specific rates when compared to yeasts, no commercial scale fermentations currently exist which use Z. mobilis for the manufacture of fuel ethanol. This may change with the recent announcement of a Dupont/Broin partnership to develop a process for conversion of lignocellulosic residues, such as corn stover, to fuel ethanol using recombinant strains of Z. mobilis. The research leading to the construction of these strains, and their fermentation characteristics, are described in the present review. The review also addresses opportunities offered by Z. mobilis for higher value products through its metabolic engineering and use of specific high activity enzymes.

Improving fermentation performance of recombinant Zymomonas in acetic acid-containing media

In the production of ethanol from lignocellulosic biomass, the hydrolysis of the acetylated pentosans in hemicellulose during pretreatment produces acetic acid in the prehydrolysate. The National Renewable Energy Laboratory (NREL) is currently investigating a simultaneous saccharification and cofermentation (SSCF) process that uses a proprietary metabolically engineered strain of Zymomonas mobilis that can coferment glucose and xylose. Acetic acid toxicity represents a major limitation to bioconversion, and cost-effective means of reducing the inhibitory effects of acetic acid represent an opportunity for significant increased productivity and reduced cost of producing fermentation fuel ethanol from biomass. In this study, the fermentation performance of recombinant Z. mobilis 39676:pZB4L, using a synthetic hardwood prehydrolysate containing 1% (w/v) yeast extract, 0.2% KH2PO4, 4% (w/v) xylose, and 0.8% (w/v) glucose, with varying amounts of acetic acid was examine. To minimize the concentration of the inhibitory undissociated form of acetic acid, the pH was controlled at 6.0. The final cell mass concentration decreased linearly with increasing level of acetic acid over the range 0-0.75% (w/v), with a 50% reduction at about 0.5% (w/v) acetic acid. The conversion efficiency was relatively unaffected, decreasing from 98 to 92%. In the absence of acetic acid, batch fermentations were complete at 24 h. In a batch fermentation with 0.75% (w/v) acetic acid, about two-thirds of the xylose was not metabolized after 48 h. In batch fermentations with 0.75% (w/v) acetic acid, increasing the initial glucose concentration did not have an enhancing effect on the rate of xylose fermentation. However, nearly complete xylose fermentation was achieved in 48h when the bioreactor was fed glucose. In the fed-batch system, the rate of glucose feeding (0.5 g/h) was designed to simulate the rate of cellulolytic digestion that had been observed in a modeled SSCF process with recombinant Zymomonas. In the absence of acetic acid, this rate of glucose feeding did not inhibit xylose utilization. It is concluded that the inhibitory effect of acetic acid on xylose utilization in the SSCF biomass-to-ethanol process will be partially ameliorated because of the simultaneous saccharification of the cellulose.

Studies on nutrient requirements and cost-effective supplements for ethanol production by recombinant E. coli

This article describes a systematic study of the nutritional requirements of a patented recombinant ethanologenic Escherichia coli (11303:pLO1297) and provides cost-effective formulations that are compatible with the production of fuel ethanol in fermentations of lignocellulosic prehydrolysate characterized by high xylose conversion efficiency. A complex and nutrient-rich laboratory medium, Luria broth (LB), provided the benchmark with respect to fermentation performance standard. Xylose fermentation performance was assessed in terms of the target values for operational process parameters established by the US National Renewable Energy Laboratory (NREL)-final ethanol concentration (25 g/L), xylose-to-ethanol conversion efficiency (90%), and volumetric productivity (0.52 g/L.h). Biomass prehydrolysates that are rich in xylose also contain acetic acid, and in anticipation of a need to reduce acetic acid toxicity, the fermentors were operated with a pH control set-point of 7.0 Growth and fermentation in the minimal defined salts (DS) medium was only about 15% compared to the reference medium. Amendment of the minimal medium containing 6 wt% xylose with both vitamins and amino acids resulted in improved growth, but the volume productivity (0.59 g/L.h) was still only about 54% of that with LB (1.1g/L.h). Formulations directed at cost reduction through the use of less expensive commercial complex nutritional supplements were within 90% of the NREL process target with respect to yield and provided a productivity at about 80% of the LB medium, but were not economical. Corn steep liquor (CSL) at about 7-8 g/L was shown to be a complete source of nutritional requirements and supported a fermentation performance approaching that of LB. At a cost of CSL of $50/t(dry wt), the economic impact of using this amount CSL as the sole nutritional supplement in a cellulosic ethanol plant was estimated to be about 4 cents/gal of ethanol.