Coexpression of pyruvate decarboxylase and alcohol dehydrogenase genes in Lactobacillus brevis

Utilization of agricultural wastes for co-production of xylitol, ethanol, and phenylacetylcarbinol: A review

Corn, rice, wheat, and sugar are major sources of food calories consumption thus the massive agricultural waste (AW) is generated through agricultural and agro-industrial processing of these raw materials. Biological conversion is one of the most sustainable AW management technologies. The abundant supply and special structural composition of cellulose, hemicellulose, and lignin could provide great potential for waste biological conversion. Conversion of hemicellulose to xylitol, cellulose to ethanol, and utilization of remnant whole cells biomass to synthesize phenylacetylcarbinol (PAC) are strategies that are both eco-friendly and economically feasible. This co-production strategy includes essential steps: saccharification, detoxification, cultivation, and biotransformation. In this review, the implemented technologies on each unit step are described, the effectiveness, economic feasibility, technical procedures, and environmental impact are summarized, compared, and evaluated from an industrial scale viewpoint.

Optimization of n-butanol synthesis in Lactobacillus brevis via the functional expression of thl, hbd, crt and ter

Abstract

N-butanol is an important chemical and can be naturally synthesized by Clostridium species; however, the poor n-butanol tolerance of Clostridium impedes the further improvement in titer. In this study, Lactobacillus brevis, which possesses a higher butanol tolerance, was selected as host for heterologous butanol production. The Clostridium acetobutylicum genes thl, hbd, and crt which encode thiolase, β-hydroxybutyryl-CoA dehydrogenase, and crotonase, and the Treponema denticola gene ter, which encodes trans-enoyl-CoA reductase were cloned into a single plasmid to express the butanol synthesis pathway in L. brevis. A titer of 40 mg/L n-butanol was initially achieved with plasmid pLY15-opt, in which all pathway genes are codon-optimized. A titer of 450 mg/L of n-butanol was then synthesized when ter was further overexpressed in this pathway. The role of metabolic flux was reinforced with pLY15, in which only the ter gene was codon-optimized, which greatly increased the n-butanol titer to 817 mg/L. Our strategy significantly improved n-butanol synthesis in L. brevis and the final titer is the highest achieved amongst butanol-tolerant lactic acid bacteria.

Graphic abstract

Comparison of Different Lactobacilli Regarding Substrate Utilization and Their Tolerance Towards Lignocellulose Degradation Products

Fermentative lactic acid production is currently impeded by low pH tolerance of the production organisms, the successive substrate consumption of the strains and/or the requirement to apply purified substrate streams. We identified Lactobacillus brevis IGB 1.29 in compost, which is capable of producing lactic acid at low pH values from lignocellulose hydrolysates, simultaneously consuming glucose and xylose. In this study, we compared Lactobacillus brevis IGB 1.29 with the reference strains Lactobacillus brevis ATCC 367, Lactobacillus plantarum NCIMB 8826 and Lactococcus lactis JCM 7638 with regard to the consumption of C5- and C6-sugars. Simultaneous conversion of C5- and C6-monosaccharides was confirmed for L. brevis IGB 1.29 with consumption rates of 1.6 g/(L h) for glucose and 1.0 g/(L h) for xylose. Consumption rates were lower for L. brevis ATCC 367 with 0.6 g/(L h) for glucose and 0.2 g/(L h) for xylose. Further trials were carried out to determine the sensitivity towards common toxic degradation products in lignocellulose hydrolysates: acetate, hydroxymethylfurfural, furfural, formate, levulinic acid and phenolic compounds from hemicellulose fraction. L. lactis was the least tolerant strain towards the inhibitors, whereas L. brevis IGB 1.29 showed the highest tolerance. L. brevis IGB 1.29 exhibited only 10% growth reduction at concentrations of 26.0 g/L acetate, 1.2 g/L furfural, 5.0 g/L formate, 6.6 g/L hydroxymethylfurfural, 9.2 g/L levulinic acid or 2.2 g/L phenolic compounds. This study describes a new strain L. brevis IGB 1.29, that enables efficient lactic acid production with a lignocellulose-derived C5- and C6-sugar fraction.

Lactobacillus casei as a biocatalyst for biofuel production

Microbial fermentation of sugars from plant biomass to alcohols represents an alternative to petroleum-based fuels. The optimal biocatalyst for such fermentations needs to overcome hurdles such as high concentrations of alcohols and toxic compounds. Lactic acid bacteria, especially lactobacilli, have high innate alcohol tolerance and are remarkably adaptive to harsh environments. This study assessed the potential of five Lactobacillus casei strains as biocatalysts for alcohol production. L. casei 12A was selected based upon its innate alcohol tolerance, high transformation efficiency and ability to utilize plant-derived carbohydrates. A 12A derivative engineered to produce ethanol (L. casei E1) was compared to two other bacterial biocatalysts. Maximal growth rate, maximal optical density and ethanol production were determined under conditions similar to those present during alcohol production from lignocellulosic feedstocks. L. casei E1 exhibited higher innate alcohol tolerance, better growth in the presence of corn stover hydrolysate stressors, and resulted in higher ethanol yields.

Structure and functional characterization of pyruvate decarboxylase from Gluconacetobacter diazotrophicus

BACKGROUND

Bacterial pyruvate decarboxylases (PDC) are rare. Their role in ethanol production and in bacterially mediated ethanologenic processes has, however, ensured a continued and growing interest. PDCs from Zymomonas mobilis (ZmPDC), Zymobacter palmae (ZpPDC) and Sarcina ventriculi (SvPDC) have been characterized and ZmPDC has been produced successfully in a range of heterologous hosts. PDCs from the Acetobacteraceae and their role in metabolism have not been characterized to the same extent. Examples include Gluconobacter oxydans (GoPDC), G. diazotrophicus (GdPDC) and Acetobacter pasteutrianus (ApPDC). All of these organisms are of commercial importance.

RESULTS

This study reports the kinetic characterization and the crystal structure of a PDC from Gluconacetobacter diazotrophicus (GdPDC). Enzyme kinetic analysis indicates a high affinity for pyruvate (K M 0.06 mM at pH 5), high catalytic efficiencies (1.3 • 10(6) M(-1) • s(-1) at pH 5), pHopt of 5.5 and Topt at 45°C. The enzyme is not thermostable (T½ of 18 minutes at 60°C) and the calculated number of bonds between monomers and dimers do not give clear indications for the relatively lower thermostability compared to other PDCs. The structure is highly similar to those described for Z. mobilis (ZmPDC) and A. pasteurianus PDC (ApPDC) with a rmsd value of 0.57 Å for Cα when comparing GdPDC to that of ApPDC. Indole-3-pyruvate does not serve as a substrate for the enzyme. Structural differences occur in two loci, involving the regions Thr341 to Thr352 and Asn499 to Asp503.

CONCLUSIONS

This is the first study of the PDC from G. diazotrophicus (PAL5) and lays the groundwork for future research into its role in this endosymbiont. The crystal structure of GdPDC indicates the enzyme to be evolutionarily closely related to homologues from Z. mobilis and A. pasteurianus and suggests strong selective pressure to keep the enzyme characteristics in a narrow range. The pH optimum together with reduced thermostability likely reflect the host organisms niche and conditions under which these properties have been naturally selected for. The lack of activity on indole-3-pyruvate excludes this decarboxylase as the enzyme responsible for indole acetic acid production in G. diazotrophicus.

Engineering biofuel tolerance in non-native producing microorganisms

Large-scale production of renewable biofuels through microbiological processes has drawn significant attention in recent years, mostly due to the increasing concerns on the petroleum fuel shortages and the environmental consequences of the over-utilization of petroleum-based fuels. In addition to native biofuel-producing microbes that have been employed for biofuel production for decades, recent advances in metabolic engineering and synthetic biology have made it possible to produce biofuels in several non-native biofuel-producing microorganisms. Compared to native producers, these non-native systems carry the advantages of fast growth, simple nutrient requirements, readiness for genetic modifications, and even the capability to assimilate CO2 and solar energy, making them competitive alternative systems to further decrease the biofuel production cost. However, the tolerance of these non-native microorganisms to toxic biofuels is naturally low, which has restricted the potentials of their application for high-efficiency biofuel production. To address the issues, researches have been recently conducted to explore the biofuel tolerance mechanisms and to construct robust high-tolerance strains for non-native biofuel-producing microorganisms. In this review, we critically summarize the recent progress in this area, focusing on three popular non-native biofuel-producing systems, i.e. Escherichia coli, Lactobacillus and photosynthetic cyanobacteria.

Bioethanol production by heterologous expression of Pdc and AdhII in Streptomyces lividans

Two genes from Zymomonas mobilis that are responsible for ethanol production, pyruvate decarboxylase (pdc) and alcohol dehydrogenase II (adhII), were heterologously expressed in the Gram-positive bacterium Streptomyces lividans TK24. An examination of carbon distribution revealed that a significant portion of carbon metabolism was switched from biomass and organic acid biosynthesis to ethanol production upon the expression of pdc and adhII. The recombinant S. lividans TK24 produced ethanol from glucose with a yield of 23.7% based on the carbohydrate consumed. The recombinant was able to produce ethanol from xylose, L-arabinose, mannose, L-rhamnose, galactose, ribose, and cellobiose with yields of 16.0, 25.6, 21.5, 33.6, 30.6, 14.6, and 33.3%, respectively. Polymeric substances such as starch and xylan were directly converted to ethanol by the recombinant with ethanol yields of 18.9 and 8.8%, respectively. The recombinant S. lividans TK24/Tpet developed in this study is potentially a useful microbial resource for ethanol production from various sources of biomasses, especially microalgae.

Improvement of ethanol yield from glycerol via conversion of pyruvate to ethanol in metabolically engineered Saccharomyces cerevisiae

The conversion of low-priced glycerol to higher value products has been proposed as a way to improve the economic viability of the biofuels industry. In a previous study, the conversion of glycerol to ethanol in a metabolically engineered strain of Saccharomyces cerevisiae was accomplished by minimizing the synthesis of glycerol, the main by-product in ethanol fermentation processing. To further improve ethanol production, overexpression of the native genes involved in conversion of pyruvate to ethanol in S. cerevisiae was successfully accomplished. The overexpression of an alcohol dehydrogenase (adh1) and a pyruvate decarboxylase (pdc1) caused an increase in growth rate and glycerol consumption under fermentative conditions, which led to a slight increase of the final ethanol yield. The overall expression of the adh1 and pdc1 genes in the modified strains, combined with the lack of the fps1 and gpd2 genes, resulted in a 1.4-fold increase (about 5.4 g/L ethanol produced) in fps1Δgpd2Δ (pGcyaDak, pGupCas) (about 4.0 g/L ethanol produced). In summary, it is possible to improve the ethanol yield by overexpression of the genes involved in the conversion of pyruvate to ethanol in engineered S. cerevisiae using glycerol as substrate.

Functional expression of the thiolase gene thl from Clostridium beijerinckii P260 in Lactococcus lactis and Lactobacillus buchneri

The first step of the butanol pathway involves an acetyl-CoA acetyltransferase (ACoAAT), which controls the key branching point from acetyl-CoA to butanol. ACoAAT, also known as thiolase (EC 2.3.1.9), is encoded by the thl gene and catalyzes ligation of two acetyl-CoA into acetoacetyl-CoA. Bioinformatics analyses suggest there are no thl in the genomes of lactic acid bacteria (LAB), in this study we aimed to introduce the thl gene into selected LAB strains and analyze the fermentation products. The thl gene from Clostridium beijerinckii P260 was amplified by genomic PCR using gene-specific primers designed from the published genome sequences of C. beijerinckii NCIMB 8025. The 1.2 kb thl gene was cloned into the pETBlue vector and overexpressed in Escherichia coli Tuner (DE3) pLacI cells. Functional enzyme activity was detected spectrophotometrically by measuring the decrease in absorbance at 303 nm, which reflects the change in acetoacetyl-CoA concentrations. The thl gene was subsequently introduced into Lactococcus lactis and Lactobacillus buchneri strains, and GC analysis indicated about 28 mg/L and 66 mg/L of butanol was produced in the recombinant strains, respectively. This study reports the first step toward developing a butanolgenic LAB through the introduction of the butanol pathway into butanol-tolerant strains of LAB.

Reconstructing the clostridial n-butanol metabolic pathway in Lactobacillus brevis

A Lactobacillus brevis strain with the ability to synthesize butanol from glucose was constructed by metabolic engineering. The genes crt, bcd, etfB, etfA, and hbd, composing the bcs-operon, and the thl gene encode the enzymes of the lower part of the clostridial butanol pathway (crotonase, butyryl-CoA-dehydrogenase, two subunits of the electron transfer flavoprotein, 3-hydroxybutyryl-CoA dehydrogenase, and thiolase) of Clostridium acetobutylicum. They were cloned into the Gram-positive/Gram-negative shuttle plasmid vector pHYc. The two resulting plasmids pHYc-thl-bcs and pHYc-bcs (respectively, with and without the clostridial thl gene) were transferred to Escherichia coli and L. brevis. The recombinant L. brevis strains were able to synthesize up to 300 mg l(-1) or 4.1 mM of butanol on a glucose-containing medium. A L. brevis strain carrying the clostridial bcs-operon has the ability to synthesize butanol with participation of its own thiolase, aldehyde dehydrogenase, and alcohol dehydrogenase. The particular role of the enzymes involved in butanol production and the suitability of L. brevis as an n-butanol producer are discussed.