From flavors and pharmaceuticals to advanced biofuels: Production of isoprenoids in Saccharomyces cerevisiae

Isoprenoids denote the largest group of chemicals in the plant kingdom and are employed for a wide range of applications in the food and pharmaceutical industry. In recent years, isoprenoids have additionally been recognized as suitable replacements for petroleum-derived fuels and could thus promote the transition towards a more sustainable society. To realize the biofuel potential of isoprenoids, a very efficient production system is required. While complex chemical structures as well as the low abundance in nature demonstrate the shortcomings of chemical synthesis and plant extraction, isoprenoids can be produced by genetically engineered microorganisms from renewable carbon sources. In this article, we summarize the development of isoprenoid applications from flavors and pharmaceuticals to advanced biofuels and review the strategies to design microbial cell factories, focusing on Saccharomyces cerevisiae for the production of these compounds. While the high complexity of biosynthetic pathways and the toxicity of certain isoprenoids still denote challenges that need to be addressed, metabolic engineering has enabled large-scale production of several terpenoids and thus, the utilization of these compounds is likely to expand in the future.     

Metabolic engineering of microorganisms for biofuels production: from bugs to synthetic biology to fuels

The ability to generate microorganisms that can produce biofuels similar to petroleum-based transportation fuels would allow the use of existing engines and infrastructure and would save an enormous amount of capital required for replacing the current infrastructure to accommodate biofuels that have properties significantly different from petroleum-based fuels. Several groups have demonstrated the feasibility of manipulating microbes to produce molecules similar to petroleum-derived products, albeit at relatively low productivity (e.g. maximum butanol production is around 20 g/L). For cost-effective production of biofuels, the fuel-producing hosts and pathways must be engineered and optimized. Advances in metabolic engineering and synthetic biology will provide new tools for metabolic engineers to better understand how to rewire the cell in order to create the desired phenotypes for the production of economically viable biofuels.     

Engineering microbial biofuel tolerance and export using efflux pumps

Many compounds being considered as candidates for advanced biofuels are toxic to microorganisms. This introduces an undesirable trade‐off when engineering metabolic pathways for biofuel production because the engineered microbes must balance production against survival. Cellular export systems, such as efflux pumps, provide a direct mechanism for reducing biofuel toxicity. To identify novel biofuel pumps, we used bioinformatics to generate a list of all efflux pumps from sequenced bacterial genomes and prioritized a subset of targets for cloning. The resulting library of 43 pumps was heterologously expressed in Escherichia coli, where we tested it against seven representative biofuels. By using a competitive growth assay, we efficiently distinguished pumps that improved survival. For two of the fuels (n‐butanol and isopentanol), none of the pumps improved tolerance. For all other fuels, we identified pumps that restored growth in the presence of biofuel. We then tested a beneficial pump directly in a production strain and demonstrated that it improved biofuel yields. Our findings introduce new tools for engineering production strains and utilize the increasingly large database of sequenced genomes.     

The path to next generation biofuels: successes and challenges in the era of synthetic biology

Volatility of oil prices along with major concerns about climate change, oil supply security and depleting reserves have sparked renewed interest in the production of fuels from renewable resources. Recent advances in synthetic biology provide new tools for metabolic engineers to direct their strategies and construct optimal biocatalysts for the sustainable production of biofuels. Metabolic engineering and synthetic biology efforts entailing the engineering of native and de novo pathways for conversion of biomass constituents to short-chain alcohols and advanced biofuels are herewith reviewed. In the foreseeable future, formal integration of functional genomics and systems biology with synthetic biology and metabolic engineering will undoubtedly support the discovery, characterization, and engineering of new metabolic routes and more efficient microbial systems for the production of biofuels.     

Biofuel production in Escherichia coli: the role of metabolic engineering and synthetic biology

The microbial production of biofuels is a promising avenue for the development of viable processes for the generation of fuels from sustainable resources. In order to become cost and energy effective, these processes must utilize organisms that can be optimized to efficiently produce candidate fuels from a variety of feedstocks. Escherichia coli has become a promising host organism for the microbial production of biofuels in part due to the ease at which this organism can be manipulated. Advancements in metabolic engineering and synthetic biology have led to the ability to efficiently engineer E. coli as a biocatalyst for the production of a wide variety of potential biofuels from several biomass constituents. This review focuses on recent efforts devoted to engineering E. coli for the production of biofuels, with emphasis on the key aspects of both the utilization of a variety of substrates as well as the synthesis of several promising biofuels. Strategies for the efficient utilization of carbohydrates, carbohydrate mixtures, and noncarbohydrate carbon sources will be discussed along with engineering efforts for the exploitation of both fermentative and nonfermentative pathways for the production of candidate biofuels such as alcohols and higher carbon biofuels derived from fatty acid and isoprenoid pathways. Continued advancements in metabolic engineering and synthetic biology will help improve not only the titers, yields, and productivities of biofuels discussed herein, but also increase the potential range of compounds that can be produced.     

Synthetic biology for biofuels: Building designer microbes from the scratch

The ultimate goal in the production of biofuels is to produce fuels identical or similar to petroleum-derived transportation fuels more efficiently and in commercial quantities. Synthetic biologists have been engineering microbes to synthesize biofuels, such as butanol and fatty acid- or isoprenoid-based fuels, which are nearly identical to gasoline and diesel. One of the most urgent demands along this direction is to attain a solid framework for characterizing and standardizing the biological parts and devices. It seems quite promising because biotechnologies specially based on miniaturizations have been making a big contribution to this work. Therefore, in this review, recent advances and difficulties in the biofuel field are discussed, along with the advances of synthetic biology, which will make it possible to create designer microorganisms that produce economically viable next generation biofuels, aside from bioethanol, from corn or sugar cane, and biodiesel from plant or animal oils.     

An integrated approach: advances in the use of Clostridium for biofuel

Almost 90% of our energy comes from fossil fuels, which are both limited and polluting, hence the need to find alternative sources. Biofuels can provide a sustainable and renewable source of energy for the future. Recent significant advances in genetic engineering and fermentation technology have made microbial bio-based production of chemicals from renewable resources more viable. Clostridium species are considered as promising micro-organisms for the production of a wide range of chemicals for industrial use. However, a number of scientific challenges still need to be overcome to facilitate an economically viable production system. These include the use of cheap non-food-based substrates, a better understanding of the metabolic processes involved, improvement of strains through genetic engineering and innovation in process technology. This paper reviews recent developments in these areas, advancing the use of Clostridium within an industrial context especially for the production of biofuels.     

Metabolic engineering strategies toward production of biofuels

Exacerbation of climate change and air pollution around the world have emphasized the necessity of replacing fossil fuels with clean and sustainable energy. Metabolic engineering has provided strategies to engineer diverse organisms for the production of biofuels from renewable carbon sources. Although some of the processes are commercialized, there has been continued effort to produce advanced biofuels with higher efficiencies. In this article, metabolic engineering strategies recently exploited to enhance biofuel production and facilitate utilization of non-edible low-value carbon sources are reviewed. The strategies include engineering enzymes, exploiting new pathways, and systematically optimizing metabolism and fermentation processes, among others. In addition, metabolic and bioprocess engineering strategies to achieve competitiveness of current biofuel production systems compared with fossil fuels are discussed.     

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.     

Metabolic engineering for advanced biofuels production from Escherichia coli

Global energy and environmental problems have stimulated increasing efforts toward synthesizing liquid biofuels as transportation energy. Compared to the traditional biofuel, ethanol, advanced biofuels should offer advantages such as higher energy density, lower hygroscopicity, lower vapor pressure, and compatibility with existing transportation infrastructure. However, these fuels are not synthesized economically using native organisms. Metabolic engineering offers an alternative approach in which synthetic pathways are engineered into user-friendly hosts for the production of these fuel molecules. These hosts could be readily manipulated to improve the production efficiency. This review summarizes recent progress in the engineering of Escherichia coli to produce advanced biofuels.     

Advancing oleaginous microorganisms to produce lipid via metabolic engineering technology

With the depletion of global petroleum and its increasing price, biodiesel has been becoming one of the most promising biofuels for global fuels market. Researchers exploit oleaginous microorganisms for biodiesel production due to their short life cycle, less labor required, less affection by venue, and easier to scale up. Many oleaginous microorganisms can accumulate lipids, especially triacylglycerols (TAGs), which are the main materials for biodiesel production. This review is covering the related researches on different oleaginous microorganisms, such as yeast, mold, bacteria and microalgae, which might become the potential oil feedstocks for biodiesel production in the future, showing that biodiesel from oleaginous microorganisms has a great prospect in the development of biomass energy. Microbial oils biosynthesis process includes fatty acid synthesis approach and TAG synthesis approach. In addition, the strategies to increase lipids accumulation via metabolic engineering technology, involving the enhancement of fatty acid synthesis approach, the enhancement of TAG synthesis approach, the regulation of related TAG biosynthesis bypass approaches, the blocking of competing pathways and the multi-gene approach, are discussed in detail. It is suggested that DGAT and ME are the most promising targets for gene transformation, and reducing PEPC activity is observed to be beneficial for lipid production.     

Algal biofuels

The world is facing energy crisis and environmental issues due to the depletion of fossil fuels and increasing CO₂ concentration in the atmosphere. Growing microalgae can contribute to practical solutions for these global problems because they can harvest solar energy and capture CO₂ by converting it into biofuel using photosynthesis. Microalgae are robust organisms capable of rapid growth under a variety of conditions including in open ponds or closed photobioreactors. Their reduced biomass compounds can be used as the feedstock for mass production of a variety of biofuels. As another advantage, their ability to accumulate or secrete biofuels can be controlled by changing their growth conditions or metabolic engineering. This review is aimed to highlight different forms of biofuels produced by microalgae and the approaches taken to improve their biofuel productivity. The costs for industrial-scale production of algal biofuels in open ponds or closed photobioreactors are analyzed. Different strategies for photoproduction of hydrogen by the hydrogenase enzyme of green algae are discussed. Algae are also good sources of biodiesel since some species can make large quantities of lipids as their biomass. The lipid contents for some of the best oil-producing strains of algae in optimized growth conditions are reviewed. The potential of microalgae for producing petroleum related chemicals or ready-make fuels such as bioethanol, triterpenic hydrocarbons, isobutyraldehyde, isobutanol, and isoprene from their biomass are also presented.     

Fluorescence resonance energy transfer sensors for quantitative monitoring of pentose and disaccharide accumulation in bacteria

Background  

Engineering microorganisms to improve metabolite flux requires detailed knowledge of the concentrations and flux rates of metabolites and metabolic intermediates in vivo. Fluorescence resonance energy transfer sensors represent a promising technology for measuring metabolite levels and corresponding rate changes in live cells. These sensors have been applied successfully in mammalian and plant cells but potentially could also be used to monitor steady-state levels of metabolites in microorganisms using fluorimetric assays. Sensors for hexose and pentose carbohydrates could help in the development of fermentative microorganisms, for example, for biofuels applications. Arabinose is one of the carbohydrates to be monitored during biofuels production from lignocellulose, while maltose is an important degradation product of starch that is relevant for starch-derived biofuels production.     

Metabolic engineering of microbial pathways for advanced biofuels production

Production of biofuels from renewable resources such as cellulosic biomass provides a source of liquid transportation fuel to replace petroleum-based fuels. This endeavor requires the conversion of cellulosic biomass into simple sugars, and the conversion of simple sugars into biofuels. Recently, microorganisms have been engineered to convert simple sugars into several types of biofuels, such as alcohols, fatty acid alkyl esters, alkanes, and terpenes, with high titers and yields. Here, we review recently engineered biosynthetic pathways from the well-characterized microorganisms Escherichia coli and Saccharomyces cerevisiae for the production of several advanced biofuels.     

Strain engineering for microbial production of value-added chemicals and fuels from glycerol

While the widespread reliance on fossil fuels is driven by their low cost and relative abundance, this fossil-based economy has been deemed unsustainable and, therefore, the adoption of sustainable and environmentally compatible energy sources is on the horizon. Biorefinery is an emerging approach that integrates metabolic engineering, synthetic biology, and systems biology principles for the development of whole-cell catalytic platforms for biomanufacturing. Due to the high degree of reduction and low cost, glycerol, either refined or crude, has been recognized as an ideal feedstock for the production of value-added biologicals, though microbial dissimilation of glycerol sometimes can be difficult particularly under anaerobic conditions. While strain development for glycerol biorefinery is widely reported in the literature, few, if any, commercialized bioprocesses have been developed as a result, such that engineering of glycerol metabolism in microbial hosts remains an untapped opportunity in biomanufacturing. Here we review the recent progress made in engineering microbial hosts for the production of biofuels, diols, organic acids, biopolymers, and specialty chemicals from glycerol. We begin with a broad outline of the major pathways for fermentative and respiratory glycerol dissimilation and key end metabolites, and then focus our analysis on four key genera of bacteria known to naturally dissimilate glycerol, i.e. Klebsiella, Citrobacter, Clostridium, and Lactobacillus, in addition to Escherichia coli, and systematically review the progress made toward engineering these microorganisms for glycerol biorefinery. We also identify the major biotechnological and bioprocessing advantages and disadvantages of each genus, and bottlenecks limiting the production of target metabolites from glycerol in engineered strains. Our analysis culminates in the development of potential strategies to overcome the current technical limitations identified for commonly employed strains, with an outlook on the suitability of different hosts for the production of key metabolites and avenues for their future development into biomanufacturing platforms.     

Quantifying the metabolic capabilities of engineered <Emphasis Type="Italic">Zymomonas mobilis</Emphasis> using linear programming analysis

Background  

The need for discovery of alternative, renewable, environmentally friendly energy sources and the development of cost-efficient, "clean" methods for their conversion into higher fuels becomes imperative. Ethanol, whose significance as fuel has dramatically increased in the last decade, can be produced from hexoses and pentoses through microbial fermentation. Importantly, plant biomass, if appropriately and effectively decomposed, is a potential inexpensive and highly renewable source of the hexose and pentose mixture. Recently, the engineered (to also catabolize pentoses) anaerobic bacterium Zymomonas mobilis has been widely discussed among the most promising microorganisms for the microbial production of ethanol fuel. However, Z. mobilis genome having been fully sequenced in 2005, there is still a small number of published studies of its in vivo physiology and limited use of the metabolic engineering experimental and computational toolboxes to understand its metabolic pathway interconnectivity and regulation towards the optimization of its hexose and pentose fermentation into ethanol.     

Genome-scale metabolic analysis of <Emphasis Type="Italic">Clostridium thermocellum</Emphasis> for bioethanol production

Background  

Microorganisms possess diverse metabolic capabilities that can potentially be leveraged for efficient production of biofuels. Clostridium thermocellum (ATCC 27405) is a thermophilic anaerobe that is both cellulolytic and ethanologenic, meaning that it can directly use the plant sugar, cellulose, and biochemically convert it to ethanol. A major challenge in using microorganisms for chemical production is the need to modify the organism to increase production efficiency. The process of properly engineering an organism is typically arduous.     

Sugar transporters in efficient utilization of mixed sugar substrates: current knowledge and outlook

There is increasing interest in production of transportation fuels and commodity chemicals from lignocellulosic biomass, most desirably through biological fermentation. Considerable effort has been expended to develop efficient biocatalysts that convert sugars derived from lignocellulose directly to value-added products. Glucose, the building block of cellulose, is the most suitable fermentation substrate for industrial microorganisms such as Escherichia coli, Corynebacterium glutamicum, and Saccharomyces cerevisiae. Other sugars including xylose, arabinose, mannose, and galactose that comprise hemicellulose are generally less efficient substrates in terms of productivity and yield. Although metabolic engineering including introduction of functional pentose-metabolizing pathways into pentose-incompetent microorganisms has provided steady progress in pentose utilization, further improvements in sugar mixture utilization by microorganisms is necessary. Among a variety of issues on utilization of sugar mixtures by the microorganisms, recent studies have started to reveal the importance of sugar transporters in microbial fermentation performance. In this article, we review current knowledge on diversity and functions of sugar transporters, especially those associated with pentose uptake in microorganisms. Subsequently, we review and discuss recent studies on engineering of sugar transport as a driving force for efficient bioconversion of sugar mixtures derived from lignocellulose.     

Molecular genetic improvements of cyanobacteria to enhance the industrial potential of the microbe: A review

The rapid increase in worldwide population coupled with the increasing demand for fossil fuels has led to an increased urgency to develop sustainable sources of energy and chemicals from renewable resources. Using microorganisms to produce high‐value chemicals and next‐generation biofuels is one sustainable option and is the focus of much current research. Cyanobacteria are ideal platform organisms for chemical and biofuel production because they can be genetically engineered to produce a broad range of products directly from CO₂, H₂O, and sunlight, and require minimal nutrient inputs. The purpose of this review is to provide an overview on advances that have been or could be made to improve strains of cyanobacteria for industrial purposes. First, the benefits of using cyanobacteria as a platform for chemical and biofuel production are discussed. Next, an overview of cyanobacterial strain improvements by genetic engineering is provided. Finally, mutagenesis techniques to improve the industrial potential of cyanobacteria are described. Along with providing an overview on various areas of research that are currently being investigated to improve the industrial potential of cyanobacteria, this review aims to elucidate potential targets for future research involving cyanobacteria as an industrial microorganism. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:1357–1371, 2016