Design and development of synthetic microbial platform cells for bioenergy

Abstract

The finite reservation of fossil fuels accelerates the necessity of development of renewable energy sources. Recent advances in synthetic biology encompassing systems biology and metabolic engineering enable us to engineer and/or create tailor made microorganisms to produce alternative biofuels for the future bio-era. For the efficient transformation of biomass to bioenergy, microbial cells need to be designed and engineered to maximize the performance of cellular metabolisms for the production of biofuels during energy flow. Toward this end, two different conceptual approaches have been applied for the development of platform cell factories: forward minimization and reverse engineering. From the context of naturally minimized genomes,non-essential energy-consuming pathways and/or related gene clusters could be progressively deleted to optimize cellular energy status for bioenergy production. Alternatively, incorporation of non-indigenous parts and/or modules including biomass-degrading enzymes, carbon uptake transporters, photosynthesis, CO2 fixation, and etc. into chassis microorganisms allows the platform cells to gain novel metabolic functions for bioenergy. This review focuses on the current progress in synthetic biology-aided pathway engineering in microbial cells and discusses its impact on the production of sustainable bioenergy.

Keywords: bioenergy; genome reduction; metabolic engineering; microbial platform cells; synthetic biology.

FIGURE 1

Overview of the microbial pathways on the KEGG pathways using the iPath tool (Letunic et al., 2008. To date, conserved pathways known as essential are shown in red. Hypothetical proteins found as essential are excluded.

FIGURE 2

Fermentative and non-fermentative pathways for the production of biofuels in E. coli. Dashed lines represent multiple reaction steps. Red circles represent metabolic intermediates.ACP, acyl-carrier protein; DHAP, dihydroxyacetone phosphate; DMAPP, dimethylallyl pyrophosphate; F-1,6-BP, fructose-1,6-bisphosphate; G6P, glucose-6-phosphate; Gly3P, glyceraldehyde 3-phosphate; IPP, isopentenyl pyrophosphate; 1,3-PG, 1,3-diphosphoglycerate; PEP, phosphoenolpyruvate.

FIGURE 3

Illustration of photosynthesis and carbon fixation (A) and key light-driven processes in microorganisms (B). Black solid and dashed arrows represent reactions catalyzed by each enzyme and electron flows, respectively. (Top panel) Overview of oxygenic photosynthesis from cyanobacteria. (Middle panel) Overview of anoxygenic photosynthesis from α-proteobacteria. (Bottom panel) Light-driven proton pump. The major components of photosynthesis and carbon fixation including elements are depicted: S, chloroplast stroma; T, thylakoid lumen; P, periplasm; C, cytoplasm; Cyt c, cytochrome c; Cyt bc₁, cytochrome bc₁; Cyt b₆f, cytochrome b₆f; Fd, ferredoxin; FNR, ferredoxin-NADP⁺ reductase; LH, light-harvesting complex; PC, plastocyanin; PS, photosystem; PQ, plastoquinone; Q, ubiquinone; QH₂, ubiquinol.