Biofuels Production by Cell-Free Synthetic Enzymatic Technology

Biomass is the only renewable resource that can provide a sufficient fraction of both future transportation fuels and renewable materials at the same time. Synthetic biology is an emerging interdisciplinary area that combines science and engineering in order to design and build novel biological functions and systems. Different from in vivo synthetic biology, cell-free in vitro synthetic biology is a largely unexplored strategy. Cell-free synthetic enzymatic pathway engineering (SEPE) is to in vitro assemble a number of enzymes and coenzymes to implement complicated biotransformations that can mimic natural fermentation or achieve unnatural processes. Recently, a novel synthetic enzymatic pathway composed of 13 enzymes and a cofactor has been demonstrated to produce 12 molecules of hydrogen per molecule of glucose unit of starch and water (PLoS One, 2007, 2:e456). This new sugar-to-hydrogen technology promises to solve several obstacles to the hydrogen economy – cheap hydrogen production, high hydrogen storage density (14.8 H2 mass%), and costly hydrogen infrastructure, and to eliminate safety concerns about mass utilization of hydrogen. Furthermore, the advantages and limitations of producing liquid biofuels -- ethanol and butanol -- from sugars by SEPE are discussed. The research and development of SESE require more efforts, especially in low-cost recombinant thermophilic enzyme building block manufacturing, efficient cofactor recycling, enzyme and cofactor stabilization, and so on.