Branched-Chain Higher Alcohols - Biotechnology in China: Biofuels and Bioenergy - page 109

Research Program of China (973 Program) and the National High Technology
Research and Development Program of China (863 Program) are among them.
In the 863 Program of the 12th Five-Year Plan from 2011 to 2015, 11 bio-related
projects have been funded, and the higher alcohol project is one of them, with
a focus on microbial production of isobutanol from nongrain feedstocks and
systematic technology integration for improved yield and productivity. As the
national innovation system, the Chinese Academy of Sciences is also actively
involved in the projects for microbial production of BCHAs [ 32 , 33 ].
4 Update on Progress in Microbial Production of BCHAs
In the quest to find substitutes for petroleum-based fuels, several low-energy
molecules such as ethanol have been employed because of their easy production.
However, biofuels with energy density similar to that of current fuels would be
preferred, and BCHAs are among them. Other properties of these alcohols also
display more desirable features. For example, a lower miscibility with water and
lower vapor pressure are benefits for transportation fuels. For microbial production
of BCHAs, a unique approach is to employ synthetic biology and metabolic
engineering to engineer and assemble the natural or chimeric synthetic pathways
in cell factories for the production of the target compounds since a natural
microorganism only accumulates a very tiny amount of BCHAs [ 34 - 36 ]. Since
James Liao's group at the University of California, Los Angles [ 28 , 37 ] and other
groups worldwide [ 38 , 39 ] developed new strategies for producing BCHAs by
genetically modified microorganisms, microbial production of BCHAs has also
made progress in China.
4.1 Product Toxicity
Unlike ethanol, BCHAs are more toxic to microbes, although different microor-
ganisms respond differently to the toxicity owing to the differences in their tol-
erance and the length of the carbon chain, and this has been investigated with
isobutanol [ 40 , 41 ]. However, the underlying mechanisms are less well known.
The inhibitory effect of 3-methyl-1-butanol on Gram-negative Escherichia coli
and Gram-positive Corynebacterium glutamicum has been studied in the authors'
laboratory. E. coli was more sensitive than C. glutamicum to 3-methyl-1-butanol
(Fig. 1 ). Another study reported that Bacillus subtilis is more tolerant to higher
alcohols than E. coli [ 42 ], indicating B. subtilis might be a more suitable host for
microbial production of higher alcohols.
For the host, the toxicity of BCHAs has been proved to be one of the barriers
limiting the final product titer in the fermentation broth. For example, no more
than 2% (w/v) BCHAs can be achieved presently, making the downstream product