Branched-Chain Higher Alcohols - Biotechnology in China: Biofuels and Bioenergy - page 114

Fig. 4 Synthetic pathway and key enzymes for production of isobutanol, 3-methyl-1-butanol
(isopentanol), and 2-methyl-1-butanol. When 2-keto acid decarboxylase (kivd) and alcohol
dehydrogenase (ADH) are introduced, the 2-keto acid intermediates of L -valine, L -leucine,
and L -isoleucine biosynthesis are directed for the production of isobutanol, 3-methyl-1-butanol,
and 2-methyl-1-butanol
catabolic and anabolic pathways involved in de novo synthesis of branched-chain
amino acids through their biosynthetic pathways from glucose.
As an alternative to the petrochemical production, isobutanol can also be pro-
duced from renewable bio-based feedstocks by metabolically engineered microor-
ganisms. The team led by Liao [ 28 ] made great progress in constructing the synthetic
pathway for isobutanol production in E. coli as well as C. glutamicum. The
engineered strain produced 22 g/L isobutanol after 112 h [ 28 ]. This team also
engineered a C. glutamicum strain which can produce 4.9 g/L isobutanol [ 51 ]. To use
lignocellulose as the feedstock for isobutanol production, Liao and his colleagues
[ 52 ] successfully engineered Clostridium cellulolyticum for direct conversion of
cellulose to isobutanol through the process of consolidated bioprocessing. The US
Department of Energy's BioEnergy Science Center (BESC) at Oak Ridge National
Laboratory also developed the technology for isobutanol production directly from
cellulose.
Some researchers have used Cyanobacteria for isobutanol production, since
these organisms can use carbon dioxide as the feedstock. Heterologous expression
of valine synthesis enzymes has been used to convert pyruvate to ketoisovalerate
within Synechococcus elongatus PCC 7942. Intracellular ketoisovalerate is then
converted to isobutyraldehyde at a rate of 6.23 mg/L/h by coexpression of the keto
acid decarboxylase gene. Isobutyraldehyde is further converted to isobutanol by
coexpression of alcohol dehydrogenase from E. coli. Isobutanol was recovered
from the liquid phase in about 50% yield [ 53 ].
Two research groups in China have also been engaged in engineering micro-
organisms for isobutanol biosynthesis. The team led by Jianping Wen of Tianjin
University [ 42 , 54 ] engineered B. subtilis 168 for isobutanol production through