the pathway to tricarboxylic acid (TCA) cycle was activated. Under
micro-aerobic condition and sufficient aerobic condition, 5.9% and
31.0% carbon flux flowed from glycerol to TCA cycle while it was
zero in anaerobic culture. The carbon flux from glycerol flowed to
PDO was the highest (59.3%) in anaerobic condition because a big
portion of carbon source flowed to biomass and TCA cycle, which
decreased 10.3% and 92.9%, respectively, in micro-aerobic and
sufficient aerobic condition.
The flux distribution at three key metabolic nodes (Glycerol,
Pyruvate (PYR) and Acetyl Coenzyme (AcCoA)) was also
analyzed in Fig. 5. At glycerol node, compared with the
anaerobic condition, the carbon flux flowed toward phosphoenolpyruvate
(PEP) and biomass increased 18.4%, 10.1% and
77.9%, 349.9%, respectively, under micro-aerobic and sufficient
aerobic condition. However, the carbon flux flowed to PDO
decreased 10.3% and 92.9%, respectively. At PYR node, the carbon flux distributions under anaerobic and micro-aerobic
condition were almost the same. But for the sufficient aerobic
condition, the carbon flux from PYR flowed toward lactate
decreased 87.3% and to AcCoA increased only 7.9% compared
with that under anaerobic condition. At AcCoA node, 26.8% and
82.4% carbon flux flowed to TCA cycle, respectively, under
micro-aerobic condition and sufficient aerobic condition, while
it was zero under anaerobic condition (no TCA pathway). The
carbon flux flowed to acetate and ethanol decreased 23.2%,
49.1% and 82.6%, 81%, respectively, under micro-aerobic and
sufficient aerobic conditions. The results suggested that
although the carbon flux flowed to other byproducts decreased
in the presence of oxygen, it flowed to TCA cycle was also greatly
enhanced, so the carbon flux flowed to PDO did not increase.
Thus, the metabolic flux control of TCA cycle is significant to
increase the yield of PDO under aerobic condition.