Engineered Microbial Consortium for the Efficient Conversion of Biomass to Biofuels Page: 5
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by the presence of two key enzymes of the Z. mobilis metabolism: a type 2 NADH oxidoreductase
and an NADH oxidase [13]. The type 2 NADH oxidoreductase is relevant because it does not
pump protons during electron transport under aerobic respiration, unlike the more common type 1
NADH oxidoreductase [14]. Therefore, type 2 NADH oxidoreductase does not contribute to the
proton gradient of the cellular membrane, which is the driving force in generating ATP. Other
membrane proteins such as cytochrome bcl complex, electron transfer flavoprotein and
ubiquinone proteins are present to generate the proton gradient and thereby drive ATP generation
under aerobic conditions. Z. mobilis can grow under aerobic conditions at a slower rate due to
decreased supply of ATP. NADH oxidase catalyzes the oxidation of NADH (NADH + 0.5 O2-
NAD) under aerobic conditions. Because of this enzyme, the pool of NADH, which is used for
ethanol production, is decreased, resulting in a decrease in capacity to produce ethanol under
aerobic conditions [14]. In the study of the transcriptomic and metabolic profiling of Z. mobilis
during aerobic and anaerobic fermentations, Yang et al. [15] made the following observations:
1. In the absence of oxygen, ZM4 consumed glucose more rapidly, had a higher growth
rate and ethanol was the major end-product.
2. Greater amounts of other end-products such as acetate, lactate and acetoin were
detected under aerobic conditions and at 26 hours there was only 1.7% (v v-1) of the
amount of ethanol present anaerobically.
3. In the early exponential growth phase, significant differences in gene expression were
not observed between aerobic and anaerobic conditions via microarray analysis.
4. HPLC and GC analyses revealed minor differences in extracellular metabolite profiles
at the corresponding early exponential phase time point and
5. Transcripts for Entner-Doudoroff pathway genes (glk, zwf pgl, pgk and eno) and gene
pdc, encoding a key enzyme leading to ethanol production, were at least 30-fold more
abundant under anaerobic conditions in the stationary phase based on quantitative-PCR
results.5
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Anieto, Ugochukwu Obiakornobi. Engineered Microbial Consortium for the Efficient Conversion of Biomass to Biofuels, dissertation, August 2014; Denton, Texas. (https://digital.library.unt.edu/ark:/67531/metadc699973/m1/16/?rotate=270: accessed July 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; .