Biomedical Engineering Reference
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the chromosomes of the strains, and the size of the DSM1731 megaplasmid is 4 bp
smaller than that of ATCC824 [ 3 ]. The proteomic analysis of C. acetobutylicum
DSM1731 and its mutant Rh8 showed that 102 expressed proteins are significantly
down/upregulated, involved in protein folding, solvent formation, amino acid
metabolism, protein synthesis, nucleotide metabolism, transport, and others. More
than 70% of the 102 differentially expressed proteins were either upregulated or
downregulated in both the acidogenic and solventogenic phases in Rh8, whereas
they were upregulated or downregulated only in the solventogenic phase in
DSM1731. Genes encoding for 52 proteins of the 102 proteins are involved with
response to butanol stress or the transition from acidogenesis phase to solvento-
genesis. These results showed that the mutant strain Rh8 has developed a mech-
anism to prepare itself for butanol challenge before butanol is produced, which
gives rise to the increased production of butanol. Mao et al. also performed
comparative membrane proteome analysis between the wild-type and the butanol-
tolerant mutant strain Rh8 [ 34 ]. A total of 73 significantly differently expressed
proteins were identified, of which 92% were involved in ion and metabolite
transport, components of the cellular membrane or wall machinery, involved in
protein processing and stability, associated with surface coat and flagellar
formation, and involved in respiratory chain and energy metabolism. The
increased protein expression levels in membrane structure and surface stabiliza-
tion, sporulation, and lipid metabolism, and also the increased expression of
ATPase and reduced mobilization and peptide transport, might save energy to cope
with butanol challenge, showing that the butanol-tolerant mutant Rh8 might have
evolved a more stabilized membrane structure and a cost-efficient energy metab-
olism strategy to cope with the butanol challenge. Further characterizations of the
differentially expressed membrane proteins identified in this study can help to
understand and elucidate the molecular mechanisms underlying the complex
phenotype of butanol tolerance and butanol production. The study, for the
first time, reports the systematic analysis of the membrane proteome of
C. acetybutylicum. The comparative cytoplasmic proteomics and comparative
membrane proteomics gave new sights into the effect of butanol on cellular
physiology and the molecular basis of butanol tolerance in C. acetobutylicum.
For ABE fermentation, the process of solvent production often collapses when
cells are grown in pH-uncontrolled glucose medium (''acid crash'' phenomenon).
Acetic acid and butyric acid was previously believed to be the reason for this
phenomenon. Wang et al. found that formic acid plays an important role in the
''acid crash'' of ABE fermentation [ 50 ]. In pH-uncontrolled glucose medium or
glucose-rich medium, C. acetobutylicum failed to produce solvents and could
accumulate 0.5-1.24 mM formic acid in cells. By expressing formate dehy-
drogenase from Candida boidinii in C. acetobutylicum, intracellular formic acid
concentration decreased to below detection level, and the engineered strain could
restore solvent production. Hence, it was suggested that formic acid triggers the
''acid crash'' of ABE fermentation in C. acetobutylicum, rather than acetic acid
and butyric acid.
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