To obtain an efficient ethanologenic Escherichia coli strain, we reduced the functional space of the central metabolic network, with eight gene knockout mutations, from over 15,000 pathway possibilities to 6 pathway options that support cell function. The remaining pathways, identified by elementary mode analysis, consist of four pathways with non-growth-associated conversion of pentoses and hexoses into ethanol at theoretical yields and two pathways with tight coupling of anaerobic cell growth with ethanol formation at high yields. Elimination of three additional genes resulted in a strain that selectively grows only on pentoses, even in the presence of glucose, with a high ethanol yield. We showed that the ethanol yields of strains with minimized metabolic functionality closely matched the theoretical predictions. Remarkably, catabolite repression was completely absent during anaerobic growth, resulting in the simultaneous utilization of pentoses and hexoses for ethanol production.
Elementary Mode Analysis is a useful Metabolic Pathway Analysis tool to identify the structure of a metabolic network that links the cellular phenotype to the corresponding genotype. The analysis can decompose the intricate metabolic network comprised of highly interconnected reactions into uniquely organized pathways. These pathways consisting of a minimal set of enzymes that can support steady state operation of cellular metabolism represent independent cellular physiological states. Such pathway definition provides a rigorous basis to systematically characterize cellular phenotypes, metabolic network regulation, robustness, and fragility that facilitate understanding of cell physiology and implementation of metabolic engineering strategies. This mini-review aims to overview the development and application of elementary mode analysis as a metabolic pathway analysis tool in studying cell physiology and as a basis of metabolic engineering.
Zinc dipyrrin complexes with two
identical dipyrrin ligands absorb
strongly at 450–550 nm and exhibit high fluorescence quantum
yields in nonpolar solvents (e.g., 0.16–0.66 in cyclohexane)
and weak to nonexistent emission in polar solvents (i.e., <10–3, in acetonitrile). The low quantum efficiencies in
polar solvents are attributed to the formation of a nonemissive symmetry-breaking
charge transfer (SBCT) state, which is not formed in nonpolar solvents.
Analysis using ultrafast spectroscopy shows that in polar solvents
the singlet excited state relaxes to the SBCT state in 1.0–5.5
ps and then decays via recombination to the triplet or ground states
in 0.9–3.3 ns. In the weakly polar solvent toluene, the equilibrium
between a localized excited state and the charge transfer state is
established in 11–22 ps.
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