2.3. A systemic approach to identify key reactions for ethanol
production
In this research, a systemic approach was applied for S.
cerevisiae to find candidate reactions for up and down-regulation for
the overproduction of ethanol. As S. cerevisiae is the main
industrial ethanologenic microorganism and the production rate of
ethanol is dependent on pH, we assumed that by recognizing the key
reactions for ethanol production at optimal pH, not only can we maintain
the cell at optimum pH, but also increasing ethanol production
indirectly.
Initially, the optimal pH level for ethanol production was identified
based on a double robustness analysis determining the sensitivity of
growth to proton exchange and ethanol production fluxes. Then, maximum
ethanol production and flux distribution at optimal growth in the
selected range of proton exchange rate for the desired pH level were
determined. The absolute Pearson correlation coefficient of 0.95
(p-value of 0.05) between the flux of each reaction and ethanol
production rate was used to determine key reactions coupled with ethanol
production for optimal growth at the desired pH. If the coefficient was
positive (negative), the reaction was selected for up (down) regulation.
The Pearson correlation coefficient (between the flux of each key
reaction and growth rate) was also applied to determine growth
associated reactions. For evaluation of the role and importance of each
key reaction on metabolic differentiation, fluxes of the key reactions
at pH levels of 5, 6 and 7 were applied to cluster by PCA. Figure 2
indicates the flowchart of the systemic approach for finding the key
reactions for ethanol overproduction and identification of the effect of
pH on the metabolism.