(651d) Analyzing the Fundamental Role of Constraints Leading to Overflow Metabolism in Escherichia Coli | AIChE

(651d) Analyzing the Fundamental Role of Constraints Leading to Overflow Metabolism in Escherichia Coli

Authors 

Zhuang, K. H. - Presenter, University of Toronto
Mahadevan, R. - Presenter, University of Toronto


For heterotrophs, organisms that consume organic carbon, ATP can be produced through either oxidative phosphorylation or substrate-level phosphorylation. Since respiration primarily generates ATP through the highly efficient oxidative phosphorylation, it is a more attractive ATP production strategy than fermentation, which relies on the inefficient substrate-level phosphorylation. Surprisingly, when the substrate uptake rate increases above a certain threshold value, many aerobic organisms simultaneously utilize respiratory and fermentative pathways, often in presence of stoichiometrically abundant oxygen. Despite decades of research, a mechanistic understanding of all the physiological characteristics associated with the overflow metabolism or an explanation for the underlying evolutionary basis of the ubiquitous overflow metabolism was lacking (Molenaar 2009).

For a given physiological state, there are certain physical properties of the cell that remain relatively constant, such as the surface area to volume ratio and intracellular density (Molenaar 2009). Based on these physical properties, we have formulated a new metabolic constraint. We hypothesized that the existence of this constraint makes the simultaneous utilization of respiration and fermentation an attractive metabolic strategy at higher substrate uptake rate.

We used the genome-scale metabolic model of Escherichia coli (Feist 2007) as a platform to evaluate whether this theory could provide a mechanistic explanation for the the evolution of bacterial overflow metabolism. By incorporating this hypothetical constraint as well as the kinetic information of some key enzymes, we were able to accurately predict physiological characteristics associated with the overflow metabolism of E. coli, such as the changes in biomass yield, growth rate, acetate production rate, oxygen uptake rate, and TCA cycle activity, which was hitherto not possible. Our findings suggest that there is an important relationship between bacterial physiology and morphology; furthermore, these findings suggest a new mechanistic explanation for the existence of catabolic repression; lastly, these findings provide new insights into the evolution of mitochondria in eukaryotes.

Figure 1. Predicting the physiology of E. coli during overflow metabolism. Computational predictions of biomass yield, growth rate, acetate production rate, and oxygen uptake rate using our model (red line) as well as traditional FBA with high and low maintenance requirements (green and blue) are compared with experimental measurements. Our model, which assumes that the overflow metabolism is caused by enzyme crowding at the membrane, can accurately predict the overflow physiology of E. coli. On the other hand, traditional FBA model, which assumes that the overflow metabolism is due to a respiratory rate constraint, could not.

References:

Molenaar D, van Berlo R, de Ridder D & Teusink B (2009) Shifts in growth strategies reflect tradeoffs in cellular economics. Mol Syst Biol 5: 323.

Feist AM, Henry CS, Reed JL & Krummenacker M (2007) A genome-scale metabolic reconstruction for Escherichia coli K-12 MG1655 that accounts for 1260 ORFs and thermodynamic information. Mol Syst Biol