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Addition of Autotrophic Carbon Fixation Pathways to Increase the Theoretical Heterotrophic Yield of Acetate

The production of valuable biological products from sugars via fermentation is of importance in the chemical industry. One key parameter by which processes are evaluated is the product carbon yield on substrate. In the normal growth of heterotrophic organisms on fermentable sugars, some carbon is lost as CO2. In the production of acetate from hexose or pentose sugars, homoacetogens using the reductive acetyl CoA pathway have a theoretical yield of acetate on glucose of 3. We hypothesized that the addition of enzymes necessary for fixation of the lost CO2 would enhance the theoretical carbon yield of acetate in the dark in organisms without the reductive acetyl CoA pathway. Using computational flux balances of central metabolism of Escherichia coli, we show that the addition of the enzymes for the reductive pentose phosphate (Calvin Benson Bassham) cycle increases the theoretical yield of acetate from glucose or xylose. An even more significant increase in the theoretical carbon yield of acetate was obtained by adding the reactions for the reductive tricarboxylic acid cycle (rTCA), matching that of homoacetogens. However, addition of these pathways did not result in an increase in the theoretical yield of other excreted metabolites or biomass.