Question

Consider an anaerobic process where glucose is fermented to succinic acid and ethanol as products (as well as CO2 and H2O), with biomass growth occurring through acetyl-CoA (assume K=5 mol ATP/C-mol biomass). Determine through a structured approach (i.e. DRAFT) the overall stoichiometric reaction.

Answer 1

Fermentation is an anaerobic enzymatic conversion process including a series of chemical reactions to break down the glucose component of biomass material to its basic contents with the help of yeast or bacteria. Fermentation is used commercially on a large-scale in numerous countries to produce bioethanol from sugar crops and starch crops, which can be further distilled to obtain a purified alcohol to be used as an automotive fuel.

Here is the stoichiometric equation for -End products of mixed-acid and butanediol fermentation along with Succinic acid formation and acetyl-CoA pathway:-

The ATP yield of mixed-acid fermentation is about 2.5 mol of ATP per mole of glucose.

Clostridium butyricum ferments glucose producing butyrate, acetate, carbon dioxide and hydrogen with the following stoichiometry:

(a) Crotonyl/butyryl-CoA cycle with butyrate as end product and (b) coupling of the butyryl-CoA dehydrogenase/electron transferring flavoprotein (BCD/ETF) complex to production of a proton motive force. The BCD/ETF complex utilizes NADH for the concomitant conversion of crotonyl-CoA to butyryl-CoA and the production of reduced ferredoxin (FdR).

Reduced ferredoxin can be reoxidized to FdO via two pathways: production of hydrogen by a soluble ferredoxin-dependent hydrogenase or H+ translocation via the membrane-bound NADH:ferredoxin oxidoreductase complex (Rnf complex). The distribution between these pathways depends on several factors including NAD+ availability and utilization of H2 by other organisms.