The Biochemistry of ABE Fermentation

Clostridium species (C. kluyveri and C. tetanomorphum) has been shown to perform an electron bifurcation reaction [24]. In this reaction, the exergonic reduction of crotonyl-CoA by NADH is coupled with the endergonic reduction of oxidized ferredoxin to maximize energy utilization. Although Bcd/Etf in C. acetobutylicum has not been characterized, it is presumed to perform the same reaction. Butyryl-CoA can then be converted to either butyric acid to generate ATP, or reduced to generate -butanol. 

In the acidogenic phase, acetyl-CoA and butyryl-CoA are converted to acetate and butyrate, respectively, which are reassimilated in the solvenotogic phase. The enzyme at the center of acid reassimilation and solvent production is 

As acetone is the least attractive product in the ABE fermentation, many efforts have been made to engineer the bacterial cells to produce butanol with minimal or no acetone. This goal has been achieved both in C. acetobutylicum natively and in many foreign hosts. However, all these successful cases rely on the direct n-butanol synthesis from acetyl-CoA, a process that will be discussed later in this chapter, and not via the native acid reassimilation. 

---