General Microbiology - Schlegel, H. 1987

Types of Fermentation
Homoacetic fermentation. CO2 as a hydrogen acceptor

Some clostridia (Clostridium formicoaceticum, C. thermoaceticum, C. acidi-urici, and C. cylindrosporum) are capable of transferring hydrogen split off from a substrate in initial oxidation reactions exclusively to СO₂, thereby forming acetate:

Class="center">8[Н] + 2СO₂ → СН₃—СООН + 2Н₂O

The thermophilic species C. thermoaceticum and the mesophilic species C. formicoaceticum ferment hexoses primarily into acetate. The process proceeds via the fructose bisphosphate pathway, yielding nearly 3 moles of acetate per mole of hexose. Consequently, a major portion of the carbon dioxide released during Pyruvate Cleavage must be refixed to serve as a hydrogen acceptor. The formation of acetate from СO₂ and reduction equivalents (electrons) liberated in the initial oxidation reactions follows the pathway illustrated in Fig. 8.9.

Hexose is oxidized to pyruvate via the typical clostridial fructose bisphosphate pathway. Pyruvate is subsequently converted into acetate and СO₂ through the action of pyruvate:ferredoxin oxidoreductase (1), phosphotransacetylase (2), and acetate kinase (3), yielding ferredoxin-Н₂ and ATP in the process. СO₂ acts as the hydrogen acceptor. Carbon dioxide molecules are partially reduced to formate via formate dehydrogenase (4) and subsequently incorporated into the methyl group of a third acetate molecule, while another portion is reduced to CO via CO dehydrogenase (5) and further converted into the carboxyl group of acetate. The reduction of the formyl group to a methyl group proceeds with tetrahydrofolate (FH₄) serving as a coenzyme. The methyl group is then transferred to a corrinoid-protein complex

(B₁₂ coenzyme) and carboxylated via The addition of bound CO. The product of these transformations is acetyl-CoA, from which acetate is formed through standard pathways accompanied by the regeneration of ATP.

Fig. 8.9. Pathway of acetate Biosynthesis from hexose in Clostridium thermoaceticum via the acetyl-CoA pathway. See text for details. E[Со] - corrinoid protein; FH4 - tetrahydrofolate; [Н] - hydrogen equivalents as NADH₂ or FdH₂; СО - exogenous carbon monoxide, [СО] - bound carbon monoxide.

This pathway of acetyl-CoA synthesis from СO₂ and hydrogen equivalents—known as the acetyl-CoA pathway—shares many similarities with acetate synthesis in anaerobically respiring methanogenic and acetogenic Bacteria (Sections 9.4 and 9.5).

Cytochrome b has been detected in Clostridium formicoaceticum and C. thermoaceticum. It remains to be determined whether these bacteria, as well as C. aceticum and other clostridia, can generate energy via electron transport-coupled phosphorylation (see Section 9.5).