Fermentation - Food and Industrial Bioproducts and Bioprocessing - page 183

2H + + 2 e -

CO 2

CODH,
Fd

CO 2 + 2H + + 2 e -

CO + H 2 O

HCOOH

Hydrogenase

ATP, THF

2H + + 2 e - H 2

CO 2

2H + + 2 e - HCO-THF

4H + + 4 e -

Coenzyme-A

CH 3 -THF

CO

CH 3 -CO-SCoA
Acetyl-CoA

Figure 7.2 Wood-Ljungdahl pathway (CODH: carbon monoxide dehydrogenase; Fd: ferredoxin).

2 ADP+ 2 Pi 2 ATP

4 NAD +

2 Ethanol 2 Acetyl-CoA 2 Acetate

4 NAD + 4 NADH

Acetone Acetoacetyl-CoA

2 NADH

2 NADH

2 NAD +

Isopropanol

2 NAD +

n-Butanol Butyryl-CoA Butyrate

2 NAD +

2 NADH 2 ADP+ 2 Pi 2 ATP

Figure 7.3 Fermentative pathway utilizing acetyl-CoA.

precursors for the formation of the various compounds necessary for cellular metabolism. If
oxygen is not present, pyruvate can still be converted to acetyl-CoA or it can be reduced to
form various products.
There are also organisms that are capable of utilizing carbon monoxide (CO), carbon
dioxide (CO 2 ), and hydrogen (H 2 ) to form acids and alcohols instead of, or in addition to,
utilizing carbohydrates. These organisms use a metabolic pathway known as the Wood-
Ljungdahl pathway to convert carbon monoxide and/or carbon dioxide/hydrogen to acetyl-
CoA (Figure 7.2). Acetyl-CoA is then converted to organic acids and/or alcohols (Figure 7.3).
The acetyl-CoA pathway involves two branches, known as the methyl branch and the