In A. woodii, one of the best studied acetogens, this enzyme allows coupling of the
exergonic reduction of NAD
+ to the endergonic reduction of ferredoxin by oxidation of H 2 [31]. In addition, during heterotrophic growth, the enzyme provides
the reductant for the HDCR reaction, molecular hydrogen [32]. In Clostridium
ljungdahlii, an acetogen also used for biotechnological applications, the reducing
equivalents are provided by either the electron-bifurcating hydrogenase, as found in
A. woodii or by a complex consisting of a formate dehydrogenase and a NADP
+
-
specific electron-bifurcating hydrogenase [19]. Depending on different modules for
balancing redox equivalents, the overall ATP yield can vary. However, it is always
below one ATP per mol acetate [18].
Fig. 1 The Wood–Ljungdahl pathway of CO 2 reduction. Two molecules of CO 2 are reduced to
the central intermediate acetyl-CoA. Acetyl-CoA is the precursor of biomass and a wide range of
natural products (dotted arrows). CoFeSP, corrinoid/iron sulfur protein; THF, tetrahydrofolate;
CoA, coenzyme A; [H], reducing equivalent
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D. Litty and V. Müller