n-Butanol and isopropanol are the two higher alcohols which are overproduced in nature by Clostridium species. The fermentative pathway in this organism starts from acetyl-CoA. The enzyme acetyl-CoA acetyltransferase condenses two mol­ecules of acetyl-CoA to one molecule of acetyl-CoA. This molecule branches the pathway into isopropanol and n-butanol. For the biosynthesis of isopropanol, an acetoacetyl-CoA transferase transfers the CoA group away from acetoacetyl-CoA

Butyraldehyde

-NADH

adhE2

NAD*

n-Butanol

to acetate or butyrate, forming acetoacetate. Acetoacetate is decarboxylated to acetone by an acetoacetate decarboxylase. Then, acetone is reduced to isopropanol by a NADPH-dependent secondary alcohol dehydrogenase [64]. For n-butanol biosynthesis, acetoacetate has to go through four steps of NADH-dependent reduction and one step of dehydration as shown in Fig. 9.10.

Isopropanol and n-butanol are produced by Clostridium species. However, production by this procedure is difficult to handle and optimize, because of

Fig. 9.11 Schematic illustration of higher chain alcohol production via keto acid pathway. keto acid decarboxylase (KDC), alcohol dehydrogenase (ADH) complex physiological features, such as oxygen sensitivity, slow-growth rate, and spore-forming life cycles of Clostridium. Therefore, E. coli has been metabolically engineered to produce acetone, the immediate precursor of isopropanol [15] and n — butanol production by using the traditional CoA-dependent pathway originated from C. acetobutylicum [8].