Bioalcohol production by a new synthetic route in a hyperthermophilic archaeon.

Abstract

The still growing demand for bioethanol is currently met by two microbiological processes. The by far most important process is the use of first- (sugar) or second- (lignocellulosic biomass) generation raw materials and yeast as catalysts. Yeasts convert the sugars via glycolysis to pyruvate, which is decarboxylated, and the resulting acetaldehyde is then reduced to ethanol by a monofunctional ethanol dehydrogenase. The second and industrially less important is the bacterial process, in which pyruvate is oxidized to acetyl-CoA, which is then reduced by a bifunctional aldehyde dehydrogenase/ethanol dehydrogenase (AdhE) to ethanol. This pathway is found in many fermenting bacteria (Fig. 1). There is also a growing interest in longer-chain alcohols that have advantages over ethanol in certain applications; for example, butanol is superior to ethanol as an additive to gasoline. Pathways for production of butanol are found in specific groups, such as in Clostridia , and the encoding genes have been implemented in industrial production strains, such as yeast (1). Bacteria that produce even higher alcohols are rare. Fig. 1. Pathways for ethanol formation from glucose in yeasts ( A ) and bacteria ( B ). Adh1, alcoholdehydrogenase 1; AdhE, bifunctional CoA-dependent ethanol/aldehyde dehydrogenase. In PNAS, Basen et al. (2) report a completely different and novel metabolic route generated by insertion of a single gene into the archaeon Pyrococcus furiosus (Fig. 2). This hyperthermophile grows optimally near 100 °C and ferments sugars to acetate, carbon dioxide, and hydrogen. The pathway used for sugar breakdown is a modification of glycolysis in which the oxidation of glycerinaldehyde-3-phosphate is not coupled to the reduction of NAD+ but to that of ferredoxin (3), a small iron-sulfur containing redox protein with a rather low redox potential (E′ = −480 mV). Oxidation of pyruvate to acetyl-CoA and carbon dioxide is also coupled to the reduction of ferredoxin. Fig. 2. Bioenergetics and … [↵][1]1Email: vmueller{at}bio.uni-frankfurt.de. [1]: #xref-corresp-1-1