Ethanol production by engineered thermophiles

•PDC, PDH, PFL and PFOR are key enzymes in pathways used for ethanol production in thermophiles.•Of these, only the PDC, PDH and PFOR pathways have been engineered for high-yield production.•To date, there are no reports of the transfer of a high-yield pathway into a thermophilic host.•This is in contrast to the relatively easy transfer of the PDC pathway in mesophiles.

We compare a number of different strategies that have been pursued to engineer thermophilic microorganisms for increased ethanol production. Ethanol production from pyruvate can proceed via one of four pathways, which are named by the key pyruvate dissimilating enzyme: pyruvate decarboxylase (PDC), pyruvate dehydrogenase (PDH), pyruvate formate lyase (PFL), and pyruvate ferredoxin oxidoreductase (PFOR). For each of these pathways except PFL, we see examples where ethanol production has been engineered with a yield of >90% of the theoretical maximum. In each of these cases, this engineering was achieved mainly by modulating expression of native genes. We have not found an example where a thermophilic ethanol production pathway has been transferred to a non-ethanol-producing organism to produce ethanol at high yield. A key reason for the lack of transferability of ethanol production pathways is the current lack of understanding of the enzymes involved.

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