Biofuels and Bioenergy

Many anaerobic hyperthermophiles can grow on carbohydrates and peptides and produce ethanol as an end product. Alcohol dehydrogenases (ADHs) catalyze the final step of the ethanol production from acetaldehyde; however, there is a lack of understanding of the enzyme catalyzing the production of acetaldehyde at high temperatures. No homolog genes are found to encode commonly-known pyruvate decarboxylase (PDC) and CoA-dependent aldehyde dehydrogenase respectively. Anovel bifunctional PDC is present in Pyrococcus furiosus, which catalyzes both oxidative (pyruvate ferredoxin oxidoreductase, POR) and non-oxidative (PDC) decarboxylation of pyruvate, producing acetyl-CoA and acetaldehyde, respectively. Our results showed that the PDC activities were also present in hyperthermophilic archaeon Thermococcus guaymasensis (Tg) and bacteria Thermotoga maritima (Tm) and Thermotoga hypogea (Th). Coenzyme A or desulfo-CoA was required for the PDC activity. PDC and POR activities were co-eluted during different steps of chromatography. All three purified enzymes from Tg, Tm and Th were revealed to be a hetero-tetrameric protein using SDS-PAGE. The purified Tg enzyme had PDC activity of 3.8±0.22 U mg-1 with optimal pH-value of 9.5. The optimum pH for both TmPDC and ThPDC was 8.4, while specific activities of PDCs were 1.9±0.4 U/mg and 1.4±0.2 U/mg for Th and Tm, respectively. Another PDC activity (25.94±0.6 U/ mg) was also identified in T. maritima, which also had acetohydroxyacid synthase (AHAS) activity. It was concluded that the bifunctional PDC and POR enzyme is present in hyperthermophilic archaeon T. guaymasensis and bacteria T. maritima and T. hypogea, and the bifunctional PDC and AHAS is also present in T. maritima. These bifunctionalities are likely a common property of the same type of enzymes in hyperthermophiles. Further studies of these thermostable PDC enzymes are required for bioengineering a more efficient alcohol fermentation process at high temperatures.