FORMATION OF L-ALANINE AS A REDUCED END-PRODUCT IN CARBOHYDRATE FERMENTATION BY THE HYPERTHERMOPHILIC ARCHAEON PYROCOCCUS-FURIOSUS

The hyperthermophilic archaeon Pyrococcus furiosus was found to form substantial amounts of L-alanine during batch growth on either cellobiose, maltose or pyruvate. Acetate, CO2 and H-2 were produced next to alanine. The carbon- and electron balances were complete for all three substrates. Under standard growth conditions (N-2/CO2 atmosphere) an alanine/acetate ratio of about 0.3 was found for either substrate. The alanine/acetate ratio was influenced, however, by the hydrogen partial pressure. In the presence of S degrees or in coculture with Methanococcus jannaschii this ratio was only 0.07, whereas under a H-2/CO2 atmosphere this ratio could amount up to 0.8. Alanine formation was also affected by the NH4+ concentration, i.e. below 4 mM, NH4+ becomes limiting to alanine formation. Alanine formation was shown to occur via an alanine aminotransferase, which exhibited a specific activity in cell-free extract of up to 6.0 U/mg (90 degrees C; direction of pyruvate formation). The alanine aminotransferase probably cooperates with glutamate dehydrogenase (up to 23 U/mg; 90 degrees C) and ferredoxin: NADP(+) oxidoreductase (up to 0.7 U/mg, using methyl viologen: 90 degrees C) to recycle the electron acceptors involved in catabolism. Thus, the existence of this unusual alanine-forming branch enables P. furiosus to adjust its fermentation, depending on the redox potential of the terminal electron acceptor.