Central metabolic pathways of four archaebacterial organisms,
Pyrococcus sp. OT3,
Methanococcus jannaschii, Archaeoglobus fulgidus, and
Methanobacterium thermoautotrophicum, have been analyzed by using their genomic DNA sequences. Close to a full set of enzymes that are expected in the standard TCA cycle have been identified with the DNA sequence of
A. fulgidus. However, with the other three sequences, enzymes expected in half the TCA cycle, connecting citrate to α-ketoglutarate through isocitrate, are not found. It is believed that the autotrophic methanogens,
M. jannaschii and M.
thermoautotrophicum, do not need the reduced form of co-enzymes for the production of ATP. A heterotroph,
P. OT3, reduces co-enzymes mainly by degrading amino acids. These three organisms are not totally dependent on the TCA cycle. Another heterotroph,
A. fulgidus seems to be dependent on the electron transfer chain and is likely to operate the TCA cycle in the standard clockwise direction for the reduction of co-enzymes, while the autotrophs are likely to operate the cycle in the anti-clockwise direction for biosynthesis only. The glycolytic pathways of the four organisms extend closely to glycogen, are used first for gluconeogensis and then for glycolysis. The glycolysic pathway of
P. OT3 is best designed for amphibolic regulation by placing two alternative sets of enzymes at steps. The modified Embden-Meyerhof type is suggested for the pathways for glycolysis/gluconeogenesis of the four organisms.
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