96
P. K. STUMPF AND G. A. BARBER
At the time these investigations were being made Rittenberg and
Bloch (60), using isotopically labeled compounds, had already shown
that fatty acid synthesis proceeds by the condensation of active two
carbon derivatives of acetate. Barker then realized the peculiar advantages of C. kluyveri as a test organism in the study of fatty acid
metabolism, since its fatty acid interconversions are uncomplicated by
the ramifications of carbohydrate metabolism. The subsequent work
was carried out by a number of researchers, most notably E. R. Stadtman and H. A. Barker.
Their research was substantially aided by the discovery that cellfree extracts of the organism were quite active and stable. Further, the
somewhat bizarre observation was made that those extracts possessed an
active terminal oxidase system which made possible the oxidation of
butyric and caproic acids by molecular oxygen (61). Since the final
electron acceptor under those conditions was oxygen, the intermediates
in fatty acid synthesis, i.e., the normal electron acceptors, were not
reduced and tended to accumulate in the medium.
In 1951 Lynen and Reichert (62) isolated the active derivative of
acetate, acetyl-S-CoA, from yeast extracts. Until then the presence of
such a compound had been inferred, but its actual discovery made it
possible to clarify most of the obscure points remaining in this problem.
Figure 4 shows the scheme for the mechanism of fatty acid synthesis in C. kluyveri presented by Barker and later modified by Stadtman (63).
The interconversions of this scheme differ from those in animals,
aerobic bacteria, and presumably higher plants by the presence of the
enzymes phosphotransacetylase, vinylacetylisomerase, and CoA-S-transphorase and by the presence of the intermediate compound, acetyl phosphate, resulting from the activity of phosphotransacetylase. The lack of
CoA-S-transphorase prevents other organisms from catalyzing the direct
transfer of CoA groups from one fatty acid to another. On the other
hand, animals, plants, and aerobic bacteria possess the tricarboxylic acid
cycle through which CoA-SH is regenerated as a result of the condensation of acetyl-S-CoA and oxaloacetate to citrate, and ATP is produced
by oxidative phosphorylation.
The nature of the mechanisms by which C. kluyveri obtains energy
from the conversion of ethanol and acetate to butyrate and caproate is
not known. In the oxidation of ethanol, acetyl-CoA is formed and the
energy of the thioester bond is available to form ATP:
Acetyl-CoA + P t · —> Acetyl phosphate + CoA (phosphotransacetylase)
(31)
Acetyl phosphate + ADP —> Acetate + ATP (acetokinase)
(32)
P. K. STUMPF AND G. A. BARBER
At the time these investigations were being made Rittenberg and
Bloch (60), using isotopically labeled compounds, had already shown
that fatty acid synthesis proceeds by the condensation of active two
carbon derivatives of acetate. Barker then realized the peculiar advantages of C. kluyveri as a test organism in the study of fatty acid
metabolism, since its fatty acid interconversions are uncomplicated by
the ramifications of carbohydrate metabolism. The subsequent work
was carried out by a number of researchers, most notably E. R. Stadtman and H. A. Barker.
Their research was substantially aided by the discovery that cellfree extracts of the organism were quite active and stable. Further, the
somewhat bizarre observation was made that those extracts possessed an
active terminal oxidase system which made possible the oxidation of
butyric and caproic acids by molecular oxygen (61). Since the final
electron acceptor under those conditions was oxygen, the intermediates
in fatty acid synthesis, i.e., the normal electron acceptors, were not
reduced and tended to accumulate in the medium.
In 1951 Lynen and Reichert (62) isolated the active derivative of
acetate, acetyl-S-CoA, from yeast extracts. Until then the presence of
such a compound had been inferred, but its actual discovery made it
possible to clarify most of the obscure points remaining in this problem.
Figure 4 shows the scheme for the mechanism of fatty acid synthesis in C. kluyveri presented by Barker and later modified by Stadtman (63).
The interconversions of this scheme differ from those in animals,
aerobic bacteria, and presumably higher plants by the presence of the
enzymes phosphotransacetylase, vinylacetylisomerase, and CoA-S-transphorase and by the presence of the intermediate compound, acetyl phosphate, resulting from the activity of phosphotransacetylase. The lack of
CoA-S-transphorase prevents other organisms from catalyzing the direct
transfer of CoA groups from one fatty acid to another. On the other
hand, animals, plants, and aerobic bacteria possess the tricarboxylic acid
cycle through which CoA-SH is regenerated as a result of the condensation of acetyl-S-CoA and oxaloacetate to citrate, and ATP is produced
by oxidative phosphorylation.
The nature of the mechanisms by which C. kluyveri obtains energy
from the conversion of ethanol and acetate to butyrate and caproate is
not known. In the oxidation of ethanol, acetyl-CoA is formed and the
energy of the thioester bond is available to form ATP:
Acetyl-CoA + P t · —> Acetyl phosphate + CoA (phosphotransacetylase)
(31)
Acetyl phosphate + ADP —> Acetate + ATP (acetokinase)
(32)