3. MECHANISMS FOR FATTY ACID OXIDATION
97
However, acetyl-CoA is the precursor of butyrate and caproate and also
is used to form the electron acceptors, acetoacetyl-CoA and crotonylCoA, without which ethanol and acetaldehyde cannot be oxidized. Another potential mechanism that has been considered by Barker is the
coupling of an electron transport reaction with phosphorylation. Thus
the ethanol-aldehyde and crotonyl-CoA-butyryl-CoA system (AF'
= —18kcal.) with a simultaneous coupling of phosphorylation has sufficient energy capacity in theory but has never been demonstrated.
Recently Shuster and Gunsalus (64) have re-examined the C. kluyveri system (crotonyl-CoA + H 2 -> butyryl-CoA) and have noted a
formation of high-energy phosphate coupled with the electron transport
process. Approximately 1 mole of ATP is detected for each mole of
2-alkenoic thioester reduced. Pi is esterified to ADP to form ATP.
The aerobic oxidation of butyrate has been studied with extracts of
two other Clostridia, C. saccharobutyricum and C. acetobutylicum. Their
metabolic patterns have been found to be similar, if not identical, with
those of C. kluyveri (64a).
^-Oxidation has also been observed in the fatty acid metabolism of
the methane bacteria, another anaerobic group, with certain variations
of the fundamental scheme. In fact, the first demonstration of the biochemical conversion of higher fatty acids to acetate was shown by
Barker to occur in a member of this group (65). Energy is obtained by
these organisms through the oxidation of fatty acids to acetate and the
concomitant reduction of C0 2 to methane. One species, Methanobacterium propionicum, oxidizes propionic acid at the alpha carbon, decarboxylates the intermediate formed, and reduces the C0 2 evolved to
methane (66).
Oxidation: 4CH 3 CH 2 COOH + 8H 2 0 -> 4CH 3 COOH + 4C0 2 + 24H
(33)
Reduction: 3C0 2 + 24H -> 3CH 4 + 6H 2 0
(34)
Observed: 4CH 3 CH 2 COOH + 2H 2 0 -* 4CH 3 COOH + C0 2 3CH 4
(35)
Limited ^-oxidation occurs in M. suboxydans, which converts valeric to
propionic and acetic acids and in a similar manner oxidizes butyric and
caproic acids to acetate (66) with the reduction of 1 mole of C0 2 to
methane:
2CH 3 CH 2 CH 2 COOH + 2H 2 0 + C0 2 -> 4CH 3 COOH + CH 4
(36)
Other groups of anaerobes also utilize fatty acids as reducing agents
in energy-yielding reactions. Oxidants vary according to the genus and
species (67). It should be emphasized that in these cases just discussed
the fatty acids act as reducing agents rather than oxidants, as in C.
kluyveri, but growth is still under anaerobic conditions.
97
However, acetyl-CoA is the precursor of butyrate and caproate and also
is used to form the electron acceptors, acetoacetyl-CoA and crotonylCoA, without which ethanol and acetaldehyde cannot be oxidized. Another potential mechanism that has been considered by Barker is the
coupling of an electron transport reaction with phosphorylation. Thus
the ethanol-aldehyde and crotonyl-CoA-butyryl-CoA system (AF'
= —18kcal.) with a simultaneous coupling of phosphorylation has sufficient energy capacity in theory but has never been demonstrated.
Recently Shuster and Gunsalus (64) have re-examined the C. kluyveri system (crotonyl-CoA + H 2 -> butyryl-CoA) and have noted a
formation of high-energy phosphate coupled with the electron transport
process. Approximately 1 mole of ATP is detected for each mole of
2-alkenoic thioester reduced. Pi is esterified to ADP to form ATP.
The aerobic oxidation of butyrate has been studied with extracts of
two other Clostridia, C. saccharobutyricum and C. acetobutylicum. Their
metabolic patterns have been found to be similar, if not identical, with
those of C. kluyveri (64a).
^-Oxidation has also been observed in the fatty acid metabolism of
the methane bacteria, another anaerobic group, with certain variations
of the fundamental scheme. In fact, the first demonstration of the biochemical conversion of higher fatty acids to acetate was shown by
Barker to occur in a member of this group (65). Energy is obtained by
these organisms through the oxidation of fatty acids to acetate and the
concomitant reduction of C0 2 to methane. One species, Methanobacterium propionicum, oxidizes propionic acid at the alpha carbon, decarboxylates the intermediate formed, and reduces the C0 2 evolved to
methane (66).
Oxidation: 4CH 3 CH 2 COOH + 8H 2 0 -> 4CH 3 COOH + 4C0 2 + 24H
(33)
Reduction: 3C0 2 + 24H -> 3CH 4 + 6H 2 0
(34)
Observed: 4CH 3 CH 2 COOH + 2H 2 0 -* 4CH 3 COOH + C0 2 3CH 4
(35)
Limited ^-oxidation occurs in M. suboxydans, which converts valeric to
propionic and acetic acids and in a similar manner oxidizes butyric and
caproic acids to acetate (66) with the reduction of 1 mole of C0 2 to
methane:
2CH 3 CH 2 CH 2 COOH + 2H 2 0 + C0 2 -> 4CH 3 COOH + CH 4
(36)
Other groups of anaerobes also utilize fatty acids as reducing agents
in energy-yielding reactions. Oxidants vary according to the genus and
species (67). It should be emphasized that in these cases just discussed
the fatty acids act as reducing agents rather than oxidants, as in C.
kluyveri, but growth is still under anaerobic conditions.