propionyl-CoA formed in the transcarboxy-
lase reaction undergoes transesterification
with succinate yielding succinyl-CoA and
propionate. The transesterification reaction
eliminates the thiokinase reaction. Once
primed, the randomizing pathway produces
an ATP and consumes two NADH,H+per
propionate produced.
The acrylate pathway converts
pyruvate to propionate and acetate in a
coordinated and stoichiometric manner.
Pyruvate is reduced to lactate, consuming a
reducing equivalent. The lactate is
energized to lactyl-CoA by a transesterifica-
tion reaction with acetyl-CoA. The acetyl-
CoA generation was discussed in the
acetate section. The lactyl-CoA is phos-
phorylated using acetyl-phosphate as the
donor. The phosphate is a much better
leaving group than a hydroxyl, and sets up
the next reaction which is phosphate
elimination. In effect, the phosphoryla-
tion/phosphate elimination dehydrates
lactyl-CoA to acrylyl-CoA. Acrylyl-CoA is a
terminal electron acceptor of an electron
transport chain, and an ATP is generated as
acrylyl-CoA is reduced to propionyl-CoA.
The propionyl-CoA is transesterified with
acetate yielding propionate and acetyl-
CoA. Overall, two pyruvates are converted
to a propionate, a CO 2 and an acetate while
generating an ATP and consuming an
NADH,H+. Considering the pathway with-
out the contribution of pyruvate to acetate,
there are two units of reducing power
consumed per propionate produced and no
change in ATP.
Production of CO 2 , in effect, was
described concurrently with the pathways
of VFA production. Glycolysis and produc-
tion of acetate generate reducing power,
and production of propionate and butyrate
consumes reducing power. Acetate is
the dominant end-product of ruminal
fermentation. The VFA ratios normally
encountered produce more reducing power
than is consumed by VFA production
alone. Species that generate excess reduc-
ing power balance the excess by using a
proton as a terminal electron acceptor,
which in combination with a hydride ion
generates H 2 gas. This system can only be
effective if there is a means of consuming
the H 2. The methanogens use H 2 to reduce
CO 2 to methane. The methanogenic path-
way is a series of oxidation–reduction reac-
tions of a series of electron transport chains
that ultimately use CO 2 or its reduction
products as terminal electron acceptors.
ATP is generated concurrently with
electron transport. A branch from the
methanogenic pathway also fixes carbon
for anabolic processes (Fig. 6.5). In addi-
tion to fumarate, crotonyl-CoA, acrylyl-
CoA, H+and CO 2 , other compounds that
serve as significant terminal electron
acceptors are SO 4 and NO 3. Methanogens
generate virtually all of the ATP from
electron transport-coupled phosphoryla-
tion, but there is a wide range in non-
methanogenic species, e.g. B. succinogens,
50%; B. ruminicola, 33%; B. fibrisolvens,
25%, and S. bovis, ~0%.
The summary in Fig. 6.3 represents
the whole of the ruminal ecosystem and
not the metabolism of individual species.
There is interspecies metabolite transfer.
End-products of the metabolism of one
species are used as substrates for meta-
bolism of another species. The inter-
species hydrogen transfer to methanogens
may prove to be quite interesting. There
are indications that methanogens physic-
ally attach themselves to the cells of larger
microorganisms. Typically, the metabolic
characteristics expressed by micro-
organisms in monoculture are quite
different from those in mixed culture. This
effect is very dramatic in ruminal micro-
organisms grown in the absence of
methanogens. Although methanogens may
be politically incorrect, they are vital to
the ruminal ecology. Methane produced by
ruminants worldwide is significant (~12%
of total world production) but must be
kept in context. Ruminants produce the
same amount of methane as rice fields.
Methane production from landfills is half
that from ruminants. The major generator
of methane is natural wetlands (42%)
(Crutzen, 1995).
Organic acid end-products of meta-
bolism of individual microbes (VFAs and
metabolic intermediates for interspecies138 R.W. Russell and S.A. Gahr