redox imbalance. The redox could be
balanced by reducing pyruvate to lactate as
occurs in mammalian metabolism. This
works chemically, but such metabolism
eliminates many places where micro-
organisms can gain production of ATP.
Keep in mind that due to management
errors, the ruminal ecosystem may switch
to generating lactate as its major end-
product. When this happens, the ruminant
is likely to be dead within hours.
In the production of acetate, pyruvate
is first oxidatively decarboxylated to yield
CO 2 , a reducing equivalent, and acetyl-
CoA. Formate also could be released
instead of CO 2 , but the reducing equiva-
lent eventually would be generated as
the formate was oxidized to CO 2.
Transesterification of acetyl-CoA yields
acetyl-phosphate and CoA. A phospho-
transferase catalyses formation of acetate
and ATP from acetyl-phosphate and ADP.
The production of acetate from pyruvate
produces one unit each of CO 2 , ATP and
reducing power per acetate.
Butyrate is produced by the condensa-
tion of two units of acetyl-CoA forming
CoA and acetoacetyl-CoA and consuming
an ATP. The acetoacetyl-CoA is reduced
to -hydroxybutyryl-CoA followed by
dehydration to crotonyl-CoA. Crotonyl-
CoA serves as a terminal electron acceptor
of an electron transport chain coupled to
phosphorylation of ADP (Fig. 6.4). The
electron transport chain transfers reducing
power from NADH,H+/FADH 2 and reduces
crotonyl-CoA to butyryl-CoA. The energy
of the butyryl-CoA thioester is captured
in the form of ATP via transesterification
and phosphotransferase, similarly to that
described for the acetate pathway. The
production of butyrate from acetyl-CoA
produces an ATP and consumes two units
of reducing power per unit of butyrate
formed.
Propionate is produced by two path-
ways, the randomizing and acrylate
pathways. The randomizing pathway pre-
dominates, producing 90–95% of the
propionate when ruminants are fed forage-
based diets and 60–70% of the propionate
when fed mixed diets. The acrylate pathway
predominates in grain-fed ruminants,
accounting for 70–90% of the propionate
produced after a meal of grain. The acrylate
pathway is more tolerant of the acid condi-
tions induced by grain feeding (more rapid
production of acid coupled with reduced
stimulus for rumination and therefore less
buffer from saliva) with increases in
propionate and decreases in methane
production.
The randomizing pathway is initiated
by a carboxytransphosphorylase reaction in
which PEP reacts with CO 2 and Piforming
oxaloacetate (OAA) and PPi. This may or
may not be equivalent to expenditure of an
ATP, depending on the efficiency of
coupling, because phosphofructokinase in
propionate-producing bacteria can use PPi
as the phosphate donor in forming fructose-
1,6-bisphosphate. The OAA is reduced to
malate, consuming a unit of reducing
power. The malate is dehydrated to
fumarate. Fumarate, like crotonyl-CoA, is
the terminal electron acceptor in an
electron transport chain, resulting in the
reduction of fumarate to succinate and
production of ATP. Initially, the succinate is
converted to succinyl-CoA by a thiokinase,
consuming an ATP. Once succinyl-CoA is
formed, the pathway becomes autocatalytic,
eliminating the need for the thiokinase and
carboxytransphosphorylase reactions. These
reactions are necessary only for priming
the pathway. The succinyl-CoA is mutated
to methymalonyl-CoA by a vitamin B 12 -
catalysed reaction. Methylmalonyl-CoA is
epimerized from the (R) to the (S) stereo-
isomer. The next reactions, catalysed by
a biotin-containing transcarboxylase, are
central to the randomizing pathway.
Methylmalonyl-CoA donates a CO 2 , forming
carboxybiotin on the enzyme and propionyl-
CoA. The carboxybiotin form of the en-
zyme reacts with pyruvate, carboxylating
pyruvate to OAA and reforming the uncar-
boxylated enzyme. Thus the initial CO 2 of
the transcarboxyphosphorylase becomes
catalytic, and pyruvate rather than PEP is
the subsequent source of propionate carbon.
Conversion of PEP to pyruvate captures
energy in the form of ATP when the trans-
carboxyphosphorylase is eliminated. The136 R.W. Russell and S.A. Gahr