11.Lipolytic, e.g. Anaerovigrio lipolytica,
Butyrivibrio fibrisolvens, Treponema
bryantii, Eubacterium sp., Fusocillus sp.
and Micrococcussp.
After solubilizing substrates, the
soluble nutrients are transported across the
plasma membrane of the microbial cell.
The major soluble substrates derived from
dietary carbohydrates are glucose, cello-
biose, xylose and galacturonic acid. The
soluble carbohydrates enter microbial cells
via an ATP-driven active transport system
similar to that described previously or via a
phosphoenolpyruvate (PEP)-dependent
phosphotransferase system (PTS) (Fig. 6.2)
(Konings et al., 1986; Erni, 1992). High-
energy phosphate is transferred from PEP
in glycolysis through a series of phospho-
protein intermediates to a sugar being
transported across the plasma membrane.
The first two proteins, EI and HPr, are
cytosolic and are intermediates common to
all sugars being transported. The other two
protein intermediates are sugar-specific,
EIIIsugarxand EIIsugarx. EIII may be either
cytosolic or membrane-bound. The dephos-
phorylated form of EIII (dephosphorylated
because sugar x is being transported
rapidly from the medium into the cell)
binds to ATP-driven permeases in the cell
membrane inhibiting them. The EIII~P
form (which accumulates when sugar x is
unavailable in the medium) activates
adenylate cyclase to generate cAMP to turn
on synthesis of other sugar transport pro-
teins. EII is a transmembrane protein that
binds sugar x on the outside surface of the
membrane, phosphorylates the sugar, and
releases the phosphorylated sugar into the
cytosol of the cell. Coupling transport with
phosphorylation conserves ATP. If the
sugar is cellobiose, even more ATP savings
are realized. The PTS will release
cellobiose phosphate (on C-6 of the reduc-
ing sugar) on the interior of the cell.
Phosphorylysis then will yield glucose-6-
phosphate and a glucose-1-phosphate.
The ATP-dependent systems usually
are symport transporters, with the specific
sugars being the co-solute as in SGLT1. The
primary solute is a proton or Na+. As with
SGLT1, both solutes must bind on the
exterior prior to protein conformational
change in order to bring the solutes to the
interior. The Kms of solutes when the bind-
ing sites are on the interior of the mem-
brane are high, causing release of the
solutes and preventing transport to the
exterior. The primary solute is pumped out
of the cell at the expense of ATP.
The Embden–Meyerhof–Parnas glyco-
lytic pathway is the basis for metabolism of
the phosphorylated sugars under anaerobic
conditions (Fig. 6.3) to pyruvate. Pentoses,
dietary and derived from decarboxylation
of galacturonic acid of pectins, enter
glycolysis via the pentose phosphate shunt.
An alternative route of metabolism of
pentoses is the pentose phosphate phospho-
ketolase reaction. In this reaction, ribulose-
5-phosphate (an intermediate of the
pentose phosphate shunt) is phosphory-
lated (cleavage of a carbon–carbon bond by
addition of phosphate) to glyceraldehyde-
3-phosphate (an intermediate in glycolysis)
and acetyl-phosphate. With this exception
noted, virtually all carbon entering the
ruminal anaerobic fermentation is con-
verted to pyruvate. In conversion of 1 mol
of hexose to pyruvate there are 2–5 mol of
ATP generated by substrate-level phos-
phorylation, depending on the transport
system used in getting the carbohydrate
into the cells, and 2 mol of reducing
power generated (NAD+→NADH,H+). The
NADH,H+must be oxidized for glycolysis
to continue. Thus reactions that produce
NADH,H+must be balanced with reactions
that consume NADH,H+.
The secondary degradation of pyruvate
in ruminal fermentation differs from that of
mammalian glycolysis. Lactate, although
an intermediate in ruminal fermentation, is
not an end-product of a normal, healthy
fermentation. The major end-products of a
normal fermentation are acetate, propionate,
butyrate, CO 2 and CH 4. A typical molar
ratio of acetate:propionate:butyrate is
66:20:14. Balancing this molar production
ratio of volatile fatty acids (VFAs) for redox
potential (Wolin, 1960), the molar ratio for
major end-products is 34:10:8:30:18, for
acetate, propionate, butyrate, CO 2 and CH 4 ,
respectively. Expressing these values asGlucose Availability and Associated Metabolism 133