24
Chapter 2
In secondary active transport the compound is transported across the cytoplasmic membrane at the
expense of a previously established gradient of another substance. If the two substances are
transported in the same direction, the transport process is called
symport
(as illustrated in Fig. 2.3);
if they are transported in opposite directions, it is called
antiporf,
and if an electrochemical potential
drives the flow of ions, it is called
uniport.
Often, secondary active transport is coupled to the pH
gradient across the cytoplasmic membrane, and in order to keep the intracellular pH constant it is
necessary to pump protons out of the cells by means of ATPase. In eukaryotes there are specific
ATPases - different from the F
0
Fr ATPase (see Fig. 2.3), which are located in the cytosolic
membrane. Since Gibbs free energy is used in this process, the overall effect is that a secondary
active transport process requires free energy generated inside the cell. Examples of secondary active
transport are the uptake of sugars in microorganisms by so-called permeases, where the sugar is
transported into the cytoplasm together with a proton - i.e., there is proton symport, A well-studied
system is lactose permease in
E. coli,
where a stoichiometric ratio of 1:1 in the lactose proton
transport has been found. A similar simple stoichiometric ratio is not necessarily found for other
transport processes.
In group translocation the transport process is coupled to a concomitant conversion of the
transported substance. The best-known example of group translocation is the
phosphotransferase
system
(PTS), which is used by many bacteria for uptake of different sugars. In this system the
sugar
is
phosphorylated
upon
uptake
and
the
phosphate
group
is
donated
from
phosphoenolpyruvate (PEP), which is an intermediate in the Embden-Meyerhof-Pamas pathway
(see Section 2.1.2.1). The transfer of the phosphate group involves at least four separate proteins
(see Fig. 2.2), o f which the last member of the chain also serves as the carrier protein that transports
the sugar across the cytoplasmic membrane. The last two proteins in the chain are specific to the
particular sugar, whereas the first two are identical in different PTSs. When glucose is transported
to the cell by means of a PTS, it is directly converted to glucose-
6
-phosphate (G
6
P). The
high-energy phosphate bond originally present in PEP is therefore conserved, and the uptake
process is more economical from an energy point of view than glucose uptake by a permease.
Furthermore, the PTSs may operate at very high rates of sugar uptake compared with other uptake
systems. This may explain why the PTSs are predominant in fermentative bacteria, where the ATP
generation resulting from sugar metabolism is less than in respirative bacteria; i.e., strict aerobes
such as
Azotobacter
do not possess PTSs, whereas anaerobes and facultative anaerobes such as
Lactococcus
and
Escherichia
possess PTSs for several different sugars. One may finally speculate
why PTS systems use an apparently complicated transfer of the free energy to the membrane bound
protein by a chain of 3 cytosolic proteins. But this multi step transfer process serves as a fine-tuned
regulator for the energy status of the cell-see Lengeler
et al.
(1999) for more details of this
fascinating biological system.
2.1.3 Catabolism
When the substrates have been transported into the cytoplasm, they are converted to metabolic
products and biomass components in a large number of biochemical reactions. As mentioned earlier
the first reactions are the fueling reactions where the substrate (carbon and energy source) is
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