electrical potential (ΔΨ) for attracting H
+ from the outer surface membrane. This
creates for each H
+ an enhanced ATP synthase driving force.
Faster pumping of H
+ by the respiratory complex may require an increased
oxygen uptake and high level of electron donor (NADH). The respiratory and
NADH activities of an alkaliphilic Bacillus have been studied [132]. The results
indicate that the oxygen uptake and NADH oxidation activities increase with the rise
in cultivation pH. The oxygen consumption studies revealed that the Bacillus grown
at pH 7.2 and 9.8 has consumed 1.17 and 2.43 μmol oxygen atom/min/mg cell
protein, respectively [132]. In addition to viable cells, the authors have also studied
the activities of membrane vesicles prepared from the cell envelope of the
alkaliphilic Bacillus cells. The trend was the same. When the cells were grown at
pH 7–9 and 9.9, the oxygen uptake was 1.1–1.4 and 2.5 pmol oxygen atom/min/mg
of the cell envelope protein used to make the membrane vesicles, respectively.
Similarly, vesicles prepared from the Bacillus cells cultivated at pH 7–8.5 and 9.9
were able to oxidize 1.4–1.7 and 6.3 pmol NADH/min/mg cell envelope protein,
respectively. On the other hand, an approximately 2.5 times lower oxygen consumption rate was reported for B. clarkii DSM 8720(T) cells at pH 10 than that of
B. subtilis IAM 1026 cells at pH 7. This is despite the alkaliphilic B. clarkii 7.5
times higher rate of ATP synthesis than the neutralophilic B. subtilis [160]. Such
a discrepancy regarding oxygen consumption is also reflected among different
alkaliphiles [132], which suggests the high rate of oxygen uptake by NADH
oxidation activities is not universal. In fact, it seems that at low level of aeration,
the electron retention capacity of cytochrome c is more important in maintaining the
transmembrane potential that drives the synthesis of ATP [177].
As it has been suggested, alkaliphiles generate pmf during the respiratory electron
transport events [179]. At least theoretically, the rapid pumping of H
+ forms the high
pmf before it gets equilibrated with bulk phase of the extracellular environment.
However, the potential equilibration with the bulk phase should be minimized to tap
the pmf for ATP synthesis. The high level of cytochrome c and cardiolipin that
retains the H
+ close to the surface of the membrane seems to play a crucial role in
preventing the dissipation of the pmf created by the respiratory complexes. Moreover, the retention of H
+ by the cytochromes and cardiolipin forms H
+ pool. If the
cells have an effective means to shuttle the H
+ from the pool to the ATP synthase, it
can generate ATP efficiently. This is expected to be more effective if the ATP
synthases are located near to the respiratory complex, a task believed to be accomplished by cardiolipin. To this end, the presence of microcircuits that facilitate the
transfer of H
+ to ATP synthase by connecting the surface of the H
+ pumping
respiratory complexes and ATP synthases has been speculated [27]. The presence
of specific interaction between cytochrome c oxidase and ATP synthase, which has
been demonstrated in a reconstituted system [180], supports the speculation to some
extent. The physical interaction between the respiratory and the ATP synthase
complexes can efficiently sequester H
+ transfers during OXPHOS at high pH.
It seems that several factors contribute to enhance the efficiency of alkaliphiles’
OXPHOS based ATP production. Among these factors, probably, the most important features include the presence of high amount of:
110
G. Mamo
+ from the outer surface membrane. This
creates for each H
+ an enhanced ATP synthase driving force.
Faster pumping of H
+ by the respiratory complex may require an increased
oxygen uptake and high level of electron donor (NADH). The respiratory and
NADH activities of an alkaliphilic Bacillus have been studied [132]. The results
indicate that the oxygen uptake and NADH oxidation activities increase with the rise
in cultivation pH. The oxygen consumption studies revealed that the Bacillus grown
at pH 7.2 and 9.8 has consumed 1.17 and 2.43 μmol oxygen atom/min/mg cell
protein, respectively [132]. In addition to viable cells, the authors have also studied
the activities of membrane vesicles prepared from the cell envelope of the
alkaliphilic Bacillus cells. The trend was the same. When the cells were grown at
pH 7–9 and 9.9, the oxygen uptake was 1.1–1.4 and 2.5 pmol oxygen atom/min/mg
of the cell envelope protein used to make the membrane vesicles, respectively.
Similarly, vesicles prepared from the Bacillus cells cultivated at pH 7–8.5 and 9.9
were able to oxidize 1.4–1.7 and 6.3 pmol NADH/min/mg cell envelope protein,
respectively. On the other hand, an approximately 2.5 times lower oxygen consumption rate was reported for B. clarkii DSM 8720(T) cells at pH 10 than that of
B. subtilis IAM 1026 cells at pH 7. This is despite the alkaliphilic B. clarkii 7.5
times higher rate of ATP synthesis than the neutralophilic B. subtilis [160]. Such
a discrepancy regarding oxygen consumption is also reflected among different
alkaliphiles [132], which suggests the high rate of oxygen uptake by NADH
oxidation activities is not universal. In fact, it seems that at low level of aeration,
the electron retention capacity of cytochrome c is more important in maintaining the
transmembrane potential that drives the synthesis of ATP [177].
As it has been suggested, alkaliphiles generate pmf during the respiratory electron
transport events [179]. At least theoretically, the rapid pumping of H
+ forms the high
pmf before it gets equilibrated with bulk phase of the extracellular environment.
However, the potential equilibration with the bulk phase should be minimized to tap
the pmf for ATP synthesis. The high level of cytochrome c and cardiolipin that
retains the H
+ close to the surface of the membrane seems to play a crucial role in
preventing the dissipation of the pmf created by the respiratory complexes. Moreover, the retention of H
+ by the cytochromes and cardiolipin forms H
+ pool. If the
cells have an effective means to shuttle the H
+ from the pool to the ATP synthase, it
can generate ATP efficiently. This is expected to be more effective if the ATP
synthases are located near to the respiratory complex, a task believed to be accomplished by cardiolipin. To this end, the presence of microcircuits that facilitate the
transfer of H
+ to ATP synthase by connecting the surface of the H
+ pumping
respiratory complexes and ATP synthases has been speculated [27]. The presence
of specific interaction between cytochrome c oxidase and ATP synthase, which has
been demonstrated in a reconstituted system [180], supports the speculation to some
extent. The physical interaction between the respiratory and the ATP synthase
complexes can efficiently sequester H
+ transfers during OXPHOS at high pH.
It seems that several factors contribute to enhance the efficiency of alkaliphiles’
OXPHOS based ATP production. Among these factors, probably, the most important features include the presence of high amount of:
110
G. Mamo