Contrary to the thermodynamically unfavored condition, ATP synthesis by
prokaryotic alkaliphiles is known to be more efficient at alkaline condition than
in the near neutral range [47, 50, 51, 160]. Moreover, often aerobic alkaliphiles
have higher growth rate and yield than neutralophiles [48, 161], which specifies
that alkaliphiles are efficient in producing ATP. Studies have also shown that
alkaliphiles, with few exceptions of anaerobes, depend on H
+
-coupled ATP synthase
to produce ATP and satisfy their energy requirement [20]. Thus, the fact that
alkaliphiles do not exhibit energy shortage despite the thermodynamic hurdle of
generating ATP in alkaline habitats marks that alkaliphiles have devised unique
adaptive strategies to efficiently generate energy carriers at elevated pH. A number
of experimentally supported and speculative adaptations have been forwarded to
substantiate how prokaryotic alkaliphiles accomplish H
+
-coupled ATP synthesis
under the unfavorable low pmf. These adaptations which allow alkaliphiles to
effectively generate ATP during high pH growth are discussed below.
3.3.1 ATP Synthase
Two types of ATP synthases are known in bacteria, those that are coupled to H
+ and
those coupled to Na
+ [162]. Although it seems disadvantageous for alkaliphiles
which are thriving in alkaline (low proton) environment to couple their energy
carrier generating system to H
+
, surprisingly non-fermentative aerobic alkaliphiles
are entirely dependent on H
+
-coupled ATP synthase [20, 98, 133, 163, 164]. Several
studies have tried to decipher the reason behind why aerobic alkaliphiles couple their
OXPHO-based ATP synthesis to H
+ . Some of these studies have been focused on
adaptation of alkaliphiles ATP synthase and able to identify certain unique features
of the enzyme that seem to be correlated to high pH adaptation.
ATP synthase in non-alkaliphilic organisms is known to mediate both the synthesis and degradation of ATP. The ATPase activity that breaks down ATP to ADP
and P i is linked to pumping out H
+ from the cytoplasm. As discussed in Sect. 3.1.2,
one of the phenomenal adaptations of this enzyme is inactivation of its ATPase
activity. This inactivation, in addition to pH homeostasis, may contribute to energy
saving. However, as revealed by several studies, ATP synthase mainly contributes to
high pH adaptation through enhanced level of expression and specific adaptations of
its subunits.
High Level Expression of ATP Synthase
One of the ATP synthase contributions to high pH adaptations seems to be related to
the level of activity. Transcriptome and mutagenic studies revealed an increased
expression and activity of ATP synthase at high pH. As pH increases, the energy
demand to fuel cellular activities is also expected to rise [27], and hence an increase
in the level of expression and activity of the synthase will compensate the high
energy demand and contribute to minimize the low pmf effect on H
+
-coupled ATP
106
G. Mamo
prokaryotic alkaliphiles is known to be more efficient at alkaline condition than
in the near neutral range [47, 50, 51, 160]. Moreover, often aerobic alkaliphiles
have higher growth rate and yield than neutralophiles [48, 161], which specifies
that alkaliphiles are efficient in producing ATP. Studies have also shown that
alkaliphiles, with few exceptions of anaerobes, depend on H
+
-coupled ATP synthase
to produce ATP and satisfy their energy requirement [20]. Thus, the fact that
alkaliphiles do not exhibit energy shortage despite the thermodynamic hurdle of
generating ATP in alkaline habitats marks that alkaliphiles have devised unique
adaptive strategies to efficiently generate energy carriers at elevated pH. A number
of experimentally supported and speculative adaptations have been forwarded to
substantiate how prokaryotic alkaliphiles accomplish H
+
-coupled ATP synthesis
under the unfavorable low pmf. These adaptations which allow alkaliphiles to
effectively generate ATP during high pH growth are discussed below.
3.3.1 ATP Synthase
Two types of ATP synthases are known in bacteria, those that are coupled to H
+ and
those coupled to Na
+ [162]. Although it seems disadvantageous for alkaliphiles
which are thriving in alkaline (low proton) environment to couple their energy
carrier generating system to H
+
, surprisingly non-fermentative aerobic alkaliphiles
are entirely dependent on H
+
-coupled ATP synthase [20, 98, 133, 163, 164]. Several
studies have tried to decipher the reason behind why aerobic alkaliphiles couple their
OXPHO-based ATP synthesis to H
+ . Some of these studies have been focused on
adaptation of alkaliphiles ATP synthase and able to identify certain unique features
of the enzyme that seem to be correlated to high pH adaptation.
ATP synthase in non-alkaliphilic organisms is known to mediate both the synthesis and degradation of ATP. The ATPase activity that breaks down ATP to ADP
and P i is linked to pumping out H
+ from the cytoplasm. As discussed in Sect. 3.1.2,
one of the phenomenal adaptations of this enzyme is inactivation of its ATPase
activity. This inactivation, in addition to pH homeostasis, may contribute to energy
saving. However, as revealed by several studies, ATP synthase mainly contributes to
high pH adaptation through enhanced level of expression and specific adaptations of
its subunits.
High Level Expression of ATP Synthase
One of the ATP synthase contributions to high pH adaptations seems to be related to
the level of activity. Transcriptome and mutagenic studies revealed an increased
expression and activity of ATP synthase at high pH. As pH increases, the energy
demand to fuel cellular activities is also expected to rise [27], and hence an increase
in the level of expression and activity of the synthase will compensate the high
energy demand and contribute to minimize the low pmf effect on H
+
-coupled ATP
106
G. Mamo