Bacillus strains [170, 172, 173]. Neutralophilic Bacillus spp. have GxGxGNG motif
in the TMH1. But, in alkaliphiles, this motif is substituted with 16AxAxAVA, which
possibly indicates its potential importance in high pH adaptation. In the second
conserved motif, the residue 51P is specific to alkaliphiles, and it is positioned close
to the ion-binding residue E54. Mutational, structural, and sequence analysis of the
c-subunit has shown features that may be recognized as high pH adaptations.
Mutation of all the alanine of the 16AxAxAVA motif to glycine led to ATP
synthesis activity loss by more than 80% [173]. P51 has been mutated to alanine
and glycine. The mutant P51A exhibited a dramatic loss of ATP synthesis and
non-fermentative growth at pH 10.5, whereas the P51G mutation did not affect the
ATP synthesis capacity although it had growth problem at high pH and exhibits H
+
leakage [173]. Studies made on the mutant E54G revealed that it leaks H
+ and a 90%
drop on non-fermentative growth. Although it needs further studies to expound how
these identified motifs contribute to the high pH adaptation exactly, the results
generated so far are valuable and indicate the involvement of the c-subunit in the
adaptation of ATP synthesis at high pH. However, it is imperative to expand the
study to other alkaliphilic genera as well to see the whole picture of the c-subunit
high pH adaptation role.
3.3.2 Cytochrome
The adaptation of alkaliphiles respiratory system is believed compensating the pmf
lost by the reversed transmembrane pH gradient [51]. To unravel this adaptive
mechanism, different components of the respiratory system have been isolated and
characterized. Analysis of the cytochrome content has shown that it increases with
the cultivation pH [51, 161, 174]. This correlation possibly shows that the high pH
adaptation of alkaliphiles involves cytochromes. In fact, characterization of isolated
cytochrome c from alkaliphiles and neutralophiles revealed that the midpoint redox
potential of alkaliphiles cytochrome c is much lower (<+100 mV) than that of
neutralophiles (+220 mV) [161, 175]. However, the redox potential of cytochrome
c oxidase, the terminal oxidase that accepts electron from cytochrome c, is similar
between those of alkaliphiles and neutralophiles, +250 mV (cytochrome a)
[161, 176]. The high midpoint redox potential difference between the terminal
oxidase (cytochrome a) and cytochrome c drives the flow of H
+ and e
À faster across
the membrane of alkaliphiles. This can create a H
+ gradient close to the membrane
surface, especially the membrane part embedded with the respiratory system.
The H
+ gradient created by the respiratory complex activity is further enhanced
and maintained by another unique feature of alkaliphiles cytochrome c, high electron
retention capacity [177]. Studies made on soluble cytochrome c-552 of the Gramnegative facultative alkaliphilic Pseudomonas alcaliphila strain revealed that at
alkaline pH, it has high electron retention ability and serves as an electron reservoir
in the periplasmic space [174, 178]. A similar observation of electron retention is
made for cytochrome c-550 of Bacillus clarkii, an obligate alkaliphile. The retention
of electrons attracts H
+
, and this contributes to the formation of high membrane
Challenges and Adaptations of Life in Alkaline Habitats
109
in the TMH1. But, in alkaliphiles, this motif is substituted with 16AxAxAVA, which
possibly indicates its potential importance in high pH adaptation. In the second
conserved motif, the residue 51P is specific to alkaliphiles, and it is positioned close
to the ion-binding residue E54. Mutational, structural, and sequence analysis of the
c-subunit has shown features that may be recognized as high pH adaptations.
Mutation of all the alanine of the 16AxAxAVA motif to glycine led to ATP
synthesis activity loss by more than 80% [173]. P51 has been mutated to alanine
and glycine. The mutant P51A exhibited a dramatic loss of ATP synthesis and
non-fermentative growth at pH 10.5, whereas the P51G mutation did not affect the
ATP synthesis capacity although it had growth problem at high pH and exhibits H
+
leakage [173]. Studies made on the mutant E54G revealed that it leaks H
+ and a 90%
drop on non-fermentative growth. Although it needs further studies to expound how
these identified motifs contribute to the high pH adaptation exactly, the results
generated so far are valuable and indicate the involvement of the c-subunit in the
adaptation of ATP synthesis at high pH. However, it is imperative to expand the
study to other alkaliphilic genera as well to see the whole picture of the c-subunit
high pH adaptation role.
3.3.2 Cytochrome
The adaptation of alkaliphiles respiratory system is believed compensating the pmf
lost by the reversed transmembrane pH gradient [51]. To unravel this adaptive
mechanism, different components of the respiratory system have been isolated and
characterized. Analysis of the cytochrome content has shown that it increases with
the cultivation pH [51, 161, 174]. This correlation possibly shows that the high pH
adaptation of alkaliphiles involves cytochromes. In fact, characterization of isolated
cytochrome c from alkaliphiles and neutralophiles revealed that the midpoint redox
potential of alkaliphiles cytochrome c is much lower (<+100 mV) than that of
neutralophiles (+220 mV) [161, 175]. However, the redox potential of cytochrome
c oxidase, the terminal oxidase that accepts electron from cytochrome c, is similar
between those of alkaliphiles and neutralophiles, +250 mV (cytochrome a)
[161, 176]. The high midpoint redox potential difference between the terminal
oxidase (cytochrome a) and cytochrome c drives the flow of H
+ and e
À faster across
the membrane of alkaliphiles. This can create a H
+ gradient close to the membrane
surface, especially the membrane part embedded with the respiratory system.
The H
+ gradient created by the respiratory complex activity is further enhanced
and maintained by another unique feature of alkaliphiles cytochrome c, high electron
retention capacity [177]. Studies made on soluble cytochrome c-552 of the Gramnegative facultative alkaliphilic Pseudomonas alcaliphila strain revealed that at
alkaline pH, it has high electron retention ability and serves as an electron reservoir
in the periplasmic space [174, 178]. A similar observation of electron retention is
made for cytochrome c-550 of Bacillus clarkii, an obligate alkaliphile. The retention
of electrons attracts H
+
, and this contributes to the formation of high membrane
Challenges and Adaptations of Life in Alkaline Habitats
109