6 The Link Between Respiration and ATP Synthases
Despite the low bulk PMF generated in high pH environments, alkaliphilic Bacillus
species exclusively use proton-coupled F 1 F 0 -ATP synthases, contrary to some
anaerobic and fermentative bacteria which use Na
+
-coupled ATP synthases. Evidence showing that respiration supports maximal ATP synthesis at high pH, while
artificially imposed diffusion gradients fail to energize the ATP synthase, suggests
that the cytochrome complexes involved in the generation of bulk electrochemical
gradient during respiration play a crucial role in the oxidative phosphorylation in
alkaliphiles [39]. Models suggest that physical interactions between the terminal
oxidases of the respiration chain, such as cytochrome-caa 3 , and the ATP synthase
would allow for protons to be retained in the membrane instead of dissipating into
the extracellular environment [59]. The operon coding for the ATP synthase in
B. pseudofirmus OF4, the atp operon, contains an extra gene upstream of the
conventional genes coding for the ATP synthase complex and the putative chaperone atpI. This gene, atpZ, has several homologues in other Firmicutes and, together
with atpI, is hypothesized to form a divalent cation channel for the import of Mg
2+ .
Magnesium is required for the formation of the transition state between ADP and
ATP in the F 1 catalytic moiety of the ATP synthase and might contribute to charge
compensation during pH cytoplasmic homeostasis in alkaliphiles [60, 61]. In addition, structural studies have shown that proton-coupled synthases from alkaliphiles
contain specific amino acid compositions that allow ATP synthesis at high pH
values. For example, a lysine residue at position 180 of the alpha subunit of the
ATP synthase from Bacillus sp. TA2.A1 was shown to facilitate proton capture at
high pH [62]. Multiple alignment studies between the alpha subunits of alkaliphiles
and neutrophiles identified additional motifs that are distinct in alkaliphiles, in
particular two conserved methionines (M171 and M184) in the membrane portion
close to the proton pathway that affect growth on malate when replaced by the
neutrophilic counterparts [63]. The ATP synthase of B. pseudofirmus OF4 contains a
consensus alanine-rich motif AxAxAxA in the N-terminal helix of the c-subunit that
is conserved across alkaliphilic Bacilli and is suggested to play an important role in
the correct assembly of the c-subunit rotor, as indicated by the inability of mutants
with a glycine GxGxGxG substitution to produce ATP [64]. Another example of
how the respiration chain is linked with ATP production in alkaliphiles comes from
the cyanobacterium A. platensis, in which the F 0 F 1 -ATP synthase is co-localized
with the photosynthetic complex in thylakoid vesicles in the cytoplasm [65, 66].
Since the cytoplasmic pH of this cyanobacterium is actively maintained at neutral pH
through the activity of Na
+
/H
+ antiporters, the PMF across the thylakoid membranes
is much more akin to that found in neutral environments and thus favourable for
oxidative phosphorylation [67, 68].
Due to the “reverse” transmembrane pH characteristic of alkaline habitats, it was
expected that alkaliphiles would express sodium-coupled ATP synthases to take
advantage of the greater SMF generated during pH homeostasis. However, only
alkaliphilic anaerobes such as Clostridium paradoxum have been found to contain a
148
P. H. Lebre and D. A. Cowan
Despite the low bulk PMF generated in high pH environments, alkaliphilic Bacillus
species exclusively use proton-coupled F 1 F 0 -ATP synthases, contrary to some
anaerobic and fermentative bacteria which use Na
+
-coupled ATP synthases. Evidence showing that respiration supports maximal ATP synthesis at high pH, while
artificially imposed diffusion gradients fail to energize the ATP synthase, suggests
that the cytochrome complexes involved in the generation of bulk electrochemical
gradient during respiration play a crucial role in the oxidative phosphorylation in
alkaliphiles [39]. Models suggest that physical interactions between the terminal
oxidases of the respiration chain, such as cytochrome-caa 3 , and the ATP synthase
would allow for protons to be retained in the membrane instead of dissipating into
the extracellular environment [59]. The operon coding for the ATP synthase in
B. pseudofirmus OF4, the atp operon, contains an extra gene upstream of the
conventional genes coding for the ATP synthase complex and the putative chaperone atpI. This gene, atpZ, has several homologues in other Firmicutes and, together
with atpI, is hypothesized to form a divalent cation channel for the import of Mg
2+ .
Magnesium is required for the formation of the transition state between ADP and
ATP in the F 1 catalytic moiety of the ATP synthase and might contribute to charge
compensation during pH cytoplasmic homeostasis in alkaliphiles [60, 61]. In addition, structural studies have shown that proton-coupled synthases from alkaliphiles
contain specific amino acid compositions that allow ATP synthesis at high pH
values. For example, a lysine residue at position 180 of the alpha subunit of the
ATP synthase from Bacillus sp. TA2.A1 was shown to facilitate proton capture at
high pH [62]. Multiple alignment studies between the alpha subunits of alkaliphiles
and neutrophiles identified additional motifs that are distinct in alkaliphiles, in
particular two conserved methionines (M171 and M184) in the membrane portion
close to the proton pathway that affect growth on malate when replaced by the
neutrophilic counterparts [63]. The ATP synthase of B. pseudofirmus OF4 contains a
consensus alanine-rich motif AxAxAxA in the N-terminal helix of the c-subunit that
is conserved across alkaliphilic Bacilli and is suggested to play an important role in
the correct assembly of the c-subunit rotor, as indicated by the inability of mutants
with a glycine GxGxGxG substitution to produce ATP [64]. Another example of
how the respiration chain is linked with ATP production in alkaliphiles comes from
the cyanobacterium A. platensis, in which the F 0 F 1 -ATP synthase is co-localized
with the photosynthetic complex in thylakoid vesicles in the cytoplasm [65, 66].
Since the cytoplasmic pH of this cyanobacterium is actively maintained at neutral pH
through the activity of Na
+
/H
+ antiporters, the PMF across the thylakoid membranes
is much more akin to that found in neutral environments and thus favourable for
oxidative phosphorylation [67, 68].
Due to the “reverse” transmembrane pH characteristic of alkaline habitats, it was
expected that alkaliphiles would express sodium-coupled ATP synthases to take
advantage of the greater SMF generated during pH homeostasis. However, only
alkaliphilic anaerobes such as Clostridium paradoxum have been found to contain a
148
P. H. Lebre and D. A. Cowan