Abstract
In Mitchell’s chemiosmotic theory, a proton (H+) motive force across the membrane (Δp), generated by the respiratory chain, drives F1Fo-ATPase for ATP production in various organisms. The bulk-base chemiosmotic theory cannot account for ATP production in alkaliphilic bacteria. However, alkaliphiles thrive in environments with a H+ concentrations that are one-thousandth (ca. pH 10) the concentration required by neutralophiles. This situation is similar to the production of electricity by hydroelectric turbines under conditions of very limited water. Alkaliphiles manage their metabolism via various strategies involving the cell wall structure, solute transport systems and molecular mechanisms on the outer surface membrane. Our experimental results indicate that efficient ATP production in alkaliphilic Bacillus spp. is attributable to a high membrane electrical potential (ΔΨ) generated for an attractive force for H+ on the outer surface membrane. In addition, the enhanced F1Fo-ATPase driving force per H+ is derived from the high ΔΨ. However, it is difficult to explain the reasons for high ΔΨ formation based on the respiratory rate. The Donnan effect (which is observed when charged particles that are unable to pass through a semipermeable membrane create an uneven electrical charge) likely contributes to the formation of the high ΔΨ because the intracellular negative ion capacities of alkaliphiles are much higher than those of neutralophiles. There are several variations in the adaptation to alkaline environments by bacteria. However, it could be difficult to utilize high ΔΨ in the low aeration condition due to the low activity of respiration. To explain the efficient ATP production occurring in H+-less and air-limited environments in alkaliphilic bacteria, we propose a cytochrome c-associated “H+ capacitor mechanism” as an alkaline adaptation strategy. As an outer surface protein, cytochrome c-550 from Bacillus clarkii possesses an extra Asn-rich segment between the region anchored to the membrane and the main body of the cytochrome c. This structure may contribute to the formation of the proton-binding network to transfer H+ at the outer surface membrane in obligate alkaliphiles. The H+ capacitor mechanism is further enhanced under low-aeration conditions in both alkaliphilic Bacillus spp. and the Gram-negative alkaliphile Pseudomonas alcaliphila.
Highlights
Bacteria that thrive under extreme environmental conditions, such as low or high temperatures and high or low pH, are called extremophiles
In the case of alkaliphilic Bacillus spp., a large is indispensable for adaptation at high pH. This high value compensates for the deficient pH and attracts H+ moieties that are translocated by the respiratory chain and affect the redox potential of the cytochrome c bound to the outer surface membrane
We have not determined the of alkaliphilic Pseudomonas spp., is not believed to greatly influence the cytochrome c in the periplasmic space
Summary
Bacteria that thrive under extreme environmental conditions, such as low or high temperatures and high or low pH, are called extremophiles. Alkaliphilic Bacillus spp. have been reported to produce acid to reduce the pH when the ambient pH is too high for metabolism (Horikoshi, 2006). This acid production can often be observed even in media lacking sugars. These bacteria create an alkaline environment when the ambient pH is too low for metabolism. We demonstrated the importance of a large in alkaliphiles by showing the contribution of to the retention of H+ in the vicinity of the outer surface of the membrane in the vertical direction and the contribution of efficient ATP production under conditions involving H+ scarcity (Yoshimune et al, 2010). We highlight “a high-potential H+ capacitor mechanism” based on the existence of membrane-bound or periplasmic cytochrome c in alkaliphiles
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