Na+-coupled ATP synthesis in Propionigenium modestum

Chemiosmotic systems in bioenergetics: H(+)-cycles and Na(+)-cycles

The development of membrane bioenergetic studies during the last 25 years has clearly demonstrated the validity of the Mitchellian chemiosmotic H+ cycle concept. The circulation of H+ ions was shown to couple respiration-dependent or light-dependent energy-releasing reactions to ATP formation and performance of other types of membrane-linked work in mitochondria, chloroplasts, some bacteria, tonoplasts, secretory granules and plant and fungal outer cell membranes. A concrete version of the direct chemiosmotic mechanism, in which H+ potential formation is a simple consequence of the chemistry of the energy-releasing reaction, is already proved for the photosynthetic reaction centre complexes. Recent progress in the studies on chemiosmotic systems has made it possible to extend the coupling-ion principle to an ion other than H+. It was found that, in certain bacteria, as well as in the outer membrane of the animal cell, Na+ effectively substitutes for H+ as the coupling ion (the chemiosmotic Na+ cycle). A precedent is set when the Na+ cycle appears to be the only mechanism of energy production in the bacterial cell. In the more typical case, however, the H+ and Na+ cycles coexist in one and the same membrane (bacteria) or in two different membranes of one and the same cell (animals). The sets of delta mu H+ and delta mu Na+ generators as well as delta mu H+ and delta mu Na+ consumers found in different types of biomembranes, are listed and discussed.

The electrochemical proton potential of Bacillus alcalophilus

Bacillus alcalophilus strain ATCC 27647 showed usual growth characteristics, when inoculated at pH 10.4. The cells entered the logarithmic phase at pH 10.3, and as growth continued, the pH dropped further to a value of 8.8 in the stationary phase. B. alcalophilus strain DSM 485 showed comparable growth only in the initial phase after the addition to fresh medium. The small initial growth period was succeeded by a long lag phase, where the pH continuously dropped. The cells resumed growth after the pH was about 10.0 and continued to grow accompanied by a further decrease of external pH. The bioenergetic parameters measured in the initial growth phase of the two strains at high pH (10.1-10.3) were nearly the same, i.e. delta pH = +97 to +110 mV, delta psi = -206 to -213 mV and delta microH+ = -109 to -103 mV. The inverted proton gradient of about 1.7-1.9 at high pH decreased, as the external pH dropped during growth. This led to an increase of the proton motive force (delta microH+), although the membrane potential (delta psi) also declined. The ATP/ADP ratio of strain DSM 485 was high (4.5-5.5) at fast growth during the initial and second growth period. The ratio declined to about 1.5 at the end of the lag phase. At the initial growth phase and at the end of the lag phase, the delta microH+ was, however, the same (approximately -106 mV) and considerably lower than in the middle of the second growth period (approximately -140 mV). Fast growth, therefore, correlates with a high ATP/ADP ratio but not necessarily with a high delta microH+. Addition of gramicidin or carbonylcyanide m-chlorophenylhydrazone stopped growth of B. alcalophilus strain DSM 485 at pH 10.3 or 9.5 and gramicidin immediately decreased the internal ATP/ADP ratio from 4.5 to 1.2 at pH 10.3.

The ATPase of Bacillus alcalophilus. Reconstitution of energy-transducing functions

The purified ATPase of Bacillus alcalophilus (F1F0) was reconstituted into proteoliposomes by gradual removal of the detergent Triton X-100 with Amberlite XAD-2. The reconstitution was apparently highly asymmetric with nearly 100% of the F1 portion of the ATPase becoming oriented to the outside. Similar to results obtained with the soluble enzyme, the membrane-bound ATPase required Mg2+ and methanol for maximum activity. With Ca2+ or Mg2+ without methanol, 25% and 1%, respectively, of the maximum activity were observed. The ATPase was unable to pump Na+ ions but catalyzed the translocation of protons into the reconstituted proteoliposomes. Optimum proton translocation required the presence of Mg2+, not Ca2+, as divalent metal ion. The proton pump was inhibited by dicyclohexylcarbodiimide, venturicidin and NaN3. On incubation of the reconstituted ATPase with [14C]dicyclohexylcarbodiimide, subunit c of the enzyme complex became specifically labeled. The proteoliposomes catalyzed the Mg2(+)-dependent incorporation of [32P]phosphate into ATP by ATP/[32P]phosphate exchange. This exchange was little affected by monensin, but was completely abolished by the uncoupler carbonyl cyanide m-chlorophenylhydrazone. Protons and not Na+ are thus the coupling ions of the ATPase of B. alcalophilus.

The ATPase of Bacillus alcalophilus. Purification and properties of the enzyme

The ATPase of Bacillus alcalophilus was extracted from the bacterial membranes with Triton X-100 and purified by hydroxyapatite column chromatography. SDS gel-electrophoresis of the purified protein indicated the typical subunit pattern of an F1F0 structure with five F1 subunits (alpha, beta, gamma, delta, epsilon) and three F0 subunits (a,b,c). The alpha and beta subunits were antigens for an antiserum against the corresponding subunits of the ATPase of Escherichia coli. Subunit c was extracted from the bacterial membranes with chloroform/methanol. Its amino acid composition was in the range of subunits c from other ATPases. Maximal ATPase activity was observed in the presence of 2-5 mM MgCl2, an ATP/Mg2+ ratio of 2:1 and 25% methanol. In the absence of methanol, only about 1% of the maximal activity was observed. The enzyme was also activated by Ca2+ (in the absence of methanol), reaching about 30% of the maximal activity. The dependence of initial velocity versus ATP of the Ca2(+)-activated but not of the Mg2+/methanol-activated enzyme indicted cooperativity with three strongly cooperative binding sites.