Anaerobes grow with small amounts of energy, and syntrophically cooperating anaerobes are extremely skilled in the exploitation of minimal energy spans. Synthesis of ATP under the conditions prevailing in an actively growing cell requires +49 kJ per mol (Thauer et al. 1977). Since part of the total energy is always lost in irreversible reaction steps as heat (on average about 20 kJ per mol ATP) a total of about 70 kJ per mol ATP synthesized has to be calculated for ATP synthesis in a living cell (Schink 1990). One may argue that (especially under conditions of energy limitation) an organism may waste less energy in heat production, or that it may operate at an energy charge considerably lower than that quoted for well-growing cells. Nonetheless, one cannot expect the energy requirement for irreversible ATP synthesis to go substantially below about +60 kJ per mol.
According to the Mitchell theory of respirative ATP synthesis, ATP formation is coupled to a vectorial transport of charged groups, typically protons, across a semipermeable membrane. If the ratio of proton translocation over ATP synthesized is 3, the smallest quantum of metabolically convertible energy, equivalent to the transport of a monovalent cation across the charged cy-toplasmic membrane, is equivalent to one-third of an ATP unit. This means that a bacterium needs a minimum of about -20 kJ per mol reaction to exploit a reaction's free energy change (Schink and Thauer 1988; Schink and Stams 2001).
On the basis of studies on the structure and function of F1-F0 ATPases in recent years, the stoichiometry of ATP synthesis versus proton translocation appears not to be as strictly fixed as assumed so far. Rather, the system may operate like a sliding clutch, meaning that at very low energy input, the energy transfer into ATP synthesis may be substoichiometric. Moreover, the stoichiometry is not necessarily three protons per one ATP, but is determined by the number of subunits arranged in the F0 versus the F1 complex. This concept would allow also stoichiometries of 4 to 1, perhaps even 5 to 1
Engelbrecht and Junge 1997; Cherepanov et al. 1999; Stock et al. 1999; Dimroth 2000; Seelert et al. 2000). As a consequence, the minimum energy increment that can still be used for ATP synthesis may be as low as -15 or -12 kJ per mol reaction. In some cases, to make their living, bacteria cooperating in syn-trophic fermentations are limited to this range of energy; Hoehler et al. (2001) calculated from metabolite concentrations in natural habitats for the partner bacteria cooperating in syntrophic conversions minimum amounts of exploitable energy in the range of -10 to -19 kJ per mol reaction.
The postulate that there is a minimum amount of approximately -12 to -15 kJ per reaction needed to drive ATP synthesis has been questioned recently in a paper where the remnant energy in starving syntrophically fermenting bacteria had been determined to be as low as -4 kJ per mol (Jackson and McInerney 2002). However, the authors showed only that the "battery" can burn down to such low values at low energy supply; they did not prove that the system can produce ATP under these conditions and thus they did not disprove the concept of a minimum amount of energy for ATP synthesis in the range discussed above.
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