We define a molecular heat engine as a molecule that can convert heat into free energy when thermally cycled or when placed in a thermal gradient.
A detailed model for a molecular heat engine is constituted by the pF1 enzyme, a proposed progenitor of the homologous (1) alpha and beta sub units of the F1 moiety of the contemporary ATP synthase enzyme (2-4). In the thermosynthesis model for the origin of life, pF1 synthesizes peptide and phosphate bonds by creating a local environment in its enzymatic cavity that permits dehydration reactions. The product containing the bond has a higher free energy in water (5), and can therefore, because of energy conservation, not directly be released. Instead, it remains strongly bound, entering the medium only upon a thermal unfolding of pF1. The binding change mechanism as effected by contemporary ATP synthase (6) is identified as a relic of this so-defined thermosynthesis mechanism.
In the model the thermal cycling is attributed to - macroscopic - convection of the medium in which the pF1 is suspended.
Consider as example present day F1 ATP synthase as a heat engine. The presence of nucleotides increases its unfolding temperature by ~10°C. The delta H for the unfolding of the individual alpha and beta subunits is ~660 kJ/mole (7-8). At an unfolding temperature of 60°C (330°K) the Carnot ratio predicts an available work of
T / T) delta H = (10 / 330) 660 = 20 kJ/mole,
a value comparable with the free energy of a peptide and phosphate bond.
Using only one enzyme to begin with, thermosynthesis by a molecular heat engine allows a very simple model for the emergence of the bioenergetic chemiosmotic machinery and, more generally, for the origin of life (2-4).
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(7) Z.-Y. Wang et al. (1993) Influence of nucleotide binding site occupancy on the thermal stability of the F1 portion of the chloroplast ATPsynthase. J. Biol. Chem. 268, 20785-20790.
(8) J. Villaverde et al. (1998) Nucleotide and Mg2+ dependency of the thermal denaturation of mitrochondrial F1-ATPase. Biophys. J. 75, 1980-1988.