Gear Shifting in Biological Energy Transduction

Abstract

Confronted with thermodynamically adverse output processes, free-energy transducers may shift to lower gears, thereby reducing output per unit input. This option is well known for inanimate machines such as automobiles, but unappreciated in biology. The present study extends existing non-equilibrium thermodynamic principles to underpin biological gear shifting and identify possible mechanisms. It shows that gear shifting differs from altering the degree of coupling and that living systems may use it to optimize their performance: microbial growth is ultimately powered by the Gibbs energy of catabolism, which is partially transformed into Gibbs energy (‘output force’) in the ATP that is produced. If this output force is high, the cell may turn to a catabolic pathway with a lower ATP stoichiometry. Notwithstanding the reduced stoichiometry, the ATP synthesis flux may then actually increase as compared to that in a system without gear shift, in which growth might come to a halt. A ‘variomatic’ gear switching strategy should be optimal, explaining why organisms avail themselves of multiple catabolic pathways, as these enable them to shift gears when the growing gets tough.

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“…Yet, none of these three proposed solutions solve the thermodynamics problem: these catalytic mechanisms cannot increase the Gibbs energy difference between the organic matter and the Fe(II)/hematite couple. Perhaps the recently proposed gear shifting mechanism offers a solution to this predicament [ 205 ], with a transfer of two electrons coupled to the pumping of a single proton or with an increase of the H +→ /ATP stoichiometry of the proton translocating ATPase.…”

“…the Fe(II)/hematite couple. Perhaps the recently proposed gear shifting mechanism offers a solution to this predicament [205], with a transfer of two electrons coupled to the pumping of a single proton or with an increase of the H +→ /ATP stoichiometry of the proton translocating ATPase.…”