BIOCHEMISTRY - L. Stryer - 1984

VOLUME 2

PART II GENERATION AND STORAGE OF METABOLIC ENERGY

CHAPTER 14. OXIDATIVE PHOSPHORYLATION

14.18. Proton-Motive Force Transmission by Proton Gradients is a Central Motif of Bioenergetics

The core concept presented in this chapter is that Mitochondrial Electron Transport and ATP synthesis are coupled by a transmembrane proton gradient. ATP Synthesis in Bacteria and METABOLISM/14.html">Chloroplasts (sec. 19.13) is likewise driven by proton gradients. The proton gradient serves as the driving force for a multitude of energy-requiring processes, such as The Active Transport of Ca²⁺ by Cell/35.html">Mitochondria, the uptake of Certain Amino Acids and sugars by bacteria, the rotation of bacterial flagella, and the transfer of electrons from NАDН to NADPH. Proton gradients can also be utilized for heat generation, for instance, during hibernation. Clearly, proton gradients occupy a central place in the interconversion of different forms of energy (Fig. 14.17).

Class="center">Fig. 14.17. The proton gradient as an interconvertible form of Free energy

Why were proton gradients selected for this role during the course of evolution? Mitchell (1976) offers the following explanation:

Energy Transduction via proton currents is remarkably simple and efficient, since all it requires is a thin, topologically closed, insulated lipid membrane situated between two proton-conductive aqueous phases. A proton current generator applied across the membrane at any point acts as a source of proton-motive force that can be 'tapped' by a suitable consumer at any other point on the membrane, even when the producer and consumer are widely separated on a molecular scale. This chemiosmotic system of proton-motive force transmission in biology is well known to electrophysiologists studying animal Plasma Membranes, where proton-motive force is transmitted via activity gradients of Na⁺ and K⁺ ions (e.g., during Nerve Impulse propagation or intestinal nutrient absorption). However, owing to the relatively high reactivity and mobility of the proton, which enable it to function at very low concentrations, The Use of proton fluxes to transmit proton-motive force is unique in biology for its ubiquitous distribution and broad range of Applications... It is instructive to recall that these microminiature proton devices arose by chance and evolved to a stage of astounding perfection through natural Selection millions of years before the advancements of modern technology led to the invention of microminiature electronic devices. Elucidating the detailed mechanisms of natural proton systems may thus provide food for thought and insight not only for biochemists in their theoretical pursuits, but also for technologists in their inventions.