Description
The protonmotive force, βmFH+, is known as Ξp in Peter Mitchellβs chemiosmotic theory [1], which establishes the link between electric and chemical components of energy transformation and coupling in oxidative phosphorylation. The unifying concept of the protonmotive force ranks among the most fundamental theories in biology. As such, it provides the framework for developing a consistent theory and nomenclature for mitochondrial physiology and bioenergetics. The protonmotive force is not a vector force as defined in physics. This conflict is resolved by the generalized formulation of isomorphic, compartmental forces, βtrF, in energy (exergy) transformations [2]. Protonmotive means that there is a potential for the movement of protons, and force is a measure of the potential for motion.
The protonmotive force is generated in oxidative phosphorylation by oxidation of reduced fuel substrates and reduction of O2 to H2O, driving the coupled proton translocation from the mt-matrix space across the mitochondrial inner membrane (mtIM) through the proton pumps of the electron transfer system (ETS), which are known as respiratory Complexes CI, CIII and CIV. βmFH+ consists of two partial isomorphic forces: (1) The electric part, βelFH+ (corresponding to βΞ¨)Β§, is the electric potential differenceΒ§, which is not specific for H+ and can, therefore, be measured by the distribution of any permeable cation equilibrating between the negative (matrix) and positive (external) compartment. (2) The chemical part, βdFH+, relates to the diffusion (d) of uncharged particles and contains the chemical potential differenceΒ§ in H+, βΒ΅H+, which is proportional to the pH difference, βpH. Motion is relative and not absolute (Principle of Galilean Relativity); likewise there is no absolute potential, but isomorphic forces are stoichiometric potential differencesΒ§.
The total motive force (motive = electric + chemical) is distinguished from the partial components by subscript βmβ, βmFH+. Reading this symbol by starting with the proton, it can be seen as pmf, or the subscript m (motive) can be remembered by the name of Mitchell,
βmFH+ = βelFH+ + βdFH+
With classical symbols, this equation contains the Faraday constant, F, multiplied implicitly by the charge number of the proton (zH+ = 1), and has the form [1]
βp = βΞ¨ + βΒ΅H+βF-1
A partial electric force of 0.2 V in the electrical format, βelFeH+pos, is 19 kJβmol-1 H+pos in the molar format, βelFnH+pos. For 1 unit of βpH, the partial chemical force changes by -5.9 kJβmol-1 in the molar format, βdFnH+pos, and by 0.06 V in the electrical format, βdFeH+pos. Considering a driving force of -470 kJβmol-1 O2 for oxidation, the thermodynamic limit of the H+pos/O2 ratio is reached at a value of 470/19 = 24, compared to the mechanistic stoichiometry of 20 for the N-pathway with three coupling sites.
Abbreviation: βmFH+, pmf, Ξp, Ξpmt
Reference: Mitchell 2011 Biochim Biophys Acta
Communicated by Gnaiger E 2018-10-15
Template:Keywords Membrane potential
References
- Mitchell P (1966) Chemiosmotic coupling in oxidative and photosynthetic phosphorylation. Glynn Research, Bodmin. Biochim Biophys Acta Bioenergetics 1807:1507-38. - Β»Bioblast linkΒ«
- Gnaiger E (1993) Nonequilibrium thermodynamics of energy transformations. Pure Appl Chem 65:1983-2002. - Β»Bioblast linkΒ«
Footnote
- Β§ Superscript βΒ§β indicates throughout the text those terms, where potential differences provide a mathematically correct but physicochemically incomplete description and should be replaced by stoichiometric potential differences [2]. Appreciation of the fundamental distinction between differences of potential versus differences of stoichiometric potential may be considered a key to critically evaluate the definitions of the protonmotive force.
MitoPedia concepts: MiP concept, Ergodynamics