H+/ATP ratio during ATP hydrolysis by mitochondria: modification of the chemiosmotic theory.
The stoichiometry of H+ ejection by mitochondria during hydrolysis of a small pulse of ATP (the H+/ATP ratio) has been reexamined in the light of our recent observation that the stoichiometry of H+ ejection during mitochondrial electron transport (the H+/site ratio) was previously underestimated. We show that earlier estimates of the H+/ATP ratio in intact mitochondria were based upon an invalid correction for scaler H+ production and describe a modified method for determination of this ratio which utilizes mersalyl or N-ethylmaleimide to prevent complicating transmembrane movements of phosphate and H+. This method gives a value for the H+/ATP ratio of 2.0 without the need for questionable corrections, compared with a value of 3.0 for the H+/site ratio also obtained by pulse methods. A modified version of the chemiosmotic theory is presented, in which 3 H+ are ejected per pair of electrons traversing each energy-conserving site of the respiratory chain. Of these, 2 H+ return to the matrix through the ATPase to form ATP from ADP and phosphate, and 1 H+ returns through the combined action of the phosphate and adenine nucleotide exchange carriers of the inner membrane to allow the energy-requiring influx of Pi and ADP3- and efflux of ATP4-. Thus, up to one-third of the energy input into synthesis of extramitochondrial ATP may be required for transport work. Since other methods suggest that the H+/site significantly exceeds 3.0, an alternative possibility is that 4 h+ are ejected per site, followed by return of 3 H+ through the ATPase and 1 H+ through the operation of the proton-coupled membrane transport systems.
D000227:Adenine Nucleotides / Q000378:metabolism; D000251:Adenosine Triphosphatases / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone / Q000494:pharmacology; D002414:Cations, Monovalent; D005033:Ethylmaleimide / Q000494:pharmacology; D006859:Hydrogen / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D008930:Mitochondria, Liver / Q000378:metabolism*; D008954:Models, Biological; D009994:Osmolar Concentration; D010085:Oxidative Phosphorylation / Q000187:drug effects; D010710:Phosphates / Q000378:metabolism; D051381:Rats
D000227:Adenine Nucleotides; D002414:Cations, Monovalent; D010710:Phosphates; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone; D006859:Hydrogen; D000255:Adenosine Triphosphate; D000251:Adenosine Triphosphatases; D005033:Ethylmaleimide