Relationship between phosphorylation potential and electrochemical H+ gradient during glycolysis in Streptococcus lactis.

Assays of intracellular ATP, ADP, and inorganic phosphate allowed calculation of the phosphorylation potential (delta G'ATP/F) maintained during glycolysis by Streptococcus lactis. At the same time, the electrochemical H+ gradient (delta mu-H+/F) was evaluated by distribution methods, using radioactive tetraphenylphosphonium bromide as a probe for the membrane potential and salicylic acid as a probe for the pH gradient. Detailed comparisons were made at pH 5, when the reaction mediated by the proton-translocating ATPase (BF0F1) was likely to have been poised near equilibrium; for those conditions, the ratio delta G'ATP/delta mu-H+ was used to estimate stoichiometry for BF0F1 during ATP hydrolysis. At an external pH of 5, in the presence or absence of valinomycin, this ratio was close to 3, over a range of 370 to 510 mV (8.5 to 11.7 kcal/mol) for delta G'ATP/F and a range of 128 to 167 mV for delta mu-H+/F. Other work suggested that delta G'ATP/delta mu-H+ increased from its minimum value of 3 to 4.3 as the external pH changed from pH 5 to 7.