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

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Abstract
Assays of intracellular ATP, ADP and Pi allowed calculation of the phosphorylation potential (.DELTA.G''ATP/F) maintained during glycolysis by S. 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 370-510 mV (8.5-11.7 kcal/mol) for .DELTA.G''ATP/F and a range of 128-167 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 5 to 7.