Proton Motive Force and Phosphorylation Potential in Thylakoids

Abstract

The relationship between phosphorylation potential and the transthylakoid pH difference (ΔpH) was explored in class C pea chloroplasts under two conditions: (a) in the light, in a steady state following photophosphorylation, where ΔpH was varied with addition of nigericin, and (b) in the dark after a light period in the presence of dithioerythritol to promote ΔTP-hydrolase activity. In experiments of type (b), the phosphorylation potential was varied through the concentrations of adenylates. In both conditions there was a correlation between the phosphorylation potential and ΔpH. The slope of the relationship between the two in the light was taken as giving the product n.q of the mechanistic stoichiometry, n with q, the degree of coupling of the ATP-synthetase reaction with the proton flux. This product was 1.9 � 0.1, implying n=2 if q=0.95, or n=3 if q=0.66. The data from both types of experiment required a transthylakoid potential difference of up to + 155 mV for the phosphorylation potential to be poised against the proton motive force as in a macrochemiosmotic theory.