The critical electric potential difference for photophosphorylation. Its relation to the chemiosmotic hypothesis and to the triggering requirements of the ATPase system.

TLDR: It will be demonstrated that, at first sight, the critical electric potential difference can be interpreted from the equilibrium thermodynamic formulation of the chemiosmotic hypothesis, but an inspection of the basic assumption for this interpretation, the reversibility of the ATPase system, reveals inconsistency with the apparent irreversibbility of the enzyme system.

Abstract: The physical meaning of the observed critical electric potential difference necessary for photophosphorylation is discussed. An interpretation of this phenomenon on the basis of the equilibrium thermodynamic formulation of the chemiosmotic hypothesis requires a reversible ATPase system. Contrary to this, ATP hydrolysis experiments seem to confirm that the Mg++dependent ATPase system of class 11-chloroplasts is irreversible in the absence of SH-compounds. This sheds doubt on the equilibrium thermodynamic interpretation of the critical electric potential difference for phosphorylation. However it can be shown, that the seeming irreversibility may be due to some trigger requirements of an intrinsically reversible ATPase system. This leads to an identification of the critical electric potential difference with the triggering level of the enzyme system. Two operational models for an ATPase system, whose activity is modulated by the electric potential difference, are derived. These account quantitatively for a set of experiments on the critical electric potential difference. In the preceding paper it has been shown that a certain electric potential difference across the thylakoid membrane is a necessary condition for photophosphorylation in short flash groups. Moreover it has been demonstrated that three protons have to be translocated across their electrochemical potential difference for any molecule of ATP which is synthesized [I]. Both results favour strongly the chemiosmotic hypothesis of Mitchell [2], who has postulated that an electric potential difference provides part of the free energy for the formation of ADP N P bonds. However, the mechanism which gives rise to a critical level of the electric potential difference has still to be interpreted. An attempt has been made to see whether this phenomenon can be understood from the same role of the electric potential difference, as an energy source for phosphorylation. It will be demonstrated that, at first sight, the critical electric potential difference can be interpreted from the equilibrium thermodynamic formulation of the chemiosmotic hypothesis. However, an inspection of the basic assumption for this interpretation, the reversibility of the ATPase system, reveals inconsistency with the apparent irreversibbility of the enzyme system. The latter has been concluded from hydrolysis experiments on class 11chloroplasts. This apparent irreversibility deserves a