Native and artificial energy-conserving sites in cyclic photophosphorylation systems

The influence of the plastoquinone antagonist, dibromothymoquinone, on cyclic photophosphorylation systems with various cofactors was studied in isolated chloroplasts. Three different groups of cofactors emerged: Lipophilic compounds insensitive to dibromothymoquinone, lipophilic quinones and lipid insoluble compounds with low redox potentials which were sensitive to dibromothymoquinone and lipophilic quinones with high redox potentials which were only active if N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) was present. In the presence of TMPD all cyclic systems were insensitive to dibromothymoquinone. The TMPD effect on the systems is explained by a bypass of a rate-limiting step on the inside of the thylakoid membrane. It is concluded that cyclic photophosphorylation might be coupled to ATP formation either via the native energy-conserving site at plastoquinone or via an artificial energy-conserving site. An artificial energy-conserving site operates when the cofactor is a lipophilic compound, which carries hydrogens across the membrane and generates a proton gradient upon its oxidation inside the thylakoid, thus replacing native proton translocation.