Re-assessment of the Stoichiometry of Photophosphorylation

Abstract

THE light-induced phosphorylation of adenosine diphosphate to adenosine triphosphate by chloroplasts is associated with, and presumably coupled to, a transfer of electrons. In most of the systems investigated water has been the electron (or hydrogen) donor, and flavin mononucleotide, vitamin K₃, ferricyanide or triphosphopyridine nucleotide has been the electron (or hydrogen) acceptor. The early work was carried out on systems involving flavin mononucleotide or vitamin K₃ or both. Unfortunately the reduced forms of these substances are readily re-oxidized by molecular oxygen, and this re-oxidation of the electron acceptor by the oxidized electron donor resulted in a cyclic process in which the electron flux could not be measured. Consequently the stoichiometry of the reaction or reactions forming adenosine triphosphate could not be established. However, with the discovery that phosphorylation also accompanied the reduction of substances the reduced forms of which were not re-oxidized, this difficulty disappeared. Both Arnon and his associates¹ and Avron and Jagendorf² reported that one mole of adenosine triphosphate was formed for every two moles of electrons transferred in the reduction of ferricyanide or triphosphopyridine nucleotide. This stoichiometry has been widely quoted.