Bound Nucleotides and Conformational Changes in Photophosphorylation

Abstract

The mechanism by which energy-transducing membranes utilize energy from oxidation-reduction reactions to promote the synthesis of ATP remains a major unsolved problem in molecular biology. Hypotheses of energy coupling assume the existence of either a common high-energy intermediate or an energized state of the membrane linked, on the one hand, to electron transport reactions and, on the other, to the hydration-dehydration reactions of ATP formation. Investigations during the last decade have provided ample evidence that electron transport is coupled to the vectorial translocation of ions (particularly protons) across the membrane. The electrochemical gradient resulting from these ion activities, the so-called “protonmotive force” of the chemiosmotic hypothesis (Mitchell, 1966) may be indeed directly linked to the dehydration of ADP and P_i to form ATP (for a comprehensive review see Jagendorf, 1975). However, based on current knowledge, we are still unable to propose a molecular mechanism for energy transfer from an electrochemical gradient of protons across the membrane to ATP formation.