Photophosphorylation as Function of ADP Concentration at Varying Transmembrane Proton Gradients

R ates o f pho tophosphory lation were m easured at constant sa tu rating phosphate concen tra tion , varying A D P concen tra tion , and varying light intensity. As the transm em brane p ro ton grad ien t is d ecreased by phosphoryla tion to d ifferent extents depending on the concentra tion o f A D P . rates o f A T P form ation ob tained at the d ifferent A D P concentrations w ere p lo tted versus the actual steady sta te A pH (in the absence of Ai|>) during the course of the reaction . A pH was m onito red by the calib ra ted 9-am inoacridine fluorescence technique. In secondary plots phosphoryla tion as function o f A D P concentra tion at d ifferent constant A pH values w ere ob tained . T he results indicate M ichaelis-M enten kinetics. The true K m for A D P is virtually unaffected by A pH w hereas (a t A D P sa tu ration ) strongly depends on ApH. The results are discussed in the fram ew ork of a sim ple enzym e kinetic m odel which considers the in trathylakoidal p ro ton (at constant external pH ) as a th ird substra te for A T P form ation. The m odel is capable o f explaining the reported results as well as a variety of im portan t results from the literatu re.