Does AMP participate in photosynthetic phosphorylation?

Chloride ion transport and its inhibition in thylakoid membranes

Cl- translocation across energized and nonenergized thylakoid membranes was found to be inhibited by piretanide, an inhibitor of active Cl- transport in fish intestinal epithelia. Piretanide has no effect on photophosphorylation catalyzed by phenazine methosulfate or on Ca2+-dependent ATPase activity of isolated chloroplast coupling factor (CF1). Light-dependent Cl- uptake, contrary to H+ uptake, is severalfold greater at pH 8.0 than at pH 6.7.

AMP photophosphorylation and binding by chloroplasts

Stoichiometric amounts of chloroplast thylakoids photophosphorylate free AMP to tightly bound ADP. Free ADP is a poor competitor for this AMP photoreaction, which saturates below 16 micronAMP. The inhibitor, diadenosine pentaphosphate, abolishes AMP photophosphorylation, and inhibits dark ADP binding. Taken together, these data imply that this photoreaction involves the high affinity nucleotide binding site(s) of chloroplast coupling factor CF1, and that little mixing with free nucleotides occurs.

Initial events of light-dependent ATP synthesis in spinach subchloroplast particles

The kinetics of 32Pi incorporation into adenine nucleotides by subchloroplast particles in the light is studied with a continuous flow apparatus allowing measurements between 3 and 200 ms. After a short lag time from 1 to 3 ms ATP synthesis proceeds with a constant rate. During the first few milliseconds a faster labelling of ADP is detected. This labelling of ADP reaches a constant level up to 1 molecule ADP labelled per molecule of coupling factor present. The labelling pattern in ATP indicates that the labelled ADP does not equilibrate with free ADP. The addition of 32Pi to a phosphorylating system during the light phase (32Pi pulse) exhibits unchanged kinetic characteristics for labelling of ATP and ADP. These results indicate a phosphorylation of AMP to ADP being an intermediate step in photophosphorylation. In experiments carried out in the dark no label is found in ATP within the time analysed. However the labelling of ADP occurs in the same way as in the light.