The phosphorylation site associated with the oxidation of exogenous donors of electrons to Photosystem I

1. The Photosystem I-mediated transfer of electrons from diaminodurene, diaminotoluene and reduced 2,6-dichlorophenolindophenol to methylviologen is optimal at pH 8–8.5, where phosphorylation is also maximal. In the presence of superoxide dismutase, the efficiency of phosphorylation rises from ? 0.1 at pH 6.5 to 0.6–0.7 at pH 8–8.5, regardless of the exogenous electron donor used.2. The apparent K_m (at pH 8.1) for diaminodurene is 6·10^?4 M and for diaminotoluene is 1.2·10^?3 M. The concentrations of diaminodurene and diaminotoluene required to saturate the electron transport processes are > 2 mM and > 5 mM, respectively. At these higher electron donor concentrations the rates of electron transport are markedly increased by phosphorylation (1.5-fold) or by uncoupling conditions (2-fold).3. Kinetic analysis of the transfer of electrons from reduced 2,6-dichlorophenolindophenol (DCIPH₂) to methylviologen indicates that two reactions with very different apparent K_m values for DCIPH₂ are involved. The rates of electron flux through both pathways are increased by phosphorylation or uncoupling conditions although only one of the pathways is coupled to ATP formation. No similar complications are observed when diaminodurene or diaminotoluene serves as the electron donor.4. In the diaminodurene → methylviologen reaction, ATP formation and that part of the electron transport dependent upon ATP formation are partially inhibited by the energy transfer inhibitor HgCl₂. This partial inhibition of ATP formation rises to about 50% at less than 1 atom of mercury per 20 molecules of chlorophyll, then does not further increase until very much higher levels of mercury are added.5. It is suggested that exogenous electron donors such as diaminodurene, diaminotoluene and DCIPH₂ can substitute for an endogenous electron carrier in donating electrons to cytochrome f via the mercury-sensitive coupling site (Site I) located on the main electron-transporting chain. If this is so, there would seem to be no reason for postulating yet another coupling site on a side branch of the electron transport chain in order to account for cyclic photophosphorylation.