Photophosphorylation Associated with Photosystem II: IV. KINETIC ANALYSES OF PHOTOSYSTEM II CYCLIC PHOTOPHOSPHORYLATION ACTIVITY: EVIDENCE FOR TWO CYCLIC REACTIONS

Photosystem II-dependent cyclic photophosphorylation activity produced by addition of p-phenylenediamines to KCN-Hg-NH₂OH-inhibited chloroplasts is the product of two separate reactions when a proton/electron donor is the catalyst. The activity observed with an electron donor as catalyst consists of a single reaction. One of the cyclic reactions, evoked by low (≤40 micromolar) concentrations of a proton/electron donor is sensitive to dibromothymoquinone and to perturbation of membrane organization by sonication. The second reaction, requiring higher catalyst concentrations, is less sensitive to either dibromothymoquinone or membrane perturbation. These results indicate that at low concentrations, proton/electron or electron donor catalysts act to produce a photosystem II cyclic reaction which is dependent on membrane-bound electron carriers. High concentrations of proton/electron donors, on the other hand, can produce a phosphorylation reaction in which the catalyst itself is largely responsible for cyclic activity.