Studies on the structural and functional organization of system II of photosynthesis. The use of trypsin as a structurally selective inhibitor at the outer surface of the thylakoid membrane.

The effect of trypsin on the photosynthetic electron transport has been investigated in the presence of various electron acceptors (benzyl viologen, p-benzo-quinone, K3[Fe(CN)6]) by measurements of flash-induced oxygen evolution and of the absorption changes at 334 nm, indicating the primary electron acceptor of System II, X 320, and at 515 nm, indicating via electrochromism the electrical potential gradient across the thylakoid membrane. It was found that the effect of trypsin is strongly dependent on the nature of the electron acceptor: (1) Oxygen evolution is completely inhibited in the presence of p-benzo-quinone, but remains nearly unaffected by K3[Fe(CN)]6. (2) The initial amplitude deltaAO of the 334 nm absorption change is insensitive to trypsin in the presence of K3[Fe(CN)6], but the absorption change is abolished if benzyl viologen is used as acceptor. (3) The initial amplitude deltaAO of the 515 nm absorption change decreases by trypsin down to 50% with K3[Fe(CN)6] and is completely suppressed with benzyl viologen. (4) In trypsinated chloroplasts, the above-mentioned activities appear to be rather insensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea, in contrast to normal chloroplasts. On the basis of these results it is inferred that the primary electron acceptor of System II, X 320, is covered by a proteinaceous component susceptible to tryptic digestion. In addition, it is postulated that this component acts as well as an allosteric protein responsible for the regulation of the electronic interaction between X 320 and the plastoquinone pool, as for the inhibitory effect of 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Various other possible effects caused by the proteinaceous shield and its modification by trypsin are discussed. The present results are in complete agreement with asymmetric membrane models postulating a zig-zag arrangement of the electron transport chain with the reducing side located towards the outer phase and the oxidizing side near the inner phase of the thylakoids.