Properties of the proteinaceous component acting as apoenzyme for the functional plastoquinone redox groups on the acceptor side of system II

The electron-transfer reactions between the plastoquinone molecules of the acceptor side of photosystem II have been inferred to be regulated by a proteinaceous component (apoenzyme), which additionally contains the receptor site for DCMU-type inhibitors (Renger, G., (1976) Biochim. Biophys. Acta 440, 287–300). In order to reveal the functional properties of this apoenzyme, the effect of procedures which modify the structure of proteins on the photosystem II electron transport have been investigated in isolated spinach chloroplasts by comparative measurements of O₂ evolution and absorption changes at 334 nm induced by repetitive flash excitation and of fluorescence induction curves caused by continuous actinic light. It was found that: (1) The release of blockage of O₂ evolution by the DCMU-type inhibitor SN 58132 due to mild tryptic digestion correlates kinetically with the deterioration of the binding properties. (2) Glutaraldehyde fixation of chloroplasts does not markedly modify the reoxidation kinetics of the reduced primary plastoquinone acceptor component, X320^?, of photosystem II, but it greatly reduces the fluorescence yield of the antenna chlorophylls and slightly retards the ADRY effect. Furthermore, it prevents the attack of trypsin on the apoenzyme. (3) Incubation of chloroplasts in ‘low’ salt medium markedly diminishes the ability of trypsin to release the blockage of O₂ evolution by SN 58132 and completely presents the effect on inhibition by DCMU. Based on these results and taking into account recent findings of other groups, the functional mechanism of the electron transport on the acceptor side of photosystem II is discussed. Assuming a tunnel mechanism, the apoprotein is inferred to act as a dynamic regulator rather than changing only the relative levels of the redox potentials of the plastoquinone molecules involved in the transfer steps. It is further concluded that salt depletion does not only cause grana unstacking and a change of the excitation energy transfer probabilities, but it additionally modifies the orientation of functional membrane proteins of photosystem II and their structural interaction within the thylakoid membrane.