INTERACTION OF PHOTOSYSTEM-II PROTEINS WITH NON-AGGREGATED MEMBRANES CONSTITUTED OF PHOSPHATIDYLGLYCEROL AND THE ELECTRICALLY NEUTRAL PHOSPHATIDYLCHOLINE ENHANCES THE OXYGEN-EVOLVING ACTIVITY

This work investigates the role of the relative content of phosphatidy lglycerol (PG) and the electrically neutral phosphatidylcholine (PC) o n the oxygen evolution (OE) in the photosystem II (PSII) complex in th e pH range 4.5-9.5. For this purpose, PSII particles were incorporated into non-aggregated membranes constituted of PC (PSII-PC) or PG (PSII -PG), and mixtures of PC and PG (PSII-PCPG). The major results are: (i ): In solutions at pH 6.5, the PSII-PCPG membranes having a low PC con tent, e.g. 0.3 mol PC/mol PG, display only a slight OE augmentation in relation to the oxygen evolution observed in PSII particles in the ab sence of lipids. This contrasts with the OE activity observed in PSII- PCPG Vesicles containing a high PC concentration, e.g. 1 or 3 mol PC/m ol PG, where the oxygen evolution increases to 132 or 143% of the OE r ate observed in PSII in the absence of lipids. Moreover, these OE leve ls are of the order of those observed previously in membranes prepared with the nonionic digalactosyldiacylglycerol (PSII-DGDG) [Fragata, M. , Menikh, A., Nenonene, E.K., 1994. Functional and structural aspects of the thylakoid lipids in oxygen evolution in photosystem II. Trends Photochem. Photobiol. 3, 201-210.]: (ii) In solutions at pH < 6.5 or p H > 6.5, the PSII-PCPG membranes containing a low PC content: e.g. 0.3 mol PC/mol PG, display systematically a significant decrease of oxyge n evolution to levels below those observed in PSII-PG; The cooperative effect of PC and PG on the enhancement of oxygen evolution is only ap parent in PSII-PCPG vesicles prepared with a high PC concentration, e. g. 1 or 3 mol PC/mol PG. In addition, the loss of oxygen-evolving acti vity observed in PSII particles in the absence of lipids is reversed i n PSII-PCPG membranes having a high PC content. That is, the OE activi ty increases from 18% at pH 4.5 or 13% at pH 9.5, to about 60% at eith er pH 4.5 or 9.5. An important outcome in this study is that in mixed- lipid membranes constituted of phosphatidylglycerol and the electrical ly neutral phosphatidylcholine, the hydrophobic effect or the formatio n of hydrogen bonds have a more preponderant role than the electrostat ic interactions in the enhancement of the oxygen evolution in the PSII complex. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.