Lipid environmental modulation of activity of photosynthetic membrane proteins

In this chapter, studies on the influence of lipid environment on photosynthetic protein organization and functionality are reported. The study was carried out on two organisms representing different evolutionary levels: higher plants (spinach) and photosynthetic bacteria (Rhodobacter sphaeroides). In particular in the first part investigations on the influence of hyperosmotic stress on the lipid content in spinach at different plant organization levels (whole leaves, protoplasts, cells) and in R. sphaeroides (cells and sphaeroplasts) are reported. In the second part instead the chapter focuses on the effect of the lipid environment on PSII and bacterial reaction center (RC) activity. The main results can be summarized as follows:oHyperosmotic stress causes changes in the membrane lipid content for both spinach and R. sphaeroides; in particular a significant CL increment is observed for both species.oThe analysis of the lipids from PSII enriched membranes and from R. sphaeroides membranes is discussed; in particular CL structures are obtained by ESI-MS studies.oThe addition of exogenous phospholipids PG and CL better preserves PSII complexes from thermal inactivation, as indicated by Fluorescence Emission, RLS and OER measurements.oThe lipid environment has a deep impact on the quinone acceptor complex properties of R. sphaeroides RC. In particular, charge recombination kinetics show that specific phospholipids are responsible for the stabilization of the charge-separated state D+QB -.oEndogenous and exogenous negatively charged phospholipids (like PG, CL, PI, and PS) can increase the lifetime of the charge-separated state of a factor of 3-5 with respect to zwitterionic phospholipids, mainly destabilizing the QA - state.oThe slow component of the electron transfer reaction D+QA -QB -> D+QAQB - is slower in PG proteoliposomes than in detergent and PC vesicles, and hence charge compensation effects are deduced to be faster in PC than in PG environment