Search

Recently we have introduced the use of choline / fatty acid derived compounds, in particular lauroylcholine chloride (LCC), to probe selectively Photosystem II (PS II) structure and function (Wydrzynski, T. and Huggins, B.J. (1983) in The Oxygen-Evolving System of Photosynthesis (Inoue, Y., Crofts, A.R., Govindjee, Murata, N., Renger, G. and Satoh, K., eds.), pp. 265–272, Academic Press Tokyo, Japan). In this paper we report an unusual condition in thylakoid membrane samples at relatively low amounts of LCC in which detectable O 2 evolution cannot be measured, yet electron flow through PS II is near normal without added electron donors. LCC does not appear to interfere with the O 2 yield measurements directly nor act as an electron donor itself after the Tris block. Under this condition, steady state and flash O 2 yield measurements show no O 2 release or uptake, while steady-state ferricyanide photoreduction and the variable component of the chlorophyll a fluorescence transient remains at more than 50% of the control. The photoreduction of the primary quinone acceptor, Q A , measured by microsecond range chlorophyll a fluorescence continues for a minimum of 200 single turnover excitation light flashes. Most importantly, the yield of the 35 |gms component of the chlorophyll a delayed fluorescence remains at approx. 65% of the control and oscillates with a normal period four over two cycles, indicating the normal cycling of the S-state transitions in PS II. Thus, it appears that PS II can operate normally without detectable O 2 evolution. The question remains as to whether water is still being photooxidized under this condition without the release of the dioxygen product, or whether there is another source of electrons. The results are interpreted in terms of the possible existence of an additional water binding component (termed ‘H’) in PS II and a concerted oxidation reaction mechanism for photosynthetic water splitting.

Publisher Summary This chapter discusses the use of lauroyl choline chloride (LCC) to probe photosystem II (PS II) function. LCC treatment is extremely mild, causing the loss of only a few percent of the chlorophyll from the membrane while removing a number of protein components. LCC treatment of photosynthetic membrane samples specifically perturbs PS II function inhibiting electron flow from water splitting site to Q reduction. LCC has some effect on Photosystem I (PS I) but does not inhibit electron flow through the intersystem chain and the PS I reaction center. LCC treatment does not disrupt membrane structure to a large degree and under certain conditions its effects can be reversed using polyethylene glycol. As LCC extracts out a number of protein components, LCC offers a useful approach to probe the structure function relationships in water splitting activity and PS II photochemistry. The absorption spectra of LCC treated thylakoid pellets show the same features as control thylakoids absorption spectra except for a 2nm blue shift in the red band.