Characterisation of a PS II reaction centre isolated from the chloroplasts of Pisum sativum

Abstract

A photosystem II reaction centre has been isolated from peas and found to consist of D1, D2 polypeptides and the apoproteins of cytochrome b‐559, being similar to that reported for spinach by Nanba and Satoh [(1987) Proc. Natl. Acad. Sci. USA 84, 109–112]. The complex binds chlorophyll a, pheophytin and the haem of cytochrome b‐559 in an approximate ratio of 4:2:1 and also contains about one molecule of β‐carotene. It binds no plastoquinone‐9 or manganese but does contain at least one non‐haem iron. In addition to a light‐induced signal due to Pheo− seen under reducing conditions, a light‐induced P680+ signal is seen when the reaction centre is incubated with silicomolybdate. In the presence of diphenylcarbazide, the P680+ signal is partially inhibited and net electron flow to silicomolybdate occurs. This net electron flow is insensitive to o‐phenanthroline, 3‐(3,4‐dichlorophenyl)‐1,1‐dimethyl urea and 2‐(3‐chloro‐4‐trifluoromethyl)anilino‐3,5‐dinitrothiophene but is inhibited by proteolysis with trypsin and by other treatments. Fluorescence, from the complex, peaks at 682 nm at room temperature and at 685 nm at 77 K. This emission is significantly quenched when either the P680+Pheo or P680Pheo− states are established indicating that the fluorescence emanates from the back reaction between P680+ and Pheo−.

Is the 9 kDa thylakoid membrane phosphoprotein functionally and structurally analogous to the ‘H’ subunit of bacterial reaction centres?

“…The isolated reaction centre contains only three proteins; the Dl and D2 proteins and cytochrome b-559 [28]. This reaction centre preparation lacks plastoquinone and is unable to catalyse stabilised electron transfer unless supplemented with an artificial electron acceptor which can reoxidise the photoreduced pheophytin [29]; exogenous quinones are unable to support electron transport. Full photochemical activity can be obtained with less-pure 'core' PS II preparations which contain bound plastoquinone and the 9 kDa phosphoprotein [30].…”

Two sites of primary degradation of the D1‐protein induced by acceptor or donor side photo‐inhibition in photosystem II core complexes

“…It is now widely agreed that the in vivo rapid turnover [1,2] of the Dl-protein of the photosystem II (PHI) reaction centre [3,4] is a consequence of the unique and highly reactive redox chemistry of the light-driven reactions of water oxidation and plastoquinone reduction [5-71. The damaged Dl-protein has to be replaced by a new protein copy to re-establish PSI1 electron transfer activity.…”

Similarity of primary radical pair recombination in photosystem II and bacterial reaction centers

“…Only recently the preparation of stable photoactive RCs of PS II was achieved [1,2] which allow spectroscopic measurements with high resolution. These D,D,-preparations contain 4-6 chlorophylls a (Chl), 2 pheophytins a (Pheo), 1 p-carotene, but no quinones [3]. Electron transfer from the primary donor P680 to a Pheo was reported to occur in a few picoseconds even at low temperatures [4,5], although measurements with better time resolution point to a slightly slower charge separation preceded by energy transfer processes [6].…”