Photosystem II of green plants: topology of core pigments and redox cofactors as inferred from electrochromic difference spectra

ABSTRACT: Three electrochromic difference spectra induced by the deposition of (1) a negative charge on the primary quinone acceptor, QA, (2) a positive charge on (or near) Tyr161 of the D1 subunit (YZ), and (3) a positive charge on the manganese cluster were determined at room temperature in photosystem II (PSII) core particles from pea. They were deconvoluted into Gaussian components by Powell’s numerical optimization procedure. All three spectra were fitted by four components, which we assigned to the Qy absorption bands of two chlorophyll a molecules of the primary donor P, the accessory chlorophyll a, and the pheophytin a molecules on the D1 subunit. On the basis of the electrochromic properties of chlorins and our data, we suggest an arrangement of pigments and redox cofactors in PSII that differs from current structural models, which have been shaped like the reaction centers (RC) of purple bacteria. Our model is compatible with sequence data, with the spectroscopic and electrochemical properties of chlorophyll a and pheophytin a, and with the extremely positive redox potential of water oxidation. We conclude the following: (1) P is formed from two orthogonally oriented chlorophyll a molecules that peak at 681 and 677 nm. (2) The accessory chlorophyll a on D1 is oriented perpendicular to the membrane, with ring V pointing to QA. It is presumably attached to His118 of D1. (3) The mutual arrangement of pheophytin a on the D1 subunit and QA differs from that of their counterparts in bacterial RC. (4) Th