PHOTOSYSTEM-II OF GREEN PLANTS - TOPOLOGY OF CORE PIGMENTS AND REDOX COFACTORS AS INFERRED FROM ELECTROCHROMIC DIFFERENCE SPECTRA

Three electrochromic difference spectra induced by the deposition of ( 1) a negative charge on the primary quinone acceptor, Q(A), (2) a posi tive charge on (or near) Tyr161 of the D1 subunit (Y-Z), and (3) a pos itive charge on the manganese cluster were determined at room temperat ure in photosystem II (PSII) core particles from pea. They were deconv oluted into Gaussian components by Powell's numerical optimization pro cedure. All three spectra were fitted by four components, which we ass igned to the Q,. absorption bands of two chlorophyll a molecules of th e primary donor P, the accessory chlorophyll a, and the pheophytin a m olecules on the D1 subunit. On the basis of the electrochromic propert ies of chlorins and our data, we suggest an arrangement of pigments an d redox cofactors in PSII that differs from current structural models, which have been shaped like the reaction centers (RC) of purple bacte ria. Our model is compatible with sequence data, with the spectroscopi c and electrochemical properties of chlorophyll a and pheophytin a, an d with the extremely positive redox potential of water oxidation. We c onclude 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 r ing V pointing to Q(A). It is presumably attached to His118 of D1. (3) The mutual arrangement of pheophytin a on the D1 subunit and Q(A) dif fers from that of their counterparts in bacterial RC. (4) The manganes e cluster is located out of the axis that is formed by Y-Z (Tyr161 of D1), P, and Y-D (Tyr161 of D2).